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October 24, 2020, 16:17:02 pm

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737 Max

Started by SteveBB, September 20, 2020, 20:43:46 pm

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SteveBB


A sort of interesting account of the report on the above. Boeing has announced imminent sim testing on the mods they've made, and some lucky pilot will be taking it out to have a fly around to see if it works in the real world. Boeing and the FAA are reprehensible and Boeing are trying to stop lawsuits being brought against them by the families of those killed by the two crashes. But Covid aside, who is going to trust that it is airworthy when the 'new' version is certified?
Rimmer: Step up to Red Alert!
Kryten: Sir, are you absolutely sure? It does mean changing the bulb.

meharibear

As I've said before, ANY semi competent aeromodeller could explain the issue.  Underslung engines with upthrust is an immediate "no no"!  And then to bodge a fix with software is criminal.  They need to fix the thrust-line - that can be done with a new longer undercarriage to raise the intakes to a safe level above the runway (the reason the thrust-line was deliberately set wrong in the first place).  That will need telescopic legs to be able to retrofit it and will enable to software bodge to be thrown away.

Even better, bin the thing as "Not fit for purpose"!  we flew transatlantic in them several times before the crash and they are utterly dreadful and ill suited to long haul flights from a passenger viewpoint.
Inside every old person is a young person wondering what happened.  (Terry Pratchett)

FrankS

From what I've read it's not the thrust line, but the fact that the engines are moved forward so contribute different lift, this changed the control characteristics over the previous models. The software fix was introduced to restore those so pilots could move from the earlier versions to this version with a very simple (cheap) conversion.

Without this software fix the aircraft isn't intrinsically any less safe, but pilots from earlier versions couldn't fly it without more training, so it would cost an airline flying older 737s as much to convert to a Max as an A320 NEO.


PDR

Quote from: meharibear on September 20, 2020, 21:50:34 pmAs I've said before, ANY semi competent aeromodeller could explain the issue.
  Underslung engines with upthrust is an immediate "no no"!

And any competent aeromodeller would be wrong - it's much more complicated than that.

The problem is a conjunction of multiple effects - some aerodynamic (different wing downwash locus impinging on the tailplane yielding different transient trim response to airspeed changes, different engine efflux/stream-tube patterns massively changing the transient pitch response to thrust changes - especially rapid ones, a whole different set of pitch polars due to different aerodynamic body shapes around the inner wing area, all of which also feed to differnet downwash changes with pitch attitude and throttle etc etc), some kinematic (very different polar moments of intertia, especially about the pitch axis, different  masses, different airspeeds etc etc) which then produce a wholly new set of aerodynamic/kinematic characterisitics (aka "handling").

But none of this was un-manageable. The crew could be trained to recognise and address all of these issues as part of the type rating course - as they do with every other aeroplane. What turned "handling quirks" into "instant death on a stick" was the apparently over-riding requirement that 73*-max conversion had to be a 1-hour set of slides and a test, with no sim training (let alone actual aircraft time) because that's how they'd sold it to the airlines. So when it became clear that this was going to be difficult they bodged it with an external band-aid. A band-aid that ignored certification rules, ignored the principles of sound engineering design, added a primary pitch control to an FBW aeroplane which the FBW system was unaware of, and which was generally a complete lash up.

Those at the top who demanded it should be flayed, rolled in saly and lemon juice and then lowered slowly into a deep-fat fryerm (pour encourager les autres). But those engineers who stood by and failed to blow the whistle should all be sacked and barred from the profession for life. It's what we Professional Engineers call an "E5 issue" (Professional Engineers have whistle-blowing obligations even if it costs us our jobs - the "E5" refers to UKSpec Competence E5 on professional integrity - see also https://e5group.org.uk/). Professionals would be expected to resign and blow the whistle in such circumstances.

PDR
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