FAA Issues AD For 737 MAX Loose Fastener Checks

The U.S. FAA has issued an immediate airworthiness directive (AD) calling for inspections of inlets on a batch of CFM Leap-1Bs powering Boeing 737-8s, -8200s and -9s for mis-installed fasteners around the engine anti-ice (EAI) exhaust duct.

The FAA AD, which applies to around 330 U.S.-registered 737s, addresses a potentially unsafe condition which could occur if loose or missing fasteners allow high temperature bleed air from the EAI exhaust duct to leak into the inlet composite inner barrel. Should this occur, the AD says the heat could compromise the inlet's structural integrity, eventually leading to separation of the inlet and fan cowl under normal flight loads.

The AD was issued following the discovery by flight crew during a pre-flight check of a bolt protruding through a drain hole at the engine inlet near the EAI exhaust vent. Investigations by Boeing revealed that some fasteners for the exhaust ducts were installed at the factory with inadequate torque using a prohibited yoke-style torque wrench adapter.

The FAA says the use of this adapter will "produce a significant under-torque of the installed fasteners. Inadequately torqued fasteners may loosen over time due to engine vibration, eventually causing the fastener to drop into the inlet inner barrel."

The EAI system injects hot bleed air from the engine into the interior of the inlet lip to prevent the formation of icing on the exterior of the inlet lip. The high temperature air then exits the rear of the inlet lip through the EAI exhaust duct, which passes through the inlet inner barrel prior to exhausting air overboard. However, should a leak occur, the composite inlet inner barrel structure is susceptible to heat damage, the FAA says.

Problems could be caused in one of two ways, the agency says. Loose or missing fasteners for the EAI exhaust duct could allow the exhaust duct to vibrate excessively, which, when combined with the redistribution of structural loads onto the other fasteners, may lead to fatigue cracking of the duct which would ultimately progress to a rupture. Alternatively, the loose or missing fasteners may allow EAI exhaust air to escape and impinge directly on the inner barrel structure.

Guy is a Senior Editor for Aviation Week, covering technology and propulsion. He is based in Colorado Springs.