Thread: 737 MAX future
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Old 11th May 2019, 08:59
  #59 (permalink)  
Join Date: Mar 2006
Location: England
Posts: 842
The modified MCAS - (17Apr - any later version) would only require two sensors to disable MCAS.
The acceptability of this depends the ease of flight for ‘abnormal conditions’ and the probability of occurrence. This should not be a problem, MCAS is only used in a small part of the flight envelope, not likely to be encountered often. However, given that previous 737s are sensitive to nose up pitch with high thrust - GA mode, then the acceptability of ‘abnormal’ flight in the 737 Max without MCAS might be more questionable.
Also consider the extent of the differences between the 737 Max and previous models - ‘it will be the same’, except for MCAS, etc. Are the normal flight handling qualities of the MAX sufficiently similar to previous aircraft - margin to certification limit - this is not the same judgement as ‘same type rating’.

The confidence in the modified design might be further questioned by the discovery of inoperative AoA Disagree elements in some aircraft. The effect of this depends on interpretation - either a simple pin program error (customer option), or a software error preventing the option from working, N.B. where - AoA Disagree is now a major aspect of the modification. (In the ‘continued’ section)

The objective of the modification is to prevent large trim changes which could reduce the ability to fly the aircraft. If this is not assured, then MCAS would require much higher integrity - flight with MCAS disabled is judged unacceptable, or the software has insufficient integrity.
Then the system might require three vanes or alternative means of cross checking / use of sensors. Such integration will take time and may involve complex certification assessment given the history of events.

Re preflight vane checks; the validity of electrical connections might be checked, but not the accuracy of the AoA value, which like an aircraft requires the vane to be ‘flying’.

Re stick shake; AoA for Vsw is an alerting function - cross check with triple speed display.
AoA for MCAS is high-order safety action function - system disabling, no alternative.

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