Originally Posted by
Airbubba
Here's an explanation of the EASA position on the observation that the yoke trim switches on the MAX don't work throughout the entire speed envelope (with some highlighted text):
Explanatory Note to TCDS IM.A.120 – Boeing 737 Issue 10
EQUIVALENT SAFETY FINDING:
B-05/MAX: Longitudinal trim at Vmo
APPLICABILITY: Boeing B737-7/-8/-9 REQUIREMENTS: CS 25.161(a), CS 25.161(c)(3), CS 25.1301(a) and CS 25.1309(a) ADVISORY MATERIAL: N/A
STATEMENT OF ISSUE
The aisle stand trim switches (WHEEL) 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 (EACH FOR ITSELF OR SOME OR ALTOGETHER?).
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 (SAID IN 1ST SENTENCE ALREADY).
In addition, the autopilot has the authority to trim the airplane in these conditions.(OK, BUT WILL IT? WHAT DOES THIS MEAN?)
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. (THIS IS WERE THE DINNER WAS BOUGHT. WHEN THERE ARE MULTIPLE METHODS OF TRIM, EACH MAY JUST COVER A BIT OF THE ENVELOPE?? THAT'S RIDICULOUS)
Boeing did not initially consider this to be a compliance issue because trim could always be achieved, even during the conditions AS STATED ABOVE whereuse of the aisle stand trim switch (WHEEL) was is 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. (STH BEING "UNUSUAL" IS STRANGE FOR A CERTIFICATION COMMENT)
The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides (THEY MEAN "REQUIRES") 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 SHALL 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 (THEY MEAN "THUMB SWITCHES" !!!).
The trim systems on the 737Max provide an appropriate level of safety (SHOULD SAY: COMPLY WITH CS25....) relative to longitudinal trim capability.
Page 15 of 114
EASA aircraft certification document written by an intern.
Never challenged by his/her/its boss to explain what the writing means.
Four terms for the same subject (wheel), totally confusing
Confusing wheel and thumb switch in the most important sentence.
Aviation document quality.