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Comment by adamjb | Hacker News Reader

edit: I was labouring under an illusion. Comment left up as a monument to my hubris

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>The airframe with the engines mounted differently did not have adequately stable handling at high AoA to be certifiable. Boeing decided to create the MCAS system to electronically correct for the aircraft's handling deficiencies.

While accurate, it omits that part of the certification Boeing were chasing was a common type rating with previous 737s on the basis of it having similar handling characteristics. MCAS did not exist because the 737 Max was unsafe, just different.

The reason they were chasing fleet commonality is because it would massively reduce adoption costs for airlines by making expensive training for existing 737 pilots unnecessary.

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This is a common miss conception, planes can behave quite differently, they already do depending on loading. Then you have common type ratings, e.g. A330, A340 and A350 or 777 and 787 which only requires a short difference training to move between types.

The real reason is that the aircraft does not comply with stick force regulations when approaching stalls.

The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.

http://www.b737.org.uk/mcas.htm

This is the aerospace equivalent of trying to correct boomy room acoustics with an equalizer: Yeah you can do that, but like painting over a cracked wall it wont fix the underlying issue.

And like the painted cracked wall, it might be okay in 99 out of 100 cases, but that one case where it isn't might kill a hundred people.

There are lots of commercial aircraft that don’t meet certification requirements without some form of artificial stability enhancement (yaw dampers being the classic example). There’s nothing inherently wrong with this kind of thing - you just have to get it right.

Out of curiosity, which airliners are like this?

The Boeing 727 is one example where the yaw damping systems are an important feature.

Yaw dampers in general are very common. Most (all?) airliners have them in one form or another. The reason for this is that swept wings inherently tend to increase susceptibility to Dutch roll. And yet we don't conclude from this that it's inherently irresponsible of aircraft manufacturers to make airliners with swept wings.

Interesting, thanks!

Hundreds (thousands?) of commercial pilots flew the Max and none of them even noticed MCAS until the crashes. It seems to have served its purpose remarkably well. The unforgivable sin was the brain dead engineering decisions around not sanity checking the AOA sensor.

I seem to recall reports of pilots disabling MCAS because it was acting up. They would disable autotrim or something, not sure it they even heard of MCAS.

> MCAS did not exist because the 737 Max was unsafe, just different.

That's arguable:

https://www.forbes.com/sites/petercohan/2019/04/02/mit-exper...

https://www.eetimes.com/software-wont-fix-boeings-faulty-air...

The placement of the MAX's engines due to ground clearance requirements lead to a flight behavior at low speeds which would be, if maybe not outright dangerous, at least very finicky if not for the MCAS.

Fortunately, Boeing's fix will just disable MCAS in several new situations and pilots will be left to figure out how to fly the finicky planes on short notice.

The scenarios where it is disabled without a sensor failure are also outside the flight regimes where MCAS should be activating.

If there is a sensor failure, the aircraft legally should not be dispatched (not meeting MEL), so its on the airline to make sure they don't push pilots into bad situations.

If the sensors are more than 5.5 degrees in disagreement, MCAS is disabled. This could obviously happen after takeoff.

> MCAS did not exist because the 737 Max was unsafe, just different.

You're saying the plane is "safe, just different" without MCAS? But Wikipedia says:

"The MCAS flight control law was implemented on the 737 MAX to mitigate the aircraft's tendency to pitch up [...]"

I'm a layman but it sure doesn't sound like safe or normal behavior for a passenger airplane to be fundamentally designed so that it "tends to pitch up"... is it? Are there other airliners that do such a thing?

The placements of the engines affected tendencies at some pretty extreme parts of the flight envelope as far as I understood, and MCAS helped make these tendencies similar to the previous 737. I guess an analogy could be that depending on the weight distribution and drive train in your car, it will have tendencies to oversteer or understeer when you go too fast into a turn in bad weather, or something. All cars have different behaviours and tendencies in these extreme circumstances.

So the 737Max would not "pitch up" in most normal flying, this is under high AoA situations, like a tight overspeed banking turn I think.

All jet airliners have a tendency to pitch up under thrust, because of the low position of the engines. The 737 Max has a whole other issue separate from that, caused by the aerodynamic effects of the engines placed far forward of the wings. Airplanes are not allowed to pitch up more unless the pilot applies progressively more and more backward stick force. The 737 Max has a tendency in certain conditions (high angle of attack) to pitch up even when the pilot keeps applying a constant force.

Some people argue that that behavior, without measures to prevent it, would disqualify the plane from certification completely. Others say that those measures are only needed to keep the Max under the same type rating as previous 737 generations. I'm inclined to the former, but I'm no expert so that's certainly not definitive.

This tendency being different from previous versions.

> The MCAS is intended to compensate in such cases [where the aircraft pitches up], modeling the pitching behavior of previous models, and meet a certain certification requirement, in order to enhance handling characteristics and thus minimizing the need for significant pilot retraining.

With proper training/re-certification pilots would be trained in the _actual_ characteristics of the plane, thus being able to correct for this behaviour.

Except Boeing also aggressively pitched that pilots didn't need to be extensively re-trained for the 737

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