GPS and ground speed

Franklin "Franklin wrote:

On Mon, 27 Jul 2009 20:32:52 -0700 (PDT), D Ramapriya wrote:

Since

(a) Ground Speed can be determined by GPS, (b) the relationship
between the aircraft's attitude and the angle of attack should
theoretically be unvarying and (c) the attitude indicator is a
gyroscopic instrument,

is it possible to assume with any correctness that during cruise, an
aircraft can only be at *a* particular ground speed at *a* particular
altitude at *a* particular attitude? If the answer is Yes, is it
possible to develop some formula where the pilot can at least arrive
at a rough airspeed figure using the GPS should his pitot tubes get
blocked or iced for whatever reason? I acknowledge that this will
only be a rough estimate since headwind and tailwind can't be
measured.

I'm thinking about the recent Air France crash and wondering if at
all it could've been prevented using such a calculation to roughly
estimate the actual airspeed instead of having to rely on entirely
inaccurate IAS...

Thanks in advance for your views,

Ramapriya

Obviously you are a troll.


Ramapriya, please excuse my namesake's behavior. Unfortunately he's
chosen to antagonise you in the hope that you retaliate against me.
He's got a long history and lots of names. The sock master spends his
day setting up elaborate trolls with carefully disguised identities.

(1) In alt.comp.freeware he's well-known as "Bear Bottoms", "Ari", David
W" and literally dozens of other names.

(2) In the aviation groups he's known as Maxwell, Payton Byrd and other
names which posters here know better than I do.

(3) He hangs out on music groups such as Beatles, Zeppelin, Doors,
Genesis, and Jethro Tull.

(4) Unsurprisingly a k00k like him also hangs out on the 2600 and
hacking groups.

He has fantasies about being a pilot or special forces operative which
is why you see a lot of him. The only thing he can fly is a toy
aircraft.

He tries to be irritating:

http://groups.google.ru/group/alt.comp.freeware/msg/a46d34973796b1f6
http://groups.google.ru/group/alt.comp.freeware/msg/672961a0af8ac7d6
http://groups.google.ru/group/alt.comp.freeware/msg/5ae56dab596230fa
http://groups.google.ru/group/alt.comp.freeware/msg/fd54722dd2020e57
http://groups.google.ru/group/alt.comp.freeware/msg/e522b0bedbe2bb0a

On Mon, 27 Jul 2009 20:32:52 -0700 (PDT), D Ramapriya
wrote:

Since

(a) Ground Speed can be determined by GPS,
(b) the relationship between the aircraft's attitude and the angle of
attack should theoretically be unvarying and
(c) the attitude indicator is a gyroscopic instrument,

is it possible to assume with any correctness that during cruise, an
aircraft can only be at *a* particular ground speed at *a* particular
altitude at *a* particular attitude?

dont think so. the attitude and speed that the aircraft will return to
unaided is dictated by the decalage and that will vary with the
elevator trim position. each different combination of thrust and trim
position will result in a different speed.


If the answer is Yes, is it
possible to develop some formula where the pilot can at least arrive
at a rough airspeed figure using the GPS should his pitot tubes get
blocked or iced for whatever reason? I acknowledge that this will only
be a rough estimate since headwind and tailwind can't be measured.

I'm thinking about the recent Air France crash and wondering if at all
it could've been prevented using such a calculation to roughly
estimate the actual airspeed instead of having to rely on entirely
inaccurate IAS...


if you were forced to use a gps unit for speed you could correct the
apparent value using the metrological forecast winds for the altitude.
it is a simple speed triangle calculation that could be done on a
circular sliderule like the jeppeson cr5.

in all of aviation the experience has been that a properly maintained
minimal component count in the systems has resulted in better
serviceability than a myrriad number of interconnected poorly
maintained systems. you quest should be to perfect the existing.

you just have to accept that at the leading edge of a technology you
sometimes find leading edge problems that no one has an answer for.
for those moments you just hope that the guy up front has a name like
Bartels or Sullenberger or the thousands of other pilots who are just
as capable but havent been called on to become famous.

Stealth Pilot

On Jul 28, 1:03*pm, Stealth Pilot
wrote:

is it possible to assume with any correctness that during cruise, an
aircraft can only be at *a* particular ground speed at *a* particular
altitude at *a* particular attitude?

dont think so. the attitude and speed that the aircraft will return to
unaided is dictated by the decalage and that will vary with the
elevator trim position. each different combination of thrust and trim
position will result in a different speed.

oops, okay.

in all of aviation the experience has been that a properly maintained
minimal component count in the systems has resulted in better
serviceability than a myrriad number of interconnected poorly
maintained systems. you quest should be to perfect the existing.

you just have to accept that at the leading edge of a technology you
sometimes find leading edge problems that no one has an answer for.


I'm very sure that being an experienced pilot, you know exactly what
you're talking about. I look at it all only from the (admittedly
uneducated and possibly naive) outside and what I see makes me want
newer and better technology all the time. I cite two recent examples,
both involving the A340 and incorrect Vr, both curiously involving
Emirates, one in Jo'burg and the other recently in Melbourne. In both
the cases, but for the inbuilt cutting-edge technology that prevented
rotation earlier than prior to attaining an adequate enough airspeed,
the aircraft would've tried a lift-off with potentially catastrophic
consequences. And then there's the wonderful TCAS - who knows how many
mid-airs have been averted singularly by it?

Off the top of my head, I can't think of a major event that's occurred
*because* of the cutting-edge automation.


for those moments you just hope that the guy up front has a name like
Bartels or Sullenberger or the thousands of other pilots who are just
as capable but havent been called on to become famous.


Excellently put.

Ramapriya

In article ,
Stealth Pilot wrote:

if you were forced to use a gps unit for speed you could correct the
apparent value using the metrological forecast winds for the altitude.
it is a simple speed triangle calculation that could be done on a
circular sliderule like the jeppeson cr5.

The GPS unit could also derive the actual winds if you can fly a couple
of different headings at the same speed, or make a circle.
Glider-oriented GPS units typically do this automatically since it's so
useful to have and we tend to fly in circles all the time anyway.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

"Mike Ash" wrote in message
...
In article ,
Stealth Pilot wrote:

if you were forced to use a gps unit for speed you could correct the
apparent value using the metrological forecast winds for the altitude.
it is a simple speed triangle calculation that could be done on a
circular sliderule like the jeppeson cr5.

The GPS unit could also derive the actual winds if you can fly a couple
of different headings at the same speed, or make a circle.
Glider-oriented GPS units typically do this automatically since it's so
useful to have and we tend to fly in circles all the time anyway.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

I did not think that "glider oriented GPS units" computed the wind. It's the
software that correlates a reasonably constant diameter circle to compute a
center point of the circle and then correlates the movement (drift) of the
center point over the ground to compute a "best guess wind" from the
drifting. Poor pilot thermaling techniques will result in poor wind
estimates from the software.

??
BT

In article ,
"BT" wrote:

"Mike Ash" wrote in message
...
In article ,
Stealth Pilot wrote:

if you were forced to use a gps unit for speed you could correct the
apparent value using the metrological forecast winds for the altitude.
it is a simple speed triangle calculation that could be done on a
circular sliderule like the jeppeson cr5.

The GPS unit could also derive the actual winds if you can fly a couple
of different headings at the same speed, or make a circle.
Glider-oriented GPS units typically do this automatically since it's so
useful to have and we tend to fly in circles all the time anyway.

I did not think that "glider oriented GPS units" computed the wind. It's the
software that correlates a reasonably constant diameter circle to compute a
center point of the circle and then correlates the movement (drift) of the
center point over the ground to compute a "best guess wind" from the
drifting. Poor pilot thermaling techniques will result in poor wind
estimates from the software.

I believe there are a couple of techniques depending on the data
available. A pure GPS unit is going to be stuck doing what you describe.
A unit which also has access to pitot/static data will be able to
compute airspeed as well as groundspeed and direction, and will be able
to derive wind from a couple of straight legs flown at different angles.
I believe both techniques can be found in practice, but I'm not familiar
enough with actual devices to say what does what.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

On 28 July, 04:32, D Ramapriya wrote:
Since

(a) Ground Speed can be determined by GPS,
(b) the relationship between the aircraft's attitude and the angle of
attack should theoretically be unvarying and
(c) the attitude indicator is a gyroscopic instrument,

is it possible to assume with any correctness that during cruise, an
aircraft can only be at *a* particular ground speed at *a* particular
altitude at *a* particular attitude? If the answer is Yes, is it
possible to develop some formula where the pilot can at least arrive
at a rough airspeed figure using the GPS should his pitot tubes get
blocked or iced for whatever reason? I acknowledge that this will only
be a rough estimate since headwind and tailwind can't be measured.

I'm thinking about the recent Air France crash and wondering if at all
it could've been prevented using such a calculation to roughly
estimate the actual airspeed instead of having to rely on entirely
inaccurate IAS...

I am not a pilot but maybe I can add something here.

Regarding GPS.

Obviously you can estimate airspeed with a gps and a weather forecast.
This estimate is not good enough to keep flying in an airliner
at 35,000 feet in a thunderstorm. There is only a small range
of airspeed over which the aircraft can safely operate. Maybe you
can look it up, I don't know any numbers. The problem is I believe
that the reducing air density, as altitude increases, *increases*
the stalling speed and the reducing temperature *lowers*
the speed of sound. The aircraft is limited
to an ever smaller range of safe operating speeds as
altitude increases. Look up terms like mach buffet, coffin corner.
The U2 spyplane I believe had only a 5 or 10 knot range when operating
at maximum altitude. It's more than that for airliners

So, the airspeed estimate from the GPS is not good enough
due to the strong swirling winds in a thunderstorm.

Unreliable Airspeed recovery

The airbus A330 has a specific "unreliable airspeed" flying
technique that should be used if the pilots think that the ASI
is not working. The A330 has 3 seperate ASI systems
and so it is expected that they will display different
readings if something is wrong with one or more of them.
You set an power level on the engines and
fly a particular attitude. (Or is it angle of attack?).

As you know aircraft are equipped with gyroscopic attitude
indicators. Many aircraft also have an angle of attack indicator
and I think the airbus A330 is one of them. The sensor is simply
a small weathercock operating on a lateral axis.

Google for Images with [angle of attack sensor] and you will
get loads of hits.