Discussion:
Angle of Attack Indicator Failures
(too old to reply)
p***@hotmail.com
2019-07-14 06:43:25 UTC
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The Wright brothers first aircraft were equipped with a simple angle of attack
indicator consisting of a length of yarn trailing in the airstream where
the pilot could see it. These airplanes were not stable in pitch and the
pilot had to actively control the elevator to maintain proper angle of
attack, which was within human capability at the low airspeeds used.

From what I've read, angle of attack indicators were first used by
military services in carrier-capable jet aircraft. This was part
of a set of innovations, including the gyroscopically stabilized
optical guide beam and the angled flight deck, developed to allow
jet aircraft to land routinely on carriers. Since then angle of
attack indicators have been found useful in other flight regimes
and are now standard equipment on military airplanes, as well as
many airliners, corporate jets, and private planes.

I have looked at a dismounted angle of attack transducer with its
movable vane. It incorporates a heater circuit, so one failure mode
would seem to be freezing in place when encountering icing conditions.
I am interested in any information about angle of attack indicator
failures, including failure rate, accidents caused by failures,
emergency procedures for failures, and how pilots recognize AOA failures.
For example, has an AOA indicator ever failed during a carrier landing?
With what result? What is the procedure for this?

Thanks in advance for all replies.

Peter Wezeman
anti-social Darwinist
Jeff Crowell
2019-07-18 13:24:26 UTC
Permalink
Post by p***@hotmail.com
I am interested in any information about angle of attack indicator
failures, including failure rate, accidents caused by failures,
emergency procedures for failures, and how pilots recognize AOA failures.
For example, has an AOA indicator ever failed during a carrier landing?
With what result? What is the procedure for this?
I can tell you that I was taught in US Navy flight school in the 80s to cross-check airspeed against the AOA gauge every time I dropped the wheels and got ready to land. In the jet world, every time you land you are practicing the carrier approach. Carrier approaches are flown "on-speed", which simply means you work toward, then sustain, optimum AOA. Flying on-speed, power then controls rate of descent. That's important, because flying on-speed sets the correct hook-to-eye distance.

The first thing you do when you slow to approach speed is check your fuel weight (fuel gauges are calibrated in pounds), add it to any other stores (e.g. bombs) you may be carrying, then compute what on-speed airspeed will be for that weight condition. Each aircraft type and model has a characteristic airspeed for on-speed at a base weight (for example, the TA-4F base weight was about 12,000 pounds), this varies from individual aircraft to aircraft, so it is painted on the nose gear door, or at least it was back then.

You add x number of knots (dependent on aircraft type) to base airspeed for each 1000 pounds of fuel and ordnance, then make sure that your AOA shows on-speed at that airspeed. And if it does not agree, you have to figure out which instrument is wrong.

As mentioned above, flying on-speed sets a certain hook-to-eye distance. That is a vertical distance between the pilot's eye and the hook point (the part of the hook that engages the arresting gear pendant). If you think about it, when you are flying the ball properly, you are maintaining your eye in the wedge of light that represents a centered ball. But it's the hook that catches the wire. A centered ball subtends something like 18 inches at the ramp (the ass end of the ship). In other words, there is a window 18 inches high at that point in space if the ball is centered. A centered ball at touchdown puts the hook on the deck just short of the 3 wire, ***but only if you are on-speed***.

If you are slow (cocked up, too-high AOA), your hook point is lower in space and may engage the wire while you are airborne. Since you are cocked up, that slaps the airplane onto the deck and may well collapse the nose gear, always the weak point in the system (the mains are stressed to take that big impact).

If you are fast (flat, low AOA), the hook point is higher in space and may miss the wires, or at least won't be pressed down against the deck as well by the snubber system. A hook-skip bolter is likely as a result.

On-speed is usually something like 1.2 times stall speed, so it's not like you are going to fall out of the sky if AOA is a couple knots off. If all else fails, you ask the LSO how you're looking. Those guys can tell if you're slow or fast just by looking at the picture as you come in.



Jeff
--
9mm is just a .45 set on 'stun.'
Paul Adam
Jeff Crowell
2019-07-18 19:37:45 UTC
Permalink
Post by p***@hotmail.com
I am interested in any information about angle of attack indicator
failures, including failure rate, accidents caused by failures,
emergency procedures for failures, and how pilots recognize AOA failures.
For example, has an AOA indicator ever failed during a carrier landing?
With what result? What is the procedure for this?
Oh, and:

It's much more likely that that string the Wrights used was a yaw string, not an AOA gauge (though I will cheerfully accept knowledgeable correction on this).

And as for strings used as yaw indicators... the F-14 Tomcat had a yaw string as well!


Jeff
--
Gun control allows for quick follow-up shots.
p***@hotmail.com
2019-07-21 01:54:03 UTC
Permalink
Post by Jeff Crowell
Post by p***@hotmail.com
I am interested in any information about angle of attack indicator
failures, including failure rate, accidents caused by failures,
emergency procedures for failures, and how pilots recognize AOA failures.
For example, has an AOA indicator ever failed during a carrier landing?
With what result? What is the procedure for this?
It's much more likely that that string the Wrights used was a yaw string, not an AOA gauge (though I will cheerfully accept knowledgeable correction on this).
And as for strings used as yaw indicators... the F-14 Tomcat had a yaw string as well!
From the descriptions I've read of the trailing yarn device used by
the Wright brothers the yarn was free to move in elevation and azimuth,
and so would indicate both angle of attack and yaw.

The Wrights are credited with the discovery of adverse yaw and how
correct it with a movable rudder. From _Aerodynamics, Stability and
Control of the 1903 Wright Flyer_ by Fred Culick and Henry Jex:

Wilbur made his fundamental discovery of adverse yaw,
during his flights in 1901. He noted in his diary on
August 15, "Upturned wing seems to fall behind, but at first
rises." Then in a letter to Chanute on August 11, be wrote,
"The last week was without very great results though we proved
that our machine does not turn (i.e. circle) toward the lowest
wing under all circumstances, a very unlooked for result and
one which completely upsets our theories as the causes which
produce the turning to right or left." These are the first
observations of adverse yaw.


The Wright brothers' use of angle of attack instruments seems to be
widely accepted in the aviation community. The Wrights kept meticulous
records of their work. Documentation should be found in their papers.
I'll try to talk to some of the librarians involved.

Peter Wezeman
anti-social Darwinist
p***@hotmail.com
2019-07-21 01:08:36 UTC
Permalink
Post by Jeff Crowell
Post by p***@hotmail.com
I am interested in any information about angle of attack indicator
failures, including failure rate, accidents caused by failures,
emergency procedures for failures, and how pilots recognize AOA failures.
For example, has an AOA indicator ever failed during a carrier landing?
With what result? What is the procedure for this?
I can tell you that I was taught in US Navy flight school in the 80s to cross-check airspeed against the AOA gauge every time I dropped the wheels and got ready to land. In the jet world, every time you land you are practicing the carrier approach. Carrier approaches are flown "on-speed", which simply means you work toward, then sustain, optimum AOA. Flying on-speed, power then controls rate of descent. That's important, because flying on-speed sets the correct hook-to-eye distance.
The first thing you do when you slow to approach speed is check your fuel weight (fuel gauges are calibrated in pounds), add it to any other stores (e.g. bombs) you may be carrying, then compute what on-speed airspeed will be for that weight condition. Each aircraft type and model has a characteristic airspeed for on-speed at a base weight (for example, the TA-4F base weight was about 12,000 pounds), this varies from individual aircraft to aircraft, so it is painted on the nose gear door, or at least it was back then.
You add x number of knots (dependent on aircraft type) to base airspeed for each 1000 pounds of fuel and ordnance, then make sure that your AOA shows on-speed at that airspeed. And if it does not agree, you have to figure out which instrument is wrong.
As mentioned above, flying on-speed sets a certain hook-to-eye distance. That is a vertical distance between the pilot's eye and the hook point (the part of the hook that engages the arresting gear pendant). If you think about it, when you are flying the ball properly, you are maintaining your eye in the wedge of light that represents a centered ball. But it's the hook that catches the wire. A centered ball subtends something like 18 inches at the ramp (the ass end of the ship). In other words, there is a window 18 inches high at that point in space if the ball is centered. A centered ball at touchdown puts the hook on the deck just short of the 3 wire, ***but only if you are on-speed***.
If you are slow (cocked up, too-high AOA), your hook point is lower in space and may engage the wire while you are airborne. Since you are cocked up, that slaps the airplane onto the deck and may well collapse the nose gear, always the weak point in the system (the mains are stressed to take that big impact).
If you are fast (flat, low AOA), the hook point is higher in space and may miss the wires, or at least won't be pressed down against the deck as well by the snubber system. A hook-skip bolter is likely as a result.
On-speed is usually something like 1.2 times stall speed, so it's not like you are going to fall out of the sky if AOA is a couple knots off. If all else fails, you ask the LSO how you're looking. Those guys can tell if you're slow or fast just by looking at the picture as you come in.
Thank you very much for your reply. I get the implication that if the
AOA was not working they would still go ahead and land. Is this correct?

Peter Wezeman
anti-social Darwinist
Jim Wilkins
2019-07-21 02:27:57 UTC
Permalink
..
Thank you very much for your reply. I get the implication that if the
AOA was not working they would still go ahead and land. Is this
correct?

Peter Wezeman
anti-social Darwinist
-----------------

The proper procedure is to circle until a technician arrives to fix
it.

https://aviationhumor.net/pilots-vs-maintenance-engineers/
John Weiss
2019-07-21 02:55:35 UTC
Permalink
Post by p***@hotmail.com
I get the implication that if the
AOA was not working they would still go ahead and land. Is this correct?
Yes.

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