johnh wrote:
But if you don’t know not to hold the stick right back for any reason at all (except acro or practising stalls), you should not be flying any aircraft at all.
Aren’t Airbus pilots taught to do just that in a windshear/microburst situation, relying on the FBW system to get maximum performance without stalling the aircraft?
I’m not sure AF447 has a great deal to teach this debate on pitch trim runaway.
Having read the report and watched both the subsequent investigation and followed the court case, it would appear that a great many “holes in the Swiss cheese” lined up to cause this tragedy. All parties AF, Airbus other airlines and Boeing learnt a lot and have made quite a few changes, especially SOPs.
I have a feeling there is still more to learn.
Airborne_Again wrote:
Aren’t Airbus pilots taught to do just that in a windshear/microburst situation, relying on the FBW system to get maximum performance without stalling the aircraft?
It´s not the FBW flight control system in itself that prevents the aircraft from stalling – it´s the Flight Envelope Protection, more specifically the Alpha Floor protection that limits the aircraft pitch angle (irrespective of pilot stick handle positing (say full aft/up) to just below critical (1.0 AoA) Angle of Attack, thus avoiding the stall. Obviously this is not the best climb angle, but the philosophy is that wind-shear and terrain escape maneuvers are time critical and for the average airline pilot training (and as per FM) – it´s deemed the best initial action.
In some wind-shear situations there´s a lot of turbulence involved and the ability (of the pilots) to read the instrumentation can be severely impaired due to the shaking of the aircraft, so flying a specific speed or even reading the pitch attitude instrument can be a challenge, where´s pulling full aft on the stick and letting the Alpha Floor keep the aircraft above the stall speed (below crit AoA) is deemed the most effective overall procedure.
Yeager wrote:
It´s not the FBW flight control system in itself that prevents the aircraft from stalling – it´s the Flight Envelope Protection,
That’s part of the FBW system, is it not?
Airborne_Again wrote:
That’s part of the FBW system, is it not?
No, FBW is simply what it says – Fly By Wire (as opposed to conventional fly by cable). It´s not the FBW (flight controls) that generates the AoA limiting command – it´s the envelope protection system (operating in normal mode).
If I recall correctly, and as an example, when the Airbus in operating in any other mode than “normal mode”, the FBW is of cause still fully functioning (otherwise it would be the same as a fly by cable airplane without cables…), but the envelope protection is not active and you can stall the Airbus just like a conventional aircraft. In other words – it´s not the FBW that prevents the Airbus from stalling (in normal mode). You could possible say that the FBW is part of the Envelope Protection System, but not the other way around – in my view.
Airborne_Again wrote:
That’s part of the FBW system, is it not?
Hardly. Even GA autopilots can have some form of flight envelope protection.
Ultranomad wrote:
Even GA autopilots can have some form of flight envelope protection.
But that is a completely different thing! On a GA aircraft there is still a direct relation between yoke/stick position and control surface position. That’s not necessarily the case on FBW aircraft.
Yeager wrote:
You could possible say that the FBW is part of the Envelope Protection System, but not the other way around – in my view.
I don’t see how the AoA protection can be made to work except by limiting what control surface movements the FBW system can make,
You can’t have a strict AoA protection on an aircraft where there is a direct connection between yoke/stick position and control surface position. You can discourage the pilot from stalling using a stick pusher but that’s it. Even the MCAS system can’t prevent the pilot from stalling the aircraft if the yoke is pulled back fast enough.
Airborne_Again wrote:
I don’t see how the AoA protection can be made to work except by limiting what control surface movements the FBW system can make,
That´s correct, but that doesn´t change the fact that it´s not the FBW flight control design that allows for full aft/back stick pilot input – it´s the Envelope Protection system that manages it – not the FBW.
Yeager wrote:
That´s correct, but that doesn´t change the fact that it´s not the FBW flight control design that allows for full aft/back stick pilot input – it´s the Envelope Protection system that manages it – not the FBW.
They may conceptually be different systems, but it’s all done in the Flight Control System computers is it not?
