Originally Posted by
PEI_3721
Takwis, #2469, IFF789, #2467,
It is difficult to understand how MCAS is wrong in principle. As a fix for a stability problem identified during flight test, the design (theory) was adequate (expedient, quick, low cost) although an ‘inelegant patch’ compared with changing the underlying aerodynamics - aircraft structure.
The weakness of MCAS was in its implementation - engineering, and thence safety assessment and certification.
An outstanding puzzle is why MCAS is based on AoA opposed to speed in order to cure what appears to be deficient trim-speed stability. From this arises the (false) association with stall issues, yet the existing stall alerting and protection systems remain unchanged.
Why use AoA opposed to speed?
Modifying MCAS to provide engineering and certification integrity, even if the first proposal was inadequate, should not be an onerous task within the current timescale; thus the additional issue of high trim-wheel forces could be a considerable challenge resulting in further delay.
The original reason for MCAS was found in the engineering sims long before the actual aircraft was built. It was used to compensate for the control forces in a constant speed high bank angle and load factor level turn. IIRC around 250 kts. It was based off a G switch and an AOA.
It was later co-opted for the low speed regime once they started flight testing. The speed is irrelevant, it is the AOA that matters.
Also the STS does have a AND trim function related to speed. But it is separate from mcas.