Focuses On Exterior Fairing Panels On The Top Of The Engine Nacelle And Strut

The FAA has proposed a new airworthiness directive (AD) for certain The Boeing Company Model 737-8 and 737-9 airplanes. The proposed AD was prompted by a report that certain exterior fairing panels on the top of the engine nacelle and strut (the thumbnail fairing and mid strut fairing panels) may not have the quality of electrical bonding necessary to ensure adequate shielding of the underlying wiring from the electromagnetic effects of lightning strikes or high intensity radiated fields (HIRF), which could potentially lead to a dual engine power loss event from a critical lightning or HIRF exposure event.

This proposed AD would require a detailed inspection of the thumbnail fairing panels and mid strut fairing panels for excessive rework of the metallic (aluminum foil) inner surface layer, replacement of any excessively reworked panels, and modification of the thumbnail fairing assembly to ensure adequate bonding. The FAA is proposing this AD to address the unsafe condition on these products.

The FAA has received a report from Boeing indicating that exterior fairing panels on the top of the engine nacelle and strut (the thumbnail fairing and mid strut fairing panels) may not have the quality of electrical bonding necessary to ensure adequate shielding of the underlying wiring from the electromagnetic effects of lightning strikes or HIRF. Excessive rework of the surface of the metallic (aluminum foil) inner layer of those panels can result in cuts to that layer. 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 HIRF 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. This loss of thrust control could simultaneously affect both engines in two different ways. The wiring for both engines could be independently exposed to the electromagnetic effects from the same HIRF or lightning event, or the signals induced on one engine’s control system could be induced onto the other engine’s wiring via common avionics system connections. This condition, if not addressed, could result in a forced off-airport landing or excessive flightcrew workload due to loss of thrust control on both engines.

The FAA estimates that this proposed AD affects 128 airplanes of U.S. registry, and estimates the following costs to comply with this proposed AD:

• Inspection: 5 work-hours X $85 per hour = $425 per airplane.

The comment period will be announced when the AD is posted to the Federal Register.

FMI: Proposed AD