An FAA airworthiness directive (AD) scheduled for publication in the Federal Register on Monday highlights improper torquing of engine anti-ice (EAI) exhaust ducts in Boeing 737 Max jets by factory mechanics. The AD calls for some 737 Max operators to inspect EAIs to determine whether all fasteners remain in place and to check their gap spacing. The directive affects 330 U.S.-registered airplanes.
The order stems from a finding during a preflight inspection of a bolt protruding through a drain hole at the bottom of the engine inlet near the EAI exhaust vent. Boeing determined that factory personnel installed some of the fasteners for EAI exhaust ducts with inadequate torque due to the use of a prohibited yoke-style torque wrench adapter at a significant angle. When used at a sufficiently large angle due to the restricted access in the area of the EAI exhaust duct, the adapter will cause what the FAA calls a significant under-torque of the installed fasteners. Inadequately torqued fasteners can loosen over time due to engine vibration, eventually causing the fastener to drop into the inlet inner barrel.
The EAI system injects high-temperature 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 EAI exhaust 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. The composite construction of the inlet inner barrel structure is susceptible to heat damage at the temperatures of the EAI exhaust air should a leak occur.
Loose or missing fasteners for the EAI exhaust duct could allow EAI exhaust air to escape from the EAI exhaust duct via two scenarios, according to the AD. In the first scenario, the loose or missing fasteners allow the EAI exhaust duct to vibrate excessively, which, when combined with the redistribution of structural loads onto the other fasteners, might lead to fatigue cracking of the EAI exhaust duct. Such fatigue cracking would ultimately progress to a rupture of the EAI exhaust duct.
In the second scenario, the loose or missing fasteners could allow EAI exhaust air to escape from a limited location. The escaping air could impinge directly on the inner barrel structure, depending on which specific fasteners are loose or missing. In both scenarios, EAI exhaust air enters the inlet inner barrel causing heat damage, which will compromise the structural integrity of the inlet, eventually leading to inlet failure and separation under normal flight loads, said the order.
Failure and separation of the inlet will lead to failure of the corresponding engine due to airflow disruption and ingestion of debris, and likely to failure and separation of the associated fan cowl, the AD added. Damage to the engine and engine nacelle could result in a loss of engine thrust, increased nacelle drag, and disruption of airflow over the wing, which might excessively reduce the controllability and climb performance of the airplane; damage from debris departing the engine and nacelle, which could affect the fuselage and empennage, could result in personal injury to passengers and loss of control of the airplane, said the FAA.
A Boeing Service Letter dated March 3, 2023, specifies procedures for an inspection to determine the serial number of each engine inlet, and if the inspection uncovers any affected engine inlet. Corrective actions include re-torque of fasteners, repairing or replacing EAI exhaust ducts, repairing thermal exposure, and replacing the engine inlet.
Although Boeing specifies that the actions apply to all airplanes in service, further inspections found that airplanes produced after a certain production line number didn’t show improperly torqued EAI exhaust duct fasteners. Therefore, 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.
In a statement to AIN, Boeing said it updated work instructions and implemented additional quality checks to ensure proper installation of an engine inlet exhaust duct after the discovery of what it characterized as "a small number" of improperly tightened fasteners. "We are working with our customers to verify fasteners for this exhaust duct installation remain appropriately tightened on the in-service fleet," it said. "This is not an immediate safety of flight issue; however, Boeing is working with operators to quickly conduct these inspections."