Airlines flying the 737 MAX will face new restrictions on the operation of some autopilot-related systems when the type returns to service. The changes are detailed in revision 2 of the Master Minimum Equipment List (MMEL), published by the Federal Aviation Administration on 10 April 2020. The new rules remove exemptions for several systems, in effect increasing the required redundancies for dispatching the MAX when it returns to service.
The revised MMEL includes 12 changes to required equipment or redundancies on board. Previously, for example, a 737 MAX could be dispatched with no working autopilot systems so long as en route operations and approach minimums did not require the system, and the total amount of flying was acceptable to the flight crew. Under the new rules at least one of the two autopilot computers on board must be functional for an aircraft to depart.
The audible warning system for autopilot disengage and warning lights tied to Speed Trim Fail and Stabilizer out of Trim must now also be functional for a MAX to depart. Under the prior rules these were not required so long as autopilot was not in use or the the trim system was “verified to operate normally.”
Similarly, at least one of the autopilot command switches and lights must now function, tied to the working autopilot system on board. Ditto for the autopilot disengage light and Control Wheel Autopilot Disengage Switches. As the autopilot requirements increase these warning and control systems can no longer be exempted.
Finally, the Flight Controls section of the document was updated to require both of the Control Wheel Trim Switch Systems be operating properly. Previously the MMEL allowed for one “inoperative on non-flying pilot’s side provided control wheel trim switch operates normally on flying pilot’s side.”
The pilots of both the LionAir and Ethiopian Airlines fought against incorrect trim settings as the Maneuvering Characteristics Augmentation System (MCAS) erroneously pushed the aircraft into the ground. Given the critical nature of trim control in both accidents this change increases the odds that pilots could overcome another runaway trim issue. Or at least ensures that both have the necessary control systems in place to do so.
All of the MMEL changes are secondary, however, to the other revisions Boeing is making to the MCAS system and other facets of autopilot as it works to bring the MAX back to the skies. These include not allowing MCAS to repeatedly override pilot inputs.
It will also include as standard (previously only an add-on feature) an alert for angle-of-attack (AOA) disagree between the redundant sensors on board. MCAS depends on the AOA system to trigger its controls. In both crashes it activated when one of the AOA sensors indicated problems while the other reported normal flight. Lack of notification regarding that disagreement between the AOA sensors is believed to have contributed to the pilots’ inability to regain control in both crashes.
The new rules apply to both the MAX 8 and MAX 9 models. Presumably the MAX 10 will include similar requirements when its MMEL is released as part of the type’s certification.