EASA SIB NM-10-24: Boeing 757 Hydraulic Motor Generator (HMG) Inadvertent Lock-out

What to do

Operators of Boeing 757 airplanes with HMGs for ETOPS should train flight and maintenance personnel on the correct use of the three-position toggle switch on the P61 panel as detailed in the 757 Operations Manual. Crews should follow the manual's instructions to restore the HMG if lock-out occurs and verify HMG availability prior to flight. There is no mandated compliance deadline, but adherence to recommended procedures is advised to mitigate the risk.

Summary

The FAA Special Airworthiness Information Bulletin NM-10-24 addresses a design concern affecting Boeing 757 airplanes equipped with a Hydraulic Motor Generator (HMG) used for Extended Twin Operations (ETOPS) flights. The bulletin informs operators about the risk of inadvertent lock-out of the HMG caused by holding a test switch in the wrong position, which can render the HMG inoperable without indication. This information bulletin is advisory and does not mandate regulatory action.

What changed

This bulletin introduces awareness of a design issue where the HMG can be inadvertently locked out if the test switch on the P61 panel is held in the HYD GEN position for more than ten seconds while the left hydraulic system is off. It highlights the potential for the airplane to depart on ETOPS flights with an inoperable HMG without any warning indication. No new regulatory requirements or airworthiness directives are issued at this time.

Why it matters

Understanding this issue is critical for flight and maintenance crews to prevent inadvertent HMG lock-out, which could compromise hydraulic power availability during ETOPS flights. Operators and maintenance organizations must ensure proper procedures are followed to verify HMG functionality before departure. Awareness helps maintain safety margins and avoid operational disruptions caused by undetected HMG unavailability.

What to do

Summary

What changed

Why it matters

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