A320 SIM SCENARIO

[MD83FO]

While cruising at 390 and max weight, Being given a 70 knot instant tailwind gain, simulating jet-stream crossing, airspeed drops to alpha prot autopilot drops off.
instructor is asking for thrust to be reduced to idle.
what is the logic here?

[Check Airman]

Stall recovery after encountering a mountain wave?

[sonicbum]

Originally Posted by MD83FO

While cruising at 390 and max weight, Being given a 70 knot instant tailwind gain, simulating jet-stream crossing, airspeed drops to alpha prot autopilot drops off.
instructor is asking for thrust to be reduced to idle.
what is the logic here?

None, but primarily because the instructor did not tell You why You should do A, B or C and that's where he failed, prompting You to go and look for answers elsewhere.
Reference (Airbus) is in FCTM -> Abnormal and emergencies procedures -> MISC -> Upset prevention and recovery.
Your operator will probably have something tailored.

[vilas]

Originally Posted by MD83FO

While cruising at 390 and max weight, Being given a 70 knot instant tailwind gain, simulating jet-stream crossing, airspeed drops to alpha prot autopilot drops off.
instructor is asking for thrust to be reduced to idle.
what is the logic here?

The recovery is to push the nose down to reduce the AoA. But with the tail wind as the speed drops and ATHR will drive the thrust to Max cruise thrust. That may cause a pitch up preventing nose down movement. In that case close thrust and push the nose down.

[nervous_novice]

Originally Posted by vilas

The recovery is to push the nose down to reduce the AoA. But with the tail wind as the speed drops and ATHR will drive the thrust to Max cruise thrust. That may cause a pitch up preventing nose down movement. In that case close thrust and push the nose down.

Yes, that right. Nicely put

[TheBat]

Originally Posted by vilas

The recovery is to push the nose down to reduce the AoA. But with the tail wind as the speed drops and ATHR will drive the thrust to Max cruise thrust. That may cause a pitch up preventing nose down movement. In that case close thrust and push the nose down.

vilas, every post that I have seen from you is always to the point and spot-on. I feel the need to say: Bravo!!!

[Roj approved]

Originally Posted by TheBat

vilas, every post that I have seen from you is always to the point and spot-on. I feel the need to say: Bravo!!!

I only read this thread to learn more about the Airbus from the worldwide community, I read the whole thread, and everytime it is vilas that provides the concise answer.

I would also like to say thank you to vilas 🙏

[compressor stall]

The SOP of THR LVRs to idle was not mentioned at the Vienna Airbus safety conference that analysed the speed decay in high altitude cruise scenario. There was another one 8 years ago. Both over the same airspace (Iran) interestingly, suggesting local effects.

With speed decay at high weight max altitude, it is likely that as near as maximum thrust is set anyway. There will likely be no further pitch up moments from increased thrust as there won't be much. The most important thing with speed decay in cruise below GD is to descend. Point the nose down to gain speed and decrease the AoA and turn offroute/Mayday as required. Sure, IF the nose does not come down, THEN reduce the thrust but that's full UA territory, and questionable whether the OP's sim has that fidelity.

Refer FCTM>PR>NP>SOP>150>Speed Decay during cruise.

"If the aircraft speed goes below GD with max thrust available in use, ... descend."

For those who have access - Airbus world - Safety Conferences>2018>Day 1>Climb and Cruise events. P28

"If airspeed reduces below GreenDot with the maximum available thrust in use, the only way to avoid the risk of increasing the angle of attack is to descend"

Alpha prot or not, pointing the nose down to reduce the AoA is the most important thing. Yes, reducing thrust may help that in an extreme upset, but it's not the standard SOP every speed decay and I would argue that there's negative training going on in the OP's situation.

[FlightDetent]

At high alt the thrust is within 2-4 % N1 of max available, so any increase would have a negligible pitch effect if any at all.

Teaching students to close the trust is negative training, self-made procedure. The instructor should either explain what specific trick is he trying to teach and carefully insist the students understand its limited scope, or stick to the book script.

A general comment, all devout followers please note that vilas argues to close the thrust specifically in the presence of such an engine pitch-up moment that a nose down movement cannot be achieved.

[Fursty Ferret]

While cruising at 390 and max weight, Being given a 70 knot instant tailwind gain, simulating jet-stream crossing, airspeed drops to alpha prot autopilot drops off.

The first thing you should be doing is nothing (IMHO, of course), provided that the aircraft is still in normal law. The flight control law will apply full down elevator if necessary to protect the AoA. Since the idea is that this happens out of the blue, you need to control the startle effect and take a moment to evaluate what's happened to the aircraft - ie, genuine jetstream, mountain wave, instrumentation problem etc etc.

Screwing around with thrust levers adds another variable to the picture which you don't need.

Only then can you make the decision to descend, and the appropriate way to do it (ie get out of alpha protection then use VS etc).

[compressor stall]

Screwing around with thrust levers adds another variable to the picture which you don't need.

Yes, plenty of very very untidy events have come from pulling the thrust levers back due to a mild overspeed due sudden increasing headwinds. It’s actually the more common scenario features in reports and analyses.

[vilas]

The first thing you should be doing is nothing (IMHO, of course), provided that the aircraft is still in normal law. The flight control law will apply full down elevator if necessary to protect the AoA.

Not true! Alpha prot is latching condition. Once alpha prot is activated if you do nothing it will maintain alpha prot by doing what ever it takes yes even climbing. In the airprox incident between A330 and A340 over the Atlantic same think happened pilot disconnected the AP but didn't break the alpha lock by pushing on the stick. The aircraft trying to maintain the alpha lock zoomed up past incoming A340. To accelerate past Alpha lock it needs to be broken by pushing forward.

[Fursty Ferret]

Originally Posted by vilas

Not true! Alpha prot is latching condition. Once alpha prot is activated if you do nothing it will maintain alpha prot by doing what ever it takes yes even climbing. In the airprox incident between A330 and A340 over the Atlantic same think happened pilot disconnected the AP but didn't break the alpha lock by pushing on the stick. The aircraft trying to maintain the alpha lock zoomed up past incoming A340. To accelerate past Alpha lock it needs to be broken by pushing forward.

I’m not suggesting that someone should sit there and expect the aircraft to fly itself out of the situation, but taking 10 seconds or so to evaluate the problem will not make things worse. If the aircraft climbs at 200fpm it doesn’t hurt anyone.

If the AoA continues to increase (increasing tailwind for example) then the aircraft will descend if it approaches the critical AoA. Flying around in alpha protection is not the best thing to do, but it is not intrinsically unsafe - GPWS, TCAS, windshear can all see the pilot make a deliberate decision to enter alpha prot.

[vilas]

FF
What you said is general principle of handling with any abnormality.

Screwing around with thrust levers adds another variable to the picture which you don't need.

The airprox incident was triggered by this very action. The A340 speed excursions were very violent fluctuating between .85 to .88. tripping the AP. The PF closed thrust disconnecting ATHR. However as you say it wasn't 200ft/mt gentle climb but despite the fact that aircaft was close to it's max it climbed at 6000ft/mt that's what triggered TCAS RA. The pilot was not aware he needs to break it by pushing forward. Once alpha prot is triggered the system may not apply full elevator or commence descent. It will maintain the alpha unless you come out of it.

[Fursty Ferret]

Ah, yes. I see what you mean.

[MD83FO]

Thank you for the valuable interventions.
I believe the sidestick displacement needs to be forward of 10 degrees to unlatch alpha prot.
now on the other side of the spectrum, 70 knot increase, would you just use speedbrake to avoid traffic conflict?

[compressor stall]

I might be missing something here.

All this talk of Alpha Prot is interesting, but "should" not be relevant to significant short term wind change at high altitude leading to underspeed or overspeed.

As in the OP's scenario of increasing tailwind, the SOPs are to descend as soon as sustained below GD and trending south with MAX THR. There should be no training of THR IDLE to counter Alpha floor. It shouldn't happen. Sure pilots should be able to recover from Alpha Floor, but that's nought to do with teaching underspeed SOPs, it shouldn't get that far.

Conversely, with sudden increasing headwind - as I alluded to above - there are three events I know of, make that 4 with Vilas' example above and I am sure many more - of an aircraft exceeding MMO due sheer and ending up in Alpha Floor. So a brief overspeed leads to the protections preventing a stall? Something is wrong there, and it all seems to stem from pulling THR LVRs to IDLE and in some cases pulling backstick to keep the speed below MMO.

In the overspeed situation, the 330/340 bird is tested structural integrity of Vdive of 0.93 and all aircraft are certified for short term excursions over Mmo. The wings will not fall off as soon as you go into the red.
SOPs say in the overspeed if the AP drops out (Vmo+15) fly manually. Leave the A/THR ON, select a lower speed and use speed brakes if required - even if the HSP activates giving a small pitch up.
From the FCTM - "ATHR engaged or setting manual thrust has the same effect on the overspeed recovery".

FCTM>PR>AEP>MISC>OVERSPEED.

I make no comment on the crew concerned as it was 20 years ago and I don't know what was in the A340 FCOM then regarding overspeed recovery, but in the modern day context that example would suggest a mishandling of the overspeed by the A340 crew. Overspeed condition>THR LVR IDLE>Alpha Floor in <10 seconds. Not SOP.

[Check Airman]

That's a good point compressor stall . It's a natural reaction to want to keep out of the red band. I guess most pilots just overreact a bit when they see the speed in the red and increasing. Airbus Safety first published the VD for each model, and as you you stated, the margins are generous. 381/.89 for the A320 series.

https://safetyfirst.airbus.com/contr...eed-in-cruise/

[vilas]

As the safety first explains at high level VD is almost impossible because of the drag. It's achieved in dive at full thrust. So there's no need for over reaction. With AP& ATHR leave AP on the thrust will come back just monitor if not happy use speed brakes. If unsatisfied with ATHR then use manual thrust. There was another more recent TCAS event in A320 where the pilot disconnected the AP(cardinal error) and pulled the nose up causing RA. These panic reactions are the result of not knowing that it's not that serious. After control your speed series in Safety First the procedue was introduced in FCOM/FCTM.

[compressor stall]

I also think there's a FOQA/FDA fear going on causing some of these events. Any red must mean tea and biccies with the CP and they avoid the overspeed at all costs...

Unfortunately the fear sets up a chain of events that leads to Alpha Floor and worse. That becomes more than one round of tea and biccies.