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Comment by mhandley | Hacker News Reader

The 737 Max is not actually unstable without MCAS. The requirement is for increasing angles of attack to require increasing back pressure on the yoke up until the edge of the flight envelope. The lift from the larger nacelles placed further forward is enough to violate this requirement at high angles of attack. It won't take itself to a stall unless you keep pulling, but it doesn't take extra back pressure to get it there either.

Other airliners have had undesirable parts of the flight envelope. The 727 for example has a high T tail, and it's possible to get into a deep stall, which is not recoverable because at very high angles of attack the elevators are in the turbulent air behind the wings. In the UK, the CAA required 727s to be equipped with stick pushers to prevent them ever getting close to this regime. This was due to their experience with a BAC 111 that deep-stalled and crashed in 1963. I don't think the FAA ever required this though.

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> It won't take itself to a stall unless you keep pulling, but it doesn't take extra back pressure to get it there either.

It won't "take itself" but possibly just a very light pull would stall the plane in some situations, and that is the very safety problem which Boeing tried to solve with MCAS, while keeping silent on its existence, because the plane is behaving differently than the one for which the pilots trained. Which is the opposite of how the plane was advertised and sold.

So any time when the MCAS is off the plane behaves differently than the one for which the pilots trained, therefore "just keep it turned off" was also not an option.

And we don't know how "light" is light, it can really be the case that it's even dangerously light.

I'm not trying to excuse Boeing's shoddy implementation of MCAS, nor their hiding its existence from pilots - both of those are inexcusable in my opinion. But my understanding is that when working properly, MCAS would not kick in except with angles of attack that should not be seen in normal flight conditions. It's quite likely that most pilots would never have experienced MACS in operation during their career. Having said that, the full flight envelope should be available to pilots should they need it, without it potentially biting them. Without MCAS, that's not the case with the Max.

> my understanding is that when working properly, MCAS would not kick in except with angles of attack that should not be seen in normal flight conditions.

Was the need for MCAS really so infrequent ("never during the pilot's career" -- are you aware how seldom it is?) the planes would simply have MCAS completely disabled and fly as we speak. That would be the full content of the Boeing's "software fix.

It's obvious that MCAS has to be kept turned on as soon as no autopilot is activated. (I'd also like to know how non-redundant (i.e. max 2) faulty AoA sensors affect the autopilot, by the way -- having just read that the modern Airbuses have four of combined AoA-speed sensors).

"But it's just because of the regulations" -- don't forget these regulations are there to minimize the number of crashes. And the regulations eventually in this case reduce to as simple as "the pilots have to be trained for the plane if the plane behaves differently." Which they weren't, and the latest Boeing claimed on that topic, after the two crashes was that the "one hour iPad training" will be enough(!?) And we now even know that Boeing was even allowed to do the certification process for themselves instead of FAA doing this.

Main question for me is, why doesn't it use gyro meters for figuring out a rough approximation of AoA? So it would only kick in if gyro is above a certain boundaryb and the air speed sensor is below threshold. Then double up both the gyros and the air speed sensors, if anything disagrees give errors instead of activating. Even outside of aviation I find it incredible how naive the current design is.

DC-9;MD-[89]0;B717 have the same T-tail design, the deep stall was address by changes to the wing leading edge, not sure if they also have a stick pusher.

From a quick look at both planes it might be that going into deep stall in the 727 might be easier because the AOA needed is smaller, but that's just my quick guess

(For those who don't know, a Deep Stall happens when the horizontal stabilizer gets in the "air shadow" of the wings, hence you can't get airflow there to move the plane)

Some interesting discussion on t-tails and deep stalls http://www.rbogash.com/Safety/deep_stall.html

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