Originally Posted by
PEI_3721
fdr,
Clarifying my point, scepticism that the Max can be flown safely, has nothing to do with MCAS.
If the trim, for whatever reason, exceeds a particular deviation then the likelihood of being able to recover is low. This is based on the two accidents and questionable, but likely correct simulation of the stick / trim forces in the NG.
PEI, we are close but still off concurrence on this, which may always be the case.
ET302 uncovered glaringly the loss of industry knowledge on the limitations of manual trim. MCAS put the aircraft in a condition that exposed the underlying problem. (JT610 was somewhat different in the consequences of the MCAS issue, highlighting the cognitive issues that can arise). Fixing MCAS is needed and is a straight forward engineering task to undertake, with regulator oversight involved to ensure success. The manual trim issue remains, and is a continued problem that can only be mitigated by redesign or training. The redesign would need to remove the possibility of exceeding the manual trim inout torque loads at all times, and that probably looks like another electric trim jack incorporated in the system. I doubt that is a simple solution. Aerodynamic mods to the elevator may increase the range of off trim that the elevator can directly counter, however that would logically increase the resultant load on the manual trim, and still requires a specific manoeuvre to be undertaken to recover the aircraft to a position where manual trim input can be effective. The operational reliability of the trim system prior to MCAS has been very high, but the out of trim remains as a possibility, but the consequences still need to be viable. Expecting a crew to develop a technique under duress was not viable, however with training the recovery would be achievable. The manual trim issue is a global issue for any type that has a secondary trim system dependent on manual force input, and that can only be reinforced by wholesale design change to a powered secondary system or by training to make manual trim viable.
The two possible mis trim cases are either ANU, or AND. The ANU is the type that the Falcon 7x encountered on descent into KL. The copilot of that aircraft understood the issue and effectively responded to the ANU trim error with bank angle. The AND is the MCAS type of problem, and is harder to cope with for the flight crew at low altitude. At high altitude the margins before encountering some nasty transonic effects are also problematic but the earth is not rushing up in the window complicating things quite so much. Non Max aircraft gain a pitch up moment from the speed brake, which the Max has reduced through its FBW system of the speed brake. The remaining simple pitch authority that remains is from thrust which is considerable at low altitude, and less so at high altitude.The sole remaining trim change arises from load movement inside the cabin, and that is surprisingly effective. On a calibration flight test a single person moving from the front of the pax cabin to the rear of the cabin was detectable as a stabiliser trim change. Getting pax to move would be a whole lot of crowd control, the current SLF generation has little interest in their being a component of their own safety in flight... Those are the tools that exist, and the technique to unload the trim to gain in trim condition again is able to be taught.
Mitigation of the Max MCAS issue starts with the knowledge of the MCAS problem and how it works, and then how to manage the cutouts to permit the return to an in trim case before finally isolating the MCAS. The potential for any manual trim case to end up out of trim to the point of exceeding manual torque input suggests doing mitigation training to give the crews confidence in the aircraft and a fighting chance to survive such events on all variants. We need to make up for the loss on 50 years of awareness