PPRuNe Forums - View Single Post - Boeing 737 Max Recertification Testing - Finally.
Old 7th Oct 2022, 11:56
  #760 (permalink)  
fdr
 
Join Date: Jun 2001
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Originally Posted by WillowRun 6-3
It was once the case that FAA was considered the "gold standard" insofar as certification was concerned (and if I've been misled by everything I've read or heard about the good old days, I'm sure someone here will straighten it out).

Now where we're at is that the MAX crew alerting system is at a technology level of something like "your father's Oldsmobile". (That is, if I've understood the discussions of this factor here.) Leave to the side the current question of whether the Congressionally imposed deadline will be retained in place or extended.

The point I'm interested in is, with Transport Canada and EASA stepping up their game, so to speak, and possibly having leveraged previous certification practices into higher fidelity with current levels of technology, will FAA return the courtesy, and begin to see the work of those CAAs as now constituting the gold standard? I mean, this isn't to disrespect or disparage the FAA whatsoever . . . but just as elections have consequences, so also do certification debacles, especially ones involving 346 fatalities.

And on the engineering and/or technology isn't it widely accepted by people who do understand the aerodynamics (I'm not among them) that the MAX still has questionable, or at least subpar, stability in some parts of its flight regime? It seems unlikely either TC or EASA would have allowed this.

Also, if TC and EASA gain higher levels of recognition for setting the standard, will China's CAAC follow suit?
History...

The B737 as it stands with the Max meets the requirements of Subpart B to Part 25... however, the inherent issue of the stabilizer-elevator problems when manual trim is applied with a substantial miss-trim condition is not a pleasant feature of the aircraft. The 737 is the last of the manual trim wheel backups to the stabilizer.

early on with the Max acceptance by EASA, the ELS related to stab trim was B-05/MAX: Longitudinal trim at Vmo
It covered:
  • CS 25.161(a),
  • CS 25.161(c)(3),
  • CS 25.1301(a) and
  • CS 25.1309(a)
The unfortunate matter was missed due to lack of information on system architecture and the pertinent comment was:

"The need to use the trim wheel is considered unusual, as it is only required for manual flight in those corners of the envelope".

As the Max introduced a second electrical circuit to the trim motor, EASA was slightly surprised. However, to the best of my awareness, the cut tout switches take out both yoke and pedestal trim switches, which left the manual trim as a necessity following a stab runaway, in order to comply with §25.161. Unfortunately, the fact that the manual stab trim can be defeated where there is a severe out of trim condition even within the envelope was either unknown or overlooked, it being a latent issue of the 737 design, which followed prior types that had accepted the issue through training and crew awareness.

The trim system could have the ratios changed which would be a significant design change, or have a second trim motor/power/control circuit to give full redundancy, that would then have full trim authority, like a B777, B744, or B747 (levers v alternate trim switches).

It still is a poor system and questionably compliant as it stands to §25.161 IMHO.
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