B737 vs A320 4

[rb1957]

Given that both A320neo and B737max have the same (or similar) engines, why does the B737 have such trouble (presumably with the larger engine intake driving the need for the MCAS) whilst the A320 doesn't ?

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[IRstuff]

The 320 is 20 years newer, but I've read that Airbus has had similar issues with their full FBW early on, including a couple of crashes.

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[rb1957]

yes, but the issues are supposed to be due to the larger intakes of the new engine, catching more air at high AoA, causing the side force to be non-linear, requiring the MCAS to linearise (though I would have thought that a smart feel unit could've accomplished the same end … but probably more work).

So maybe it is the difference between powered (manual) controls and FBW (ie FBW easier to linearise stick force) ?

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[SwinnyGG]

It has nothing to do with the control scheme.

The Airbus platform has taller landing gear- meaning when they added the new geared turbofan and its much larger cowling, there was room underneath for the engine in the same position as the old one(s).

The 737 has shorter gear and is closer to the deck. The new engine did not fit in the 'conventional' location. In order to fit the larger engine and cowling, they had to move the engine up and forward on the wing.

The end result is that while both the Airbus and Boeing airframes experience some changes in lift due to the engine cowlings (larger magnitude lift vector), on the Airbus the location of this additional lift vector, relative to airframe Cg and Cp, did not move.

On the Boeing, the lift vector generated by the engine cowling is applied in a different location AND is higher in magnitude than the old engine cowling lift vector; this means it has much more impact on the airframe's handling characteristics in certain situations.

The idea of "why didn't they just extend the gear" is a non-starter. Lengthening the landing gear would have necessitated structural changes to the airframe which would have triggered the recertification they were trying to avoid.

 

[debodine]

Some information that may help with understanding is the the pitch control (old style control wheel, side joystick in FBW, etc) controls the elevators for adjusting the pitch attitude. MCAS affects the horizontal stabilizer for pitch trim. Two completely different flight controls, although they are interrelated because the primary purpose of trim is to relieve the flight crew of the need to hold pressure on the control wheel or joystick to maintain a specific attitude/speed.

The typical sequence of action: The pilot moves the control wheel/applies pressure to the joystick to establish a new pitch attitude. This deflects the elevators, and the elevators experience dynamic air load that tries to force them back to neutral (air load balanced on both sides of the elevator). The pilot has to hold the pressure against this air load to maintain the desired pitch attitude.

So the pilot adjusts the trim to move the horizontal stabilizer to do two things: 1) take over the air load to maintain the attitude, and by doing so, 2) remove the air load from the elevators so the pilot can release the pressure on the control wheel/joystick.

Now for a bit of speculation on my part as I am not an aerospace engineer. My guess is that Boeing, during their flight testing, decided that moving the elevator did not provide enough authority to get the nose down in the specific critical situation, so they decided to have MCAS control the trim. The horizontal stabilizer is far larger than the elevators, so it has far more authority to adjust the aircraft flight patch compared to the elevators.

Of course because it has far more authority, if the controller moving it (human or machine) moves it incorrectly, the situation gets more critical MUCH faster.

Hope this helps.

 

[SwinnyGG]

Boeing, during their flight testing, decided that moving the elevator did not provide enough authority to get the nose down in the specific critical situation, so they decided to have MCAS control the trim.

I see how you got to that hypothesis, but it's not quite right. The elevator has equal-or-better authority to the full stabilizer, depending on pitch attitude/speed etc. It is much smaller, yes, but has a MUCH larger range of travel. The full authority of the elevator is capable of counteracting the rear stab, but the control forces required to hold the elevator in that position are extremely high.

MCAS exists for one reason: because the additional lift vector from the larger, relocated engine cowlings causes control force non-linearity under certain conditions.

Regulations state that as the airframe approaches stall, there must be a linear increase in control force. Every plane with a type rating (i.e. every plane) operates this way, Cessna on up. Pilots learn quickly, and it becomes ingrained, that increasing control forces mean increasing angle of attack and that their aircraft is approaching some edge of the flight envelope.

The new engines created a situation where under certain conditions, increased angle of attack would result in reduced rate-of-change of the control column forces. That's a counter-intuitive behavior for every pilot in the world, and also a dangerous control system behavior in general.

MCAS' sole purpose is this: when the flight computer thinks the aircraft is approaching pitch attitude that will result in a stall, it adds nose-down trim so that the pilot's control forces continually increase up to the point where the aircraft enters a stall.

Electric, automatic adjustment of rear stab for the purpose of trim already existed; MCAS is just a little piece of software that activates the automatic trim system in a very specific situation, in order to provide control force linearity.

 

[rb1957]

oh what a tangled web we weave ! nice post.

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[GregLocock]

"The new engines created a situation where under certain conditions, increased angle of attack would result in a reduction in stick force. That's a counter-intuitive behavior for every pilot in the world, and also a dangerous control system behavior in general."

Oddly that's exactly how cars are set up. As you approach the limit of grip the steering goes light (for reasons to do with how tires work). You can mask it or even I suppose obliterate the natural signal, but it's what drivers are used to.

Cheers

Greg Locock


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[3DDave]

The larger engines don't create a reduction in force - they create a reduction in the rate of stick force vs AoA.

No doubt it's why the addition of the software was given such a low priority for safety review - it's designed effect is really small. I still blame the Systems Engineers who thought this up as a work-around for not evaluating all the failure modes on the aircraft that would cause MCAS to operate.

 

[GregLocock]

Yes, that's one FMEA that's going to receive a lot of attention.

Cheers

Greg Locock


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[verymadmac]

Commercially the biggest difference is that the 737 can't take containers in the hold. The 737 has a much small fan diameter than the same engine on the A320.

They have lengthened the gear on the 737, the Max 10 (I think), is fitted with a strut compressor as they need to length the gear to prevent tail strike. All in all this is clearly one stretch to far for the 737.

The cause of the issue with the MCAS appears to be incredibly simple, they considered it to be part of the trim system, when in fact MCAS acts as a crude FBW system (it changes the stick force gradient) as such subject to much stricter certification requirements. Its a pretty fundamental failure of certification requirements that should have been caught by a quite a few people before it got anywhere near the FAA.

 

[SwinnyGG]

The larger engines don't create a reduction in force - they create a reduction in the rate of stick force vs AoA

This is a better wording than what I used, and Dave is correct.

Thanks for the correction, I've edited my post to clarify.

 

[rb1957]

they lengthened the NLG … using a "shortening link" … a link that pivots during retraction to shorten the length of the stowed LG. Harrier has one.

another day in paradise, or is paradise one day closer ?

 

[3DDave]

Interesting about the landing gear:

https://www.flightglobal.com/news/a...etails-737-max-10-landing-gear-design-451546/

This seems limited to the MAX 10. On the rest of the MAX line only the nose gear is longer, to provide engine clearance.


On a side note, it looks like someone cross-connected the roll-spoilers on a German government plane leading to a plane that was nearly uncontrollable in roll. The pilots apparently struck both wingtips before getting it stopped on the ground. Unlike a simple reversal, the roll-spoilers work in conjunction with ailerons, which were hooked up correctly. So, up to a small roll input everything is good. A tiny bit more and it all goes to hell. Unfortunately no one can really check roll authority on the ground, so there's a cockpit display that shows deployment vs input. However, unfortunately, some pilots are apparently checking to see the surfaces reported moving, not that the correct ones were doing so. Gahhh.

https://www.spiegel.de/video/berlin...landung-von-regierungsjet-video-99026533.html

(The video ending is archival footage.)

 

[debodine]

jgKRI and 3DDave:

I really appreciate the clarifications as I am trying to advance my own systems and aeronautical knowledge whenever I post. So if I take what you have shared and apply it to the Ethiopian aircraft accident, I am wondering if my thought process is reasonable.

If I recall correctly from the report, the flight crew should have reduced thrust to a specified level and established a specified pitch attitude as part of the memory items for airspeed problems, which began at lift-off along with the stick shaker. Per your explanation, after flaps full up and autopilot off (the conditions necessary to allow MCAS to operate), as the trim ran AND under the control of MCAS, the elevator would have the authority to counter-act the trim with what I believe would be two caveats:

1) The control force to hold the elevator would increase as the trim moved AND;
2) It may have been possible for the aircraft to reach a region of the flight envelope at very high speed (I think the overspeed clacker was operating too, likely due to the high thrust that established an overspeed condition) where the force to hold the elevator was greater than the pilots could maintain while still attempting to troubleshoot and manage other systems on the flight deck.

Is this reasonable or am I still off-base?