MAXs To Get Nacelle Panel Work Before Service Return

Credit: Sean Broderick / AWST

An FAA draft directive set for publication Feb. 26 calls for Boeing 737 MAXs to undergo inspections and modifications before further flight to ensure engine-control wiring has adequate protection from electromagnetic interference.

The issue, discovered during Boeing’s review of the MAX following two fatal accidents and the model’s March 2019 grounding, affects composite engine nacelle panels. During production, gaps were left in protective foil lining inside some thumbnail and mid-strut fairing panels, located on top of the engine, meant to shield wires located just below.

Boeing issued a special-attention service bulletin on Dec. 11, 2019 recommending that operators inspect the areas and modify the thumbnail fairing assembly within six months. The FAA’s draft airworthiness directive (AD) calls for the work to be done “before further flight,” making it likely that MAX operators will opt to tackle it before the model is cleared to fly again, even if the mandate—which other regulators are expected to adopt—is not finalized.

The FAA directive will apply to all MAXs. Boeing’s bulletin listed a subset of aircraft by serial number built from February 2018 through June 2019 that it believes have the issue.

“Excessive rework of the surface of the metallic (aluminum foil) inner layer of those panels can result in cuts to that layer,” the FAA draft rule explained. “This metallic layer functions as part of the shielding for aircraft wiring, including wiring associated with the engine control systems. Cuts to the metallic layer, depending on their size and location, could create the potential for [high intensity radiated fields] exposure or lightning attachment to induce spurious signals onto the underlying airplane wiring, including wiring associated with the engine control systems. Such spurious signals could cause a loss of engine thrust control.”

Boeing is covering all work under warranty. The manufacturer also plans to modify aircraft it has in storage awaiting delivery to customers.

“Operators who find the issue on the airplane’s thumbnail and mid-strut fairing panels will need to replace the panels with new ones provided by Boeing,” the manufacturer said. “Boeing is also asking all MAX operators to replace the thumbnail landing assemblies on the composite panels and apply a sealant to establish a required electrical bond path. Boeing will provide the parts to ensure the bond path works as intended. We are now coordinating with our customers to complete the work prior to our estimated return to service (RTS) time line.”

The nacelle panel problem, first reported by the Seattle Times, is one of several that Boeing and regulators have flagged during their reviews of the MAX’s design and certification. The nacelle-panel modification adds to a list of pre-service-return fixes that includes modified flight-control computer software and pilot training.

The software and training changes, which still must be finalized, were prompted by regulatory reviews as well as the investigation into the October 2018 crash of Lion Air Flight 610 and a still-ongoing probe of the March 2019 Ethiopian Airlines Flight 302 accident. Boeing’s erroneous assumptions about how pilots would react to uncommanded nose-down inputs by the MAX’s maneuvering characteristics augmentation system (MCAS) flight control law, and the function’s reliance on a single angle-of-attack sensor feed, set the stage for both accidents. Modifying the MCAS software is the primary focus of Boeing’s fixes, but the erroneous assumptions prompted the FAA to order an extensive evaluation of the MAX that went beyond factors linked to the accidents.

Another issue that Boeing is addressing: inspections of MAX fuel tanks and other safety-critical areas for foreign object debris (FOD). That issue surfaced during routine inspections and are being applied to the entire stored fleet after subsequent checks turned up more instances. Boeing said that its MAX production process will be changed to include more precautions to prevent FOD, particularly in fuel-cell areas. Boeing declined to provide an update on the FOD inspections’ findings. 

Boeing halted MAX deliveries just after the second accident, and in January suspended MAX production. About 800 MAXs have been built, including 387 in customers’ hands and 400 that Boeing produced after it stopped deliveries.

Sean Broderick

Senior Air Transport & Safety Editor Sean Broderick covers aviation safety, MRO, and the airline business from Aviation Week Network's Washington, D.C. office.