Author Topic: Pitch attitude when airspeed indicator fails  (Read 2268 times)

Offline Danny Dot

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Pitch attitude when airspeed indicator fails
« on: 07/16/2018 06:21 pm »
In the shuttle we had pitch attitude limits on the attitude indicator that could be used to maintain a safe airspeed if the airspeed indicator failed.  It was called Theta Limits where theta is the Greek symbol for pitch attitude.  Basically if the high pitch limit is flown for a bit, the shuttle would fly close to its minimum airspeed.  If the low limit is flown, the shuttle would fly close to its maximum airspeed limit.

Do airliners or other aircraft have pitch limits displayed on the pilot's attitude indicator?  I don't think so.  I have looked into a couple of airliner crashes where the root cause was loss of airspeed indicator in the cockpit.  Maybe this very useful display should be used in aircraft so they can be flown safely if the airspeed indicator fails.

Just my thought for the day.
Danny Deger

Offline Jim

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Re: Pitch attitude when airspeed indicator fails
« Reply #1 on: 07/17/2018 02:06 pm »
In the shuttle we had pitch attitude limits on the attitude indicator that could be used to maintain a safe airspeed if the airspeed indicator failed.  It was called Theta Limits where theta is the Greek symbol for pitch attitude.  Basically if the high pitch limit is flown for a bit, the shuttle would fly close to its minimum airspeed.  If the low limit is flown, the shuttle would fly close to its maximum airspeed limit.

Do airliners or other aircraft have pitch limits displayed on the pilot's attitude indicator?  I don't think so.  I have looked into a couple of airliner crashes where the root cause was loss of airspeed indicator in the cockpit.  Maybe this very useful display should be used in aircraft so they can be flown safely if the airspeed indicator fails.

Just my thought for the day.

Shuttle was a glider, airliners have engines, so not applicable.

Offline Danny Dot

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Re: Pitch attitude when airspeed indicator fails
« Reply #2 on: 07/20/2018 04:09 am »
Shuttle has a speedbreak and speedbreak setting is input to Theta Limits.  Why is it impossible for an airliner Theta Limits to use throttle setting as an input?
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Offline Stan-1967

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Re: Pitch attitude when airspeed indicator fails
« Reply #3 on: 07/20/2018 04:55 am »
Shuttle has a speedbreak and speedbreak setting is input to Theta Limits.  Why is it impossible for an airliner Theta Limits to use throttle setting as an input?

Wasn't the theta limit tied to the energy management of the shuttle trajectory to it's landing point?  ( defined as E/W, E=energy, W=weight)  It was not a physical limit of the airframe aerodynamics?   The theta limits are meaningful to a glider, as the total (potential & kinetic) energy of the vehicle are constantly being bled from the system, and the goal is to arrive at the runway at more or less the right amount of energy to bleed off the excess in the landing flare & rollout.

Theta limits would have no meaning to a powered aircraft.  When I fly a nominal approach, I first set the correct pitch & airspeed, then I manipulate the throttle to follow the decent rate for the landing.  The point being I can add or subtract energy from the aircraft by using the throttle if I am high or low on my approach.  A pilot can fly a pitch prioritized approach, but it doesn't change the fact that a powered aircraft can manage the total energy at the landing point with throttle control in ways simply not relevant to a glider like the shuttle.


Offline Jim

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Re: Pitch attitude when airspeed indicator fails
« Reply #4 on: 07/20/2018 05:03 pm »
Shuttle has a speedbreak and speedbreak setting is input to Theta Limits.  Why is it impossible for an airliner Theta Limits to use throttle setting as an input?

Speed brake reduces energy.  Throttle adds energy.

Offline DarkenedOne

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Re: Pitch attitude when airspeed indicator fails
« Reply #5 on: 07/20/2018 06:21 pm »
Shuttle has a speedbreak and speedbreak setting is input to Theta Limits.  Why is it impossible for an airliner Theta Limits to use throttle setting as an input?

The Theta limits for an aircraft depends on a number of factors including thrust, weight, and wind speed.

For the space shuttle the weight is a given, the thrust is zero, and for the shuttle moderate wind speeds probably have a negligible effect.  Since all these variables are constant throughout the landing of the shuttle, the theta values are constant. 

For a powered aircraft the weight, thrust, and wind speed are constantly changing. It would have to be recalculated for every combination of weight, thrust, and wind speed.  I suppose you could have a computer continuously calculate it.  I am not sure what the point would be since if you don't like the value you can just increase thrust. 

Now aircraft often do have a Pitch Limit Indicator that does something similar, but not is not really the same. 
« Last Edit: 07/20/2018 06:36 pm by DarkenedOne »

Offline ThereIWas3

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Re: Pitch attitude when airspeed indicator fails
« Reply #6 on: 07/20/2018 07:38 pm »
Regardless of whether the indicator is marked as to these limits, all pilots are taught about this, and not just glider pilots.  In the famous case of Air France flight 447, if they had stopped trying to chase the airspeed indicator and altimeter (which were indicating incorrectly due to icing) and flown a constant pitch, they would all be alive today.  In fact, the Airbus flight computer would have done exactly this if they had not overridden it.  But an under trained first officer who had no business being in that cockpit killed them all.  This was a case of an aircraft with all engines operating properly where "flying the horizon" could have saved them.

Offline TripleSeven

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Re: Pitch attitude when airspeed indicator fails
« Reply #7 on: 07/20/2018 07:58 pm »
In the shuttle we had pitch attitude limits on the attitude indicator that could be used to maintain a safe airspeed if the airspeed indicator failed.  It was called Theta Limits where theta is the Greek symbol for pitch attitude.  Basically if the high pitch limit is flown for a bit, the shuttle would fly close to its minimum airspeed.  If the low limit is flown, the shuttle would fly close to its maximum airspeed limit.

Do airliners or other aircraft have pitch limits displayed on the pilot's attitude indicator?  I don't think so.  I have looked into a couple of airliner crashes where the root cause was loss of airspeed indicator in the cockpit.  Maybe this very useful display should be used in aircraft so they can be flown safely if the airspeed indicator fails.

Just my thought for the day.

All Boeing EFIS Aircraft have "similar" indications, for high and low speed stall as well as pitch indications for complete engine failure...in large measure for windmill relight.

.the more modern the airplane is the more sophisticated they are...in addition all pilots of Boeing since well the 707 and B52 have been taught pitch and power settings for approximate speeds in various configurations and mass assuming failure of airspeed indications or sensing systems.  in post 1990 Boeings that failure is nearly impossible.

Unreliable airspeed indication and pilot control  for high speed flight and approach operations are routinely taught and evaluated in simulator training.

I am a little tired having just gotten back from  three day, but next time I fly (and it will be a week or so...days off) I'll get you a picture if you are interested.  I might have a couple here at the house ...when I wake up :) I'll try and find some.

Offline TripleSeven

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Re: Pitch attitude when airspeed indicator fails
« Reply #8 on: 07/20/2018 08:37 pm »
Regardless of whether the indicator is marked as to these limits, all pilots are taught about this, and not just glider pilots.  In the famous case of Air France flight 447, if they had stopped trying to chase the airspeed indicator and altimeter (which were indicating incorrectly due to icing) and flown a constant pitch, they would all be alive today.  In fact, the Airbus flight computer would have done exactly this if they had not overridden it.  But an under trained first officer who had no business being in that cockpit killed them all.  This was a case of an aircraft with all engines operating properly where "flying the horizon" could have saved them.

I am not an airbus person, never flown one :) but the accident is baffling in some ways to  myself and my friends who fly large commercial transports and train and evaluate others who do the same thing and in other ways it is depressingly familiar.

you're evaluation is pretty close.  Most ocean crossing (or what we call "long haul") airplanes consist of three pilots....there is a "Captain commander" ie the senior pilot who is in all respects in command of the flight, a first officer, and what is called a "cruise captain".  In the case of this accident, the Captain Commander had gotten the airplane to cruising altitude, done all the checks and as is common left the flight deck turning control over to the cruise captain for crew rest.

In the industry cruise captains are called "the golden voice" as their role is to command the airplane in cruise where in theory the decision making process is "slower" and not as intensive.  the key to the accident is that really the Captain commander left the flight deck at the normal time (about three hours) but with the airplane  approaching some "serious weather" activity and some significant weather deviations (with little or no ground support...ie the weather radar larger picture) were going to be required.

It is important to grasp however that the airplane was, due to its mass and flight level chosen flying at nearly the limit of its performance.  The Captain had made that choice, and that choice is made a lot of times a day, but in large measure had left the flight deck due to his belief that the flight path ahead was clear of weather.  Its not .  As they go into weather The cruise captain makes some serious mistakes as to evasion techniques, radar management,  and well they get into some massive weather issues..then the pitots ice over...and then the airmanship mistakes come.

There are serious mistakes made in terms of flying pitch power and airspeed..mistakes that go back to the early 60s when a TWA crew took off and FAILED to manually turn the pitot heaters on...and they iced up.  Having said that, the airplane does not have a lot of "flying" margin (this is common) and it would have taken a lot of experience to hand fly it absent the airspeed information and with the information that the pilot was receiving as the computers reconfigured themselves for airplane control and flight display.



Airbus and Boeing fly by wire airplanes differ substantially as to how the pilot interacts with the controls and the fly by wire systems and the displays that they have in both normal and failure modes.  I cannot with any substance discuss the actions the crew made in terms of the flight computers...because they are simply not in my realm of experience or knowledge, EXCEPT to say this the actions they took were in my view not very well thought out in terms of Airbus procedures and what my airbus friends tell me.

When the CC leaves the flight deck is of course part of why he/she is paid the big bucks...today we were in a similar situation to the Airbus, ie we left at near max gross mass about 100KG below max takeoff limit for structure...and the big decision was either to climb to FL300 where we had a lot of stall margin or go on up to 360 where there was a 258 knot tail wind...the "fingers" ie the margin between high and low speed stall was the required plus 8 on the high end and minus 15 on the low end...it was all well within what the triple will do (and I have over 9000 hours in the plane and was a test pilot in it) but the forecast was for some turbulence...so I sat the seat for the first 5 hours of the 14 hour flight...by then we had burned down to where "the fingers" were back into the red...and I got my dinner and took crew rest :)

Its all about decision making :)
« Last Edit: 07/20/2018 08:49 pm by TripleSeven »

Offline Danny Dot

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Re: Pitch attitude when airspeed indicator fails
« Reply #9 on: 07/21/2018 05:17 pm »
Quote from: ThereIWas3 link=topic=46010.msg1839211#msg1839211 date=153211553we9
Regardless of whether the indicator is marked as to these limits, all pilots are taught about this, and not just glider pilots.  In the famous case of Air France flight 447, if they had stopped trying to chase the airspeed indicator and altimeter (which were indicating incorrectly due to icing) and flown a constant pitch, they would all be alive today.  In fact, the Airbus flight computer would have done exactly this if they had not overridden it.  But an under trained first officer who had no business being in that cockpit killed them all.  This was a case of an aircraft with all engines operating properly where "flying the horizon" could have saved them.

You are 100% correct.  Fly a reasonable pitch profile, and you will not overspeed or stall when the airspeed indicator fails.  In the shuttle, theta limits were a function of bank angle and speed break setting.  In an aircraft, throttle setting would need to be an input to algorithm.  If you are pitched up 20 degrees nose high, you are not overspeeding the aircraft.  A theta limit display on the attitude indicator would be very, very useful when the maintenance guy forgets to remove the tape from the static ports.
Danny Deger

Offline ThereIWas3

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Re: Pitch attitude when airspeed indicator fails
« Reply #10 on: 07/21/2018 06:13 pm »
Books have been written about this incident.  My knowledge of Airbus cockpits comes from many hours in PC flight simulators.  :)

The Captain did come onto the flight deck somewhere in there, but the Airbus cockpit is arranged in such a fashion that from his position behind the pedestal he could not see the hand controllers and they are not in a place where he could just reach over the FOs shoulder and grab the wheel like in a Boeing.  All he could see was the instruments and might not have realized that the FO was actually causing the problem.

You can't forget to turn on the pitot heat in the Airbus, and I expect in most modern airliners - it is automatic.  But there was a flaw in the design of these particular pitots that when the incoming moisture was at just the right wetness and temperature it would jam into the tube and freeze there.  The manufacturer already knew this and there was a maintenance order out on it, but this aircraft had not had the work done yet.

In my opnion, the big flaw was Air France's policy of taking zero-time job applicants and training them to fly the Airbus computer, rather than first teaching them how to fly.

Offline TripleSeven

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Re: Pitch attitude when airspeed indicator fails
« Reply #11 on: 07/21/2018 06:21 pm »
Books have been written about this incident.  My knowledge of Airbus cockpits comes from many hours in PC flight simulators.  :)



In my opnion, the big flaw was Air France's policy of taking zero-time job applicants and training them to fly the Airbus computer, rather than first teaching them how to fly.

I dont agree with that at all.  I am familiar with Air France, and several other European airlines ab initio training and then jet upgrade ....both from my current job and my time as a test  pilot for a major US airplane manufacturer...

while the foundations of flying are constant even into high performance military aircraft (and to some extent the shuttle) the notion that pilots are not taught to "fly" today is really not valid.  flying modern fly by wire airplanes is really in a league by itself in terms of "how to fly"...my first transport category airplane was a DC3 where rudder control was essential (as evidenced by a crash today in Texas) but is not part of the equation at all in my triple.

and if you are going to fly airbus, you have to fly it the airbus computer way...and it is unique

the abinitio training most European airlines give is really really good.

as an aside, I will be interested to see how spacecraft controls and control develops with modern fbw vehicles...docking should be a breeze :) safe flights


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Re: Pitch attitude when airspeed indicator fails
« Reply #12 on: 07/22/2018 06:47 am »
Retired for some 14 years now, but in my maritime days (military) turbulence was our daily lot and eyeball bounce an interesting side effect , one could see the instruments leaping around except they weren't".  Set power, anti icing on, hold an attitude and ignore airspeed fluctuations. 

On the F100 (civil) we were expected to remember deck angles for various configurations for the loss of ASI scenario; no doubt we practised it in the sim.

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