Ethiopian 737 - same as Indonesia ?

[John H Watson]

This is pure speculation, as static pressure is not intuitively dependent on AoA,

The block diagram in a schematic of a 744 ADC shows a line going from the AOA processor block/box to the "Static Source Error Correction block/box.
This makes sense on a 744 as the primary static sensors are on the pitot tubes, not on the side of the fuselage. Pitot tubes do change their angle to the wind (with variations in body pitch angle).

[Hardy Heinlin]

... with aerodynamic loads by increased relative wind, it is even more demanding ..."

I still don't get why the increased ram air should make manual trimming more demanding.

(1) If the surface axis is well-balanced, ram air will not produce any rotational force at all.

(2) If the surface axis is more aft (which isn't the case), ram air tries to increase any non-neutral trim and thus a brake or the trim gear friction needs to stop that "runaway" force. (Bad design, and not applied, obviously.)

(3) If the surface axis is more forward (which seems to be the case), ram air tries to center any non-neutral trim and thus a brake or the trim gear friction needs to stop that centering force. This means that increased ram air actually increases the stab trim centering force.

I conclude system (1) or (3) is applied.

If (3) is applied, the gear must incorporate enough friction to stop that self-centering force. And in that case I understand that the friction must be increased at higher airspeeds. And if so, it's not the increased airspeed per se that requires greater human muscles but it's the artificially increased gear friction. Such a design, all in all, doesn't look reasonable to me, and therefore I return to idea No. (1) -- or a mix of (1) and (3).


|-|ardy

[skelsey]

... with aerodynamic loads by increased relative wind, it is even more demanding ..."

I still don't get why the increased ram air should make manual trimming more demanding.

Don't forget that at the trailing edge of the stab is the elevator.

When deflected in an aircraft nose up position, the aerodynamic force will be in effect pushing the TE of the stab down... the same direction as a nose down trim input.

I would be unsurprised if, at even moderate airspeeds, this force renderered manual stab movement difficult to impossible without relaxing the back pressure to unload the stab.

Not much of an option at 1000ft AGL...

[Hardy Heinlin]

In my comment above I'm intentionally excluding the elevator component -- just trying to understand the principle of the rotational balance.

What force or brake keeps the stabilizer surface in its set position? (When the hydraulics are not in use.)

I guess it's basically the same as on other airliners: A "screw thread". (Not sure if that's the correct English term.) And the gear ratio is so high that the ram air cannot turn the screw. The advantages of the high gear ratio is that it makes manual control possible and it also holds the surface in its position. So that question is possibly answered. But what remains is the question on the rotational balance -- be it with or without elevator inputs.

On the 747, two hydraulic systems move the stab trim faster than 1 hyd sys does. This means one can notice a trim speed variation when the resistance force is higher than the hydraulic force. I conclude that a trim speed variation would also occur when the stab trim is far off-center, because of the increased resistance. But as far as I know (on the 747) the trim speed (of the current trim mode) is pretty constant along the entire trim range. That's why I assume the air load is very well balanced at any stab trim angle.


P.S.: When just 1 hydraulic system runs the stab trim, is the trim speed reduction by 50% really caused by the reduced hydraulic force or is there automatically a lower gear ratio in use to assure the single system can drive the trim?

[John H Watson]

What force or brake keeps the stabilizer surface in its set position? (When the hydraulics are not in use.)

On the 744, there are two types of brake. The primary brake is a purely mechanical double ratchet/clutch setup, but I don't understand how it works. The secondary brake is a hydromechanical brake. You need hydraulics to push against a spring-loaded brake to release it. The 737 stabiliser isn't driven by hydraulics. It has an electric trim actuator. It just uses the ratchet type.

P.S.: When just 1 hydraulic system runs the stab trim, is the trim speed reduction by 50% really caused by the reduced hydraulic force or is there automatically a lower gear ratio in use to assure the single system can drive the trim?

The two motors drive the jackscrew via a gearbox differential. Mechanical magic.

[John H Watson]

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.

Rumour has it that the MAX manual trim wheel is smaller than the normal NG trim wheel 

Are more turns required... or do you just have to apply more force?

[farrokh747]

Is there any word on the AOA failures on both aircraft? Apart from the MCAS not using a comparator for the L and R AOA, the problem seems to have begun with the AOA failure..  (The L one in both cases?)

It seems odd that the same unit would fail in 2 accidents, in 2 different areas, at more or less the same time of the flight phase...

Some wiring issue, or RF/EMF interference? Perhaps an issue with the A to D conversion inside the boxes? Incorrectly installed?  - the unit is the same type of synchro used at various places on the aircraft.. If i recall, the Lion air LHS sensor was replaced before the fatal flight, as the previous crew reported issues - can 2 sensors be faulty in a row? Possible...  but strange if true..

https://www.washingtonpost.com/business/economy/sensor-cited-as-potential-factor-in-boeing-crashes-draws-scrutiny/2019/03/17/5ecf0b0e-4682-11e9-aaf8-4512a6fe3439_story.html?noredirect=on&utm_term=.41b3feb6df1d

https://edition.cnn.com/2019/04/04/us/boeing-sensor-investigation/index.html

fc

[John H Watson]

If i recall, the Lion air LHS sensor was replaced before the fatal flight, as the previous crew reported issues - can 2 sensors be faulty in a row? Possible...  but strange if true..

They changed the wrong one. In regards to the MCAS function, the Left AOA feeds into the Left Flight Control Computer, the Right AOA feeds into the R Flight Control Computer. The maintenance engineers (and the pilots) didn't understand the automatic selection process for the AOA sensors.

It's probably something like the data selection process on the 744. e.g. first autopilot in command.

[Jeroen D]

P.S.: When just 1 hydraulic system runs the stab trim, is the trim speed reduction by 50% really caused by the reduced hydraulic force or is there automatically a lower gear ratio in use to assure the single system can drive the trim?

In hydraulic system, with parallel pumps, with all things being equal, the rate at which an actuator moves is a direct function of the volume of hydraulic fluid being moved by the pump (s). With two similar pumps running, switching to one pump, the actuator will move at half the speed.

Or am I missing something?

Jeroen 

[John H Watson]

Or am I missing something?

Hardy is talking about fixed hydraulic fluid/pressure driving individual hydraulic stabiliser drive motors, not hydraulic pumps moving fluid.

There is only one jackscrew which is being driven by a single shaft coming from a differential gearbox. The gearbox gearing/differential is being driven by the two hydraulic motors on either side of the differential gearbox.

[Jeroen Hoppenbrouwers]

Just asking because I am curious.

If, by defect or mistubing or miswiring, both hydraulic motors would turn their input shaft to the differential gearbox in opposite directions -- then the output shaft would remain basically static, right?  All kinds of planetary wheels whirring around inside but no actual work being done?


Hoppie

[Hardy Heinlin]

The 737 stabiliser isn't driven by hydraulics. It has an electric trim actuator. It just uses the ratchet type.

I see. So to get back to my original question, -- just referring to electromagnetic force now instead of hydraulic force -- would the trim speed decrease when the electromagnetic force remains constant while the ram air force is increasing due to the stabilizer surface rotating towards max angle?

The core of my idea was this: If the trim speed (in the current trim mode) remains constant along the entire trim range (which seems to be the case), one can assume that the stabilizer surface axis is pretty well balanced so that increasing airspeeds, i.e. ram air in general, have only little effect on the work required to turn the surface.

Or, in other words: Is the (motor driven) trim speed lower at higher airspeeds?

(Work is work, be it a working human muscle or a motor.)


|-|ardy


If there is a weathervane effect on the stabilizer surface (and no brake), I conclude:
When a higher trim force is required to turn the surface into a bad angle, a lower trim force is required to return the surface into a good angle.

If there is no weathervane effect on the stabilizer surface (and no brake), I conclude:
The required trim force is nearly the same at any angle and airspeed.

[Jeroen Hoppenbrouwers]

Lastly, if the gearbox has a ratio that is sufficiently large and the gears/jackscrew are well greased or with a ball bearing nut, then it is possible that the difference in required force at different airspeeds is low compared to the torque of the motor.

It isn't currently clear whether the Ethiopian pilot could not fight the air loads, or was "just" not used to the always heavy manual trim, pulling mechanics around by steel wires 50 meters long. I still think that manually winding 737 trim wheels may not be part of the training... and you get a very nasty surprise when the moment comes, especially when already in dire straits.

Hoppie

[Hardy Heinlin]

Just to complete the picture with Skelsey's suggestion, assuming the elevator position adds further air load to the stabilizer rotational force: If so, I conclude ...

When the elevator is in a command-nose-down position, the command-nose-down trim speed decreases.
When the elevator is in a command-nose-up position, the command-nose-up trim speed decreases.

In other words, when the elevator's tail goes down, the elevator pushes the stabilizer's tail up.

If this effect really happens and the pilots feel this effect in manual flight, the required nose-up trim force increases when the elevator is pulled and the airspeed increases.

[John H Watson]

@Hoppie

If, by defect or mistubing or miswiring, both hydraulic motors would turn their input shaft to the differential gearbox in opposite directions -- then the output shaft would remain basically static, right?  All kinds of planetary wheels whirring around inside but no actual work being done?

Sounds plausible in an unlikely"what if" situation )

Lastly, if the gearbox has a ratio that is sufficiently large and the gears/jackscrew are well greased or with a ball bearing nut, then it is possible that the difference in required force at different airspeeds is low compared to the torque of the motor.

Beyond my knowledge, but I assume the 3 phase electric motor would have sufficient force, with the aid of gearing, to move the jackscrew under all conditions. However, the aircraft was exceeding max airspeed at relatively low altitudes.

It isn't currently clear whether the Ethiopian pilot could not fight the air loads, or was "just" not used to the always heavy manual trim,

Not clear, but there are lots of posts on PPRuNe talking about how to unloading the force on the stabiliser by inputting elevator (to allow easier manual adjustment). However, no pilot is going to unload the stabiliser by pushing the stick forward at 1~2,000 feet.

[farrokh747]

https://www.youtube.com/watch?v=rxPa9A-k2xY     NG jackscrew in motion

https://www.youtube.com/watch?v=-IlsPnMPYHE   - 787 - here the motor seems to be fixed top down... and appears to move faster... ?

https://www.youtube.com/watch?v=mFq06tGuxak    - 777

for the 737:

The stabilizer trim actuator gets 115 volt, three-phase, 400 Hz
AC power. The stabilizer trim
actuator has an internal AC to DC converter that changes input
power to 270v dc necessary for the brushless DC motor.

Nor sure if the MAX is the same....

fc

[John H Watson]

Summary of new MCAS software

https://www.pprune.org/10443754-post3824.html

[Jeroen Hoppenbrouwers]

... with aerodynamic loads by increased relative wind, it is even more demanding ..."

I still don't get why the increased ram air should make manual trimming more demanding.

Don't forget that at the trailing edge of the stab is the elevator.

When deflected in an aircraft nose up position, the aerodynamic force will be in effect pushing the TE of the stab down... the same direction as a nose down trim input.

I would be unsurprised if, at even moderate airspeeds, this force renderered manual stab movement difficult to impossible without relaxing the back pressure to unload the stab.

Not much of an option at 1000ft AGL...

Oh dear... New stuff, mostly original research by Aviation Herald.
http://avherald.com/h?article=4c534c4a/0045

Excessive airloads on the stabilizer may require effort by both pilots to correct the mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually."

This procedure was first mentioned in Boeing's publication "Airliner" published in May 1961 stating (in relation to other Boeing aircraft, the 737 first flew in 1967): "To trim the stabilizer manually while holding a high stick force on control column. As the airplane changes altitude, crank in the desired trim change. Correct airplane attitude after a few seconds with elevators. Relax stick force again and crank in more trim. Repeat this procedure as necessary until proper 'trim' position of stabilizer is established."

[farrokh747]

http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/2018%20-%20035%20-%20PK-LQP%20Final%20Report.pdf

Final Report for Lion Air .....

[vnangli]

Seems like this report didn't go through some serious proof reading... Synopsis page has a date of 29th October 2019... They need to be a little more attentive than this...