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> On an unstable aircraft such as the 737 Max, if the trim fails then the nose pitches-up, precipitating a stall, unless the pilot can execute a manual trim. M

> On an unstable aircraft such as the 737 Max, if the trim fails then the nose pitches-up, precipitating a stall, unless the pilot can execute a manual trim. MCAS is a safety mechanism meant to suppress a very specific behavior in a very specific corner of the flight envelope where an airliner is not supposed to operate at all. Its mere existence doesn't mean that the MAX is an unstable plane. The 737 is still a positively stable airplane, just like basically all civilian aircraft. If the trim on such plane fails (gets stuck), it becomes annoying to fly, like a car that pulls to one direction. It doesn't do anything violent or sudden (the plane is trimmed correctly until the last moment) and it's possible to overcome the force with the control column and return safely.

jiofih 6y ago
An out of whack MCAS that pushes the nose down, to a point where you can’t overcome it with the trim control, technically puts it into the “unstable” category, doesn’t it?
- Toutouxc 6y ago
  MCAS moves the trim wheel itself, so you don't overcome it with trim, you overcome it with your control column. Also, MCAS moves the trim to an extreme position that is nowhere close to how the plane should be trimmed for normal flight. But the pilots were still able to overcome it with their control columns for quite a long time. A situation like that is usually called a "trim runaway" and it can happen for multiple reasons, MCAS being a rather exotic one, but they all have the same solution, a procedure that the pilots unfortunately weren't able to follow. The other way the trim can fail is that it gets stuck. When that happens, it usually gets stuck in a reasonable position (because the plane was flying normally before it happened), which makes the flight back annoying and renders systems that rely on trim input, like MCAS, autopilot, Speed Trim, inoperable, but it doesn't make the plane unsafe or uncontrollable.
  - jakeogh 6y ago
    If I understand correctly, ultimately they could not overcome it, and after many dives, the computer succeeded in crashing the aircraft... it's practically irrelevant long the pilots were able to overcome it. The base issue is that it was even possible for the control of humans with hundreds of lives in their hands to be overridden by a set of predefined instructions.
    - Toutouxc 6y ago
      It's very relevant that the plane was flying long enough for the pilots to go through the runaway trim checklist, which, if they went through the procedure, would have probably saved the plane. They didn't go through the checklist, because AFAIK they didn't realize immediately that it's a runaway trim situation, because of imprecise and/or conflicting indications in the cockpit.
      - jakeogh 6y ago
        It's got control of the trim wheels, and it's plunging the aircraft to the ground with nonsense trim settings. Right? Needing to consult the manual to disable that input is eh... not optimal. They physically could not trim the aircraft, and even worse (as if), could not -trivially- prevent changes to that trim. Should humans on the line have ultimate control of the surfaces or not? If so, how many steps through "complicated checklists" should be required to get control of a basic setting like trim? The base issue is that it was even possible for the control of humans with hundreds of lives in their hands to be overridden by a set of predefined instructions.

jiofih 6y ago

An out of whack MCAS that pushes the nose down, to a point where you can’t overcome it with the trim control, technically puts it into the “unstable” category, doesn’t it?

Toutouxc 6y ago
MCAS moves the trim wheel itself, so you don't overcome it with trim, you overcome it with your control column. Also, MCAS moves the trim to an extreme position that is nowhere close to how the plane should be trimmed for normal flight. But the pilots were still able to overcome it with their control columns for quite a long time. A situation like that is usually called a "trim runaway" and it can happen for multiple reasons, MCAS being a rather exotic one, but they all have the same solution, a procedure that the pilots unfortunately weren't able to follow. The other way the trim can fail is that it gets stuck. When that happens, it usually gets stuck in a reasonable position (because the plane was flying normally before it happened), which makes the flight back annoying and renders systems that rely on trim input, like MCAS, autopilot, Speed Trim, inoperable, but it doesn't make the plane unsafe or uncontrollable.
- jakeogh 6y ago
  If I understand correctly, ultimately they could not overcome it, and after many dives, the computer succeeded in crashing the aircraft... it's practically irrelevant long the pilots were able to overcome it. The base issue is that it was even possible for the control of humans with hundreds of lives in their hands to be overridden by a set of predefined instructions.
  - Toutouxc 6y ago
    It's very relevant that the plane was flying long enough for the pilots to go through the runaway trim checklist, which, if they went through the procedure, would have probably saved the plane. They didn't go through the checklist, because AFAIK they didn't realize immediately that it's a runaway trim situation, because of imprecise and/or conflicting indications in the cockpit.
    - jakeogh 6y ago
      It's got control of the trim wheels, and it's plunging the aircraft to the ground with nonsense trim settings. Right? Needing to consult the manual to disable that input is eh... not optimal. They physically could not trim the aircraft, and even worse (as if), could not -trivially- prevent changes to that trim. Should humans on the line have ultimate control of the surfaces or not? If so, how many steps through "complicated checklists" should be required to get control of a basic setting like trim? The base issue is that it was even possible for the control of humans with hundreds of lives in their hands to be overridden by a set of predefined instructions.