fredag 20 december 2019

Can FAA Authorise an Airliner for Passenger Transportation with Negative Longitudinal Stability?

The new Boeing 737 Max came out from a hastened design process, where an existing airframe dating back to 1967 was equipped with larger more fuel efficient engines mounted further forward and higher on the wings to satisfy requirements on ground clearance. The Max showed a tendency to stall in climb and turn as an effect of the new engines apparently giving extra lift thus moving the centre of lift CL ahead of the center of gravity CG into possibly negative longitudinal stability. To compensate the software MCAS was installed with the objective to automatically pitch the nose down upon input from an angle of attack sensor. For details, see Boeing 737 Max Groundings.

It appears that the idea was to hide the MCAS to not distract the pilot, but with a malfunctioning sensor that led to the fatal crashes.

But a civil airplane for passenger transportation is required to have positive longitudinal stability (see also FAA 1962 4b.150) with then CL behind CG, which pitches the nose down on increasing angle of attack and thus, without software or pilot control, maintains stable flight without stall.

Apparently, Boeing did not have the capacity to predict the tendency to stall of the new design using Computational Fluid Dynamics CFD, and so make a redesign into airplane with better stall characteristics. DFS is CFD software with the unique capability of predicting stall.

How could then FAA authorise the Max, when it was clear from the presence of MCAS that the Max design did not fully meet a requirement of longitudinal stability?  Or was the MCAS hidden also to FAA? And how will it be possible for FAA to reauthorise the Max if the basic design is kept and only the software is updated? Is it possible to guarantee that the MCAS will always work as intended, if the flight characteristics cannot be fully explored in real life testing and neither in CFD simulation? The new DFS Flight Simulator under construction can answer this question.

The Swedish military jet fighter JAS-Gripen was designed to have negative longitudinal stability to allow quick turns, which required stabilising software. After two crashes the software was modified into slower turns, which was ok since the plane was not used for combat.

First crash:

Second crash (2.23):

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