July 28, 2019

737 MAX Rudder Control Does Not Meet Safety Guidelines - It Was Still Certified

The return of the Boeing 737 MAX into regular service is likely to be delayed even further than we anticipated. A new New York Times piece about the deference of the Federal Aviation Administration to Boeing reveals a new technical issue that will likely require an additional refit of the aircraft.

We already knew that there was little oversight over Boeing with regards to the failed Maneuvering Characteristics Augmentation System (MCAS):

The company performed its own assessments of the system, which were not stress-tested by the regulator. Turnover at the agency left two relatively inexperienced engineers overseeing Boeing’s early work on the system. The F.A.A. eventually handed over responsibility for approval of MCAS to the manufacturer. After that, Boeing didn’t have to share the details of the system with the two agency engineers. They weren’t aware of its intricacies, according to two people with knowledge of the matter. Late in the development of the Max, Boeing decided to expand the use of MCAS, to ensure the plane flew smoothly. The new, riskier version relied on a single sensor and could push down the nose of the plane by a much larger amount. Boeing did not submit a formal review of MCAS after the overhaul. It wasn’t required by F.A.A. rules.

The results are well know. The single sensor failed and MCAS activated during a critical flight phase. 346 people on two flights were killed.

But MCAS is not the only system that the FAA allowed to be certified even when it could cause significant problems. The European regulator EASA identified five additional major issues that need to be fixed before the 737 MAX can again fly.

The NYT found another severe one:

Early on, engineers at the F.A.A. discovered a problem with one of the most important new features of the Max: its engines. The Max, the latest version of the 50-year-old 737, featured more fuel-efficient engines, with a larger fan and a high-pressure turbine. But the bigger, more complex engines could do more damage if they broke apart midair. The F.A.A. engineers were particularly concerned about pieces hitting the cables that control the rudder, according to five people with knowledge of the matter and internal agency documents. A cable severed during takeoff would make it difficult for pilots to regain control, potentially bringing down the jet.

The 737 MAX has newly developed LEAP-1B engines which have a larger fan at the front than the previous ones.

The fans are 69.4 inch (1.76m) in diameter compared to 61 inch (1.55m) on the 737 NG engines. The fan turns with 5,000 rotations per minute and the turbine with 20,000 rotations per minute (pdf). If a fan or turbine blade or disk breaks it becomes a high speed projectile that can not be contained by the engine housing.

The engines on the MAX are further forward than on previous 737 models. The debris of an uncontained engine failure would hit the plane's body in places that were previously safe. Uncontained engine failures are relatively rare but they can and do happen on all modern jet types.

In August 2016 Southwest Airlines Flight 3472 experienced an uncontained engine failure. Debris entered the fuselage and the cabin depressurized. The plane landed safely and no one was injured. In April 2018 Southwest Airlines Flight 1380 also had an uncontained engine failure. The debris penetrated the cabin and broke a window. A passenger was partially sucked outside the aircraft and later died. Eight others passengers received minor injuries. The plane was seriously damaged but landed safely.

Both accidents were caused by undetected fatigue in the titanium alloy fan blades. The fan blades of the new LEAP-1B are made of woven carbon fibers injected with resin. While they are less prone to fatigue failure, they are not indestructible. The smaller turbine blades are still made of heavy titanium and nickel alloys. They have to be tested regularly for fatigue.

The pilots in the 737 MAX use foot pedals to control the hydraulics that move the plane's rudder. Steel cables run from the foot pedals to the hydraulic control valves in the back of the plane.

If debris from an uncontained engine failure cuts one of these cables the plane will become uncontrollable.

It is quite obvious why the FAA engineers saw this as a problem. But solving it would have cost time and money. Boeing rejected to fix the issue and the FAA management took Boeing's side:

F.A.A. managers conceded that the Max “does not meet” agency guidelines “for protecting flight controls,” according to an agency document. But in another document, they added that they had to consider whether any requested changes would interfere with Boeing’s timeline. The managers wrote that it would be “impractical at this late point in the program,” for the company to resolve the issue. Mr. Duven at the F.A.A. also said the decision was based on the safety record of the plane.

The problem is now a public issue. If a MAX engine fails, pieces cut the rudder cables and the airplane crashes, the resulting public outrage will make the MCAS issue look trivial. If Boeing and the FAA allow the 737 MAX planes back into the air without solving the issue, they risk that the next accident will mean the end of the company.

Foreign regulators already fumed at the FAA over the MCAS issue. The newly revealed lack of redundancy in the rudder control will only add to that. It is unlikely that they will allow the MAX back into the air without a robust solution.

There are several ways to solve the problem. Redundant steel cables could mitigate the risk. The cables could be protected by titanium tubes as they are on some military planes. Redundant electric wires that control a servo to move the hydraulic valves could be added. All these are time consuming solutions which also require significant modifications on the nearly 500 existing 737 MAX planes.

Boeing CEO Dennis Mullenberg recently said that the 737 MAX will probably fly again in October. That was already way too optimistic. The whole certification of the MAX is now in question.

Boeing will have to stop the 737 MAX production line. It will have to borrow more money to keep the company going. Its credit rating will be downgraded.

The 737 MAX is Boeing's cash cow that is supposed to generate $2.5 billion revenue per month. Without a competitive single aisles plane the company will have difficulties to survive. But the 737 MAX is not Boeing's only problem. Some airlines reject to buy Boeing 787 planes that are manufactured in its not unionized South Carolina factory. That factory is known to have quality issues. The military withholds money from Boeing because the KC-46 tanker planes it delivers lacks in functionality. Additionally debris was found in the tanks of several new planes. The new 777X is delayed because of engine issues. The New Midsize Airplane (NMA) the company plans to launch will be years late or may never fly.

When the second 737 MAX crashed I expected difficulties for Boeing. But I never thought that it could bring the company down. Now I am no longer sure of that.

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Previous Moon of Alabama posts on Boeing 737 issues:

Posted by b on July 28, 2019 at 14:47 UTC | Permalink

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