Boeing has to complete four design changes on the MAX before the type will be certified for re-entry into service, the Federal Aviation Authority FAA has outlined in its Notice for Proposed Rulemaking on August 3. The document, which has been anticipated for a few weeks, has entered a 45-day public consultation period.
The modifications required are:
1 – Installing updated flight control software with new control laws for the Flight Control Computer (FCC) operational program software. This includes the new software of the Maneuvering Characteristics Augmentation System (MCAS) that has played a pivotal role in the Lion Air crash in October 2018 and the Ethiopian crash in March 2019.
Based on erroneous input from the single Angle of Attack (AOA) sensor, MCAS corrected the aircraft’s attitude by commanding nose-down trim every five seconds. Boeing had downplayed the significance of this system to FAA regulators when developing the MAX and pilots were unaware of its existence on the plane, preventing them to respond correctly to attitude issues.
The software update will address this issue as MCAS no longer can interfere as aggressively as before. Before MCAS will be activated, it will monitor the status of the two AOA sensors now required to identify if one of them is operating incorrectly. If an anomaly is detected, the FCC will disable the speed trim system. MCAS will only activate once after a high AOA event has been sensed and subsequently only after the aircraft has resumed a low-AOA position again.
The FCC also limits the magnitude at which MCAS can command the horizontal stabilizer. Not the computer but the crew can manually control the aircraft using the control column.
2 – Installing updated display processing computer (DPC) software, so that an Angle of Attack disagree alert will be visible at all times. This used to be an ‘option’ to customers (the FAA says was an ‘error’) but will now become standard. More importantly, there will be two AOA sensors now to feed the FCC instead of one, offering redundancy which the original design failed to have – in hindsight to the surprise of regulators and operators of the MAX.
3 – Revising certain Aircraft Flight Manual flight crew operating procedures. When trained for the MAX, pilots got to get familiar with the type only through a limited digital Ipad-course. A set of emails from Boeing staff published last January made clear that the OEM had done everything to prevent expensive simulator training for cockpit crew in order to reduce costs for customers and downplay the extent of modifications made between the 737NG and MAX.
With crew unaware of MCAS and other modifications, they were unable to respond correctly to any issues that involved a sudden change in attitude. Regulators and airlines across the world have called on extensive crew training that includes the simulator, a requirement that has been addressed now by the FAA: “To facilitate the flight crew’s ability to recognize and respond to undesired horizontal stabilizer movement and the effects of a potential AOA sensor failure, the FAA proposes to mandate revising and adding certain operating procedures (checklists).”
The FAA will do an operational evaluation before finalizing these checklists, which include the Airspeed Unreliable, Runaway Stabilizer, and Stabilizer Trim Inoperative checklist. Extra checklists will be added to address a situation when the stabilizer trim system and MCAS are inoperative or out of trim. These are called the Speed Trim Fail and Stabilizer Out of Trim checklist.
The FAA will issue a draft Boeing 737 Flight Standardization Board Report documenting the results of the operational evaluation on pilot training.
4 – Changing the routing of the horizontal stabilizer trim wires. Since the grounding of the MAX and a thorough review of the type, the FAA became worried about the physical separation of wiring cables in the empennage. In case of damage to one set the other could easily sustain damage as well, the FAA reasoned, potentially leading to the loss of control as the stabilizers would be inoperable. The layout doesn’t comply with a 2007 regulation and needs to be addressed by rewiring.
In addition to these four modifications, the FAA also requires MAX operators to run an AOA check on each aircraft before it returns to flight in order to verify that the sensor is working correctly. The MAX involved in the Lion Air crash had an improperly repaired AOA sensor. Each aircraft also has to perform an operational readiness flight before it can enter commercial service.
Regulators will jointly review changes
Most of the modifications outlined in the proposal have been addressed by the FAA in Airworthiness Directives and picked up by Boeing, but the notice confirms what is needed to get the MAX back in service again. More than 4.000 hours of flight testing have been done to check the updated FCC software in addition to extensive human factors and crew load testing. Three certification flights using a MAX 7 were done late June/early July.
At last week’s HY1-results presentation, Boeing CEO David Calhoun and CFO Greg Smith said they hope the MAX will be recertified again in Q4 with deliveries to resume soon afterward. There are some 450 undelivered MAX on the ground that all need the modifications requested by the regulators. This might take a full year, with some modifications to be done by either Boeing or the operators themselves.
“The FAA, through an operational evaluation, will assess the impact of the proposed aircraft design changes on pilot training. The FAA intends to conduct this evaluation jointly with three international civil aviation authorities in the Joint Operations Evaluation Board (JOEB): Agência Nacional de Aviação Civil (ANAC) Brazil, Transport Canada Civil Aviation (TCCA), and the EASA. 19 The FAA will issue a draft Boeing 737 Flight Standardization Board Report documenting the results of the operational evaluation on pilot training.” EASA and other regulators have said on numerous occasions that they will follow their own procedures before they will recertify the MAX.
