The 737 MAX 8 grounding

The Boeing 737 MAX 8, pictured here on its roll-out in 2015, has been grounded since mid-March. Boeing

Mark Broadbent reports on the Boeing 737 MAX 8 grounding, the biggest current story in the aviation world

ONE STORY dominates the aerospace industry headlines just now: the grounding of the Boeing 737 MAX 8.

The worldwide grounding followed the crash on March 10, 2019, of Ethiopian Airlines Flight 302 that killed all 157 on board. This was the second disaster to befall the 737 MAX 8 in five months, after the October 29, 2018, crash off Indonesia of Lion Air Flight 610 that killed all 189 passengers and crew.

The accidents have put numerous issues on the agenda; above all, the aircraft’s Manoeuvring Characteristics Augmentation System (MCAS), attributed as a common factor in the two accidents. Other issues at play are the process of certifying the 737 MAX and, more broadly, how the accidents might affect Boeing’s reputation with the flying public.

What is MCAS?

The MCAS is a flight control law implemented on the 737 MAX designed, Boeing says, “to enhance the pitch stability of the airplane”. It is not, as some media outlets have described it, an ‘anti-stall’ system.

Using input from two nosemounted sensors measuring airspeed, altitude and angle-ofattack (AOA), the MCAS is designed, “to activate in manual flight, with the airplane’s flaps up, at an elevated angle-of-attack”.

The system automatically trims the horizontal stabilisers on the tail when the system determines the upward pitch of the aircraft may become too steep.

The use of MCAS on the 737 MAX stemmed from the aircraft’s slightly different aerodynamic profile to the 737NG caused by the fact that the 737 MAX’s CFM International LEAP-1B engines have a much larger fan diameter than the CFM56-7Bs on the 737NG.

Preliminary investigations into the Ethiopian and Lion Air accidents suggest faulty data from a malfunctioning AOA sensor may have triggered the MCAS, which erroneously pitched down the nose of the aircraft despite crews’ repeated attempts to disengage the system.

Boeing says it has developed a software update for the system to provide “additional layers of protection if the AOA sensors provide erroneous data”.

A Boeing statement explained the flight control system will now compare inputs from both AOA sensors. The statement says: “If the sensors disagree by 5.5o or more with the flaps retracted, MCAS will not activate. An indicator on the flight deck display will alert the pilots.

“If MCAS is activated in nonnormal conditions, it will only provide one input for each elevated AOA event. There are no known or envisioned failure conditions where MCAS will provide multiple inputs.

“MCAS can never command more stabiliser input than can be counteracted by the flight crew pulling back on the column. The pilots will continue to always have the ability to override MCAS and manually control the aircraft. These updates reduce the crew’s workload in non-normal flight situations and prevent erroneous data from causing MCAS activation.” Boeing said the 737 MAX’s flight deck displays would be upgraded to give pilots a better indication of MCAS status. Pilot training materials will also be updated, “to provide 737 type-rated pilots with an enhanced understanding of the 737 MAX Speed Trim System, including the MCAS function, associated existing crew procedures and related software changes. Pilots will also be required to review [the] Flight Crew Operations Manual Bulletin, [the] Updated Speed Trim Fail Non-Normal Checklist [and the] Revised Quick Reference Handbook.”

Boeing under pressure

On April 29, 2019, there was a tense press conference at Boeing’s Chicago headquarters where Boeing Chairman and Chief Executive Dennis Muilenburg appeared in front of the media for the first time since the Ethiopian crash.

Muilenburg acknowledged MCAS was a common factor in both accidents, but said the system met “design and certification criteria” and said other factors may also have contributed to the accidents. He said: “As in most accidents, there are a chain of events that occur. It is not correct to attribute that to any single item.”

Muilenburg was repeatedly asked questions about the design of MCAS. Asked by one reporter whether he had considered resigning over the issue, Muilenburg replied: “The important thing here is, again, [that] we are very focused on safety.”

After 15 minutes, the press conference ended abruptly. A video of the event, which can be viewed online, shows Muilenburg exiting the room at the end of a response to a question, causing uproar among reporters who continued to ask questions as he left the room.

Aircraft on ground

The Boeing 737 MAX grounding obviously has a practical consequence on airlines, because operators do not have the planned number of aircraft in their fleet and therefore the expected level of available seats.

At the time the grounding was imposed, 376 examples of the 737 MAX had been delivered. Southwest Airlines has received more 737 MAX 8s than any other operator, with 34 at the time of the grounding.

The carrier told AIR International: “We were originally cancelling between 130 and 150 flights a day until we began implementing a series of schedule revisions on March 27. Then, our daily flight cancellations were reduced to about 90 flights a day.

“Beginning with our latest schedule revision, which will impact flights from June 8 through [to] August 5, we took roughly 160 daily flights out of the schedule to establish stability to our network and avoid last-minute cancellations Our goal with the revisions is to operate a reliable schedule.”

Southwest said the 737 MAX 8 grounding affects 5% of the airline’s 737 fleet, which at the start of May 2019 also consisted of 513 737-700s and 213 737-800s.

Norwegian, the initial European operator of the type, told this magazine: “We have 18 737 MAX 8 aircraft in our total fleet of around 170 aircraft. There has been minimal impact on our operation.”

Another European operator is the TUI Travel Group, whose airlines operate 15 737 MAX 8s. A spokesperson said: “Our overall fleet at TUI is 150 planes. We are talking about 15 aircraft, so it’s 10% of our capacity.”

These statements show airlines are keen to downplay the influence of the grounding. However, in the case of Southwest, although the 737 MAX ‘only’ accounts for 5% of the carrier’s total daily flights, when those daily flights number over 4,000, around 200 services are still influence.

To put it another way, assuming one of its 175-seat 737 MAX 8s has around 140 to 150 passengers on each flight (a plausible number, given the airline reported an 81% load factor for its flights in its Q1 2019 results released in April 2019), then the airline must therefore find replacement capacity for up to 30,000 passengers per day.

This indicates the significance of the aircraft-on-ground situation the grounding of the 737 MAX presents. Operators are compelled to work their remaining aircraft harder and/or lease in capacity under aircraft, crew, maintenance and insurance agreements. Norwegian said: “We’ve rerouted passengers on to other Norwegian services, combined flights or wet-leased a small number of aircraft to fill the gap.”

United Airlines, which has 14 737 MAX 9s, said: “Since March, United has gone to great lengths to minimise the impact of the grounding of the MAX aircraft on our customers’ travel plans. We’ve used spare aircraft and other creative solutions to help our customers who had been scheduled to travel on one of our 14 MAX aircraft get where they are going; but it’s harder to make those changes at the peak of the busy summer travel season.”

Mark Diamond, a Principal at the industry consultancy ICF, told AIR International: “Right now, where there’s an immediate global shortfall in capacity due to the 737 MAX groundings, the foremost consideration for lessees may simply be availability of aircraft to lease. Nearly 400 MAXs have been grounded. That’s not a small number and it extends across the globe. With the peak summer travel season approaching, there could well be pressure on availability of narrowbodies that can be leased in.”

A further consequence of the grounding is disruption to customers’ fleet plans caused by the pause in deliveries. In its Q1 results, Southwest acknowledged: “Prolonged grounding of the [737] MAX [8] aircraft could impact the company’s delivery and retirement schedule.”

AOA Disagree alert


More pressure on Boeing emerged because of an issue relating to the AOA Disagree alert in the MCAS.

The AOA Disagree alert is described by Boeing as “a software-based information feature” designed to inform pilots of significant discrepancies between the information provided by the two AOA sensors in the nose, warning flight crews when data from one sensor is incompatible with the other.

Boeing admitted in a statement issued on April 29 the alert was only enabled on aircraft in which the customers had selected the optional AOA indicator.

The statement said: “The Disagree alert was intended to be a standard, standalone feature on MAX airplanes. However, the Disagree alert was not operable on all airplanes, because the feature was not activated as intended. The Disagree alert was tied or linked into the angle of attack indicator, which is an optional feature on the MAX. Unless an airline opted for the angle of attack indicator, the Disagree alert was not operable.” In a subsequent statement on May 5, Boeing acknowledged it had identified this discrepancy in 2017, before the Lion Air accident.

The statement said: “In 2017, several months after beginning 737 MAX deliveries, engineers at Boeing identified that the 737 MAX display system software did not correctly meet the AOA Disagree alert requirements.

A telling photo taken in April 2019, with Southwest Airlines’ fleet of grounded Boeing 737 MAX 8s parked at Victorville in California.
Colin Parker/AirTeamImages

“The software delivered to Boeing linked the AOA Disagree alert to the AOA indicator, which is an optional feature on the MAX and the NG. Accordingly, the software activated the AOA Disagree alert only if an airline opted for the AOA indicator.

“When the discrepancy between the requirements and the software was identified, Boeing followed its standard process for determining the appropriate resolution of such issues. That review, which involved multiple company subject matter experts, determined that the absence of the AOA Disagree alert did not adversely impact airplane safety or operation.

“Accordingly, the review concluded, the existing functionality was acceptable until the alert and the indicator could be delinked in the next planned display system software update.

“Approximately a week after the Lion Air accident, on November 6, 2018, Boeing issued an Operations Manual Bulletin [OMB], which was followed a day later by the [Federal Aviation Administration] FAA’s issuance of an Airworthiness Directive [AD]. In identifying the AOA Disagree alert as one among a number of indications that could result from erroneous AOA, both the OMB and the AD described the AOA Disagree alert feature as available only if the AOA indicator option is installed.

“Boeing discussed the status of the AOA Disagree alert with the FAA in the wake of the Lion Air accident.

At that time, Boeing informed the FAA that Boeing engineers had identified the software issue in 2017 and had determined per Boeing’s standard process that the issue did not adversely impact airplane safety or operation.

“In December 2018, Boeing convened a Safety Review Board [SRB] to consider again whether the absence of the AOA Disagree alert from certain 737 MAX flight displays presented a safety issue. That SRB confirmed Boeing’s prior conclusion that it did not.

Boeing shared this conclusion and the supporting SRB analysis with the FAA.”

Boeing has said the revised and upgraded MCAS software will have “an activated and operable disagree alert and an optional angle of attack indicator”.

Boeing said it will issue a display system software update to implement the AOA Disagree alert as a standard, standalone feature before the 737 MAX returns to service, with all production aircraft having an activated and operable AOA Disagree alert and an optional angle of attack indicator.

It added: “All customers with previously delivered MAX airplanes will have the ability to activate the AOA Disagree alert.”

The upgraded software will be reviewed by the FAA for certification to get the 737 MAX flying again. In its first-quarter financial results, the company said: “Boeing is making steady progress on the path to final certification for a software update for the 737 MAX, with over 135 test and production flights of the software update complete. The company continues to work closely with global regulators and our airline partners to comprehensively test the software and finalise a robust package of training and educational resources.”

Boeing’s 737 MAX 8 pictured on its 2016 first flight. The company had delivered 376 at the time of the aircraft’s grounding.
Matthew Thompson/Boeing


Beyond the software upgrade work, there is a bigger issue at play: the certification process for the 737 MAX and in particular the MCAS.

The FAA has convened a Joint Authorities Technical Review (JATR) panel to review the agency’s certification of the MCAS. Chaired by former National Transportation Safety Board (NTSB) Chairman Christopher Hart, the panel comprises technical safety experts from nine civil aviation authorities worldwide, including the FAA, as well as from NASA and the US Air Force.

An FAA statement on May 3 said: “The team received extensive overviews and engaged in subsequent discussions about the design, certification, regulations, compliance, training and Organization Designation Authorization programme associated with the 737 MAX.

“Over the next few months, JATR participants will take a comprehensive look at the FAA’s certification of the aircraft’s automated flight control system.

Each participant will individually provide the FAA with findings regarding the adequacy of the certification process and any recommendations to improve the process …The JATR is separate from and not required to approve enhancements for the return of the 737 MAX to service.”

Meanwhile, the US Department of Transportation is to investigate the FAA’s certification process in a Special Committee reviewing the agency’s procedures for certifying new aircraft, including the 737 MAX.

The Special Committee’s findings will be presented to the Secretary of Transportation Elaine Chao and the FAA Administrator, and issue “advice and recommendations on issues facing the aviation community related to the FAA’s safety oversight and certification programmes and activities”.

Chao said: “Safety is the number one priority of the Department, and this review by leading outside experts will help determine if improvements can be made to the FAA aircraft certification process.”

A Senate Commerce Committee has also launched a Congressional investigation into the FAA’s process for training the inspectors involved in the certification of the 737 MAX.

A US House of Representatives subcommittee was set to hold a hearing on May 15 with FAA administrator Daniel Elwell and the NTSB Chairman Robert Sumwalt.

On May 23, a meeting of international regulatory agencies was scheduled to be held in Washington, DC to discuss the results of the FAA and multi-agency working group and determine the appropriate steps once the review of the proposed software fix is completed, laying out the path for a return to flight.

Rebuilding trust

As well as the investigations and hearings, the 737 MAX’s grounding clearly has a financial effect on Boeing. With the manufacturer having cut back monthly output of the aircraft from 52 to 42 units, completed airframes are now mounting up at the Renton final assembly plant.

In its 2019 first-quarter results issued in April, the company put the cost of the grounding at $1 billion in the first month, adding: “Due to the uncertainty of the timing and conditions surrounding return to service of the 737 MAX fleet, new guidance will be issued at a future date.”

Beyond the balance sheet, there is a wider issue at stake for Boeing: trust in the 737 MAX given the two fatal accidents in the space of five months.

In a video statement posted on Boeing’s website in April, Muilenburg said: “We’ll do everything possible to earn and re-earn that trust and confidence from our airline customers and the flying public in the weeks and months ahead.”

According to media reports in May, Boeing executives have been hosting regular conference calls with airline executives, operators, lessors and union representatives discussing the specifics of the software upgrades.

Convincing the wider public might be a bigger and longerterm task. According to a report punlished by Bloomberg in May, a survey of 1,756 passengers by Barclays showed 44% of respondents in North America and Europe said they would wait a year or more before flying the 737 MAX.