Ethiopian 737 - same as Indonesia ?

[G-CIVA]

Worth a subscription

https://youtu.be/AgkmJ1U2M_Q

[Mariano]

Interesting article.

https://www.washingtonpost.com/investigations/how-the-faa-allows-jetmakers-to-self-certify-that-planes-meet-us-safety-requirements/2019/03/15/96d24d4a-46e6-11e9-90f0-0ccfeec87a61_story.html?noredirect=on&utm_term=.e5c696b49450

Regards,

Mariano

[Phil Bunch]

I found this NY Times article on the 737 MAX problems to be one of the more informative general news media articles on this set of issues.

https://www.nytimes.com/2019/03/16/business/boeing-max-flight-simulator-ethiopia-lion-air.html

[Hardy Heinlin]

My understanding is that there is no CG shift on the 737 MAX even though these big pods sit more forward. I rather think the pitch momentum is greater because the engines are higher. The "pendulum" is shorter.

Maybe this picture explains it better:

[skelsey]

My understanding is that there is no CG shift on the 737 MAX even though these big pods sit more forward. I rather think the pitch momentum is greater because the engines are higher. The "pendulum" is shorter.

Maybe this picture explains it better:

I see what you are saying, Hardy - but don't forget that with a shorter arm the force required to achieve the same angular displacement will be much higher (imagine the difference between tightening a nut with a spanner that is, say, 5cm long and one that is 10 or 20cm long).

In any event, the requirement for MCAS as I understand it is aerodynamic rather than thrust couple related: however it is the fact that the nacelles are further forward of the CG which gives rise to the issue (at high AoA the lift created by the nacelle has a longer arm to work with ahead of the CG and this gives a greater pitch-up effect).

This is incompatible with the longitudinal static stability requirements in FAR25 which requires stick forces to increase at a given rate as airspeed is reduced away from the trimmed airspeed. Hence MCAS - in the problem region some nose down trim is introduced, the pilot has to pull back harder to maintain or reduce airspeed and everyone is happy,  until the AoA data used to compute the MCAS demand is faulty...

[Hardy Heinlin]

... don't forget that with a shorter arm the force required to achieve the same angular displacement will be much higher ...

Yes, I take this factor into account. But I have difficulties to believe that this factor will eliminate the angular force amplification by 100%? The thrust vector is much greater than the aerodynamic and inertia forces of the counter-acting aircraft structure. This engine doesn't push a drill through granite; it's a rocket that pushes a tin can through the air ...


|-|ardy

[John H Watson]

Yes, I take this factor into account. But I have difficulties to believe that this factor will eliminate the angular force amplification by 100%?

I think it would be hard to analyse unless we knew the height changes and thrust changes (between different engines).

Does engine weight and position at different body angles factor into the equation?

[Hardy Heinlin]

I haven't yet understood the effect of the more forward located engine, but if that also shifts the average CG more forward, I understand that this aircraft provides less elevator maneuverability just by that forward CG alone, no matter whether it comes from the engine weight or cockpit weight.

[cagarini]

It's my interpretation from various sources that:

.) the lower arm ( due to higher engine "center of force" ) would actually contribute to a lower pitching moment but....

.) the nacelle design itself made it more "aerodynamic" and lift generating than the older nacelles and...

.) the shift forward of the "center of force" ( because AFAIK the engine was also shifted fwd ) all contribute to
the increased pitching moment when power is increased from a steady state situation.

[skelsey]

.) the shift forward of the "center of force" ( because AFAIK the engine was also shifted fwd ) all contribute to the increased pitching moment when power is increased from a steady state situation.

That may or may not be the case, but either way it is not the reason for MCAS -- it is NOT a thrust coupling issue. The FAR25 longitudinal stability tests are done at constant thrust, so what happens to the nose when thrust is increased is irrelevant.

All aircraft with underslung engines will exhibit some degree of pitch up moment when thrust is applied; this is not unique to the Max.

[cagarini]

.) the shift forward of the "center of force" ( because AFAIK the engine was also shifted fwd ) all contribute to the increased pitching moment when power is increased from a steady state situation.

That may or may not be the case, but either way it is not the reason for MCAS -- it is NOT a thrust coupling issue. The FAR25 longitudinal stability tests are done at constant thrust, so what happens to the nose when thrust is increased is irrelevant.

All aircraft with underslung engines will exhibit some degree of pitch up moment when thrust is applied; this is not unique to the Max.

Not saying it's unique to the MAX, of course, just saying it's potentiated in the MAX due to various factors that can aggravate the pitching moments due to thrust...

[Hardy Heinlin]

In any event, the requirement for MCAS as I understand it is aerodynamic rather than thrust couple related: however it is the fact that the nacelles are further forward of the CG which gives rise to the issue (at high AoA the lift created by the nacelle has a longer arm to work with ahead of the CG and this gives a greater pitch-up effect).

Is this because the thrust vector (turbine axis) is also slightly pitched up in relation to the fuselage axis?

If it were exactly parallel to the fuselage, I wouldn't understand why the more forward position would have a greater pitch-up effect.

Edit: Of course, when the aircraft AOA is high, the engine AOA is high as well. But that engine AOA doesn't change just by shifting the engine location more forward. I mean geometrically.

I imagine an engine with zero weight located 3 miles in front of the wings. If the thrust vector is parallel, it won't pitch up the aircraft more than it would when installed just 1 mile away. But if there is a vertical force component, it will. The longer the arm, e.g. 5 miles, the more easily it can work.


|-|ardy

[John H Watson]

I believe the problem is due to the somewhat larger inlet cowls on the new gen-X engines. There will be more drag and nose up lift at higher angles of attack (as there would be close to stall speed)

[G-CIVA]

Further very informative update .....

https://youtu.be/9Ts_AjU89Qk

[skelsey]

If it were exactly parallel to the fuselage, I wouldn't understand why the more forward position would have a greater pitch-up effect.

Edit: Of course, when the aircraft AOA is high, the engine AOA is high as well. But that engine AOA doesn't change just by shifting the engine location more forward. I mean geometrically.

I imagine an engine with zero weight located 3 miles in front of the wings. If the thrust vector is parallel, it won't pitch up the aircraft more than it would when installed just 1 mile away. But if there is a vertical force component, it will. The longer the arm, e.g. 5 miles, the more easily it can work.


|-|ardy

I'm not sure I follow, Hardy; the pitching moment caused by thrust is a product of the thrust force x the vertical distance from the CG.

When the aircraft is trimmed for level flight at a given thrust setting, there will be X Nm of nose-up torque which is counteracted by the sum of the other nose-down moments on the aircraft to give 0 resultant nose-up moment.

If the AoA is then increased (classically, by pitching the nose up and reducing airspeed) and all other factors remain unchanged then the nose-up moment due to thrust is also unchanged -- the thrust force has not changed, the thrust line remains in the same place relative to the aircraft's longitudinal axis, the CG of the aircraft has not changed, therefore the vertical distance from the thrust line to the aircraft CG has not changed either, therefore the pitching moment due to thrust must also be unchanged.

It is the lift from the nacelles (not the thrust couple) which is the factor; in the case of the Max we have a different nacelle design (which in itself may produce greater lift at certain higher AoAs) and, crucially, the nacelles are located further forward of the CG than in previous variants.

This is significant because the pitch-up moment due to lift from the nacelles is a product of the nacelle lift force and the horizontal distance between the nacelles' CofP and the aircraft CofG.

Greater horizontal distance from nacelle CofP to aircraft CofG = larger pitching moment.

[Hardy Heinlin]

Ah, you mean the nacelle body alone, its pure aerodynamic function as part of the wing surface, similar to that of a leading edge flap, in the sense that the larger nacelle area and its more forward location increases the overall "wing surface" at the leading edge side?

So the effect also occurs when the engines are shutdown?

I was misunderstanding you in the discussion about "thrust increase" and "thrust couple". I did understand that you don't mean the "thrust increase" momentum. But I didn't get that you exclude the "constant thrust" idea as well.

[Will]

If I am putting this together correctly, the Lion Air MAX8 had the MCAS activate because a faulty sensor gave incorrect information about the angle of attack. Do we have any evidence that something similar happened (faulty AOA information leading MCAS to trim nose-down) in the Ethiopian MAX8?

Well, it looks like we have that evidence now. Reuters and the Wall Street Journal are reporting that the Ethiopian 737MAX was also indicating a stall in the final moments.

It sounds like both flights may have succumbed to a very aggressive MCAS action in the setting of faulty AoA data.

[John H Watson]

https://www.pprune.org/10433110-post179.html

Interesting....  MCAS overrides the stab trim column cutout switches. Another line of defence removed from the 737Max.

[United744]

https://www.pprune.org/10433110-post179.html

Interesting....  MCAS overrides the stab trim column cutout switches. Another line of defence removed from the 737Max.

I KNEW IT!! Boeing are in so much s**t.

I've been saying this from day 1, but people rubbished me...

[skelsey]

https://www.pprune.org/10433110-post179.html

Interesting....  MCAS overrides the stab trim column cutout switches. Another line of defence removed from the 737Max.

Yes and no.

Have to be careful about the nomenclature here. MCAS does bypass the control column cutouts (that stop you trimming opposite column movement) because that is the whole raison d'etre of MCAS - to provide a nose-down force to pull against. It couldn't do that if it were inhibited from trimming nose down when you were pulling back, so for it to work it has to bypass the column switches.

As far as I have seen, however, it does not bypass the cutout switches on the pedestal.