Originally Posted by
gums
Salute!
Good grief, Airbus has been flying planes since the early 1990's with zero control stick feedback from AoA or atual aero forces on the elevator, aileron etc.. I really need an explanation of how they got their 320 and following planes certified.
Gums ponders....
An earlier post by
infrequentflyer789 addressed this question (my emphasis). Is anybody in a position to comment?
MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures (My emphasis)
All (civilian) fbw a/c with neutral stability control laws (C*) are certified under special conditions starting with A320 (actually arguably with Concorde but that was such a different beast it must have been
all special conditions), those SCs, or at least the rationale, have more or less carried over since then (all later buses and other makers too now). I think even today (30+ yrs after A320 and much longer after the birds you flew) fbw is still referred to as "novel or unusual design feature" in certification
The rationale is that the control force cues for heading towards the edge of the envelope are not required if the edge of envelope is protected by hard limits / fences instead (similarly MCAS is not needed on autopilot because the autopilot is fenced/limited). Pretty sure this was well covered on the 447 threads, mind you so was everything...
Boeing took a different approach with 777/787, inventing C*U and providing a completely artificial feel on conventional controls, which I think allows them to meet the normal control force / stability reqs of part 25 without special conditions.
All these approaches fail in some way when stuff breaks and the a/c doesn't know (accurately) where it is in the envelope, AF477 ended up in a C* law without protection, C*U will inevitably reduce to C* if U is unknown (or deemed unreliable). There is a general principle (which turns into a requirement somewhere in the regs. I think) that any degradation of handling qualities must be inversely related to probability of occurrence and seriousness of failure consequences - that is the bit of the design and approvals process that MCAS seems to have got fatally wrong.