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.
STATEMENT OF ISSUE
The aisle stand trim switches can be used to trim the airplane throughout the flight envelope and fully complies with the reference regulation Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts.
In those cases, longitudinal trim is achieved by using the manual stabilizer trim wheel to position the stabilizer. The trim wheel can be used to trim the airplane throughout the entire flight envelope. In addition, the autopilot has the authority to trim the airplane in these conditions. The reference regulation and policy do not specify the method of trim, nor do they state that when multiple pilot trim control paths exist that they must each independently be able to trim the airplane throughout the flight envelope.
Boeing did not initially consider this to be a compliance issue because trim could always be achieved, even during the conditions where use of the aisle stand trim switch was required. Subsequent to flight testing, the FAA-TAD expressed concern with compliance to the reference regulation based on an interpretation of the intent behind “trim”. The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only.
EASA POSITION
Boeing set the thumb switch limits in order to increase the level of safety for out-of-trim dive characteristics (CS 25.255(a)(1)). The resulting thumb switch limits require an alternative trim method to meet CS 25.161 trim requirements in certain corners of the operational envelope.
The need to use the trim wheel is considered unusual, as it is only required for manual flight in those corners of the envelope.
The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides a compensating factor for the inability to use the thumb switches throughout the entire flight envelope. Furthermore, the additional crew procedures and training material will clearly explain to pilots the situations where use of the trim wheel may be needed due to lack of trim authority with the wheel mounted switches.
The trim systems on the 737Max provide an appropriate level of safety relative to longitudinal trim capability.