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#81
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Another stall spin
I also have wondered how an experienced pilot can spin in from
low thermalling. It must be a full spin, with the resulting steep recovery dive, that causes a spin-in accident: a stall doesn't lose much height. Even the most spin-eager gliders I've flown (Dart 17 and Puchacz) always signalled an approaching stall in plenty of time to stop an unintended spin developing. I've never accidentally spun while thermalling at normal altitudes, not once in my several thousand hours of gliding, so I wonder why it would be more likely to happen low down. One explanation might be that pilots are very stressed when circling low, and simply don't fly as well as usual. Or maybe they are circling unusually tightly, perhaps in a small thermal. I suspect experienced pilots would not make these mistakes. So I think that John Cochran's comment of August 28th may be right. There may some unknown, unexpected risk when low-and- slow that catches pilots out, even the best ones. Maybe small, strong, bubbly thermals exist low down, that can perhaps suddenly stall one wing? That would produce an immediate, uncontrollable, violent roll, somewhat like a flick manoever, without any advance warning signals. Not technically a spin, but probably ending the same way: a steep dive with insufficient height. If true, that's a risk that no amount of pilot skill can prevent, except by adopting the sensible rules I was trained with: 1. never thermal below pattern altitude, and 2. always fly at approach speed below pattern altitude. I have had one personal experience that supports John's suggestion: after a normal thermal flight in the midwest, on a day with light winds, I was on a normal final approach with wings level, at normal approach speed (60kts). At about 100ft agl, without any warning, my starboard wing was pushed rapidly and smoothly upwards, and despite immediate full opposite control input, I was put into a steep bank, I'd estimate close to 45 degrees. The surge vanished as fast as it had arrived. After it stopped, I was able to level the wings, correct the heading, and made a normal landing further down the runway. It totally surprised me. I assume a narrow thermal bubble lifted off under the starboard wing just as I passed. I estimate the surge lasted about three seconds, so at 60kts it must have been about 300ft long. I would not have believed it, except that it happened to me. I'm sure it would have been much harder to cope with, if I had not been flying at approach speed. Perhaps if I'd been flying slowly, at a higher angle of attack, the surge might have stalled the starboard wing. This roll event was also seen by an experienced pilot observer on the ground, who said he was astonished to see it, and inquired about it after I landed. A roll upset like this has only happened to me once, so (thankfully) its clearly a very rare occurrence, and maneageable at approach speed. If such bubbles are baby thermals, they are probably only in small areas, miles apart, and short lived, so would also be rarely encountered. However a pilot who is attempting to thermal low is presumably intentionally over an area where baby thermals are forming, so may have a higher chance of encountering such an effect. At 17:08 28 August 2012, John Cochrane wrote: One point not reiterated yet here -- the atmosphere down low is very different from what you're used to at 2000 feet and above. It's easy to say "I haven't unintentionally stalled /spun in a thermal in a thousand hours. How much of a dope do you have to be?" A short list of what's different down low: The atmosphere is much more turbulent. Thermals, such as they are are much smaller. In this layer, many small punchy thermals will start. Many will die. The ones we use up higher consist of many little parcels of hot air that have coalesced. Most thermals are either short lived, or basically unworkable to a modern glider. You're in the boundary layer where wind is being affected by the ground, so there is wind-induced turbulence. Punches of strong lift/gust followed by sink when you make a half turn will be the norm. The ground picture will be totally different to the pilot. If you turn downwind at altitude, you don't notice that much. If you turn downwind at 300 feet, all of a sudden the ground will rush by and, this being a high stress moment, you may pull back. Just as the gust you turned in fades, or the thermal turns to sink. And when the canopy fills with trees going by at 70 mph, the urge to pull back will be really strong. You may push forward to recover at altitude, but it's really really hard to do with the ground coming up fast. So, just because you've never unintentionally spun at altitude does not mean your chances at 300 feet are the same. Not to raise a tired subject, but why we give out contest points for thermaling at 300 feet or below remains a puzzling question to me. John Cochrane |
#82
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Another stall spin
Not exactly the same but similar are wake vortices off the large birds when you follow them in for landing at the larger airports. ATC is supposed to space you and give you reason for it. Get in one of those and it will knock off your game .. as happened to an acquaintance of mine. Not unreasonable to think that hills/mtns, buildings, tree lines and obstacles present a similar phenomenon from surface winds.
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#83
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Another stall spin
At altitude thermals flow generally vertically relatively unrestricted. At birth, even on a perfectly flat surface, thermal air must transition from a flat, shallow disk shaped zone feeding in from 360 degrees, crashing in, upwards ,which introduces a rotational component. Add orographic features and wind effects to these forces occurring in such a short period of time, over a relatively small area, and the air is very chaotic indeed. At 00:08 02 September 2012, Andrew wrote: I also have wondered how an experienced pilot can spin in from low thermalling. It must be a full spin, with the resulting steep recovery dive, that causes a spin-in accident: a stall doesn't lose much height. Even the most spin-eager gliders I've flown (Dart 17 and Puchacz) always signalled an approaching stall in plenty of time to stop an unintended spin developing. I've never accidentally spun while thermalling at normal altitudes, not once in my several thousand hours of gliding, so I wonder why it would be more likely to happen low down. One explanation might be that pilots are very stressed when circling low, and simply don't fly as well as usual. Or maybe they are circling unusually tightly, perhaps in a small thermal. I suspect experienced pilots would not make these mistakes. So I think that John Cochran's comment of August 28th may be right. There may some unknown, unexpected risk when low- and- slow that catches pilots out, even the best ones. Maybe small, strong, bubbly thermals exist low down, that can perhaps suddenly stall one wing? That would produce an immediate, uncontrollable, violent roll, somewhat like a flick manoever, without any advance warning signals. Not technically a spin, but probably ending the same way: a steep dive with insufficient height. If true, that's a risk that no amount of pilot skill can prevent, except by adopting the sensible rules I was trained with: 1. never thermal below pattern altitude, and 2. always fly at approach speed below pattern altitude. I have had one personal experience that supports John's suggestion: after a normal thermal flight in the midwest, on a day with light winds, I was on a normal final approach with wings level, at normal approach speed (60kts). At about 100ft agl, without any warning, my starboard wing was pushed rapidly and smoothly upwards, and despite immediate full opposite control input, I was put into a steep bank, I'd estimate close to 45 degrees. The surge vanished as fast as it had arrived. After it stopped, I was able to level the wings, correct the heading, and made a normal landing further down the runway. It totally surprised me. I assume a narrow thermal bubble lifted off under the starboard wing just as I passed. I estimate the surge lasted about three seconds, so at 60kts it must have been about 300ft long. I would not have believed it, except that it happened to me. I'm sure it would have been much harder to cope with, if I had not been flying at approach speed. Perhaps if I'd been flying slowly, at a higher angle of attack, the surge might have stalled the starboard wing. This roll event was also seen by an experienced pilot observer on the ground, who said he was astonished to see it, and inquired about it after I landed. A roll upset like this has only happened to me once, so (thankfully) its clearly a very rare occurrence, and maneageable at approach speed. If such bubbles are baby thermals, they are probably only in small areas, miles apart, and short lived, so would also be rarely encountered. However a pilot who is attempting to thermal low is presumably intentionally over an area where baby thermals are forming, so may have a higher chance of encountering such an effect. At 17:08 28 August 2012, John Cochrane wrote: One point not reiterated yet here -- the atmosphere down low is very different from what you're used to at 2000 feet and above. It's easy to say "I haven't unintentionally stalled /spun in a thermal in a thousand hours. How much of a dope do you have to be?" A short list of what's different down low: The atmosphere is much more turbulent. Thermals, such as they are are much smaller. In this layer, many small punchy thermals will start. Many will die. The ones we use up higher consist of many little parcels of hot air that have coalesced. Most thermals are either short lived, or basically unworkable to a modern glider. You're in the boundary layer where wind is being affected by the ground, so there is wind-induced turbulence. Punches of strong lift/gust followed by sink when you make a half turn will be the norm. The ground picture will be totally different to the pilot. If you turn downwind at altitude, you don't notice that much. If you turn downwind at 300 feet, all of a sudden the ground will rush by and, this being a high stress moment, you may pull back. Just as the gust you turned in fades, or the thermal turns to sink. And when the canopy fills with trees going by at 70 mph, the urge to pull back will be really strong. You may push forward to recover at altitude, but it's really really hard to do with the ground coming up fast. So, just because you've never unintentionally spun at altitude does not mean your chances at 300 feet are the same. Not to raise a tired subject, but why we give out contest points for thermaling at 300 feet or below remains a puzzling question to me. John Cochrane |
#84
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Another stall spin
A little over 40 years ago an event occurred which stongly influenced my
thinking about spinning accidents. At the time I was employed as Deputy Chief Instructor at a large, full time gliding club in the UK. On the day in question I was flying the tow-plane. One of our club members, a fairly experienced pilot with about 400 hours and a Gold Badge (very experienced in 1972), was flying in a club glider (Ka6e). During his approach to land, he changed his mind about where on the airfield he wanted to finish and entered a turn at very low level over the middle of the airport. He spun off the turn and the glider was comprehensively destroyed. I was in the pattern at the time and landed alongside the wreckage within less than two minutes of the accident. The pilot had already extricated himself from the pile of firewood and was standing next to it, apparently uninjured. I jumped out of the tow-plane and walked up to him; what he said is stamped unforgettably on my brain. His first remark was rather flippant, "I'm sorry, I seem to have parked it rather untidily." The next remark was very revealing (remember this was only about 2 minutes after he had spun in), "I can't understand what happened, there must have been something wrong with the elevator, I kept pulling back on the stick but the nose wouldn't come up." Five minutes later he realised he had spun in. When he wrote out an accident report the next day, he stated that he had entered a spin at low altitude with insufficient height to recover and had apparently forgotten that he did not recognise the spin at the time. He had been trained, about 8 years earlier, in a regime which did include spinning and recovery in pre-solo training, but there was no requirement for anything like a BFR or annual check so long as he remained current. In all probability he had not seen a spin from inside the cockpit for a number of years. Little wonder that he did not recognise it instantly. When a glider starts to spin, its acceleration downwards is somewhat less than that of an object in free-fall and it stabilises after about one turn. That first turn takes about 4 seconds and breaks down thus (Figures measured on tests I conducted, mainly in a Puchacz): 1st second, height loss about 20 feet, pitch down 20 - 30 degrees (recovery, by just moving the stick forward will lose another 20 - 30 feet); 2nd second pitch down to about 40 degrees, height loss total about 50 feet (recovery by moving the stick forward and perhaps some opposite rudder will lose another 75 - 100 feet); 3rd second total height loss about 80 feet, pitch down about 50 degrees (recovery, opposite rudder and stick forward height loss an additional 150 - 200 feet); 4th second, pitch down about 60 degrees, height loss a bit more than 100 feet, spin now fully developed (recovery, full opposite rudder stick forward, height loss an additional 200 - 250 feet). If you spin at 300 feet and recognise and initiate recovery in 1 or 2 seconds you will get away with it, if it takes you 3 seconds you might just get lucky, more than 3 seconds and you're gonna crash - hard. I should add that my friend in the accident described above was probably somewhat below 100 feet when he spun, so he had even less time to save himself and only about 3 seconds from departure to impact. The FAA system, which does not require any spin training until training as an instructor, cannot be expected to produce pilots who will recognise an unintentional spin, purely from description, in only one or two seconds, so departures near the ground are highly likely to result in spin-ins. Even pilots trained under the UK system (which does include spinning and recovery practice as part of the pre and post solo training), can't be expected to recognise an unintentional spin that quickly if they haven't seen and practiced one for months or years. The only thing that will work is frequent practice and only instructors who are teaching spinning regularly are really likely to get enough. The pre-stall symptoms that warn of a stall, are normally readily apparent when tou are doing a deliberate stall and looking for them, they are not so obvious when the stall is not intended or expected, and attention is elsewhere (on centering on a thermal or sorting out where you are going to land, for example). Glider pilots often tend to fly very much by attitude with the ASI as a secondary reference, when you are very close to the ground quite small hill or even just a row of trees can make the nose look further down than it is. Below 500 feet AGL, glance at the ASI every 3 - 5 seconds. At 04:10 02 September 2012, John Sullivan wrote: At altitude thermals flow generally vertically relatively unrestricted. At birth, even on a perfectly flat surface, thermal air must transition from a flat, shallow disk shaped zone feeding in from 360 degrees, crashing in, upwards ,which introduces a rotational component. Add orographic features and wind effects to these forces occurring in such a short period of time, over a relatively small area, and the air is very chaotic indeed. At 00:08 02 September 2012, Andrew wrote: I also have wondered how an experienced pilot can spin in from low thermalling. It must be a full spin, with the resulting steep recovery dive, that causes a spin-in accident: a stall doesn't lose much height. Even the most spin-eager gliders I've flown (Dart 17 and Puchacz) always signalled an approaching stall in plenty of time to stop an unintended spin developing. I've never accidentally spun while thermalling at normal altitudes, not once in my several thousand hours of gliding, so I wonder why it would be more likely to happen low down. One explanation might be that pilots are very stressed when circling low, and simply don't fly as well as usual. Or maybe they are circling unusually tightly, perhaps in a small thermal. I suspect experienced pilots would not make these mistakes. So I think that John Cochran's comment of August 28th may be right. There may some unknown, unexpected risk when low- and- slow that catches pilots out, even the best ones. Maybe small, strong, bubbly thermals exist low down, that can perhaps suddenly stall one wing? That would produce an immediate, uncontrollable, violent roll, somewhat like a flick manoever, without any advance warning signals. Not technically a spin, but probably ending the same way: a steep dive with insufficient height. If true, that's a risk that no amount of pilot skill can prevent, except by adopting the sensible rules I was trained with: 1. never thermal below pattern altitude, and 2. always fly at approach speed below pattern altitude. I have had one personal experience that supports John's suggestion: after a normal thermal flight in the midwest, on a day with light winds, I was on a normal final approach with wings level, at normal approach speed (60kts). At about 100ft agl, without any warning, my starboard wing was pushed rapidly and smoothly upwards, and despite immediate full opposite control input, I was put into a steep bank, I'd estimate close to 45 degrees. The surge vanished as fast as it had arrived. After it stopped, I was able to level the wings, correct the heading, and made a normal landing further down the runway. It totally surprised me. I assume a narrow thermal bubble lifted off under the starboard wing just as I passed. I estimate the surge lasted about three seconds, so at 60kts it must have been about 300ft long. I would not have believed it, except that it happened to me. I'm sure it would have been much harder to cope with, if I had not been flying at approach speed. Perhaps if I'd been flying slowly, at a higher angle of attack, the surge might have stalled the starboard wing. This roll event was also seen by an experienced pilot observer on the ground, who said he was astonished to see it, and inquired about it after I landed. A roll upset like this has only happened to me once, so (thankfully) its clearly a very rare occurrence, and maneageable at approach speed. If such bubbles are baby thermals, they are probably only in small areas, miles apart, and short lived, so would also be rarely encountered. However a pilot who is attempting to thermal low is presumably intentionally over an area where baby thermals are forming, so may have a higher chance of encountering such an effect. At 17:08 28 August 2012, John Cochrane wrote: One point not reiterated yet here -- the atmosphere down low is very different from what you're used to at 2000 feet and above. It's easy to say "I haven't unintentionally stalled /spun in a thermal in a thousand hours. How much of a dope do you have to be?" A short list of what's different down low: The atmosphere is much more turbulent. Thermals, such as they are are much smaller. In this layer, many small punchy thermals will start. Many will die. The ones we use up higher consist of many little parcels of hot air that have coalesced. Most thermals are either short lived, or basically unworkable to a modern glider. You're in the boundary layer where wind is being affected by the ground, so there is wind-induced turbulence. Punches of strong lift/gust followed by sink when you make a half turn will be the norm. The ground picture will be totally different to the pilot. If you turn downwind at altitude, you don't notice that much. If you turn downwind at 300 feet, all of a sudden the ground will rush by and, this being a high stress moment, you may pull back. Just as the gust you turned in fades, or the thermal turns to sink. And when the canopy fills with trees going by at 70 mph, the urge to pull back will be really strong. You may push forward to recover at altitude, but it's really really hard to do with the ground coming up fast. So, just because you've never unintentionally spun at altitude does not mean your chances at 300 feet are the same. Not to raise a tired subject, but why we give out contest points for thermaling at 300 feet or below remains a puzzling question to me. John Cochrane |
#85
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Another stall spin
I remember a sign on a flying office wall years ago about the three most
important rules of flying: Rule # 1 Maintain proper air speed! Rule # 2 Maintain proper air speed!! Rule # 3 Maintain proper air speed!!! At 09:45 02 September 2012, Chris Rollings wrote: A little over 40 years ago an event occurred which stongly influenced m thinking about spinning accidents. At the time I was employed as Deput Chief Instructor at a large, full time gliding club in the UK. On the da in question I was flying the tow-plane. One of our club members, a fairl experienced pilot with about 400 hours and a Gold Badge (very experience in 1972), was flying in a club glider (Ka6e). During his approach to land he changed his mind about where on the airfield he wanted to finish an entered a turn at very low level over the middle of the airport. He spu off the turn and the glider was comprehensively destroyed. I was in th pattern at the time and landed alongside the wreckage within less than tw minutes of the accident. The pilot had already extricated himself from the pile of firewood and wa standing next to it, apparently uninjured. I jumped out of the tow-plan and walked up to him; what he said is stamped unforgettably on my brain. His first remark was rather flippant, "I'm sorry, I seem to have parked i rather untidily." The next remark was very revealing (remember this wa only about 2 minutes after he had spun in), "I can't understand wha happened, there must have been something wrong with the elevator, I kep pulling back on the stick but the nose wouldn't come up." Five minutes later he realised he had spun in. When he wrote out a accident report the next day, he stated that he had entered a spin at lo altitude with insufficient height to recover and had apparently forgotte that he did not recognise the spin at the time. He had been trained, about 8 years earlier, in a regime which did includ spinning and recovery in pre-solo training, but there was no requiremen for anything like a BFR or annual check so long as he remained current. I all probability he had not seen a spin from inside the cockpit for a numbe of years. Little wonder that he did not recognise it instantly. When a glider starts to spin, its acceleration downwards is somewhat les than that of an object in free-fall and it stabilises after about one turn That first turn takes about 4 seconds and breaks down thus (Figure measured on tests I conducted, mainly in a Puchacz): 1st second, heigh loss about 20 feet, pitch down 20 - 30 degrees (recovery, by just movin the stick forward will lose another 20 - 30 feet); 2nd second pitch down t about 40 degrees, height loss total about 50 feet (recovery by moving th stick forward and perhaps some opposite rudder will lose another 75 - 10 feet); 3rd second total height loss about 80 feet, pitch down about 5 degrees (recovery, opposite rudder and stick forward height loss a additional 150 - 200 feet); 4th second, pitch down about 60 degrees, heigh loss a bit more than 100 feet, spin now fully developed (recovery, ful opposite rudder stick forward, height loss an additional 200 - 250 feet). If you spin at 300 feet and recognise and initiate recovery in 1 or seconds you will get away with it, if it takes you 3 seconds you might jus get lucky, more than 3 seconds and you're gonna crash - hard. I should ad that my friend in the accident described above was probably somewhat belo 100 feet when he spun, so he had even less time to save himself and onl about 3 seconds from departure to impact. The FAA system, which does not require any spin training until training a an instructor, cannot be expected to produce pilots who will recognise a unintentional spin, purely from description, in only one or two seconds, s departures near the ground are highly likely to result in spin-ins. Eve pilots trained under the UK system (which does include spinning an recovery practice as part of the pre and post solo training), can't b expected to recognise an unintentional spin that quickly if they haven' seen and practiced one for months or years. The only thing that will wor is frequent practice and only instructors who are teaching spinnin regularly are really likely to get enough. The pre-stall symptoms that warn of a stall, are normally readily apparen when tou are doing a deliberate stall and looking for them, they are not s obvious when the stall is not intended or expected, and attention i elsewhere (on centering on a thermal or sorting out where you are going t land, for example). Glider pilots often tend to fly very much by attitude with the ASI as secondary reference, when you are very close to the ground quite small hil or even just a row of trees can make the nose look further down than it is Below 500 feet AGL, glance at the ASI every 3 - 5 seconds. At 04:10 02 September 2012, John Sullivan wrote: At altitude thermals flow generally vertically relatively unrestricted. At birth, even on a perfectly flat surface, thermal air must transition from a flat, shallow disk shaped zone feeding in from 360 degrees, crashing in, upwards ,which introduces a rotational component. Add orographic features and wind effects to these forces occurring in such a short period of time, over a relatively small area, and the air is very chaotic indeed. At 00:08 02 September 2012, Andrew wrote: I also have wondered how an experienced pilot can spin in from low thermalling. It must be a full spin, with the resulting steep recovery dive, that causes a spin-in accident: a stall doesn't lose much height. Even the most spin-eager gliders I've flown (Dart 17 and Puchacz) always signalled an approaching stall in plenty of time to stop an unintended spin developing. I've never accidentally spun while thermalling at normal altitudes, not once in my several thousand hours of gliding, so I wonder why it would be more likely to happen low down. One explanation might be that pilots are very stressed when circling low, and simply don't fly as well as usual. Or maybe they are circling unusually tightly, perhaps in a small thermal. I suspect experienced pilots would not make these mistakes. So I think that John Cochran's comment of August 28th may be right. There may some unknown, unexpected risk when low- and- slow that catches pilots out, even the best ones. Maybe small, strong, bubbly thermals exist low down, that can perhaps suddenly stall one wing? That would produce an immediate, uncontrollable, violent roll, somewhat like a flick manoever, without any advance warning signals. Not technically a spin, but probably ending the same way: a steep dive with insufficient height. If true, that's a risk that no amount of pilot skill can prevent, except by adopting the sensible rules I was trained with: 1. never thermal below pattern altitude, and 2. always fly at approach speed below pattern altitude. I have had one personal experience that supports John's suggestion: after a normal thermal flight in the midwest, on a day with light winds, I was on a normal final approach with wings level, at normal approach speed (60kts). At about 100ft agl, without any warning, my starboard wing was pushed rapidly and smoothly upwards, and despite immediate full opposite control input, I was put into a steep bank, I'd estimate close to 45 degrees. The surge vanished as fast as it had arrived. After it stopped, I was able to level the wings, correct the heading, and made a normal landing further down the runway. It totally surprised me. I assume a narrow thermal bubble lifted off under the starboard wing just as I passed. I estimate the surge lasted about three seconds, so at 60kts it must have been about 300ft long. I would not have believed it, except that it happened to me. I'm sure it would have been much harder to cope with, if I had not been flying at approach speed. Perhaps if I'd been flying slowly, at a higher angle of attack, the surge might have stalled the starboard wing. This roll event was also seen by an experienced pilot observer on the ground, who said he was astonished to see it, and inquired about it after I landed. A roll upset like this has only happened to me once, so (thankfully) its clearly a very rare occurrence, and maneageable at approach speed. If such bubbles are baby thermals, they are probably only in small areas, miles apart, and short lived, so would also be rarely encountered. However a pilot who is attempting to thermal low is presumably intentionally over an area where baby thermals are forming, so may have a higher chance of encountering such an effect. At 17:08 28 August 2012, John Cochrane wrote: One point not reiterated yet here -- the atmosphere down low is very different from what you're used to at 2000 feet and above. It's easy to say "I haven't unintentionally stalled /spun in a thermal in a thousand hours. How much of a dope do you have to be?" A short list of what's different down low: The atmosphere is much more turbulent. Thermals, such as they are are much smaller. In this layer, many small punchy thermals will start. Many will die. The ones we use up higher consist of many little parcels of hot air that have coalesced. Most thermals are either short lived, or basically unworkable to a modern glider. You're in the boundary layer where wind is being affected by the ground, so there is wind-induced turbulence. Punches of strong lift/gust followed by sink when you make a half turn will be the norm. The ground picture will be totally different to the pilot. If you turn downwind at altitude, you don't notice that much. If you turn downwind at 300 feet, all of a sudden the ground will rush by and, this being a high stress moment, you may pull back. Just as the gust you turned in fades, or the thermal turns to sink. And when the canopy fills with trees going by at 70 mph, the urge to pull back will be really strong. You may push forward to recover at altitude, but it's really really hard to do with the ground coming up fast. So, just because you've never unintentionally spun at altitude does not mean your chances at 300 feet are the same. Not to raise a tired subject, but why we give out contest points for thermaling at 300 feet or below remains a puzzling question to me. John Cochrane |
#86
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Another stall spin
Anyone any idea why the system clipped the last letter off every line in my
last post? At 09:45 02 September 2012, Chris Rollings wrote: A little over 40 years ago an event occurred which stongly influenced m thinking about spinning accidents. At the time I was employed as Deput Chief Instructor at a large, full time gliding club in the UK. On the da in question I was flying the tow-plane. One of our club members, a fairl experienced pilot with about 400 hours and a Gold Badge (very experience in 1972), was flying in a club glider (Ka6e). During his approach to land he changed his mind about where on the airfield he wanted to finish an entered a turn at very low level over the middle of the airport. He spu off the turn and the glider was comprehensively destroyed. I was in th pattern at the time and landed alongside the wreckage within less than tw minutes of the accident. The pilot had already extricated himself from the pile of firewood and wa standing next to it, apparently uninjured. I jumped out of the tow-plan and walked up to him; what he said is stamped unforgettably on my brain. His first remark was rather flippant, "I'm sorry, I seem to have parked i rather untidily." The next remark was very revealing (remember this wa only about 2 minutes after he had spun in), "I can't understand wha happened, there must have been something wrong with the elevator, I kep pulling back on the stick but the nose wouldn't come up." Five minutes later he realised he had spun in. When he wrote out a accident report the next day, he stated that he had entered a spin at lo altitude with insufficient height to recover and had apparently forgotte that he did not recognise the spin at the time. He had been trained, about 8 years earlier, in a regime which did includ spinning and recovery in pre-solo training, but there was no requiremen for anything like a BFR or annual check so long as he remained current. I all probability he had not seen a spin from inside the cockpit for a numbe of years. Little wonder that he did not recognise it instantly. When a glider starts to spin, its acceleration downwards is somewhat les than that of an object in free-fall and it stabilises after about one turn That first turn takes about 4 seconds and breaks down thus (Figure measured on tests I conducted, mainly in a Puchacz): 1st second, heigh loss about 20 feet, pitch down 20 - 30 degrees (recovery, by just movin the stick forward will lose another 20 - 30 feet); 2nd second pitch down t about 40 degrees, height loss total about 50 feet (recovery by moving th stick forward and perhaps some opposite rudder will lose another 75 - 10 feet); 3rd second total height loss about 80 feet, pitch down about 5 degrees (recovery, opposite rudder and stick forward height loss a additional 150 - 200 feet); 4th second, pitch down about 60 degrees, heigh loss a bit more than 100 feet, spin now fully developed (recovery, ful opposite rudder stick forward, height loss an additional 200 - 250 feet). If you spin at 300 feet and recognise and initiate recovery in 1 or seconds you will get away with it, if it takes you 3 seconds you might jus get lucky, more than 3 seconds and you're gonna crash - hard. I should ad that my friend in the accident described above was probably somewhat belo 100 feet when he spun, so he had even less time to save himself and onl about 3 seconds from departure to impact. The FAA system, which does not require any spin training until training a an instructor, cannot be expected to produce pilots who will recognise a unintentional spin, purely from description, in only one or two seconds, s departures near the ground are highly likely to result in spin-ins. Eve pilots trained under the UK system (which does include spinning an recovery practice as part of the pre and post solo training), can't b expected to recognise an unintentional spin that quickly if they haven' seen and practiced one for months or years. The only thing that will wor is frequent practice and only instructors who are teaching spinnin regularly are really likely to get enough. The pre-stall symptoms that warn of a stall, are normally readily apparen when tou are doing a deliberate stall and looking for them, they are not s obvious when the stall is not intended or expected, and attention i elsewhere (on centering on a thermal or sorting out where you are going t land, for example). Glider pilots often tend to fly very much by attitude with the ASI as secondary reference, when you are very close to the ground quite small hil or even just a row of trees can make the nose look further down than it is Below 500 feet AGL, glance at the ASI every 3 - 5 seconds. At 04:10 02 September 2012, John Sullivan wrote: At altitude thermals flow generally vertically relatively unrestricted. At birth, even on a perfectly flat surface, thermal air must transition from a flat, shallow disk shaped zone feeding in from 360 degrees, crashing in, upwards ,which introduces a rotational component. Add orographic features and wind effects to these forces occurring in such a short period of time, over a relatively small area, and the air is very chaotic indeed. At 00:08 02 September 2012, Andrew wrote: I also have wondered how an experienced pilot can spin in from low thermalling. It must be a full spin, with the resulting steep recovery dive, that causes a spin-in accident: a stall doesn't lose much height. Even the most spin-eager gliders I've flown (Dart 17 and Puchacz) always signalled an approaching stall in plenty of time to stop an unintended spin developing. I've never accidentally spun while thermalling at normal altitudes, not once in my several thousand hours of gliding, so I wonder why it would be more likely to happen low down. One explanation might be that pilots are very stressed when circling low, and simply don't fly as well as usual. Or maybe they are circling unusually tightly, perhaps in a small thermal. I suspect experienced pilots would not make these mistakes. So I think that John Cochran's comment of August 28th may be right. There may some unknown, unexpected risk when low- and- slow that catches pilots out, even the best ones. Maybe small, strong, bubbly thermals exist low down, that can perhaps suddenly stall one wing? That would produce an immediate, uncontrollable, violent roll, somewhat like a flick manoever, without any advance warning signals. Not technically a spin, but probably ending the same way: a steep dive with insufficient height. If true, that's a risk that no amount of pilot skill can prevent, except by adopting the sensible rules I was trained with: 1. never thermal below pattern altitude, and 2. always fly at approach speed below pattern altitude. I have had one personal experience that supports John's suggestion: after a normal thermal flight in the midwest, on a day with light winds, I was on a normal final approach with wings level, at normal approach speed (60kts). At about 100ft agl, without any warning, my starboard wing was pushed rapidly and smoothly upwards, and despite immediate full opposite control input, I was put into a steep bank, I'd estimate close to 45 degrees. The surge vanished as fast as it had arrived. After it stopped, I was able to level the wings, correct the heading, and made a normal landing further down the runway. It totally surprised me. I assume a narrow thermal bubble lifted off under the starboard wing just as I passed. I estimate the surge lasted about three seconds, so at 60kts it must have been about 300ft long. I would not have believed it, except that it happened to me. I'm sure it would have been much harder to cope with, if I had not been flying at approach speed. Perhaps if I'd been flying slowly, at a higher angle of attack, the surge might have stalled the starboard wing. This roll event was also seen by an experienced pilot observer on the ground, who said he was astonished to see it, and inquired about it after I landed. A roll upset like this has only happened to me once, so (thankfully) its clearly a very rare occurrence, and maneageable at approach speed. If such bubbles are baby thermals, they are probably only in small areas, miles apart, and short lived, so would also be rarely encountered. However a pilot who is attempting to thermal low is presumably intentionally over an area where baby thermals are forming, so may have a higher chance of encountering such an effect. At 17:08 28 August 2012, John Cochrane wrote: One point not reiterated yet here -- the atmosphere down low is very different from what you're used to at 2000 feet and above. It's easy to say "I haven't unintentionally stalled /spun in a thermal in a thousand hours. How much of a dope do you have to be?" A short list of what's different down low: The atmosphere is much more turbulent. Thermals, such as they are are much smaller. In this layer, many small punchy thermals will start. Many will die. The ones we use up higher consist of many little parcels of hot air that have coalesced. Most thermals are either short lived, or basically unworkable to a modern glider. You're in the boundary layer where wind is being affected by the ground, so there is wind-induced turbulence. Punches of strong lift/gust followed by sink when you make a half turn will be the norm. The ground picture will be totally different to the pilot. If you turn downwind at altitude, you don't notice that much. If you turn downwind at 300 feet, all of a sudden the ground will rush by and, this being a high stress moment, you may pull back. Just as the gust you turned in fades, or the thermal turns to sink. And when the canopy fills with trees going by at 70 mph, the urge to pull back will be really strong. You may push forward to recover at altitude, but it's really really hard to do with the ground coming up fast. So, just because you've never unintentionally spun at altitude does not mean your chances at 300 feet are the same. Not to raise a tired subject, but why we give out contest points for thermaling at 300 feet or below remains a puzzling question to me. John Cochrane |
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Another stall spin
At 13:35 02 September 2012, Chris Rollings wrote:
Anyone any idea why the system clipped the last letter off every line in my last post? Wash out? At 09:45 02 September 2012, Chris Rollings wrote: A little over 40 years ago an event occurred which stongly influenced m thinking about spinning accidents. At the time I was employed as Deput Chief Instructor at a large, full time gliding club in the UK. On the da in question I was flying the tow-plane. One of our club members, a fairl experienced pilot with about 400 hours and a Gold Badge (very experience in 1972), was flying in a club glider (Ka6e). During his approach to land he changed his mind about where on the airfield he wanted to finish an entered a turn at very low level over the middle of the airport. He spu off the turn and the glider was comprehensively destroyed. I was in th pattern at the time and landed alongside the wreckage within less than tw minutes of the accident. The pilot had already extricated himself from the pile of firewood and wa standing next to it, apparently uninjured. I jumped out of the tow-plan and walked up to him; what he said is stamped unforgettably on my brain. His first remark was rather flippant, "I'm sorry, I seem to have parked i rather untidily." The next remark was very revealing (remember this wa only about 2 minutes after he had spun in), "I can't understand wha happened, there must have been something wrong with the elevator, I kep pulling back on the stick but the nose wouldn't come up." Five minutes later he realised he had spun in. When he wrote out a accident report the next day, he stated that he had entered a spin at lo altitude with insufficient height to recover and had apparently forgotte that he did not recognise the spin at the time. He had been trained, about 8 years earlier, in a regime which did includ spinning and recovery in pre-solo training, but there was no requiremen for anything like a BFR or annual check so long as he remained current. I all probability he had not seen a spin from inside the cockpit for a numbe of years. Little wonder that he did not recognise it instantly. When a glider starts to spin, its acceleration downwards is somewhat les than that of an object in free-fall and it stabilises after about one turn That first turn takes about 4 seconds and breaks down thus (Figure measured on tests I conducted, mainly in a Puchacz): 1st second, heigh loss about 20 feet, pitch down 20 - 30 degrees (recovery, by just movin the stick forward will lose another 20 - 30 feet); 2nd second pitch down t about 40 degrees, height loss total about 50 feet (recovery by moving th stick forward and perhaps some opposite rudder will lose another 75 - 10 feet); 3rd second total height loss about 80 feet, pitch down about 5 degrees (recovery, opposite rudder and stick forward height loss a additional 150 - 200 feet); 4th second, pitch down about 60 degrees, heigh loss a bit more than 100 feet, spin now fully developed (recovery, ful opposite rudder stick forward, height loss an additional 200 - 250 feet). If you spin at 300 feet and recognise and initiate recovery in 1 or seconds you will get away with it, if it takes you 3 seconds you might jus get lucky, more than 3 seconds and you're gonna crash - hard. I should ad that my friend in the accident described above was probably somewhat belo 100 feet when he spun, so he had even less time to save himself and onl about 3 seconds from departure to impact. The FAA system, which does not require any spin training until training a an instructor, cannot be expected to produce pilots who will recognise a unintentional spin, purely from description, in only one or two seconds, s departures near the ground are highly likely to result in spin-ins. Eve pilots trained under the UK system (which does include spinning an recovery practice as part of the pre and post solo training), can't b expected to recognise an unintentional spin that quickly if they haven' seen and practiced one for months or years. The only thing that will wor is frequent practice and only instructors who are teaching spinnin regularly are really likely to get enough. The pre-stall symptoms that warn of a stall, are normally readily apparen when tou are doing a deliberate stall and looking for them, they are not s obvious when the stall is not intended or expected, and attention i elsewhere (on centering on a thermal or sorting out where you are going t land, for example). Glider pilots often tend to fly very much by attitude with the ASI as secondary reference, when you are very close to the ground quite small hil or even just a row of trees can make the nose look further down than it is Below 500 feet AGL, glance at the ASI every 3 - 5 seconds. At 04:10 02 September 2012, John Sullivan wrote: At altitude thermals flow generally vertically relatively unrestricted. At birth, even on a perfectly flat surface, thermal air must transition from a flat, shallow disk shaped zone feeding in from 360 degrees, crashing in, upwards ,which introduces a rotational component. Add orographic features and wind effects to these forces occurring in such a short period of time, over a relatively small area, and the air is very chaotic indeed. At 00:08 02 September 2012, Andrew wrote: I also have wondered how an experienced pilot can spin in from low thermalling. It must be a full spin, with the resulting steep recovery dive, that causes a spin-in accident: a stall doesn't lose much height. Even the most spin-eager gliders I've flown (Dart 17 and Puchacz) always signalled an approaching stall in plenty of time to stop an unintended spin developing. I've never accidentally spun while thermalling at normal altitudes, not once in my several thousand hours of gliding, so I wonder why it would be more likely to happen low down. One explanation might be that pilots are very stressed when circling low, and simply don't fly as well as usual. Or maybe they are circling unusually tightly, perhaps in a small thermal. I suspect experienced pilots would not make these mistakes. So I think that John Cochran's comment of August 28th may be right. There may some unknown, unexpected risk when low- and- slow that catches pilots out, even the best ones. Maybe small, strong, bubbly thermals exist low down, that can perhaps suddenly stall one wing? That would produce an immediate, uncontrollable, violent roll, somewhat like a flick manoever, without any advance warning signals. Not technically a spin, but probably ending the same way: a steep dive with insufficient height. If true, that's a risk that no amount of pilot skill can prevent, except by adopting the sensible rules I was trained with: 1. never thermal below pattern altitude, and 2. always fly at approach speed below pattern altitude. I have had one personal experience that supports John's suggestion: after a normal thermal flight in the midwest, on a day with light winds, I was on a normal final approach with wings level, at normal approach speed (60kts). At about 100ft agl, without any warning, my starboard wing was pushed rapidly and smoothly upwards, and despite immediate full opposite control input, I was put into a steep bank, I'd estimate close to 45 degrees. The surge vanished as fast as it had arrived. After it stopped, I was able to level the wings, correct the heading, and made a normal landing further down the runway. It totally surprised me. I assume a narrow thermal bubble lifted off under the starboard wing just as I passed. I estimate the surge lasted about three seconds, so at 60kts it must have been about 300ft long. I would not have believed it, except that it happened to me. I'm sure it would have been much harder to cope with, if I had not been flying at approach speed. Perhaps if I'd been flying slowly, at a higher angle of attack, the surge might have stalled the starboard wing. This roll event was also seen by an experienced pilot observer on the ground, who said he was astonished to see it, and inquired about it after I landed. A roll upset like this has only happened to me once, so (thankfully) its clearly a very rare occurrence, and maneageable at approach speed. If such bubbles are baby thermals, they are probably only in small areas, miles apart, and short lived, so would also be rarely encountered. However a pilot who is attempting to thermal low is presumably intentionally over an area where baby thermals are forming, so may have a higher chance of encountering such an effect. At 17:08 28 August 2012, John Cochrane wrote: One point not reiterated yet here -- the atmosphere down low is very different from what you're used to at 2000 feet and above. It's easy to say "I haven't unintentionally stalled /spun in a thermal in a thousand hours. How much of a dope do you have to be?" A short list of what's different down low: The atmosphere is much more turbulent. Thermals, such as they are are much smaller. In this layer, many small punchy thermals will start. Many will die. The ones we use up higher consist of many little parcels of hot air that have coalesced. Most thermals are either short lived, or basically unworkable to a modern glider. You're in the boundary layer where wind is being affected by the ground, so there is wind-induced turbulence. Punches of strong lift/gust followed by sink when you make a half turn will be the norm. The ground picture will be totally different to the pilot. If you turn downwind at altitude, you don't notice that much. If you turn downwind at 300 feet, all of a sudden the ground will rush by and, this being a high stress moment, you may pull back. Just as the gust you turned in fades, or the thermal turns to sink. And when the canopy fills with trees going by at 70 mph, the urge to pull back will be really strong. You may push forward to recover at altitude, but it's really really hard to do with the ground coming up fast. So, just because you've never unintentionally spun at altitude does not mean your chances at 300 feet are the same. Not to raise a tired subject, but why we give out contest points for thermaling at 300 feet or below remains a puzzling question to me. John Cochrane |
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Another stall spin
On Sun, 02 Sep 2012 13:35:59 +0000, Chris Rollings wrote:
Anyone any idea why the system clipped the last letter off every line in my last post? That hasn't happened he I've just saved a copy of your post for future reference and can confirm that the saved version also has unclipped lines. I use Pan 0.135 running under RedHat Fedora 15 as my newsreader. Thanks for the post. I particularly appreciate having the time vs. attitude and height lost figures for the Puchacz, which is one of my favourite dual-seat trainers. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
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Another stall spin
Am Sonntag, 2. September 2012 14:30:03 UTC+2 schrieb Don Burns:
I remember a sign on a flying office wall years ago about the three most important rules of flying: Rule # 1 Maintain proper air speed! Rule # 2 Maintain proper air speed!! Rule # 3 Maintain proper air speed!!! I'm tempted to put a sign in my office today: Rule 1: Fly at AOA significantly below the critical AOA ! Rule 2: Fly at AOA significantly below the critical AOA !! Rule 3: Fly at AOA significantly below the critical AOA !!! Why that change? I'm not good at quickly calculating the proper air speed for every configuration of flight. In my gilder have not only a yaw string but also 2 side strings. They are 30-35° upwards at the critical AOA. As long as they are not in my field of vision, I know that I'm quite some distance away from the critical AOA. |
#90
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Another stall spin
Am Donnerstag, 30. August 2012 22:13:31 UTC+2 schrieb (unbekannt):
I like Bruno and his videos, but some of what he does and shows do not reflect examples of how we should all fly. This is such an example. So- what does Crabby UH say he did wrong? 1- Obviously exceeded the critical angle of attack of the inboard wing- gust likely a factor- could happen to any of us, and does. 2- As the wing starts to drop, adds top aileron, obviously as an automatic and likely habitual reaction. This has the effect of increasing the angle of attack on the most critical portion of the wing at exactly the wrong time. 3- No obvious use of opposite rudder. 4- No forward stick to reduce angle of attack, in fact it appears the stick is positively held back. The dumping of flaps seems to be well practiced in recovering from this maneuver- I wonder who taught him this. When I have just stalled a wing, I'm just above the critical AoA and my wing produces hardly any lift, so my AoA will increase further. But when I detect this early enough I might be able to get below the critical AoA by just moving the flaps forward. This is the most direct and the fastest way to change my AoA. If I mange to reduce my AoA below the critical AoA by moving the flaps forward, I produce more lift than with the flaps in the original position, so my increase in AoA is slower. If the gust, which caused my stalled wind, ends before I read the critical AoA in the new configuration this might be sufficient to regain control. Reducing my AoA with the elevator is only my second but long term option, because I have to rotate my ship around the lateral axis and this rotation takes time. With the flaps I can reduce the AoA for only a short period of time, but it quite often buys sufficient time to stay unstalled during the gust. If it is not sufficient, I have to use the conventional slow indirect method with the elevator. |
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