Ethiopian Airlines 737 crash discussion

[PeterDonis]

away with mcas, train the pilots and let them fly the plane

Unfortunately, that's not possible with the 737 MAX. See below.

My point of contention is you implying the plane is unstable

Under appropriate conditions, it is: the control force needed to pitch the plane up further is not supposed to decrease at increasing angle of attack. But on the 737 MAX, it does.

and in turn requires mcas

It requires some way to deal with the control force issue, because there are FAA regulations that require the control force required to pitch the plane up further to increase with increasing angle of attack. Those regulations are there for good reasons, which have been explained well in some of the online articles on this subject. MCAS might not be the only way to meet that requirement for the 737 MAX, but for the 737 MAX it is required to be done somehow, and that requires some kind of system to change the natural behavior of the plane because the natural behavior of the plane does not meet the requirement.

 

[PeterDonis]

It's quite clear that the start of the issue, in both cases, is the aoa sensor / reading.

The flight characteristics of the plane, in no way, contributed to the crash.

This is correct--the crash was caused by faulty AoA sensor data, the plane was never actually in the flight regime that MCAS was intended to deal with--but it is also irrelevant to the question of whether MCAS, or some system that performs a similar function, was needed on the 737 MAX. It was; see my previous post just now.

 

[nitsuj]

It requires some way to deal with the control force issue, because there are FAA regulations that require the control force required to pitch the plane up further to increase with increasing angle of attack. Those regulations are there for good reasons, which have been explained well in some of the online articles on this subject. MCAS might not be the only way to meet that requirement for the 737 MAX, but for the 737 MAX it is required to be done somehow, and that requires some kind of system to change the natural behavior of the plane because the natural behavior of the plane does not meet the requirement.

Ah I see. That makes sense to me; I envision that the stick feel being that way would "mask" the "traditional" "feel" of approaching stall. That would be dangerous for obvious reasons.

Thanks for the clarification and clear explanation! :D

Not sure I'd call that an unstable flight characteristic, but see how it would not be a good idea to have "very" different stick feels among passenger airliners.

 

[nitsuj]

 

[nitsuj]

Under appropriate conditions, it is [an unstable plane]: the control force needed to pitch the plane up further is not supposed to decrease at increasing angle of attack. But on the 737 MAX, it does.

In the vid I linked, it shows the regulation for plane stability and says "[when climbing] the stick force curve must have a stable slope at speed of 85-115% of the speed at which the airplane is..." then lists normal conditions, flaps up gear up trimmed ect.

And apparently sans mcas, the 737 max is certified under this regulation. I take that to mean the stick feel is "okay as per FAA".

 

[PeterDonis]

@nitsuj, whether you want to use the word "unstable" is a matter of choice of words; the term has multiple possible meanings. I was referring specifically to stick force, as I said, and I note that the regulation for "plane stability" includes a stick force requirement, so it certainly seems like the FAA thinks stick force is relevant for plane stability.

The person in the video you linked appears to be using a different definition of "stability", which basically amounts to: if you neutralize all of the controls, what does the plane do? An inherently stable plane by this definition will return to straight and level flight if you neutralize the controls; an inherently unstable plane will not. AFAIK the 737 MAX is stable by this criterion, and I think that's what the person in the video meant when he said the 737 MAX is not inherently unstable. But, as noted above, even the FAA regulation for "stability" does not limit itself to just this criterion.

apparently sans mcas, the 737 max is certified under this regulation

I'm not sure where you're getting this from. I don't see where the video says the 737 max meets the stick force requirement without MCAS. And there are a number of references in the other PF thread related to the 737 MAX (which is in the Mech Eng forum IIRC) to documents that specifically state that without MCAS the stick force required to pitch the plane up decreases with increasing angle of attack.

Also, the video does say that the 737 MAX requires MCAS to meet the type certification for the 737--which means it needs MCAS to handle similarly enough to previous 737 models to be classified as the same type of aircraft and not require additional training for pilots to be certified in it. Part of "handling" is stick force, and MCAS is required to make the stick force similar to previous 737s.

 

[Roberto Teso]

Primary function of MCAS is "to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA", and, no less important but secondary "give a steady increase in stick force as the stall is approached as required by FAA regulation" reference http://www.b737.org.uk/mcas.htm.

MCAS was introducted primarly to respect previous 737 NG flight envelope, so no new training required for pilots, and, secondarily, to respect FAA required force on stick while AoA increase, none of the two things 737 Max could respect without MCAS.

 

[nitsuj]

thank both!

I'll poke around in the other 737 thread for those references; Thanks again Peter!

 

[nitsuj]

Below Juan mentions the pitch up tendency to be a "common" flight characteristic.

I've not yet looked for FAA rules on the stick feel needing to be a specific way for ALL "commercial" jets; now would be surprised to see it exists.

He seems to state very clearly the point I've been making, well trying to make; based mostly off info from his previous vids.

 

[PeterDonis]

Below Juan mentions the pitch up tendency to be a "common" flight characteristic.

Yes, as he says, any aircraft with large, powerful engines mounted under the wings will have this tendency. (The engines also need to be forward relative to the wings, i.e., under the leading edge--look at a 757 or 767 to see how their engines are.) So when an aircraft type like this first gets certified, whatever stick feel is considered the best balance between this pitch up tendency and the desire not to have stick force to pitch the aircraft up decrease as angle of attack increases. And the manufacturer will demonstrate all this to the FAA when the aircraft type first gets certified.

But the 737 (prior to the MAX) did not have this same tendency, because its engines were smaller and mounted further back under the wings. So the original 737 type certification was based on a plane that did not have this tendency, and whose stick feel was agreed on without such a tendency being there. Then the 737 MAX got newer, more powerful engines mounted further forward on the wing and got a pitch up tendency it didn't have before. The safest thing to do at that point would have been to certify the 737 MAX as a new aircraft type; but Boeing didn't want to do that because it would cost them a lot of money and take a lot more time. So they put MCAS into make the stick feel like the previous 737 models, so they could convince the FAA to let the 737 MAX fly under the existing 737 type certification.

I've not yet looked for FAA rules on the stick feel needing to be a specific way for ALL "commercial" jets

The rules don't say the stick feel has to be exactly the same on all aircraft types. They are just general rules that say things like "the stick force required to pitch the aircraft up should not decrease with increasing angle of attack". There are lots of ways to satisfy that requirement, resulting in lots of different possible ways the stick will feel to the pilot in actual flight. When you're designing a new aircraft type for certification, you therefore have a lot of freedom to "tune" the stick feel while still meeting the requirement. Pilots then get trained in the new aircraft type so they are familiar with how it feels to fly the plane.

But once you've certified a given aircraft type, all aircraft of that type are supposed to feel the same way; that's part of what a type certification means, so that a pilot who has flown one aircraft of that type has enough familiarity with how it feels to fly any aircraft of that type without additional training. That's why Boeing had to make the 737 MAX feel the same as all other 737s in order to keep it under the existing type certification.

 

[nitsuj]

Feels a bit strawman peter.

The iteration Juan says in the vid is exactly all I've ever said in this thread.

Effectively so did you above. Which imo is odd and concerning; disappointing.

I actually took your earlier post as being true, and spoke to others about this stick feel requirement. Turns out it's "made up". If not "made up", not understood well enough to properly convey the point.

you're off on the stick feel. There is no stick feel issue with the 737 max. imo the below comment is irrelevant in this case. mcas is not there for that; at all. mcas is specifically for maintaining the flight characteristic.

"They are just general rules that say things like "the stick force required to pitch the aircraft up should not decrease with increasing angle of attack". There are lots of ways to satisfy that requirement, resulting in lots of different possible ways the stick will feel to the pilot in actual flight. "

Said it before, saying it again. no justification for mcas other than boeings economic concerns.
Zero need to have it from a "stability" perspective and ESPECIALLY not from a safety perspective.

It's an interesting perspective to see how you communicate; in a topic I know well. Now am curious about the replies you've made to my posts in the physics section.

 

[russ_watters]

I guess I need to go back and reread this discussion because I don't see a need for it to get heated. What I do see however is a lack of clarity of terms like "inherently stable" and "pitch-up tendancy" - they seem to contradict each other, but may not. I think it's important to remember that you can and should understand different factors separately while recognizing that the behavior of the plane as a whole is the sum of those factors. That means that saying the engines at high thrust give a "pitch up tendancy" doesn't necessarily mean the plane is pitching-up on it's own (without stick pressure or trim). The three factors are:
1. The center of lift is behind the center of gravity. This provides a constant "pitch-down tendancy".
2. The stabilizer exists to counter #1, but its "pitch-up tendancy" varies with speed.
3. The engines provide a "pitch-up tendancy". that varies with thrust and aoa.

And I guess we should add:
4...or maybe 2a: Trim adjusts the amount of pitch force applied for a given control stick input; in particular, the stick-neutral pitch-up force.

The feel of the stick or stick neutral behavior of the plane is the sum of all these separate behaviors. It would be interesting to see and compare some graphs.

This semantic argument is interesting but I don't think is particularly relevant.

If the argument is about whether the plane ever needs forward stick pressure and down-trim to avoid a stall and therefore violates FAA law, I highly doubt Boeing could get that past the FAA. In other words, I don't believe this is an inherent airframe flaw that would warrant scrapping the plane - if that's what is being suggested.

 

[russ_watters]

Going back over the discussion, some points:

That said I have no clue at what speed / alt / temp ect that the planes structure is at risk; or if pilots are informed of such high limits.

At the higher altitude at that airport they use a higher takeoff speed...

Primarily the pilots fly based on Indicated Airspeed, which is nothing more than a differential pressure reading scaled to speed. Altitude, temp, atmospheric pressure, wind and humidity are all in that reading so the limits don't change with weather and altitude. The Indicated Airspeed based takeoff speed, stall speed and Vne ("never exceed" speed) are constant and are marked on the dial (caveat: at least for light aircraft - for transport aircraft the first two might change with load).

 

[russ_watters]

Perhaps this is the bone of contention:

It requires some way to deal with the control force issue, because there are FAA regulations that require the control force required to pitch the plane up further to increase with increasing angle of attack. Those regulations are there for good reasons, which have been explained well in some of the online articles on this subject. MCAS might not be the only way to meet that requirement for the 737 MAX, but for the 737 MAX it is required to be done somehow, and that requires some kind of system to change the natural behavior of the plane because the natural behavior of the plane does not meet the requirement.

It sounds like you are saying you don't think the 737Max could get FAA certified at all without MCAS. I don't believe that's true because I think if it were it also couldn't be certified WITH MCAS!

Also, the "type certification" is the pilot's certification to fly the plane, not the plane's certification to fly at all (airworthiness certification). Keeping the type certification the same is a business convenience decision, whereas not being capable of achieving airworthiness certification would mean the plane couldn't fly at all. Note also: airworthiness certification is per plane, not per type of plane or model of plane.

 

[PeterDonis]

you're off on the stick feel. There is no stick feel issue with the 737 max

You said you would read through the other thread, which is in the Mechanical Engineering forum and is more focused on technical questions like this. Have you done so? There is plenty of discussion there that is highly relevant to this issue. Including references on why MCAS was done for the 737 MAX which talk about making the stick feel similar enough to previous 737 models to allow the 737 MAX to be classified as the same aircraft type. The entire discussion of that topic should really be taken to that other thread; the General Dicussion forum is not supposed to be for technical questions.

 

[PeterDonis]

It sounds like you are saying you don't think the 737Max could get FAA certified at all without MCAS

No, I'm saying the FAA wouldn't have allowed it to use the existing 737 type certification.

the "type certification" is the pilot's certification to fly the plane, not the plane's certification to fly at all (airworthiness certification).

Yes, I understand that. I have never claimed that the 737 MAX would not have received an airworthiness certification without MCAS. I have only said that MCAS was needed to allow the 737 MAX to use the existing 737 type certification--or, to put it more precisely in the light of your valid comment here, to allow pilots who had the existing 737 type certification to fly the 737 MAX without additional training.

 

[PeterDonis]

It sounds like you are saying you don't think the 737Max could get FAA certified at all without MCAS. I don't believe that's true because I think if it were it also couldn't be certified WITH MCAS!

Per my earlier post just now, I think we should take this discussion to the other thread, since we really shouldn't be delving into technical issues here in General Discussion.

 

[PeterDonis]

And for easier reference, here again is the link to the technical discussion thread:

https://www.physicsforums.com/threads/boeing-737-max-mcas-system.967958/

 

[PeterDonis]

I actually took your earlier post as being true, and spoke to others about this stick feel requirement. Turns out it's "made up".

No, it isn't. See the link I posted here in the technical discussion thread:

https://www.physicsforums.com/threads/boeing-737-max-mcas-system.967958/page-18#post-6176616

 

[PeterDonis]

It's an interesting perspective to see how you communicate; in a topic I know well. Now am curious about the replies you've made to my posts in the physics section.

You really need to check references before making a statement like this.

 

[OCR]

Also, the "type certification" is the pilot's certification to fly the plane

Russ, the pilot's certification to fly the plane, is called a type rating. . .

Also, see class rating.
They are very different than a type certificate.

.