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What damage could a co-pilot cause even with another pilot present? A steep enough descent might not be recoverable under certain conditions?

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Aircraft are designed to not enter those “certain conditions” by default - I understand the PF needs to override the flight-control-law to intentionally crash the plane, but I don’t think there’s a two-person-rule before that can happen.

Anyway - this is why I’m glad I’m not involved in safety-engineering because supposing that setting sensible defaults that reduce accidents in 99.999% of cases would lead to horrible terrifying deaths in 0.001% of cases (e.g. one-way lockable reinforced cockpit doors) would be too much on my mind.

Germanwings Flight 9525[0]

[0] https://en.wikipedia.org/wiki/Germanwings_Flight_9525

That was F/O without another pilot present

Some more modern aircraft indeed have such protections, but this was an old 737 without any such stuff. Simply pushing the control column fully forward would send the plane into a dive pretty quickly.

The controls are mechanically linked. Unless the perpetrator incapacitated the other flier, it’s a test of strength. There’s no method for disconnecting the second set of controls in a 737.

That is incorrect. There is a friction clutch between the controls in case there is a mechanical jam the other pilot can at least control half the control surfaces. It needs a significant amount of force to detach, though. It is documented on Egypt Air 990 however that both elevator surfaces turned into opposite directions.

Like @FeeJai already wrote, that's incorrect. Look up for Atlas Air Flight 3591 crash, boeing 767. Spatial disorientation, dual imputs on yokes, split elevator. https://www.pprune.org/rumours-news/628156-atlas-air-3591-nt...

I am not disagreeing with you, but I just want to point out the absolutely stunning safety of modern air travel. Using your 99.999% number against the number of FAA flights of 16,405,000[1] per year (that's just US FAA, not global). This would result in 16405 "incidents" per year. Luckily for us, the actual number is far lower than 16405.

[1] https://www.faa.gov/air_traffic/by_the_numbers/

Even if another pilot was present, from that altitude a suicidal co-pilot could easily cause an unrecovable dive and crash. If two pilots pull/push controls to opposite directions they kind of neutralize each other, and obviously the suicidal one would have element of surprise as an advantage.

In fact just that happened on EgyptAir flight 990. A suicidal copilot was able to crash the plane even with another pilot present, and that happened from a higher altitude.

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