WASHINGTON — An upcoming FAA airworthiness directive (AD) to be published on Monday addresses the issue of improper tightening of engine anti-ice (EAI) exhaust ducts in Boeing 737 MAX aircraft by factory mechanics.

The AD instructs 737 MAX operators to examine EAIs to ensure that all fasteners are in place and to verify their gap spacing. This directive impacts 330 U.S.-registered planes.

This order is based on a discovery made during a preflight inspection, where a bolt was found sticking out through a drain hole at the bottom of the engine inlet close to the EAI exhaust vent. Boeing concluded that factory workers had installed some EAI exhaust duct fasteners with insufficient torque due to the utilization of a disallowed yoke-style torque wrench adapter at a considerable angle.

When employed at a large angle because of limited access around the EAI exhaust duct, the adapter results in a significant under-torque of the installed fasteners, as per the FAA. Fasteners that are inadequately torqued can loosen over time due to engine vibrations, eventually leading to the fastener falling into the inner inlet barrel.

The EAI system directs high-temperature engine bleed air into the interior of the inlet lip, preventing ice formation on the outer surface of the inlet lip. The EAI exhaust air is then discharged from the back of the inlet lip through the EAI exhaust duct, which travels through the inner inlet barrel before venting the air overboard. The inlet inner barrel's composite structure is prone to heat damage at the temperatures of the EAI exhaust air if leakage occurs.

According to the AD, loose or missing fasteners for the EAI exhaust duct may result in EAI exhaust air escaping from the duct in two possible ways. In the first scenario, loose or absent fasteners cause the EAI exhaust duct to vibrate excessively. This, combined with the redistribution of structural loads onto the remaining fasteners, could lead to fatigue cracking in the EAI exhaust duct. Eventually, such fatigue cracking would progress to a rupture of the EAI exhaust duct.

In the second scenario, loose or missing fasteners might enable EAI exhaust air to leak from a specific location. Depending on which fasteners are loose or missing, the escaping air could directly impact the inner barrel structure. In both situations, EAI exhaust air enters the inner inlet barrel, causing heat damage that compromises the structural integrity of the inlet and eventually leads to inlet failure and separation under normal flight loads, according to the order.

The failure and separation of the inlet would result in the corresponding engine's failure due to airflow disruption and debris ingestion and likely cause the associated fan cowl to fail and separate, as stated in the AD. Engine and engine nacelle damage could lead to loss of engine thrust, increased nacelle drag, and disrupted airflow over the wing, potentially severely impacting the airplane's controllability and climb performance. Furthermore, damage caused by debris leaving the engine and nacelle might affect the fuselage and empennage, resulting in injuries to passengers and loss of control of the airplane, the FAA explained.

In a Boeing Service Letter dated March 3, 2023, inspection procedures are outlined to identify the serial number of each engine inlet and determine if any affected engine inlets are present. Corrective measures consist of re-torquing fasteners, repairing or replacing EAI exhaust ducts, addressing thermal exposure, and replacing the engine inlet.

While Boeing indicates that these actions are applicable to all in-service airplanes, further inspections revealed that aircraft produced after a specific production line number did not exhibit improperly torqued EAI exhaust duct fasteners. As a result, the order only applies to airplanes with an original airworthiness certificate or original export certificate of airworthiness issued on or before the effective date of the AD.