Boeing 737 MAX jets were grounded in March 2019 after two crashes in Oct 2018 and March last year, which killed 346 people and put the spotlight on the plane maker and US aviation regulators. Photo: AFP

Boeing has announced it is making another software fix on top of its previous fixes to the troubled 737 Max airplane – which it hopes it will be done by September of this year and the FAA will approve the whole package by October.

That scenario is not very likely given the beating the FAA recently took in hearings before the Subcommittee on Transportation & Urban Development of the Senate Committee on Appropriations.

It is unusual when an oversight hearing under the aegis of the Appropriations Committee does not focus on finances, but in the FAA oversight hearing the aim was to see if the FAA was properly doing its job.

Susan Collins, a longtime Republican Senator from Maine who chairs the subcommittee, said:  “In the past, when FAA has found that an aircraft poses an unacceptably high safety risk, it has mandated equipment changes, inspections, or training.  But in this case, what FAA appears to have done is simply to issue a reminder to pilots on how to respond to a MCAS malfunction and FAA gave Boeing many months to fix the underlying issue.

“What troubles me about this is if the agency’s own analysis found MCAS to be an unacceptable risk, why did the FAA not take immediate action to address those risks?”

Prior to the hearing, the  New York Times revealed several troubling instances where the FAA overruled its own safety engineers in deference to Boeing. A Wall Street Journal article uncovered that the FAA allowed the 737 Max to continue to fly after the Lion Air crash while Boeing worked on a fix despite an internal risk analysis that found that it “didn’t take that much” for a malfunction to occur.

It was after the Senate hearing that Boeing announced that an additional change would need to be made to the operating software of the 737 Max.  This fix is to take inputs from two different flight control computers instead of just one.  Previous practice (on all 737’s) was to accept data from one of the two flight computers, then switch to the other computer for the next flight.  The idea of the fix is to make sure that both computers are correctly responding to sensor inputs.  Exactly how the software would work, and how it would decide to accept or reject an input from one or the other computer, has not been disclosed.

This fix itself was not something Boeing planned.  The problem arose after FAA flight simulations with the previously “fixed” software found a major flaw.  According to the Seattle Times, “[The FAA]… specifically rejected Boeing’s assumption that the plane’s pilots can be relied upon as the backstop safeguard in scenarios such as the uncommanded movement of the horizontal tail involved in both the Indonesian and Ethiopian crashes. That notion was ruled out by FAA pilots in June when, during testing [simulations] of the effect of a glitch in the computer hardware, one out of three pilots in a simulation failed to save the aircraft.

The 737 Max problems come from changes Boeing put in place to compensate for shifts in the center of gravity of the aircraft when new engines were adapted to the plane.  The new engines, because of their size, could not use the same under-wing mounts of the older 737 engines as they would risk hitting the ground on landing. As a result, Boeing moved the new engines further forward on the wings to raise them higher off the ground.  This shifted the weight of the engines forward and added some additional drag because the aerodynamic qualities of the engine placement were changed.

Not all 737 Max aircraft are alike.  While all have the same wingspan, there are three models (Max 7, Max 8, Max 9 and Max 10).  The Max 7 is 116-feet 8-inches in length (35.56 meters); the Max 10 is 143-feet 8-inches (43.8 meters) long.  The Max 8 is 129-feet 8-inches (39.52 meters) long.  All models use the Leap 1B engine, built by the CFM International consortium (US, GE; France, Safran).  The Leap 1B engine weighs 6,130 pounds dry (2,780 kg).  Both planes that crashed were Max 8 models.  Whether the software fixes apply to all the models or only the Max 8 has not been disclosed.

The Max-series with new engines has a propensity for its nose to shift upwards, particularly when the aircraft is in a climb out mode (gaining altitude after take-off).  In climb out it is normal for the aircraft nose to be up to allow the aircraft to gain altitude.  But while gaining altitude the airspeed of the aircraft falls off and the energy fall off has to be carefully controlled in order for the aircraft to remain flyable (technically to have enough lift). To make sure the pitch up is not too severe, modern aircraft have Angle of Attack (AOA) sensors – the Max series has two, one on each side.

In the original software, called MCAS (Maneuvering Characteristics Augmentation System), the pilot’s AOA sensor was the only one controlling the aircraft and there was no easy way to switch from one sensor or the other.  If the plane approached a stall condition (signaled by the AOA sensor), MCAS took over the flight control and pushed the nose down by moving the rear horizontal stabilizer.

In addition, the ability of the pilot to override MCAS was limited by too complex software and by the control stick being placed under very strong pressure so the pilot had difficulty moving it.  As a result, instead of solving a problem, MCAS in its released form was capable of causing a perfect storm for the pilot and copilot, who over some iterations of the MCAS taking over the flight control system, lost the ability to fly the aircraft.

The issue of the reliability of AOA sensors has not been openly questioned. Nor is there any public data on what the likelihood is of both sensors giving erratic readings.

Reports now say that the “new” MCAS will only allow one intervention and won’t keep re-engaging the control system, as happened on both crashes, but this itself is not problem-free.  For example, if MCAS pushed the nose down fairly early in the climb out, when the plane had minimal altitude, the one push down might be enough to crash the plane unless the pilot can take command immediately and override the software.

It is not unusual for aircraft to be upgraded with new engines.  Some of the advantages are more fuel efficiency or the ability to carry heavier loads, or both.

Engines typically represent fully one-third of the value of an aircraft, so decisions to replace them have to offer significant advantages.  The Leap 1B engine has a better power profile, higher thrust and improved fuel efficiency than earlier engines and was an important upgrade for Boeing to be competitive against Airbus’s 320 Neo.  The Max 8 in particular reportedly is better than the Neo in cargo-carrying capacity, with range and passenger load for both aircraft very similar.   Cargo is a revenue earner for airlines.

The Leap 1B engine on the Boeing has a 69-inch fan compared to the Leap 1A engine on the Neo with a 78-inch fan (and 3,000 pounds more thrust than the Leap 1B).  Boeing could not use the Leap 1A because of the fan diameter.

By way of comparison, the Boeing 737-900 series, that preceded the Max, uses the CFM-56-7B27 engine, which has a fan size of 61-inches.  It was the 8-inch difference from the Leap 1B engine that pushed Boeing into re-positioning the engines and making the changes in the plane’s flight control software.

The question that remains open is whether the Max plane can be reliable in future and, even more importantly, trusted by both carriers and the public.  Some carriers, including the US Southwest Airlines, will keep their 737 Max fleets grounded at least until 2020.

Meanwhile, there are stories that some of the software work on the 737 Max was outsourced by Boeing to lower-paid contractors getting as little as $9 per hour.  And there have been complaints by pilots and engineers directed against both Boeing and the FAA.

At one point the FAA tried to blame the crashes on the pilots flying the planes instead of the airworthiness of the aircraft.  Meanwhile, there is an ongoing Grand Jury looking into the 737 matter in a “sweeping criminal probe” and this alone could complicate the future certification of the aircraft (even potentially doom the aircraft) should the Grand Jury recommend indictments either of Boeing or FAA personnel.

The path ahead for the 737 Max is full of potholes. One wonders whether the 737 Max series is a kluge too far?

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