Originally Posted by
Less Hair
Is there any draft document available about the actual changes finally intended to be submitted by Boeing? If we are as close as claimed why is there no detailed info available?
It is clear some changes have been made and accepted (in principle) and it is clear it is a secret.
There are four main changes to the B737 MAX flight control system software that have been developed to prevent future accidents like the ones that happened with the Lion Air and Ethiopian Air flights. They include the following:
1. Angle of Attack (AoA) comparison – an addition to MCAS that will now compare readings from both angle of attack sensors on the aircraft. If there is a difference of more than 5.5 degrees the speed trim system will be disabled. Also included in this change is something known as a “midvalue select” which uses data from both sensors together to create a third input that will help to filter out any AOA signal oscillatory failures or spurious sensor failures. This modification will prevent MCAS from commanding nose down trim when a single AoA sensor reports a false AoA as it happened in the two accident flights.
2. MCAS resynchronization – this change will account for manual electric trim inputs made by the pilot while MCAS is activating. It will track whatever input the pilot makes and return the pitch trim to that setting when MCAS retrims back to normal.
3. Stab trim command limit – is an addition that will limit the maximum nose down trim that the automatic flight control system can command to prevent the pitch trim from reaching an uncontrollable situation.
4. FCC monitors – software monitors have been added to the flight control computers that will cross check pitch trim commands against each other. If a difference is detected by these monitors the automatic trim functions are disabled. This protection helps prevent erroneous trim commands from a myriad of causes that could occur in the automatic flight control system.