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Comment by techslave | Hacker News Reader

basic principles? i think that’s a bit far. the plane isn’t inherently unstable is it? i thought it’s just the black box of MCAS that is the problem and MCAS is there to avoid a lengthy type certification. it’s completely explainable.

and it wasn’t seemingly insignificant. it was for fuel efficiency. it was significant and they knew what they were doing.

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Seems to me that redundancy in mission critical systems where failure means loss of several hundred lives is a basic engineering principle. MCAS apparently relied on a single AOA sensor's input. The system had no way to validate the data or handle a mismatch, and the only recourse was to light up an (optional) disagreement indicator - but still relied on just the one sensor to apply control forces.

They had to install MCAS because the plane was unstable in high AoA mode, i.e. during takeoff.

And it was unstable because larger CFM engines could not fit under the wing anymore so they had to put it further forward and a bit up. This increased the pitch up moment when a full power is applied.

Also, can't really see how the height of the fuselage above the ground affects fuel efficiency.

"Also, can't really see how the height of the fuselage above the ground affects fuel efficiency."

It doesn't. They wanted to use a larger, more fuel-efficient engine but the 737's ground clearance couldn't accomodate it. So they moved the engine forward.

> They had to install MCAS because the plane was unstable in high AoA mode, i.e. during takeoff.

Forgive me if I'm missing something, but isn't MCAS disabled while the flaps are extended, which is the case during takeoff and landing?

Yup. The real danger was during climb I believe. And it wasn’t unstable, just had a backwards feel at high angle of attacks which could lead to pilots inadvertently stalling it.

You are right though pilots prefer to retract flaps soon after liftoff since extended flaps mean more drag and less efficiency. And the danger comes from the low altitude thus no margin for error.

In the doomed ET302 flight they retracted flaps 62 seconds after liftoff at only about 1200ft above the ground. MCAS kicked in 5 seconds later. Had autopilot been engaged it'd also had saved them from MCAS trim down commands.

Well, yes: the plane is inherently unstable. That's why the MCAS system is there. The system itself is mostly fine, it does what it's intended to under ordinary conditions. The problem was that Boeing, in order to avoid costs to the buyer, declared that pilots did not require training on it and made the instrumentation that manages the system woefully inadequate to the task of handling malfunctions (plus the system relying on a single sensor is pretty brittle but adequate malfunction handling instruments would've mitigated that).

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