From Aviation Week & Space Technology magazine March 25-April 7,2019, pages 16-18:
FAA Reviews Enhanced MAX Flight-Test Data
Guy Norris Los Angeles
Boeing has completed a key certification flight test of enhanced 737 MAX flight-control computer software, marking a major step toward returning the grounded fleet to service.
The test flight, conducted using the first 737-7 variant of the MAX, validated a set of updates to the Maneuvering Characteristics Augmentation System (MCAS) flight control law as well as improved pilot displays. Although the changes are part of a set of upgrades developed in the aftermath of the Lion Air Flight 610 accident in 2018, they are also expected to address a similar set of control problems implicated in the crash of Ethiopian Airlines Flight 302 this month.
Boeing says it has been “working closely with the FAA on development, planning and certification of the software enhancement, and it will be deployed across the 737 MAX fleet in the coming weeks.” It adds, “The update also incorporates feedback received from our customers.” The FAA is expected to mandate the enhancement with an airworthiness directive at the end of March.
Although the company will not comment on the current status of flight tests, or say whether they have concluded, Boeing Chairman and CEO Dennis Muilenburg says: “[W]hile investigators continue to work to establish definitive conclusions, Boeing is finalizing its development of a previously announced software update and pilot-training revision that will address the MCAS flight control law’s behavior in response to erroneous sensor inputs.”
The software upgrade package, known as the EDFCS (enhanced digital flight control system), significantly modifies the MCAS that was introduced on the MAX to match aircraft-handling characteristics with those of the 737 Next Generation and decrease pitch-up tendency at elevated angles of attack. The MCAS changes are focused on three areas: improving activation logic, enhancing angle-of-attack (AOA) inputs and limiting stabilizer-command authority.
Boeing says the changes are designed to increase overall system redundancy, limit stabilizer trim commands in response to an erroneous AOA reading and retain elevator authority by limiting the degree of stabilizer command. The company is not yet detailing how the changes are being implemented. For example, Boeing has not clarified whether the AOA changes include adding more sensors or, as is considered more likely, revising the MCAS architecture to enable data from both the AOA vanes in the current configuration to be fed into both flight control computers.
A key modification to limiting stabilizer commands is a revision that allows only one trim application for each new trigger of the MCAS system. Under the original design, the MCAS trims the aircraft nose down by moving the horizontal stabilizer up at 0.27 deg./sec. for 9.2 sec., stops for 5 sec., then trims nose down again for 9.2 sec., and continues to do so until the trim reaches the stabilizer travel limit or the crew intervenes. Boeing says that, as before, the crew will retain the capability to override the flight control law using either electric or manual trim, or by following the existing runaway stabilizer procedure and using the cutout switches as reinforced in the Operations Manual Bulletin issued on Nov. 6, 2018.
The enhanced software was demonstrated for the FAA on March 12, the day after the Civil Aviation Administration of China announced the first in a wave of 737 MAX groundings around the world; the FAA followed suit on March 13. Aviation Week was told that the software upgrade certification load, dubbed P12.1, was flown on the first 737-7 developmental aircraft, 1E001.
The greater part of the 1-hr. 20-min. test flight was flown at medium altitude between 13,500 and 17,350 ft. in a racetrack pattern over southwest Washington state. According to data from the flight-tracking website Flightradar24, the crew performed a series of high AOA maneuvers to validate the performance of the revised MCAS. These included at least six conducted during initial ascent, followed by descents from 17,000 to around 14,250 ft., during which speed dropped from more than 330 kt. to less than 180 kt.
The aircraft was then flown to 17,350 ft. and 265 kt. before pitching steeply nose down and recovering at around 15,900 ft. and 295 kt. The maneuver was repeated at a slower speed, before a further test point was conducted during which the 737 descended steeply from around 15,550 ft. to 13,500 while speed increased from 180 kt. to almost 270 kt. Two further steep descents and recoveries were then performed before the aircraft leveled off and returned to Seattle for landing.
Along with the MCAS changes, Boeing also is updating training requirements and flight-crew manuals. The company declines to comment on reports in The Wall Street Journal that Boeing and the FAA have struggled to agree on the extent of some of these changes, particularly regarding the revised training procedure. Alterations are planned for the Airplane Flight Manual and Flight Crew Operations Manual, as well as new notes for the speed trim fail checklist in the Quick Reference Handbook. Other changes are being made to the Airplane Maintenance Manual and the Interactive Fault Isolation Manual.
Boeing has outlined updated training documents to advise pilots of the changes as well, but according to the Journal, the FAA has pushed for more extensive training, including a self-guided computer-based instruction course.