Originally Posted by
Smythe
The original MCAS was designed for 0.5 degree changes. Test pilots found out this did not work, and it was changed to the 2 cycles of 2.5 degrees.
5 degrees nose down, repeatedly, at lower altitudes is far more significant than stab runaway, and look at those effects. Wonder what the trim wheel revolution speeds are under MCAS...that has to be inspiring!
So, the flight testing showed the ac was inherently unstable in the test conditions. It would be very helpful to see what the conditions are that trigger the MCAS.
Again, no. The aircraft is not, by any stretch of the imagination, "unstable", let alone "inherently unstable" (in what way is that different from "unstable"). It just does not provide enough adverse stick force to satisfy a very specific certification requirement. The situations in which this becomes relevant are rare, and should not normally be encountered in regular flight. However, if such a situation develops (high angle of attack), it is almost always in a high-workload situation, which is why the aircraft should make it harder for the pilot to venture further into the undesired flight regime, by increasing stick forces. If the aircraft does not do so to a sufficient degree by aerodynamic forces alone, it is quite common to add a little electronic assistance function to bring it back in line, and make it easier to handle, especially under stress.
The solution floated here, which may be close to the truth, seems to be to use both AoA values as inputs to MCAS (they are physically available anyway), and only to act if both values consistently read high, and close to each other. If they disagree sufficiently, MCAS will disable itself. When the probability of getting into a real high-AoA situation (where MCAS would actually be required) and getting an AoA disagree event
at the same time (the result of which would be that MCAS were unavailable when needed) is shown to be sufficiently low, such a solution can be certified.
Since the failure mode has now been demonstrated to be of "catastrophic" severity, "sufficiently low" would be less than 1 in 10^9 flight hours),
Bernd