Comment by theclaw | Hacker News Reader

Comment by theclaw | Hacker News Reader

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> The U.S. crew tested this by setting up a 737-Next Generation simulator at 10,000 ft., 250 kt. and 2 deg. nose up stabilizer trim. This is slightly higher altitude but otherwise similar to what the ET302 crew faced as it de-powered the trim motors 3 min. into the 6 min. flight, and about 1 min. after the first uncommanded MCAS input

That's not slightly higher, that's a lot higher. The graphs in the preliminary report [0] note that the flight starts at around 7,500 ft. since Addis Ababa Bole International Airport is 7,625 ft. above sea level [1]. Their radar altitude "about 1 min. after the first uncommanded MCAS input" was ~1,000 ft. The highest they got above ground was around 6,500 ft. around 5 mins 30 seconds into the flight.

> The excessive descent rates during the first two steps meant the crew got as low as 2,000 ft. during the recovery.

If you assume they meant they were 10,000 ft. above sea level in the first quote, then this quote means they'd be 5,625 ft. under ground at their lowest point.

[0] http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+...

[1] https://en.wikipedia.org/wiki/Addis_Ababa_Bole_International...

edit: oops, refs.

So if you run into this problem at 7,500’ above ground level you might just be okay.

This is why having the ability to disable MCAS without disabling electric assisted trim is so important and such a big factor in this accident. Additionally its raised another possible accident scenario:

The runaway trim cutouts were put in place after electric trim in case the switch (or some other aspect of the electric trim) got stuck in an on position that would cause the trim to go to full deflection one direction or the other, hence runaway trim. And hence if you notice this happening you cutout the power to the electric trim completely.

But now we know you can’t manually trim the aircraft above certain speeds with some trim levels. Should there be an expectation you can? What if runaway trim happens and ran to full deflection before a pilot cut it out. That would be equivalent to the same conditions experienced in this accident - the pilots now needing to manually trim but being unable to.

So on top of needing separate cutouts for the MCAS and electric trim, should the manual trim not be investigated for installing some higher ratio gearing/pulleys to enable over coming the forces experienced here?

There is the manual trim as a backup, but it's mechanical advantage is inadequate when the trim is too far out.

Right, which is why I'm saying that means there's been a failure scenario that existed even prior to MCAS. This is how they have demo'd the problem in some flight simulators for this. The electric trim can without MCAS erroneously drive the trim into this situation before being cut out. This is why the cutout switches were put in place decades ago. If this happens in some situations it could leaving the plane in a position where the trim couldn't be manually adjusted due to forces. We now know this situation also exists. Is it so rare, or resolvable in other ways, so it shouldn't be addressed? Or does there also need to be a fix for the manual trim by adjusting the gear/pulley ratio such that the pilot could overcome these forces?

What I read and makes sense the way to adjust the manual trim if the force on the stabilizer is too high is to push the nose down to unload it and crank on the wheel. Probably when this was needed the pilots killed the electric trim long before it got too far out.

So I agree the manual trim has always been kinda scketch.

Perhaps the intent was to perform the test at the same air density, as many aerodynamic properties, including stalling speed (and also engine performance) depend on density.

Also, it is more useful to know how much height was needed to recover, than that there was not enough. The latter can easily be determined from the former.

Altitude is always with respect to sea level.

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