Ethiopian 737 - same as Indonesia ?

[Jeroen Hoppenbrouwers]

New rumours about the Ethiopian crew. Apparently they initially used the cutout switches as per procedure and then, for some reason, reactivated the electric trim system which also reactivated MCAS and drove them into the ground. It is not yet clear why the reactivated. You can manually wind the tailplane pitch up and down on the 737. They did not need the electrical trim.

Hoppie

[ScudRunner]

Possibly with a significant nose-down trim and running out of altitude they felt they would not able wind the trim off manually fast enough so in desperation put the elec system back in.  Anyone who has seen those trim wheels on the 737 fly around when electrically operated would appreciate there is quite a speed difference between elec/manual operation. 

I wonder how many turns of the trim wheel it would take to return from a full-nose down to a neutral trim position??

<<Edit>> A couple of internet sources quote 250 to 268 manual turns (on the 737-700) for full range trim movement?? Surely not!!. Even a 10th of that would be incredibly difficult to perform in such dire circumstances.

[ahaka]

Anyone know why the trim wheel in 737 was designed to have such a wide range of movement? Would it be too sensitive otherwise?

[John H Watson]

Anyone know why the trim wheel in 737 was designed to have such a wide range of movement?

It's a purely mechanical system. It's a small wheel driving a large control surface which will probably have heavy airloads on it. You need sufficient mechanical advantage. On the ground, the torque required to turn the wheel is somewhere between 22 and 62pound-inches (2.5 and 7 newton-meters). In the air, the crew may have to use excess elevator to unload the mechanism.

[skelsey]

Anyone know why the trim wheel in 737 was designed to have such a wide range of movement? Would it be too sensitive otherwise?

Gearing to make it physically possible to move the stab I would imagine - I believe the 737 trim wheels literally move the stab directly via cables and pullies (not via hydraulics) so I imagine there would need to be a substantial ratio reduction to allow the aerodynamic forces on the stab to be overcome by hand.

<Edit: I see John has beaten me to it! >

[Jeroen Hoppenbrouwers]

I recently read about 40 turns for the full range. Still a lot. That is why there is a flip-out handling knob on the trim wheels, like some truck drivers have on their steering wheel.

[ahaka]

Is the 737 the only airliner produced today that uses this kind of mechanical trim backup? In 777 the trim lever is not mechnically operated I think. And why doesn't the 744 have any? Elevators are powerful enough to fly the plane even if the trim is in extreme position?

[Jeroen Hoppenbrouwers]

737 is the oldest... cheapest... simplest...

[Jeroen D]

Prelimanary report

http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+B737-800MAX+%2C%28ET-AVJ%29.pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e

[Will]

I'm reading about the Ethiopian report saying the pilots followed Boeing's procedures. Presumably, that refers to the pilots using the stab cutout switches.

I doubt Boeing's procedures say to re-engage them after suspected runaway trim. Right?

What is taught about stab cutout switches? Are they like circuit breakers, where you can try once to re-engage them? Or is the teaching that once you've identified a problem that needs cutout switches, it's best to never turn them on again? Or is it left up to the pilots?

[skelsey]

I'm reading about the Ethiopian report saying the pilots followed Boeing's procedures. Presumably, that refers to the pilots using the stab cutout switches.

I doubt Boeing's procedures say to re-engage them after suspected runaway trim. Right?

Correct...

...BUT I've not seen it stated, definitively, that the switches were re-engaged at any point.

The report is explicit that the switches were placed to cutout. But it doesn't say anywhere that they were returned to normal. People have inferred that, but I wouldn't say it is possible to consider that to be a fact yet.

Still -- ultimately if the aircraft was uncontrollable in pitch and it was not possible to move the trim by hand (which is inferred from the FO's responses) what do you do -- fly the aircraft in to the ground with the switches in cutout because that's the SOP or attempt to use the electric trim (almost certainly having a good idea that the issue is MCAS and not the stab trim itself running away per se) to recover the aircraft?

Damned if you do, damned if you don't...

[Jeroen Hoppenbrouwers]

This is the most worrying part:

At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working.

What exactly did he try? Did he really attempt to wind the mechanical wheel? If so, @#%$@%#$@. But what is the chance that, as a low-hour FO, he never before wound the mechanical wheel and grossly underestimated the number of turns required to get there? Would the airloads on the stabilizer already be so high (with the increasing forward airspeed) that manual winding was as good as impossible?

If the times above are correct, the FO tried for about four seconds to manually trim. Only a fully jammed wheel would cause this near-immediate response that it is not working, I believe.

All the rest, though alarming, is exactly what you expect when your left AoA goes haywire. Their decision to cut the stab trim was timely and right on. The missing bit is why the manual winding up apparently didn't work. And it is such a pity that they did not keep trimming up electrically -- MCAS clearly waited 5 full seconds every time before undoing the nose-up. They could have won, by simply persisting. Up... up... up... up... would have stopped MCAS logic. We saw the same pattern with LionAir. MCAS won because the pilots stopped. The big question is why they stopped.

Not much assurance to clear the MAX for flight, yet...


Hoppie

[Hardy Heinlin]

Why should a higher airspeed make the manual stabilizer rotation impossible? Isn't the stabilizer rotational axis more or less in the middle between the leading edge and the trailing edge? One half of the stabilizer surface rotates into the ram air, and the other half on the opposite side of the axis rotates with the ram air. If the ram air force ratio is 50:50, the ram air cannot rotate the stabilizer surface. Maybe it's not exactly 50:50. But the axis is certainly not in the leading edge, is it?


|-|ardy

[John H Watson]

The hinge is about 3/5's back from the leading edge, just forward of the elevator hinge line.
However, the horizontal stabiliser is swept back. I'm not sure where the centre of pressure would be.

737NG Stabiliser

I don't know if the surface is a negative airfoil like the 744.

[John H Watson]

The report is explicit that the switches were placed to cutout. But it doesn't say anywhere that they were returned to normal. People have inferred that, but I wouldn't say it is possible to consider that to be a fact yet.

At  05:43:11,  about  32 seconds  before  the  end  of  the  recording,  at  approximately 13,402ft,  two momentary manual electric trim inputs are recorded in the ANU direction.  The stabilizer moved in the ANU [airplane nose up] direction from 2.1 units to 2.3 units.

Strange. This suggests the cutout switches were returned to the non-cutout position.

At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim  command occurred and  the  stabilizer  moved in  the  AND [airplane nose down] direction from  2.3 to  1.0  unit  in approximately  5  seconds.

This sounds like the MCAS system acting again. This is a second indication that the cutout switches were in the non-cutout position.

The  aircraft  began  pitching  nose  down. Additional  simultaneous  aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down.  The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.

[Hardy Heinlin]

Is the 737 the only Boeing type whose elevators cannot maintain level flight when the stabilizer is completely mistrimmed?

[farrokh747]

At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance of AND automatic trim command occurred without any corresponding motion of the stabilizer, which is consistent with the stabilizer trim cutout switches were in the ''cutout'' position

from https://avherald.com/h?article=4c534c4a/0022&opt=0

There's some ref to Master Caution Anti-Ice going off as well...

and

The last recorded pressure altitude was 5,419 ft on the left and 8,399 ft on the right.

thats a big diff between L & R

fc

[John H Watson]

There's some ref to Master Caution Anti-Ice going off as well...

... due to a Left AOA sensor malfunction. I'm not sure about the Max, but the standard 737NG AOA sensors have integral heaters for both the vanes and the cases (and the heater system has separate wiring and electrical plugs to the angle measuring system).

It sounds like the AOA was physically damaged in some way. There were theories about birdstrikes, but the CVR records didn't detect the sound of a birdstrike or comments on one.

thats a big diff between L & R

Seems a lot. Do the AOA's really affect the pitot-static system that much?

[Jeroen Hoppenbrouwers]

Wild speculation: no, they don't, but the existing air data software (not MCAS) does not stop taking garbage when the AoA goes to extreme values not obtainable in flight and keeps correcting.

If one AoA measures 75 degrees, which is complete nonsense, and the software dutifully calculates what the corrected static pressure would be given the airspeed measured at that angle of attack, it is possible you do get very silly things. This is pure speculation, as static pressure is not intuitively dependent on AoA, but once computers get stupid data and don't throw in the towel by themselves, all bets are off.

Hoppie

[Jeroen Hoppenbrouwers]

From AV Herald:

On Apr 4th 2019 The Aviation Herald received a video to demonstrate, how long it takes to trim from full nose down (trim position 0 units) to a normal trim setting (around about 5 units) via manual trim: the captain needed about 30 seconds to get the trim back into normal range and reported being tired afterwards, the first officer tried and failed needing one minute for one unit and then being exhausted. The source added: "This video was taken on a parked B738 which is the same physical centre console system as the B38M. The importance was to demonstrate how acutely aware crews must be of their Stab Trim position. In the case of ET302, with the trim passing 2.8 Units towards 0 due to suspected MCAS faults, with aerodynamic loads by increased relative wind, it is even more demanding, time and energy consuming and thus dangerous to recognise a mis-trim too late. In case of low level flight or severe CFIT (controlled flight into terrain) risk, the outcome is predictable."

http://avherald.com/h?article=4c534c4a/0023