Concerns Expressed Over 737 MAX Redundancy, Manual Trim
Boeing’s proposed fixes including software modifications that use data from both MAX angle-of-attack (AOA) vanes to activate the maneuvering characteristics augmentation system (MCAS) flight-control law implicated in two MAX fatal accidents that led to the grounding. If they disagree by more than a defined range, the MCAS, which provides nose-down horizontal stabilizer commands, will not activate.
“This is clearly an improvement on the original design” that used one sensor’s feed and meant MCAS could push the MAX’s nose down based on one feed of faulty AOA data, the British Airline Pilots’ Association (BALPA) wrote in comments to the FAA. “[But] it would be preferable for the system to utilize three AoA sensors (as per the Airbus A320 family of aircraft) in which case ‘voting’ can be implemented to discard an erroneous AoA value. There are other systems onboard the aircraft requiring AOA input, so how will they deal with two sensors that disagree?”
Guy Woolman, a former Southwest Airlines pilot with 12,000 hours of 737 experience but none in the MAX, seconded BALPA’s call.
“There should be at least a third AOA and airspeed input,” he wrote in comments to the FAA. “I had several ‘IAS DISAGREE’ messages while flying [737 Next Generation variants]. There is certainly a lot of startle as the crew is forced to search for reliable airspeed ... Couple this with other seemingly dissimilar non-normal conditions and it is easy to see why things could get challenging in a hurry. Why not have a standby AOA gauge?”
EASA is considering mandating introduction of a third AOA sensor, but it is not expected to be a prerequisite to allowing the MAX back into service.
In both MAX accidents, Lion Air Flight 610 in October 2018 and Ethiopian Airlines Flight 302 (ET302) in March 2019, faulty AOA data triggered the MCAS, which commanded unneeded nose-down horizontal stabilizer movements. Neither crew reacted as Boeing thought pilots would, which included using cutout switches to disable the stabilizer motor and, if needed, manually turn a vertically-mounted trim wheel in the cockpit. Instead, they were confused by a cascading series of alerts and warnings, including a SPEED TRIM FAIL, and lost control of the airplane. The MCAS functionality was added to the 737’s speed trim system’s logic, and Boeing elected not to highlight it in pilot manuals or add a new alert light on the flight deck.
“The caution and warning system in the 737 is as archaic as the airframe design. I flew jets made far earlier than the first 737 with a better system,” said Woolman, a former B-52 pilot and Rockwell B-1 bomber flight instructor. “Properly analyzing aircraft failures can be like hunting for Easter eggs, especially if struggling with basic aircraft control.”
BALPA also raised concern about Boeing’s proposed manual-trim techniques. Investigators believe the ET302 pilots attempted to use manual trim, per Boeing’s instructions, to direct their 737-8’s nose up after the MCAS was disabled. But aerodynamic forces acting on the stabilizer made it too difficult to maneuver with the manually-linked wheel.
In its updated MAX training, Boeing emphasizes that both pilots may have to crank the wheel, a spool-shaped device mounted with round sides vertical between them to generate enough force to move the stabilizer. A draft training aid distributed in June illustrates the concept, with each pilot using one hand to turn the wheel, and the other to fly the aircraft.
“Requiring both crew members to turn the trim wheel simultaneously in a non-normal scenario is extremely undesirable and goes against all philosophies of having one pilot fly and one run the” quick reference handbook, or QRH, BALPA said. “No flight control system should require both pilots to operate it at any stage, let alone in an emergency.”
BALPA added that the newer 737’s smaller trim wheel compared to the 737 Classics, which created room on the flight deck for new displays, renders the manual-trim scenario even more challenging. Moving a 737 Classic horizontal stabilizer one degree, or unit, required about 10 turns of the trim wheel. On newer 737s, including the MAX, moving the stabilizer the same amount requires about 15 turns of the smaller-diameter wheel. Full nose down equates to a stabilizer at 4.2 deg nose down, meaning pilots would need to rotate a MAX’s trim wheel about 60 times to bring the stabilizer to neutral.
The public-comment period on the FAA’s proposals runs through Sept. 21. As of late in the afternoon on deadline day, more than 200 comments had been received, including many from members of the public that do not favor the MAX’s re-approval.
Comments will be considered by the FAA and, if appropriate, incorporated into a final directive outlining what changes the MAXs must undergo before they can fly again, and what training pilots will receive. The FAA plans to mandate a flight control computer software upgrade, some writing modifications, and new training. Specifics of the training programs are being reviewed and finalized in a separate process.
Pilot groups and at least one regulator have raised concerns about several non-normal pilot procedures being changed following a review of the grounded Boeing 737 MAX that also apply to older versions of the venerable narrowbody.
Boeing’s proposed modifications to the MAX in response to two fatal accidents affect the model’s flight control computer software, manuals, and pilot training. While the software changes apply only to the MAX variant, several non-normal checklists being updated are the same for the MAX and the 737 Next Generation (NG). Two in particular—runaway stabilizer and airspeed unreliable—have been highlighted by several commenters that weighed in on the FAA’s proposed requirements that would codify Boeing’s changes.
The United Arab Emirates’ General Civil Aviation Authority (GCAA) told the FAA it is concerned about a lack of understanding around factors that affect manual movement, or trimming, of the horizontal stabilizer. “The manual wheel trim forces were certified by analysis and not by flight testing (or tested on non-B737MAX aircraft),” the GCAA told the FAA in comments. “Heaviness on the manual wheel trim following a failure, like runaway stabilizer, must be fully understood and experienced by crew during training and test.”
Used when automatic stabilizer trim motors fail or are de-activated by pilots troubleshooting an issue—and, crucially, a key step on the runaway stabilizer checklist—applying manual trim requires pilots to rotate a crank attached to a spool-shaped wheel in the cockpit. Analysis of factors highlighted in the MAX accidents revealed that aerodynamic forces can make the wheel difficult to turn.
The FAA conducted flight tests in mid-2019 to evaluate the issue, and flagged it as needing further review. One of the results will be updates to Boeing’s 737 flight crew operations manual (FCOM) and training documents that highlight, in general, possible difficulties with manually trimming in certain situations.
The GCAA expressed frustration with the lack of specifics that could help industry better understand the issue.
“The least FAA and Boeing can do is to assist the authority and the operator by providing necessary data associated to this certification and manual trim techniques,” the UAE regulator said.
A representative from one U.S. pilots’ group told Aviation Week that its concern over the same issues led it to ask the FAA for data from the 2019 trials not long after they were completed. The agency has not provided it.
The Allied Pilots Association (APA) that represents American Airlines pilots is among those concerned about a related issue—teaching pilots how to reduce forces on the stabilizer so the trim wheel is easier to turn manually. If pilots facing a runaway stabilizer do not immediately counteract uncommanded inputs using yoke-mounted trim switches before disconnecting the trim motor, forces on the stabilizer can make it more difficult to adjust manually.
“The checklist should include a note that if the method of reducing airspeed to reduce air loads on the stabilizer fails to allow manual trimming (as a result of excessive stabilizer loads created by elevator pressure), slowly relaxing the control column pressure can reduce the load making manual trimming possible,” APA said.
Boeing included details of a similar procedure, which it called the “roller coaster” technique, in manuals for some of its early jets, including the 707 and the 737-100/200, but current manuals do not discuss it in detail. APA and the Air Line Pilots Association (ALPA) are among those pushing to get similar instructions put back in the 737 manual. Boeing’s proposed change to the 737 FCOM is a general note advising pilots to consider reducing airspeed to lessen aerodynamic forces on the stabilizer.
“This note does not provide any information for the flight crew to consider how much of an airspeed decrease will be necessary,” said comments filed by ALPA, which represents pilots at several 737 MAX operators, including Air Canada, United Airlines, and WestJet Airlines. “For horizontal stabilizers out of trim by a large magnitude, aircraft can quickly become difficult to manage at high airspeeds. ALPA believes guidance should be provided to the flight crew as to a specific targeted reduced airspeed.”
ALPA also expressed concern over Boeing’s language on the runaway stabilizer and stabilizer inoperative checklists that says both pilots may need to turn the manual trim wheel simultaneously to generate enough leverage to move the stabilizer.
“ALPA believes that a scenario where both pilots are required to provide manual inputs to a safety-critical flight control system during a non-normal event is not an ideal response to that event,” the association said. “During non-normal events it is commonly trained that one pilot continues to maintain the safe flight of the aircraft while the other pilot conducts the completion of related checklists, such as the [quick-reference handbook]. To interrupt this paradigm by requiring a two-pilot intervention on a safety-critical flight system cannot maintain the same level of safety.”
The British Airline Pilots Association expressed similar concerns in comments it filed earlier this month.
ALPA added that “if scenarios exist where the two-pilot intervention is not deemed extremely improbable,” Boeing should be forced “to implement design changes so that a two-pilot intervention is not required.”
Manual-trim procedures are especially crucial during one rare but long-acknowledged failure scenario. The yoke-mounted electric trim switches are designed so that one cannot override the other. If one fails, such as by shorting out while commanding nose-down stabilizer inputs, the other can be used to stop those inputs, but not reverse them.
“This has the potential to leave pilots with much heavier stick forces arising from greater air loads on the horizontal stabilizer, thereby increasing the effort required to trim manually, as directed later in the procedure,” APA said. “This unique malfunction should be noted in the [runaway stabilizer] checklist, as the ability to use the main electric stabilizer trim to reduce control column forces is not available in all runaway stabilizer events.”
Boeing’s revamp of the MAX flight control computer software began following the October 2018 crash of Lion Air Flight 610, a 737-8, and focused on the maneuvering characteristics augmentation system (MCAS) flight control law. Following the March 2019 crash of Ethiopian Airlines Flight 302, another 737-8, and the MAX fleet’s global grounding, the FAA ordered Boeing to expand its work beyond the MCAS—quickly identified as a common link between the accidents—and examine related failure scenarios such as runaway stabilizer as well as how pilots are trained to manage them. Each of the checklist changes stem from the expanded work, but only two of the eight modified checklists were changed to align with the MAX-specific software alterations.
While the FAA is focused on reviewing Boeing’s proposed changes in light of how they affect the 737 MAX, the agency has said it will consider expanding any beneficial changes to the rest of the 737 fleet.
“Ancillary changes that can enhance the 737NG will also be reviewed by Boeing,” the FAA said in its “preliminary summary” of its 737 MAX review released in early August. “The FAA will work with Boeing to ensure that any issues related to the 737 MAX design change that may apply to the 737NG will be addressed as applicable.”
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