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Comment by JshWright | Hacker News Reader

The MAX has flight envelopes that are inherently unstable. The plan can get into a state where the aerodynamics of the plane make a stall worse, not better.

For instance, the fact that the center of thrust is further forward (compared to other 737s) means that the plane pitches up when you give it more throttle. The default response to a stall is to increase the power, which pitches the nose up, which raises the angle of attack, which makes the stall worse. This inherent instability is why the MCAS was added to the MAX.

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Not a pilot, but I thought stall recovery is done through nose down first. Turbine engines take some time to respond to throttle up command and thrust on a loaded commercial plane is limited anyhow.

An airframe is supposed to have a nose down tendencies upon stalling, which would make it recoverable.

The Max may be more prone to stalling, but no one in their right mind would design a commercial airliner which can’t be recovered from stall.

> I thought stall recovery is done through nose down first

Exactly, which is why the nose-up tendency of the MAX is an issue.

Pitch up tendencies under power to my understanding don’t make it an unstable airframe. If you stall, cut power, let go of the controls and the plane responds with a recoverable dive, it’s aerodynamically stable.

Some airplanes can’t be (reliably) recovered from stall, that’s another issue altogether.

That's great at FL30. For lower altitude stalls, cutting the power isn't an option.

Except it's not because you don't sit around waiting for the engines to spool going "oh deer, we're still stalling, if this continues I'll have to push the nose down". You nose down and power up in quick succession. This is basically muscle memory that every pilot has going back to their first day in a 172. And as the person you're replying to mentioned, nose down is the first thing. You don't put the throttle all the way forward until after you've put the nose down.

The issue here is MCAS. If they just trained the pilots on the handling characteristics of the MAX the increased stall tendency compared to a normal 737 would be a non-issue and they wouldn't need MCAS as the handling is still within the bounds of "perfectly reasonable".

I'm not suggesting that dropping the nose isn't the first action to be taken. My point that increasing the thrust (which is going to try to pitch the nose up) is part of the standard response to a stall (particularly at lower altitudes).

Of course but if you're recovering from a stall you're going to be very aware of where the aircraft is pointing. You're not going to let the aircraft pitch itself up on you if you're in the middle of telling it to nose down. The danger with pitching up is that the pilot and copilot might be absorbed in some other tasks during a climb and not notice it leading to a stall. That's not really a big of a risk in grand scheme of things but it's still a risk and that kind of maneuvering gets customers complaining. The solution was using MCAS to detect the conditions prior to that kind of stall and deal with it.

I do not believe aerodynamic stability depends on whether the inputs change or not. For instance, stall a Cessna 172 and it will tend to pitch down on its own even if you keep pulling on the yoke.

Also an aerodynamically unstable plane isn't a plane in which a stall is irrecoverable. It's a plane that, left to its own devices, will not make things better for you.

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