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2 Aug 2019 - 737 MAX FCC Redesign

Following on from a review of MCAS software changes, a further problem was discovered by the FAA around 17 June 2019. The FAA crew were simulating a Runaway Stabiliser during tests in Boeing’s MAX engineering flight simulator, also known as e-cab.

The FAA said that it had identified a new risk that would need to be addressed before the plane could be ungrounded. Under a scenario where a specific fault in a microprocessor caused an uncommanded movement of the plane’s horizontal tail, it took pilots too long to recognize a loss of control known as runaway stabilizer.

The FAA determined that:

  1. Line pilots would need more time to correctly diagnose the failure and execute the appropriate checklist.
  2. A computer chip malfunction could lead to uncommanded stabilizer movement during the emergency procedure.

One problem is that a specific flight control computer chip is processing data too slowly. It was uncertain whether the chip needs to be upgraded or if a software update can increase speed enough. All models of the 737 have two flight control computers (FCCs) but use only one per flight, flip-floping to the other FCC between flights. It is now believed that the redesign involves using and receiving input from both FCCs rather than one on every flight.

Another problem can be caused by cosmic rays hitting circuitry and randomly changing binary code from 0 to 1 or vice versa, are addressed by FAA certification standards but their effects were tested by FAA pilots in relation to the function of the MCAS by flipping five of the binary switches. Although the perfect storm of simulated failures would be an extremely rare occurrence in the air, the FAA said it had to be addressed. “While it’s a theoretical failure mode that has never been known to occur, we cannot prove it can’t happen, So we have to account for it in the design.” Double teaming the flight control computers mitigates that risk because each computer is constantly checking the performance of the other. If either detects a problem, neither will move the flight controls and the aircraft will have to be flown manually.

Boeing is hoping to complete the software redesign by the end of September to submit to the FAA for approval.

For more details on the FCC follow this link.

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*** Updated 18 Apr 2019 ***

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