Low-altitude flight

[danlj]

A recent post, in response to a stall-spin fatal crash, was the sort
of things folks usually say:
":Generally it is OK to make 180 or 360 degree turns in a modern
gliders, as long as you start at at least 300ft and keep the airspeed
up to a safe manoeuvring speed. The failure to do the latter seems to
have been the probably cause of this accident."

My experience is that it's safe to manoeuver very close to the ground,
in airplanes or gliders, but that it's NOT safe to do this without
practice.

I have long pondered why so many pilots get into trouble with low-
altitude manoeuvers. Here's a little analysis.

1: Motion seems different down low.

Remember the "pivot altitude"?
When we learned to do 'turns about a point', we were told that this is
the altitude at which the down wing can be kept on one apparent ground
point. Above this altitude, the wing seems to move backward over the
ground as we turn; below this altitude, the down wing seems to sweep
forward across the ground as we turn.

The pivot alttitude for a typical slow airplane or glider is well
below 1000 ft agl. Most of us don't "live" there while flying. The
FARs oppose it, the desire to soar conflicts with it, good sense
discourages it.

But we all have to descend below this altitude in order to land.

In the pattern, usually only one maneuver, the turn from base to
final, occurs below the pivot altitude.
At this point, our attention is almost always on the runway ahead, so
we don't notice the low wing sweeping forward over the ground.

Guess what? Most stall-spin accidents occur in the turn from base to
final.

2: At all speeds we FEEL we're going faster when we're low than when
we're high, even in straight flight.

3: Our 'instinctive' responses to the aircraft are based on our habits
of perception.

Unfortunately for safe low-altitude flying, our habits of perception
are all wrong below the pivot altitude.

A: We misjudge the turn because the wing sweeps forward across the
ground, making us feel as though we are turning much less effectively
than we really are.

B: We misjudge our speed because the scenery zooms past, making us
feel as though we are moving at a much higher airspeed than we really
are.

So... If we 'instinctively' follow our habits of perception when we're
below the pivot point, we will (A) rudder the turn to make the wing
stop moving over the ground, and (B) slow down to make the scenery
pass a the 'right' rate. Presto! We've stalled and are spinning. Just
because we tried to correct that excessive speed and that ineffective
turn 'better'.

If we're going to reduce low-altitude stall-spin accidents, I believe,
we're going to have to train and practice low-altitude maneuvering.

This is *not* dangerous if we plan it intelligently -- If we follow
the same precautions we follow when high: speed protects from stall;
and always have a safe landing option available. (Therefore it can't
be practiced at every airport!)

Consider: at an altitude of 300 ft agl, an ancient 20:1 glider can
expect, at best L/D, to go nearly a mile before it touches the ground
in level flight, and once it gets into ground effect, drag is about
halved.
For example, once upon a time, years ago, I experienced a rope break
in a Blanik L-13 at about 300 ft agl. I was a bit more than 20 kt
above stall, almost halfway down the runway. I did a 180, slowing
just a bit to 'save' altitude. Then I flew at approximately minimum L/
D downwind past the beginning of the runway, and now, below 200 ft
agl, I did another 180. I finished this turn was well below 100 ft
agl, just over the touchdown zone. I accidentally extended the Fowler
flaps instead of deploying the spoilers. I realized the mistake
immediately, because the glider's descent slowed. Because I had 4000
ft of runway still ahead, I just started laughing at my mistake and
left the flaps down. We drifted in ground effect for more than 2500
feet before landing.

Ironically, I felt safe doing this because at this field we are
blessed with having landable terrain in every direction, so if any
manoeuver didn't work as I expected, I had an an off-runway
alternative. Even when we're in a less favorable situation, this will
work - but we can know it will only if we've practiced it in
safe,planned and understood conditions. And the anxiety of uncertainty
and of the unknown seriously distracts from judgement and creates a
sense of emergency when none actually exists.

In a glider with a glide ratio of 40:1, from 300 ft agl we can expect
to go 2 miles, plus whatever altitude we can get from the excess
kinetic energy we have kept, to be at a safely above stall sped, plus
whatever we gain from ground effect when *really* low.

So... let's define, at airfields where we can do so safely,
maneuvering routes and altitudes in which pilots can become skilled at
manoeuvering below the pivot altitude.

I realize that I've written heresy. Get out the tar and feathers.

Dan Johnson

[Bruce Hoult]

Well, yes. We did rope breaks at 200 - 250 ft in Blaniks and were
expected to do at least enough of a circuit for a crosswind landing,
and usually into wind was easily possible.

But, what I'm thinking as I read your post is: do you guy not ridge
soar?

[[email protected]]

While maneuvering close to the ground can be done safely (ridge
soaring, landing...) one must not forget wind gradient effects. I
tell my students that "200 feet" is the altitude below which
maneuvering a sailplane can become risky due in part to some of the
perceptual effects you mentioned but also to altimeter error, unseen
obstacles, and most of all, wind gradient.

[[email protected]]

Dan,

Thank you for your insightful post on low-altitude maneuvering.

Just yesterday during an instructional flight, my student entered the
pattern too low (especially for someone with low experience). After
some prodding from Mr. Backseater, the student started looking outside
better. After recognizing that we were low, the student acknowledged
that we could not fly a normal traffic pattern. Instead, the pattern
would have to be abbreviated.

The student turned base to final at 100 feet AGL with a rather steep
bank, influenced by her changed depth perception and altered sense of
speed during this unplanned low-altitude maneuvering. Indeed, the
inside wing seemed as if it was racing ahead of the terrain.
Therefore, I respectfully add one more comment to your good analysis
on low-altitude maneuvering: There may be a strong desire to use
inside rudder to slow the perceived speed of the inside wing, and to
reduce the radius of turn (resulting in a skidding turn).

Due to your post, not only will I be more aware of this low-altitude
hazard, but my students will, too.

Raul Boerner

[Bob]

On Jun 28, 11:56*am, danlj > wrote:
> A recent post, in response to a stall-spin fatal crash, was the sort
> of things folks usually say:
> ":Generally it is OK to make 180 or 360 degree turns in a modern
> gliders, as long as you start at at least 300ft and keep the airspeed
> up to a safe manoeuvring speed. The failure to do the latter seems to
> have been the probably cause of this accident."
>
> My experience is that it's safe to manoeuver very close to the ground,
> in airplanes or gliders, but that it's NOT safe to do this without
> practice.
>
> I have long pondered why so many pilots get into trouble with low-
> altitude manoeuvers. Here's a little analysis.
>
> 1: Motion seems different down low.
>
> Remember the "pivot altitude"?
> When we learned to do 'turns about a point', we were told that this is
> the altitude at which the down wing can be kept on one apparent ground
> point. Above this altitude, the wing seems to move backward over the
> ground as we turn; below this altitude, the down wing seems to sweep
> forward across the ground as we turn.
>
> The pivot alttitude for a typical slow airplane or glider is well
> below 1000 ft agl. Most of us don't "live" there while flying. The
> FARs oppose it, the desire to soar conflicts with it, good sense
> discourages it.
>
> But we all have to descend below this altitude in order to land.
>
> In the pattern, usually only one maneuver, the turn from base to
> final, occurs below the pivot altitude.
> At this point, our attention is almost always on the runway ahead, so
> we don't notice the low wing sweeping forward over the ground.
>
> Guess what? Most stall-spin accidents occur in the turn from base to
> final.
>
> 2: At all speeds we FEEL we're going faster when we're low than when
> we're high, even in straight flight.
>
> 3: Our 'instinctive' responses to the aircraft are based on our habits
> of perception.
>
> Unfortunately for safe low-altitude flying, our habits of perception
> are all wrong below the pivot altitude.
>
> A: We misjudge the turn because the wing sweeps forward across the
> ground, making us feel as though we are turning much less effectively
> than we really are.
>
> B: We misjudge our speed because the scenery zooms past, making us
> feel as though we are moving at a much higher airspeed than we really
> are.
>
> So... If we 'instinctively' follow our habits of perception when we're
> below the pivot point, we will (A) rudder the turn to make the wing
> stop moving over the ground, and (B) slow down to make the scenery
> pass a the 'right' rate. Presto! We've stalled and are spinning. Just
> because we tried to correct that excessive speed and that ineffective
> turn 'better'.
>
> If we're going to reduce low-altitude stall-spin accidents, I believe,
> we're going to have to train and practice low-altitude maneuvering.
>
> This is *not* dangerous if we plan it intelligently -- If we follow
> the same precautions we follow when high: speed protects from stall;
> and always have a safe landing option available. (Therefore it can't
> be practiced at every airport!)
>
> Consider: at an altitude of 300 ft agl, an ancient 20:1 glider can
> expect, at best L/D, to go nearly a mile before it touches the ground
> in level flight, and once it gets into ground effect, drag is about
> halved.
> For example, once upon a time, years ago, I experienced a rope break
> in a Blanik L-13 at about 300 ft agl. I was a bit more than 20 kt
> above stall, almost halfway down the runway. *I did a 180, slowing
> just a bit to 'save' altitude. Then I flew at approximately minimum L/
> D downwind past the beginning of the runway, and now, below 200 ft
> agl, I did another 180. I finished this turn was well below 100 ft
> agl, just over the touchdown zone. I accidentally extended the Fowler
> flaps instead of deploying the spoilers. I realized the mistake
> immediately, because the glider's descent slowed. Because I had 4000
> ft of runway still ahead, I just started laughing at my mistake and
> left the flaps down. We drifted in ground effect for more than 2500
> feet before landing.
>
> Ironically, I felt safe doing this because at this field we are
> blessed with having landable terrain in every direction, so if any
> manoeuver didn't work as I expected, I had an an off-runway
> alternative. Even when we're in a less favorable situation, this will
> work - but we can know it will only if we've practiced it in
> safe,planned and understood conditions. And the anxiety of uncertainty
> and of the unknown seriously distracts from judgement and creates a
> sense of emergency when none actually exists.
>
> In a glider with a glide ratio of 40:1, from 300 ft agl we can expect
> to go 2 miles, plus whatever altitude we can get from the excess
> kinetic energy we have kept, to be at a safely above stall sped, plus
> whatever we gain from ground effect when *really* low.
>
> So... let's define, at airfields where we can do so safely,
> maneuvering routes and altitudes in which pilots can become skilled at
> manoeuvering below the pivot altitude.
>
> I realize that I've written heresy. Get out the tar and feathers.
>
> Dan Johnson

Forget the heresy, tar and feathers.... Good perspective and comments
Dan.

Regardless of where you are, or what kind of aircraft you are flying,
the cardinal rule of "FLY THE PLANE FIRST" and airspeed, airspeed,
airspeed needs to be paramont in a pilot's head. It sure worked out
over the Hudson River.

Unfortunately, we seem to be able to talk / write about safety til we
are blue in the face, and pilots, including some VERY experienced
pilots, seem to make some terrible mistakes when faced with unexpected
situations. Regardless, it is important to keep on talking / writing
about safety concerns. Every time we lose another pilot (or
passenger), that is one more too many.

Bob T.

[[email protected]]

Brings to mind this amusing quip of unknown author:


1. Try to stay in the middle of the air.
2. Do not go near the edges of it.
3. The edges of the air can be recognized by the appearance of ground,
buildings, sea, trees and interstellar space. It is much more
difficult
to fly there.

[John Smith]

wrote:

> Indeed, the
> inside wing seemed as if it was racing ahead of the terrain.
> Therefore, I respectfully add one more comment to your good analysis
> on low-altitude maneuvering: There may be a strong desire to use
> inside rudder to slow the perceived speed of the inside wing, and to

I don't know what you teach your students. In a turn I'm looking at the
horizon, the yaw string and possibly the ASI, but certainly not at the
wingtip.

[Lars P. Hansen]

On 29 Jun., 09:23, John Smith > wrote:
> wrote:
> > Indeed, the
> > inside wing seemed as if it was racing ahead of the terrain.
> > Therefore, I respectfully add one more comment to your good analysis
> > on low-altitude maneuvering: There may be a strong desire to use
> > inside rudder to slow the perceived speed of the inside wing, and to
>
> I don't know what you teach your students. In a turn I'm looking at the
> horizon, the yaw string and possibly the ASI, but certainly not at the
> wingtip.

Dan has touched upon something very, very important here. Something I
am convinced can help explain many of those “unexplainable” stall-spin
accidents when down low.

However it is important to pay some attention to how our brain handles
information. There is the conscious part (logical thinking) and the
“reptilian brain” (instincts, automatic and very quick reactions) The
reptile brain is the one that will have your hand up to protect your
eyes when someone throws something at you, long before you had time to
“think” about it

But the reptile brain was never intended for flying, and it WILL make
wrong judgments!

The problem is that the reptile brain will act on input by means of
sending very direct commands to our arms and legs, totally bypassing
the logical thinking part. So even if you are thinking “I'm looking at
the horizon, the yaw string and possibly the ASI” your reptile brain
may very well be reacting in an instinctive way to the input from the
peripheral sight saying “wow that wingtip moves fast forward over the
ground” –Incidentally the reptile brain reacts extra strongly to input
from the edges of our field of sight. Probably those that noticed that
predator out there early lived longer and got to be our forefathers.
Unfortunately, when flying a glider it may have the opposite effect.


Be safe out there.

Lars Peder
DG-600 EE, Denmark

[Jim Logajan]

"Lars P. Hansen" > wrote:
> However it is important to pay some attention to how our brain handles
> information. There is the conscious part (logical thinking) and the
> "reptilian brain" [...]

Um, shouldn't that be "mammalian brain?" Or more likely you mean there is
the frontal lobe and the parietal lobe?

> But the reptile brain was never intended for flying, and it WILL make
> wrong judgments!

I thought birds were technically considered to be reptiles? So is the
information on this link wrong?:

http://www.ucmp.berkeley.edu/diapsids/avians.html

[sisu1a]

> Dan has touched upon something very, very important here. Something I
> am convinced can help explain many of those “unexplainable” stall-spin
> accidents when down low.

While I don't disagree with this analysis- or the original post, I
wonder how many other 'unexplainable' stall-spin accidents were a
result of training where primary students were either flat out taught
to lead turns with the rudder, or even taught that it's OK to lead
with the rudder (a previous religious debate here...
http://tinyurl.com/nrgsh7 and http://tinyurl.com/kk8xdp ), a practice
which I personally despise for many reasons.

In the case of the recent event that prompted this useful discussion,
and in quite a rare situation, we may someday be able to actually ask
the PIC what happened, and about his previous training. I for one
would *really* like to know if he falls into the lead with rudder
crowd. Q: Will a ship spin without rudder input? Q: Will a ship turn
without rudder input?

-Paul

[Rory Oconor]

I seem to spend a lot of time scratching (due to incompetance), but I
rarely get below 500ft and almost never below 400ft. When scratching below
800ft coordination (yaw string) and airspeed (ASI) are alway high on my
agenda, and the workload goes up.

When mountain flying (much of flight can be within 500ft of surface), and
circling up the side of cliffs, not hitting the mountain and control
responsiveness also become important.

On the rare occasions when I mess up and find myself doing a ground level
turn, I do look down the wing in order to ensure that I dont stick the end
in a tree or the ground, I expect my rudder control and coordination is
maintained due to the many hours of scratching.

When doing some two seater xcountry flying recent, my complaint to all the
P2s was about their coordination.

I must try to work out about pivot heights. My wing always goes backwards.
What is the pivot height for a glider flying a 45 deg banked turn at 55
knots? A formula would be nice.

Rory

[David Chapman]

Rory,

Good points, the one I wanted to reinforce, .... you have to get both
speed and yaw seriously wrong to spin most gliders. For a start, at
least one wing needs to stall, so "lack of speed kills", ... not a small
error with the the rudder. A small speed error, plus rudder, yes.

Of course, good coordination all the time is better, but that can be
practiced at any height, why do it at 100', 5knts above stall, with a
good wind gradient? I am not sure I would thank an instructor that let me
try that.

Finally, if you are close enough to terain to need to check the wing tip,
I wonder if you are looking to see if anything is missing, or looking to
see the collision, or can you navigate the glider this way? I imagine it
is a "callibration check", takes a split second to complete, and will
not interfere with the aviation work load which always comes first.

David.

At 19:30 29 June 2009, Rory Oconor wrote:
>I seem to spend a lot of time scratching (due to incompetance), but I
>rarely get below 500ft and almost never below 400ft. When scratching
below
>800ft coordination (yaw string) and airspeed (ASI) are alway high on my
>agenda, and the workload goes up.
>
>When mountain flying (much of flight can be within 500ft of surface),
and
>circling up the side of cliffs, not hitting the mountain and control
>responsiveness also become important.
>
>On the rare occasions when I mess up and find myself doing a ground
level
>turn, I do look down the wing in order to ensure that I dont stick the
end
>in a tree or the ground, I expect my rudder control and coordination is
>maintained due to the many hours of scratching.
>
>When doing some two seater xcountry flying recent, my complaint to all
the
>P2s was about their coordination.
>
>I must try to work out about pivot heights. My wing always goes
backwards.
> What is the pivot height for a glider flying a 45 deg banked turn at 55
>knots? A formula would be nice.
>
>Rory
>

[bildan]

On Jun 29, 1:30*pm, Rory Oconor > wrote:
> I seem to spend a lot of time scratching (due to incompetance), but I
> rarely get below 500ft and almost never below 400ft. When scratching below
> 800ft coordination (yaw string) and airspeed (ASI) are alway high on my
> agenda, and the workload goes up.
>
> When mountain flying (much of flight can be within 500ft of surface), and
> circling up the side of cliffs, not hitting the mountain and control
> responsiveness also become important.
>
> On the rare occasions when I mess up and find myself doing a ground level
> turn, I do look down the wing in order to ensure that I dont stick the end
> in a tree or the ground, I expect my rudder control and coordination is
> maintained due to the many hours of scratching.
>
> When doing some two seater xcountry flying recent, my complaint to all the
> P2s was about their coordination.
>
> I must try to work out about pivot heights. My wing always goes backwards..
> *What is the pivot height for a glider flying a 45 deg banked turn at 55
> knots? A formula would be nice.
>
> Rory

My experience is that airspeed and coordination need to be monitored
when flying low. If those are within safe bounds, there's no need to
fear stall/spins. There are many optical illusions that can trick you
when flying below pivotal altitude - your instruments can be your best
friend.

Long ago a retired WWII Marine Corp Instructor Pilot taught me
something he had learned landing F4U Corsairs on straight deck
carriers. If you try, he said, you can train your brain to use a part
of your peripheral vision to monitor airspeed even while your gaze is
outside the cockpit.

At first, with your eyes out of the cockpit, all you can do is locate
the ASI but not read it. Still, that helps guide quick glances to the
instrument. Soon, those quick glances will not only pick up airspeed,
they will also see the trend. In time, it will seem as if you are
seeing the ASI full-time with your eyes outside and maybe you are.
Even with sunglasses, your pupils are very small in bright light
giving you what photographers call a large "depth of field". What
you are learning to do is focus your eyes on the "hyper-focal"
distance so both the distant horizon and the panel are just within the
"depth of field".

Try it while thermalling. Human brains are very plastic and can be
trained to do this. It's useful not only in landings but also when
the natural horizon isn't visible as in canyons or near a ridge.

[BT]

> Therefore, I respectfully add one more comment to your good analysis
> on low-altitude maneuvering: There may be a strong desire to use
> inside rudder to slow the perceived speed of the inside wing, and to
> reduce the radius of turn (resulting in a skidding turn).
>
> Due to your post, not only will I be more aware of this low-altitude
> hazard, but my students will, too.
>
> Raul Boerner

Hopefully Raul meant to say.. on the comment of a little inside rudder..

DON'T DO IT.. steep skidding turn with inside rudder = spin entry
BT

[Bruce Hoult]

On Jun 30, 7:30*am, Rory Oconor > wrote:
> When mountain flying (much of flight can be within 500ft of surface), and
> circling up the side of cliffs, not hitting the mountain and control
> responsiveness also become important.

Absolutely. And pretty soon you're not letting the closeness of
terrain and changed view put you off normal coordinated flying.


> I must try to work out about pivot heights. My wing always goes backwards..
> *What is the pivot height for a glider flying a 45 deg banked turn at 55
> knots? A formula would be nice.

I'm not sure it's important, but it's pretty trivial to work out.

The condition will be tan(bank_angle) = pivot_height / turn_radius

Rearranging: pivot_height = turn_radius * tan(bank_angle)

Now of course turn_radius is related to bank_angle by a=v^2/r
(centripedal force) and a=tan(bank_angle)*g (g = gravity = 9.8 m/s^2)
giving:

turn_radius = v^2 / (tan(bank_angle) * g)

Substituting:

pivot_height = (v^2 / (tan(bank_angle) * g)) * tan(bank_angle)
= v^2/g

That's pretty remarkable actually. The pivot height depends only on
speed, not on bank angle.

To convert to knots and feet:

pivot_height = speed^2 / 11.3

At 40 knots it is 142 ft.
At 55 knots it is 268 ft..
At 75 knots it is 498 ft.
At 100 knots is it 885 ft.
At 150 knots it is 1991 ft.