Boeing's "Battle over blame": reduce power or not?

[photofly]

https://www.bbc.co.uk/news/extra/sd9LGK ... over_blame

It appears the aircraft was simply going too fast, and the aerodynamic forces building up on the stabilisers were too strong for the pilots to overcome with muscle power. They turned the electronics back on and ultimately lost control altogether.

This is where Graves’ main criticisms come in. He points out that throughout the flight, the pilots failed to reduce power from the levels used immediately after take-off, allowing the plane to continue accelerating to the point where it was moving too quickly to be trimmed manually.

“Once they set those throttles to full power, they never retarded them,” he told the committee. “They accelerated right through the certified maximum speed… and just kept on accelerating.”

The criticism is that the pilots should have reduced thrust, and didn't. That seems back-assward to me, because with too much nose-down trim, reducing thrust would just make the airplane descend faster. But...

Does what Sam Graves is saying make sense to anyone?

[telex]

https://www.bbc.co.uk/news/extra/sd9LGK ... over_blame

It appears the aircraft was simply going too fast, and the aerodynamic forces building up on the stabilisers were too strong for the pilots to overcome with muscle power. They turned the electronics back on and ultimately lost control altogether.

This is where Graves’ main criticisms come in. He points out that throughout the flight, the pilots failed to reduce power from the levels used immediately after take-off, allowing the plane to continue accelerating to the point where it was moving too quickly to be trimmed manually.

“Once they set those throttles to full power, they never retarded them,” he told the committee. “They accelerated right through the certified maximum speed… and just kept on accelerating.”

The criticism is that the pilots should have reduced thrust, and didn't. That seems back-assward to me, because with too much nose-down trim, reducing thrust would just make the airplane descend faster. But...

Does what Sam Graves is saying make sense to anyone?

Makes sense to me. TOGA was not the best option given the circumstance.

[pelmet]

Understand what he is saying but what do the manuals say. If no directions on this were given, there may be a problem from the certification point of view.

[stone69]

Seems to me that basic airmanship would tell you to reduce power and control the airspeed.... does that have to be spelled out in a manual or checklist ?

[CpnCrunch]

Seems to me that basic airmanship would tell you to reduce power and control the airspeed.... does that have to be spelled out in a manual or checklist ?

Wouldn't basic airmanship tell you not to reduce the power if you're struggling to keep the nose up, and you know that reducing thrust will cause the nose to drop?

Does the checklist mention reducing thrust or airspeed?

[telex]

Seems to me that basic airmanship would tell you to reduce power and control the airspeed.... does that have to be spelled out in a manual or checklist ?

Wouldn't basic airmanship tell you not to reduce the power if you're struggling to keep the nose up, and you know that reducing thrust will cause the nose to drop?

Does the checklist mention reducing thrust or airspeed?

Which checklist are you asking about?

[kevind]

So if the checklist does not say to stay under redline, then that is not something you need to do?

[Eric Janson]

https://www.bbc.co.uk/news/extra/sd9LGK ... over_blame

It appears the aircraft was simply going too fast, and the aerodynamic forces building up on the stabilisers were too strong for the pilots to overcome with muscle power. They turned the electronics back on and ultimately lost control altogether.

This is where Graves’ main criticisms come in. He points out that throughout the flight, the pilots failed to reduce power from the levels used immediately after take-off, allowing the plane to continue accelerating to the point where it was moving too quickly to be trimmed manually.

“Once they set those throttles to full power, they never retarded them,” he told the committee. “They accelerated right through the certified maximum speed… and just kept on accelerating.”

The criticism is that the pilots should have reduced thrust, and didn't. That seems back-assward to me, because with too much nose-down trim, reducing thrust would just make the airplane descend faster. But...

Does what Sam Graves is saying make sense to anyone?

The crew ended up in a flight regime where successful recovery was very unlikely. My understanding is that the trim wheel on this aircraft is much smaller than on previous 737 models meaning it will require more force to move by hand.

The only way to reduce speed would be to reduce power.

You are correct that this will cause a nose down pitch.

It's not clear whether restoring electric trim would allow the Yoke trim switches to trim nose up in this situation. If you are pulling back on the Yoke with both hands and all your strength I'm not sure you'd be able to operate the trim with your thumb.

The final report will hopefully clarify events.

[photofly]

The only way to reduce speed would be to reduce power.

You are correct that this will cause a nose down pitch.

Eric,

The "rule" for small planes is that you control airspeed with pitch, and rate of climb with power. If you reduce power in a "small" plane, the nose drops but without a corresponding elevator input the speed doesn't change (or if it does, very little. In a single engine Cessna, reducing the power without a corresponding pitch input causes the speed to increase).

I know nothing much about big planes specifically. If your big plane is trimmed for (I don't know) 300kts, with TOGA power, and you reduce thrust, absent a pitch input (which you're already trying to make, but largely failing, because of the out-of-trim condition) - would the speed actually decrease?

[complexintentions]

Of course it will reduce the speed, but a key difference between large and small aircraft is how long it takes to slow down due to inertia and aerodynamic efficiency, which is why large aircraft (or small high-performance ones) are fitted with speedbrakes.

The point is the situation was already out of hand with a thrust setting totally inappropriate for the situation - an uncontrolled descent. Because one thing's for certain: leaving it at TOGA thrust WILL result in a massive speed increase in a descent. It's ridiculous to suggest that if you're descending uncontrollably that you should keep large amounts of thrust on out of worry the nose will drop if you reduce it. The nose-down moment of the engines on thrust reduction is completely inconsequential compared to impacting the ground with both engines bellowing away. If an aircraft is going somewhere you don't want it to, the last thing you want is to get there faster. You have too much energy, take some out of the equation.

I'm not sure why it seems so hard to comprehend that if you have two main ways to control your speed - pitch and power - that if you can't control pitch, and you're 300kt and increasing, you may want to reduce power.

[digits_]

If an aircraft is going somewhere you don't want it to, the last thing you want is to get there faster. You have too much energy, take some out of the equation.

I'm not sure why it seems so hard to comprehend that if you have two main ways to control your speed - pitch and power - that if you can't control pitch, and you're 300kt and increasing, you may want to reduce power.

Your reasoning would make sense if you are concerned about airspeed. If you are descending *uncontrollably*, I would be more concerned about my vertical speed. With that in mind, adding power in the descend, is not that illogical.

The real question with this MCAS thing -if you are looking to blame the pilots-, once again, is how much time did the pilots have to reduce the power before they reached a speed where recovery was impossible.

[shimmydampner]

Your reasoning would make sense if you are concerned about airspeed. If you are descending *uncontrollably*, I would be more concerned about my vertical speed. With that in mind, adding power in the descend, is not that illogical.

I beg to differ. The whole point is that they should have been concerned with both.

If an aircraft is going somewhere you don't want it to, the last thing you want is to get there faster. You have too much energy, take some out of the equation.

I'm not sure why it seems so hard to comprehend that if you have two main ways to control your speed - pitch and power - that if you can't control pitch, and you're 300kt and increasing, you may want to reduce power.

This shouldn't be so hard to understand.

[digits_]

Your reasoning would make sense if you are concerned about airspeed. If you are descending *uncontrollably*, I would be more concerned about my vertical speed. With that in mind, adding power in the descend, is not that illogical.

I beg to differ. The whole point is that they should have been concerned with both.

If your elevator is jammed (which is the impression they had), then both options become mutually exclusive.

You can either add energy, arrest your descent rate and have your speed go up, at least initially, and hope that you add enough energy to get you climbing again, while in the process possibly exceeding the Vne red line. You're uncontrolled descending at low level, so who cares about the Vne at that point.

Or, you can take energy away, staying below Vne and increase your rate of descent, thereby keeping everything within limits and giving up all hope that you will recover from a stuck elevator and a crash. But hey, the black box will show you stuck to your limits till the bitter end. Great!

If an aircraft is going somewhere you don't want it to, the last thing you want is to get there faster. You have too much energy, take some out of the equation.

I'm not sure why it seems so hard to comprehend that if you have two main ways to control your speed - pitch and power - that if you can't control pitch, and you're 300kt and increasing, you may want to reduce power.

This shouldn't be so hard to understand.

Yup, it's airspeed that killed them, not their decreasing altitude.

[shimmydampner]

I think there are key points you're either not taking into consideration or not understanding. This is not a case of taking an aircraft trimmed for straight and level flight, and making it climb or descend without the use of the elevator. We've probably all seen that during ab initio training as a demonstration of the normal effects of adding/subtracting power from the stable aircraft equation. However, it sounds like this was not a case of a stable, straight and level, trimmed aircraft. Quite the opposite in fact. If you went flying and trimmed the nose of your aircraft down to the point that you could not pull against it, do you honestly believe that adding power will arrest your descent? I would say to try it the next time you go flying and let us know how that works, but I really don't think that's a good idea. Put another way, if A (nose down attitude) + P (given power setting) = X (increasing airspeed and rate of descent) you believe that (assuming A is a constant) the way to reduce X is to increase P? Furthermore, if you're in that position and the aerodynamic forces from your excessive and increasing airspeed are such that you cannot physically move a control surface, do you think that increasing those forces are somehow going to help you gain control?
Have you never trained unusual attitudes?

[digits_]

I think there are key points you're either not taking into consideration or not understanding. This is not a case of taking an aircraft trimmed for straight and level flight, and making it climb or descend without the use of the elevator. We've probably all seen that during ab initio training as a demonstration of the normal effects of adding/subtracting power from the stable aircraft equation. However, it sounds like this was not a case of a stable, straight and level, trimmed aircraft. Quite the opposite in fact. If you went flying and trimmed the nose of your aircraft down to the point that you could not pull against it, do you honestly believe that adding power will arrest your descent?

Yes. Maybe not much, but it will be someting.

I would say to try it the next time you go flying and let us know how that works, but I really don't think that's a good idea. Put another way, if A (nose down attitude) + P (given power setting) = X (increasing airspeed and rate of descent) you believe that (assuming A is a constant) the way to reduce X is to increase P?

This is mathematically such a simplification, that you can't really answer that. For starters, you are combining both opposing views into one variable X such that any reply is meaningless.

Let's say the plane is trimmed for speed X, which in nose down could be a speed that exceeds Vne. Ok, that plane will want to accelerate towards that speed. If you now add power, the plane will still want to accelerate towards that speed, but once it achieves that speed, it will/should automatically raise the nose. That will arrest the descend rate.
\

Furthermore, if you're in that position and the aerodynamic forces from your excessive and increasing airspeed are such that you cannot physically move a control surface, do you think that increasing those forces are somehow going to help you gain control?

They came out of a normal/slow climb speed regime. Then someting happened and they lost control of the elevator. The nose went down. Why would they expect for control to return if they slowed down to the speed where they lost said control? That doesn't make sense. Trying to arrest the descent rate by leaving the power where it is and praying that the nose would come up again would be, strictly aerodynamically speaking, the smartest choice.

Have you never trained unusual attitudes?

Yes, and they are always done at altitude with lots of height to recover, and with a working elevator.

[photofly]

Facts:

• An airplane is in trim when its airspeed matches the speed it's trimmed for (the trim speed).
• The closer an airplane's airspeed is to its trim speed, the lower the yoke force is, and when an airplane is in trim the yoke force is zero.
• If your trim system is jammed and you cannot re-trim, the only way to reduce the yoke force is to change the speed to be closer to the trim speed.
• If your trim is jammed nose down and the yoke forces are too strong for you to be able to think about fixing the trim system you should accelerate as much as possible towards the too-fast trim speed to reduce the yoke forces.
• it would also be sensible to arrange to be not descending, to give you as much time as possible to fix the trim.
• Flying very very fast while not descending requires a lot of thrust.
• Reducing the thrust and/or slowing down would be the wrong thing to do in this situation. The rate of descent would increase and the yoke forces would get bigger and more tiring.
• Increasing the thrust and flying as fast as possible would be the right thing to do. It might buy you enough time to fix the trim, so you can retrim for a lower speed.

This is how it would work in a small plane. i would like someone who knows about big planes to say if it applies to big planes too. I presume, perhaps wrongly, that transsonic effects like Mach tuck were not an issue here.

if you're in that position and the aerodynamic forces from your excessive and increasing airspeed are such that you cannot physically move a control surface, do you think that increasing those forces are somehow going to help you gain control?

These excessive forces didn't arise from going too fast. (Forces arising from going too fast would require a push on the yoke to prevent a climb, not a pull on the yoke to prevent a dive.)

In this case the forces actually arise from going too slow - much slower than the trimmed speed.

You should be decreasing those forces, by (in this case) accelerating the aircraft towards the trim speed. Therefore best to add thrust, not reduce it.

[shimmydampner]

Forces arising from going too fast would require a push on the yoke to prevent a climb, not a pull on the yoke to prevent a dive.

Perhaps I'm conflating this article with some scuttlebutt I've heard, but my understanding was that it was not the yoke (elevator) that was the control surface in question, rather the manual stabilizer trim which was too loaded up to be able to be moved by hand after the stab trim cutout switches were turned off.
I've heard that this scenario has been preformed recently in a simulator and reducing power was shown to allow the crew to regain control, however that's second hand info at best. Perhaps someone could unequivocally confirm or deny that. I'll admit I'm out of my depth here, knowing nothing about the aircraft in question other than it has stabilizer trim as opposed to elevator trim tabs. On other aircraft I've flown like that, faster airspeed made moving it considerably more difficult.

[photofly]

It is true that it is impossible to predict here how the trim wheel forces vary with airspeed, without knowing a lot more about the B737 trim system. It's certainly plausible that at high speed the manual trim becomes very hard to operate.

But in a small aircraft, reducing the power wouldn't reduce the airspeed, nor would it make it easier to pull on the yoke to raise the nose (which is something that would reduce the airspeed). I am interested to know if this is the case for a B737. Do the hydraulic powered controls behave differently than simple reversible push-rod or cable operated ones?

If your scuttlebutt is correct, there appears to be a catch-22 situation: at a high airspeed the manual trim becomes very hard to move, but with full forward trim, the yoke is too heavy to raise the nose to reduce the airspeed to allow the trim to be run back manually. If this is the case, it would be a very unpleasant situation in which to find oneself.

The reason I posted the thread is because Congressman Graves seems to think it's "obvious" that because the plane was going too fast, the pilots should have reduced thrust which would cause the aircraft to slow down and everything would then have been fine. That they didn't do so renders them blameworthy and absolves Boeing. This appears facially to be neither correct aerodynamics nor fair to the pilots.

Is there a Boeing SOP for a full forward stuck trim? Does it say to reduce power? I would like to hear from people familiar with the type.

In the longer term I imagine we will read about simulator studies that will answer the question about what the pilots should or could have done. But I think Congressman Graves is being unfair at this time.

[shimmydampner]

there appears to be a cleft stick: at a high airspeed the manual trim becomes very hard to move, but with full forward trim, the yoke is too heavy to raise the nose to reduce the airspeed to allow the trim to be run back manually. If this is the case, it would be a very unpleasant situation in which to find oneself.

Indeed. If my understanding is correct, once they were far enough gone, to get any purchase, it seems as though they needed the powered system, which unfortunately was trying to drive them into the ground.

[photofly]

I am surprised (if in fact it is so) that an aircraft trim system that can result in a situation which cannot be recovered without powered assistance would pass certification. I would hope that it would be required that the aircraft should always be recoverable by hand from any trim situation in which a runaway stabilizer can result, after the powered trim had been disabled. That would include full forward trim.

EDIT: I see (FAR25.255(f)) that recovering from an out-of-trim dive limits apply only to as much trim as occurs with three seconds of trim input at the normal rate. So I guess a manual recovery from full nose down trim is not anticipated.

EDIT again:

Here is more from someone who write authoritatively on the subject of large aircraft stability and trim, and the 737 in particular:
https://leehamnews.com/2019/02/08/bjorn ... ty-part-9/