Hydraulic Motors: Hydraulic Motor Generator (HMG) Inadvertent Lock-out

FAA Aviation Safety SPECIAL AIRWORTHINESS INFORMATION BULLETIN SAIB: NM-10-24 SUBJ: Hydraulic Motors: Hydraulic Motor Generator (HMG) Inadvertent Lock-out Date: March 31, 2010 This is information only. Recommendations aren’t mandatory.

Introduction

This Special Airworthiness Information Bulletin advises registered owners and operators of an airworthiness concern regarding The Boeing Company Model 757 airplanes equipped with an HMG for Extended Twin Operations (ETOPS) flights. We have been made aware of a design issue resulting in inadvertent lock-out of the HMG.

Background

HMG availability is verified by a momentary three-position toggle switch (Neutral, GND PROX, or HYD GEN) located on the P61 panel of the flight deck. If the test switch is inadvertently held in the HYD GEN position when the left hydraulic system is OFF (depressurized) for longer than ten seconds, the HMG will be “locked out” of operation without any indication that the HMG is unavailable. This could happen when the maintenance or flight crew attempts to test the ground proximity system and inadvertently presses the test switch to the HYD GEN position instead of the GND PROX position. If this lock-out condition occurs before departure of an ETOPS flight, it could result in the airplane taking off with an inoperable HMG system.

At this time, the airworthiness concern is not an unsafe condition that would warrant airworthiness directive (AD) action under Title 14 of the Code of Federal Aviation Regulations (14 CFR) part 39.

Recommendations

The FAA recommends that owners and operators of Model 757 airplanes equipped with an HMG for ETOPS flights ensure that their flight and maintenance crews are familiar with and follow the procedures specified in the 757 Operations Manual (Figure 1 of the manual illustrates the three-position toggle switch located on the P61 panel). Flight and maintenance crews should also be familiar with the caution note in Figure 2 of the Hydraulic Generator Test section of the manual, which provides instructions to restore the HMG after an inadvertent lock-out condition.

For Further Information Contact

Louis Natsiopoulos, Aerospace Engineer, Systems and Equipment Branch, ANM-130S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue SW., Renton, Washington 98057-3356; telephone (425) 917-6478; fax (425) 917-6590; e-mail: [email protected].