Today, we look at Boeing’s design flaws in the 737 MAX family of airplanes and how software was allowed to act on the planes in such a way as to make it impossible for the pilots to keep them in the air. How did the FAA’s regulatory oversight break down to such a degree? And why didn’t Boeing communicate better after the first crash?

Your Host: Fabian A. Scherschel


Comments on Broadcast 1: A Perfect Phone Call:

Mike The Dane says:

I think you are focusing too much on the transcript, as there were other activities undertaken by Trump and Guiliani that support the quid pro quo/bribe/ransom. E.g. as the hearings attested, several people were aware that Trump wanted a public statement by the Ukranian president announcing the investigations into the Bidens before they would get their aid and a state visit to the White House. State department officials who queried why all normal diplomatic channels were being superseeded by a non-government person (Guiliani) were removed (e.g. Ambassador Yovanovitch). There was a clear and concerted effort to target the Bidens specifically for Trumps personal gain, not for the benefit of the US.

I think it is very simple, the US President went through a lot of effort and dispatched his personal aide (who is not a diplomat and did not have state department credentials – he is a “lawyer” and therefore a private citizen) to force a foreign government under bribe/ransom to set up a narrative to support his re-election campaign. There was nothing beneficial for the US in those acts and it was done with intent to conceal their efforts and remove the officials that warned against it. Again, nothing of benefit to the US – the people on the floor of the administration could not see why these activities were beneficial. Whatever happened before has nothing to do with this actual breach of the US constitution. I agree that Nixon and Clinton were just as dirty as many other US Presidents. That I think Trump is an order of magnitude worse, much worse. However, the US Constitution is specifically set up for removing a President that solicits aid from foreign governments, and I think what has been done here fits that interpretation.

He also points to the transcript possibly being redacted and that Congressional witnesses under oath might have more weight.

Boeing 737 MAX Basics

The 737 is currently Boeing’s only narrow-body airliner, with the 737 Next Generation (-700, -800, and -900ER), and the re-engined and updated 737 MAX variants in production. Envisioned in 1964, the initial 737-100 made its first flight in April 1967 and entered airline service in February 1968 with Lufthansa. The 737 series was the highest-selling commercial jetliner, having been surpassed in total orders by the Airbus A320 family by the end of October 2019.

The 737 MAX series has been offered in four variants. The 737 MAX 7, MAX 8 (including the denser, 200–seat MAX 200), and MAX 9 are intended to replace the 737-700, -800, and -900, respectively. Additional length is offered with the further stretched 737 MAX 10.

The CFM International LEAP engines of the 737 MAX have a higher bypass ratio and have a larger nacelle than the engines of previous Boeing 737 models, so the engines are placed higher and further forward in relation to the wing than on previous models. This destabilises the aircraft pitch at higher angles of attack (AoA). To deal with this, Boeing designed a Maneuvering Characteristics Augmentation System (MCAS) for the 737 MAX series.

The first 737 MAX performed its first flight on January 29, 2016. The series gained FAA certification in March 2017. The first delivery was a MAX 8 in May 2017, to Malindo Air (a subsediary of Lion Air), which placed the aircraft into service on May 22, 2017.

The Airbus A320neo family (neo for new engine option) is a development of the A320 family of narrow-body airliners produced by Airbus. Launched on 1 December 2010, it made its first flight on 25 September 2014 and it was introduced by Lufthansa on 25 January 2016. Re-engined with CFM International LEAP or Pratt & Whitney PW1000G engines and with large sharklets, it is 15% to 20% more fuel efficient than its predecessor. The A320neo family is based on the previous A319, A320 and A321. As of November 2019, a total of 7,200 A320neo family aircraft had been ordered by more than 115 airlines, making it the fastest selling commercial aircraft ever.

How MCAS Works

The Maneuvering Control Augmentation System (MCAS) is a flight control law built into the Boeing 737 MAX’s flight control computer, designed to help the aircraft emulate the handling characteristics of the earlier Boeing 737 Next Generation. According to an international Civil Aviation Authorities team review (JATR) commissioned by the FAA, MCAS is part of a stall identification system or a stall protection system, depending on the natural (unaugmented) stall characteristics of the aircraft. Boeing considered MCAS part of the flight control system, and elected to not describe it in the flight manual or in training materials, based on the fundamental design philosophy of retaining commonality with the 737NG.

Minimizing the functional differences between the Boeing 737 MAX and Next Generation aircraft variants allowed both variants to share the same type rating. Thus, airlines can save money by employing and training one pool of pilots to fly both variants of the Boeing 737 interchangeably.

The 737 MAX’s larger CFM LEAP-1B engines are fitted further forward and higher up than in previous models. The aerodynamic effect of its nacelles contributes to the aircraft’s tendency to pitch up at high angles of attack (AOA). The MCAS is intended to compensate in such cases, modeling the pitching behavior of previous models, and meet a certain certification requirement, in order to enhance handling characteristics and thus minimizing the need for significant pilot retraining. When activated, MCAS directly engages the horizontal stabilizer, thus is distinct from an anti-stall device, such as stick pusher, which physically moves the pilot’s control column forward and engages the airplane’s elevators when the airplane is approaching a stall.

As an automated corrective measure, the MCAS was given full authority to bring the aircraft nose down, and could not be overridden by pilot resistance against the control wheel as on previous versions of the 737. Following the Lion Air accident, Boeing issued an Operations Manual Bulletin (OMB) which outlined the many indications and effects resulting from erroneous AOA data and provided instructions to turn off the motorized trim system for the remainder of the flight, and trim manually instead. Until Boeing supplemented the manuals and training, pilots were unaware of the existence of MCAS due to its omission from the crew manual and no coverage in training.

In addition, the system acts on only one of two available AoA sensors, a single point of failure that goes against aviation requirements of robustness and integrity, for example using redundancy. An AOA “disagree” light illuminates when the two sensors read different values. Airlines could optionally add a software gauge indicator to the multifunction cockpit displays. However, scrutinization has revealed that the two safety features were interdependent, and were inoperable in both doomed flights.

AOA sensor

An angle of attack (AOA) sensor from a modern passenger jet — Wikimedia Commons

The older 737 cockpit has separate on/off control switches for independent electrically-assisted and automatic trim systems. On the 737 MAX, a combined switch is provided and the pilot cannot turn off the MCAS without also disabling electrically-assisted trim. A manual trim wheel is provided, but is not powerful enough to adjust the stabilizer in all flight conditions. Activating the powered trim system can be necessary and this also activates the MCAS.

737 MAX console

Centre console of a Boeing 737 MAX 8; note the CUTOFF switches (centre) and manual trim wheels on both sides of the console — Dimas Ardian / Bloomberg

MCAS exists in a different form on the Boeing KC-46 Pegasus, an aerial refueling tanker that is fundamentally a Boeing 767-2C. The system takes input on dual redundant angle of attack sensors; it will disengage with stick input by the pilot.

The Lion Air Crash

The aircraft involved was a Boeing 737 MAX 8, registration PK-LQP, line number 7058. The aircraft was delivered new to Lion Air on 13 August 2018.

The aircraft took off from Jakarta on 29 October 2018 at 6:20 a.m. local time (28 October 2018, 11:20 p.m. UTC) and was scheduled to arrive at Depati Amir Airport in Pangkal Pinang at 7:20 a.m. It took off in a westward direction before circling around to a northeast heading, which it held until crashing offshore northeast of Jakarta in waters estimated to be up to 35 metres (115 ft) deep. The flight crew had requested clearance to return to the Jakarta airport 19 nautical miles (22 mi; 35 km) into the flight. At 7:30 a.m., the Indonesian National Search and Rescue Agency received reports that Flight 610 had crashed a few kilometres from an offshore oil platform. Workers on the platform reportedly saw the aircraft crash with a steep nose-down angle.

The aircraft was used on a flight from Ngurah Rai International Airport, Bali to Soekarno-Hatta International Airport, Jakarta the night before the crash. Detailed reports from that flight revealed that the aircraft had suffered a serious incident, which left many passengers traumatized. Passengers in the cabin reported heavy shaking and a smell of burnt rubber inside the cabin. A recording of air traffic control communications indicated that the pilot had called a “pan-pan.” The crew later decided to cancel the pan-pan and continue the flight to Jakarta. The aircraft’s maintenance logbook revealed that the aircraft suffered an unspecified navigation failure on the captain’s side, while the First Officer’s side was reported to be in good condition.

Further examination of the aircraft’s instruments revealed that one of the aircraft’s airspeed indicators had malfunctioned for its last four flights, including the flight to Denpasar. On 7 November, the NTSC confirmed that there had been problems with Flight 610’s angle of attack (AoA) sensors. Thinking that it would fix the problem, the engineers in Bali then replaced one of the aircraft’s AoA sensors, but the problem persisted on the penultimate flight, from Denpasar to Jakarta. Just minutes after takeoff, the aircraft abruptly dived. The crew of that flight, however, had managed to control the aircraft and decided to fly at a lower than normal altitude. They then managed to land the aircraft safely and recorded a twenty-degree difference between the readings of the left AoA sensor and the right sensor.

The NTSC released its final report into the accident on 25 October 2019: The assumption made by Boeing (approved by FAA) on pilots response were incorrect. Reliance of MCAS on single AOA sensor was deemed appropriate (based on the assumption above), but this decision of using single AOA sensor caused it to be vulnerable to erroneous sensor input. The absence of MCAS guidance (in pilot’s manual and training material) made it difficult (impossible) for crew to properly respond to uncommanded trim caused by MCAS. The AOA DISAGREE alert was not enabled; as a result it was not documented by the previous flight’s crew, and was therefore not identified by maintenance.

The replacement AOA sensor installed on the airplane had been mis-calibrated by the repair shop (Xtra Aerospace of Florida). On both the previous and the accident flight the left AOA sensor was reading 21 degrees too high since the beginning of the takeoff.

The pilots did not respond appropriately during the accident flight. There was a lengthy delay for the UNRELIABLE AIRSPEED non-normal checklist. Neither captain nor FO executed it from memory immediately, despite it is an industry standard that all NNCs classified as memory items should be executed without delay from memory. The FO had difficulties finding the UNRELIABLE AIRSPEED non-normal checklist in the Quick Reference Handbook (QRH).

On 7 November, on the basis of preliminary information gathered in the investigation of the Lion Air accident, the Federal Aviation Administration (FAA) issued an emergency Airworthiness Directive (AD) requiring that amended operating limitations and procedures relating to erroneous data from an AoA sensor be inserted into the aircraft flight manual of each 737 MAX aircraft, and urged all airlines operating Boeing 737 MAX 8s to heed the warnings.

During difference training, pilots of American Airlines and Southwest Airlines converting from earlier Boeing 737 Next Generation models to the 737 MAX were not informed of the MCAS linked to the fatal crash, leaving them concerned that they were possibly untrained with respect to other differences. In November 2018, Aviation Week reviewed the 737 MAX flight crew operations manual and found that it did not mention the MCAS. The Wall Street Journal reported that Boeing had “decided against disclosing more details to cockpit crews due to concerns about inundating average pilots with too much information.”

In an internal message on 19 November 2018, Boeing CEO Dennis Muilenburg defended the Flight Crew Operations Manual as describing the relevant function of MCAS. On 20 November, Boeing was to hold a conference call with 737 Max operators to detail the new MCAS not present in the Next Generation models. The conference call was cancelled later, to be replaced by a series of regional calls to allow more questions. Lion Air co-founder and former CEO Rusdi Kirana reportedly considered cancelling Lion Air’s outstanding 190 Boeing aircraft orders – worth some $22 billion at list prices – over what he viewed as an attempt by Boeing to blame Lion Air for the crash.

Boeing decided in November 2017 to defer a software update to correct the so-called AOA Disagree alert defect until 2020, three years after discovering the flaw, U.S. Reps. Peter DeFazio and Rick Larsen said in a press release on Friday. Boeing only accelerated this schedule after the Lion Air accident in Indonesia, they added. Boeing spokesman Gordon Johndroe said by email that a company safety review found the absence of the AOA Disagree alert did not adversely impact airplane safety or operation. “Based on the safety review, the update was scheduled for the MAX 10 entry into service in 2020,” Johndroe said.

The captain had accumulated 6,000 hours of total flight experience, the first officer 5,000 hours. On Nov 1st 2018 the airline confirmed one of their maintenance engineers was on board of the aircraft during the accident flight. This was an “anticipatory measure” in the event of technical problems with the new aircraft. As such, “the presence of the technician has nothing to do with the condition of the aircraft before taking off.”

The KNKT (Indonesian transport safety agency) provides a key analysis which indicates that normal, intuitive and logic crew reactions actually trigger the accident sequence when they state: In the event of an MCAS activation with manual electric trim inputs by the flight crew, the MCAS function will reset which can lead to subsequent MCAS activations. With an MCAS command due to an erroneous high AOA signal, and flight crew inputs that do not fully return the aircraft to a trimmed state, subsequent MCAS commands can result in the aircraft becoming significantly miss-trimmed. In the event of an MCAS activation with manual electric trim inputs by the flight crew, the MCAS function will reset which can lead to subsequent MCAS activations.

The Ethiopian Airlines Crash

The aircraft was a Boeing 737 MAX 8, registered ET-AVJ (manufacturer’s serial number 7243). The aircraft was manufactured in October 2018 and delivered on 15 November 2018, making it around four months old at the time of the accident.

Flight 302 was a scheduled international passenger flight from Addis Ababa to Nairobi. The aircraft took off from Addis Ababa at 08:38 local time (05:38 UTC) on 10 March 2019 with 149 passengers and 8 crew on board. One minute into the flight, the first officer, acting on the instructions of the captain, reported a “flight control” problem to the control tower. Two minutes into the flight, the plane’s MCAS system activated, pitching the plane into a dive toward the ground.

The pilots struggled to control it and managed to prevent the nose from diving further, but the plane continued to lose altitude. The MCAS then activated again, dropping the nose even further down. The pilots then flipped a pair of switches to disable the electrical trim tab system, which also disabled the MCAS software. However, in shutting off the electrical trim system, they also shut off their ability to trim the stabilizer into a neutral position with the electrical switch located on their yokes. The only other possible way to move the stabilizer would be by cranking the wheel by hand, but because the stabilizer was located opposite to the elevator, strong aerodynamic forces were pushing on it. As the pilots had inadvertently left the engines on full takeoff power, which caused the plane to accelerate at high speed, there was further pressure on the stabilizer. The pilots’ attempts to manually crank the stabilizer back into position failed. Three minutes into the flight, with the aircraft continuing to lose altitude and accelerating beyond its safety limits, the captain instructed the first officer to request permission from air traffic control to return to the airport. Permission was granted, and the air traffic controllers diverted other approaching flights. Following instructions from air traffic control, they turned the aircraft to the east, and it rolled to the right. The right wing came to point down as the turn steepened. At 8:43, having struggled to keep the plane’s nose from diving further by manually pulling the yoke, the captain asked the first officer to help him, and turned the electrical trim tab system back on in the hope that it would allow him to put the stabilizer back into neutral trim. However, in turning the trim system back on, he also reactivated the MCAS system, which pushed the nose further down. The captain and first officer attempted to raise the nose by manually pulling their yokes, but the aircraft continued to plunge toward the ground.

The aircraft disappeared from radar screens and crashed at almost 08:44, six minutes after takeoff. Flight tracking data showed that the aircraft’s altitude and rate of climb and descent were fluctuating. Several witnesses stated the plane trailed “white smoke” and made strange noises before crashing. The aircraft impacted the ground at nearly 700 mph. Both the cockpit voice recorder and the flight data recorder were recovered from the crash site on 11 March. On 13 March 2019, the FAA announced that new evidence found on the crash site and satellite data on Flight 302 suggested that the aircraft might have suffered from the same problem which the aircraft operating Lion Air Flight 610 had suffered from. Investigators discovered the jackscrew that controlled the pitch angle of the horizontal stabilizer of Flight 302, was in the full “nose down” position. The finding suggested that, at the time of the crash, Flight 302 was configured to dive, similar to Lion Air Flight 610.

John Cox, previously a 737 pilot and pilots’ union safety representative, and Chesley Sullenberger, who successfully ditched US Airways Flight 1549 in the Hudson River, both did Flight Simulator replications of Flight 302. Cox described the rapid onset of unforeseen events as a “…breeding ground for confusion and task saturation.” Sullenberger commented that “Even knowing what was going to happen, I could see how crews would have run out of time and altitude before they could have solved the problems.” While defending the pilots’ actions, Sullenberger was also highly critical of allowing someone with only 200 hours of flight experience to be first officer.

According to Ethiopian transport minister Dagmawit Moges, the crew “performed all the procedures repeatedly provided by the manufacturer but was not able to control the aircraft”. Boeing’s CEO Dennis Muilenburg said on 29 April that if “you go through the checklist …it calls out actions that would be taken around power management and pitch management of the airplane. It also refers to the cutout switches, that after an activation that was not pilot-induced, that you would hit the cutout switches. And, in some cases, those procedures were not completely followed”.

The aircraft was in visual meteorologic conditions throughout the flight. The captain was with Ethiopian Airlines for 9 years and had about 8000 hours of flight experience, a first officer with 200 flight hours assisted.

In the aftermath of the Lion Air crash I had already raised a good number of questions to the FAA about how it was possible to certify MCAS with its dependence on a single AoA etc. Over the more than 100 years of aviation and aircraft accident investigations one of the principles deemed most important today emerged: the Cockpit Resource Management (CRM). The principle that everybody in the cockpit as well as anything in the cockpit should ensure that all available resources in the cockpit are being used. Is it thus not a gross violation of the CRM, committed already by the designers of the aircraft, when a system does not take a second available resource into account, like the right hand AoA? How can it be argued to be in compliance with CRM when a crew can not de-activate a stick shaker that has been identified to operate erroneously, except by pulling the circuit breaker? How can it be argued, that other than on NG aircraft, where the TRIM CUTOUT switches disable automatic trim inputs and electrical manual trim inputs separately, either of the TRIM CUTOUT switches disables ALL electrical trim inputs, both manual and automatic ones depriving the crew of possibly still well functioning, available and needed resources?

Overall at least a total of three Boeing 737 MAX flights were affected by wrong values delivered by the left hand AoA sensor prompting the MCAS system to provide automatic, repeated and large nose down trim inputs: Lion Air’s JT-43 (which did not crash), JT-610 and Ethiopian Airlines’ ET-302. Prior to flight JT-43 the left AoA sensor (which was still the one mounted by Boeing during aircraft assembly) was replaced by Lion Air maintenance due to repetitive similiar malfunctions on the flights before JT-43, the new AoA sensor on flights JT-43 and JT-610 however showed repetitive malfunctions again, on JT-43 the crew was able to correct, on JT-610 the crew however was not able to correct and crashed. And ET-302 shows a similiar pattern which the crew was not able to correct resulting in the crash of the aircraft, too. Neither of the three crews would have been forced to react under time pressure in order to prevent a crash, e.g. to find out what to do or identify the correct procedures to follow, without the technical malfunctions and the nose down trim inputs.

On Apr 27th 2019 it became known, that four independent whistleblowers, current and former Boeing employees, had called the FAA hotline for whistleblowers regarding aviation safety concerns on Apr 5th 2019. The concerns reported were wiring damage to the AoA related wiring as result of foreign object damage as well as concerns with the TRIM CUTOUT switches.

Groundings & Investigation

The worldwide grounding affected 387 MAX airplanes making 8,600 weekly flights with 59 airlines.

Although regulators typically follow guidance from the plane maker and its national certifying authority, in this case they cited safety precautions as reason to ground the aircraft, and revoked clearance of MAX aircraft from foreign airlines despite the lack of guidance from Boeing and the Continued Airworthiness Notification from the FAA.

Following the grounding orders, Boeing halted deliveries and reduced production of the MAX. Airlines do not expect to resume flights until March 2020 or later. As of November 2019, the grounding cost Boeing up to $10 billion in revenue and compensation to airlines and bereaved families. Boeing also faced lawsuits from airline pilots and families of victims.

Boeing implemented the original version of MCAS on the KC-46 tanker, a plane derived from the Boeing 767. The tanker compares the data from both AoA sensors and allows pilots to retake control in the event of large differences; without cross-checking, the MAX flight control computer activates MCAS using just one AoA sensor. In addition, some familiar pilot actions for manually controlling the pitch on other 737 types do not deactivate the MCAS. Boeing presented MCAS to the FAA as being existing technology, avoiding deeper scrutiny. MCAS was not evaluated as an individual system that was “new/novel on the MAX.” The FAA is required to be closely involved in the testing and certification of any new and novel features on an aircraft.

Just before entering certification, the functional requirements for MCAS were still changing. Boeing modified MCAS so that it intervened more strongly and at lower airspeeds than originally planned.

Problems with the angle-of-attack sensor had been reported in over 200 incident reports submitted to the FAA; however, Boeing did not flight test a scenario in which it malfunctioned.

When a significant difference is registered between a pair of Angle of Attack sensors, the message AoA Disagree is supposed to appear on the primary flight display of an aircraft. Thus, pilots get insight into sensor problems, and possible resulting control system problems; aircrew will also normally make a maintenance log entry. Boeing had charged extra for this optional safety feature. For example, Air Canada, American Airlines and Westjet had purchased the disagree alert, while Air Canada and American Airlines also purchased, in addition, the AoA value indicator, and Lion Air had neither. In November 2017, after several months of MAX deliveries, Boeing discovered that the disagree alert depended on the presence of the visual indicator software, a paid option that was not selected by most airlines. Boeing had determined that the defect was not critical to aircraft safety or operation, and an internal safety review board (SRB) corroborated Boeing’s prior assessment and its initial plan to update the aircraft in 2020. Boeing did not disclose the defect to the FAA until November 2018, in the wake of the Lion Air crash.

A top Boeing pilot working on the 737 Max said in messages from 2016 that a new automated system known as MCAS was making the plane difficult to control in flight simulators. The messages suggest that Boeing officials working on the development of the best-selling jet knew of potential issues with the automated system years before the plane was involved in two deadly crashes.

Mark Forkner 6:50 PM: Oh shocker alerT! MCAS is now active down to M .2 It’s running rampant in the sim on me. at least that’s what Vince thinks is happening

Gustavsson, Patrik H 6:51 PM: Oh great, that means we have to update the speed trim descritption in vol 2

Mark Forkner 6:51 PM: so I basically lied to the regulators (unknowingly)

Gustavsson, Patrik H 6:51 PM: it wasnt a lie, no one told us that was the case

Mark Forkner 6:51 PM: I’m levelling off at like 4000 ft, 230 knots and the plane is trimming itself like craxy. I’m like, WHAT?

Mark Forkner 6:52 PM: granted, I suck at flying, but even this was egregious

Gustavsson, Patrik H 6:52 PM: No, i think we need aero to confirm what its supposed to be doing

Gustavsson, Patrik H 6:53 PM: I don’t know, the test pilots have kept us out of the loop. It’s really only christine that is trying to work with us, but she has been too busy

The Federal Aviation Administration relied heavily on Boeing employees to vouch for the safety of the Max and lacked the ability to effectively analyze much of what Boeing did share about the new plane, according to the report by a multiagency task force. The system of delegation is now being scrutinized by lawmakers in the wake of the tragedies. Boeing employees who worked on behalf of the F.A.A. faced “undue pressures” at times during the plane’s development because of “conflicting priorities,” according to the report. Hours after the report was released, Boeing’s board stripped the company’s chief executive, Dennis A. Muilenburg, of his chairman title. The move was the most direct response yet from a board that has resisted shaking up the management team before the Max is flying again, even as pressure mounted inside Boeing to hold someone accountable.

Friday’s report, which was put together by representatives of the Federal Aviation Administration, NASA and nine international regulators, provided the first official detailed account of how federal regulators certified the Max. Lawmakers and federal investigators are still conducting their own inquiries into the design and approval of the jet. The task force said it believed that if F.A.A. technical staff had been fully aware of the details of MCAS, the agency would probably have required additional scrutiny of the system that might have identified its flaws.

When Boeing broke ground on its new factory near Charleston in 2009, the plant was trumpeted as a state-of-the-art manufacturing hub, building one of the most advanced aircraft in the world. But in the decade since, the factory, which makes the 787 Dreamliner, has been plagued by shoddy production and weak oversight that have threatened to compromise safety.

Safety lapses at the North Charleston plant have drawn the scrutiny of airlines and regulators. Qatar Airways stopped accepting planes from the factory after manufacturing mishaps damaged jets and delayed deliveries. Workers have filed nearly a dozen whistle-blower claims and safety complaints with federal regulators, describing issues like defective manufacturing, debris left on planes and pressure to not report violations. Others have sued Boeing, saying they were retaliated against for flagging manufacturing mistakes.


For the third time in two weeks, the FAA said publicly it will take all the time it needs to deem the Max safe. The FAA issued a new statement, saying, “The FAA has not completed its review of the 737 Max aircraft design changes and associated pilot training. The agency will not approve the aircraft for return to service until it has completed numerous rounds of rigorous testing.”

Boeing’s suggestion the Max is close to returning did not sit well with the FAA and Administrator Steve Dickson. Four days after Boeing’s statement, Dickson released an internal letter he sent to the FAA’s associate administrator who oversees the Max certification process. “The FAA fully controls the approval process,” Dickson wrote.

Boeing Co. engineers were nearly done redesigning software on the grounded 737 Max in June when some pilots hopped into a simulator to test a few things. It didn’t go well. A simulated computer glitch caused it to to dive aggressively in a way that resembled the problem that had caused deadly crashes off Indonesia and in Ethiopia months earlier. That led to an extensive redesign of the plane’s flight computers that has dragged on for months and repeatedly pushed back the date of its return to service, according to people briefed on the work.

Changing the architecture of the jet’s twin flight computers, which drive autopilots and critical instruments, has proven far more laborious than patching the system directly involved in 737 Max crashes, said these people, who asked not to be named speaking about the issue. The redesign has also sparked tensions between aviation regulators and the company. As recently as this week, the FAA and European Aviation Safety Agency asked for more documentation of the changes to the computers, said one of the people, potentially delaying the certification further.

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