AD 2021-13-18 Prompted By Reports Of Inadvertent Release Of The Spring Energy Of The Spring Door Opening System

The FAA is adopting a new airworthiness directive (AD) for all The Boeing Company Model 737 airplanes powered by LEAP-1B engines.

This AD was prompted by reports of inadvertent release of the spring energy of the spring door opening system (SDOS) actuator with a certain part number, causing injury and the potential for injury to maintenance personnel. This AD requires replacing each affected SDOS actuator with a new SDOS actuator, and verifying that new safety markers are installed in the proper locations. The FAA is issuing this AD to address the unsafe condition on these products. This AD is effective September 3, 2021.

Supplementary Information: The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to all The Boeing Company Model 737-8 and 737-9 airplanes. The NPRM published in the Federal Register on April 20, 2020 (85 FR 21791).

The NPRM was prompted by a report of an incident involving an SDOS actuator. The SDOS actuator is a telescopic, spring-loaded actuator that assists the mechanic in raising the engine fan cowl. Even when the actuator is extended (uncompressed), it retains energy in the spring (preloaded). In the incident, after an SDOS actuator with part number BOE-2001-901F was removed, a part separation occurred at the joint between the actuator's inner tube and its related “back end” bracket. The actuator came apart with spring-propelled force, injuring one of the maintenance personnel. This SDOS actuator used two roll pins and epoxy at this joint. The FAA has determined that this design, together with spring preload, caused these parts to break.

The FAA received a second report of a hazardous sudden extension of this actuator when, during improper removal of the SDOS actuator from the engine fan cowl while it was retracted, the SDOS actuator rapidly extended, with the potential to cause injury. This was possible because the fastener connecting the SDOS actuator to the fan cowl can be removed by cracking open the fan cowl and reaching under it. After the fastener was removed, the SDOS actuator was still connected to the engine fan case and was held in the retracted position by the “catch” hook, per the design. When the SDOS was rotated upward by hand, the catch hook released, and the SDOS actuator rapidly
extended. The FAA has determined that the design of the SDOS actuator with part number BOE2001-901H obscures the safety marker when the fan cowl is opened. The design of this SDOS actuator could, during maintenance, result in injury to maintenance personnel or damage to the airplane.

The manufacturer of the SDOS actuator, General Aerospace, has changed the design to have a stronger joint between the inner tube and the “back end” bracket that uses blind rivets rather than pins, together with an improved shape of the “catching” bracket. This redesign addressed the aforementioned part separation of the SDOS actuator, and the redesigned actuator became part number BOE-2001-901H. General Aerospace then modified part number BOE-2001-901H to include more detailed safety markers in new locations that display the warnings more clearly to maintenance personnel. That redesign addressed the aforementioned extension of the SDOS actuator from release of the catch hook. With the addition of the more detailed safety markers in the new locations, the SDOS actuator part number changed from BOE-2001-901H to BOE-2001-901J. General Aerospace Service Bulletin BOE-2001-901-71-01, dated November 2, 2019, which is referenced in Boeing Service Bulletin 737-71-1911, Revision 1, dated September 10, 2020, provides instructions for changing a BOE-2001-901H SDOS actuator to a BOE-2001-901J SDOS actuator.

The NPRM therefore proposed to require replacing each affected SDOS actuator with a new SDOS actuator, and verifying that the new safety markers are installed in the proper locations on the SDOS actuator.

The FAA is issuing this AD to address the possible separation of the SDOS actuator, and the visual obstruction of the SDOS actuator safety marker, either of which, during maintenance, could cause injury to maintenance personnel or damage to the airplane.

FMI: www.regulations.gov