Re high control forces and EASA certification document - 737 Max.
This might not generate a warm feeling about a runaway trim in any 737 version. If the text below also relates to 737 NG, does the FCTM identify these points, why there is an inability to trim, and a special procedure.
Also, is the thrust level considered; discussions re Max question ‘what if’ the AT remains engaged and is in a thrust / climb mode during the ‘dive’.
This document adds further light on the MCAS problems in the Max. Also, if the Max is similar or worse than other 737s then it might not meet the requirements due to a special procedure, if recoverable at all. #15
“This annex to the EASA TCDS IM.A.120 was created to publish selected special conditions / deviations / equivalent safety findings that are part of the applicable certification basis:”
Re line 1 typo? ‘switches’ should read ‘wheel’, consistent with ‘wheel’ later in the page.
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.
https://www.easa.europa.eu/sites/def...20ISS%2010.pdf
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