FAA 777 ACMP Pump Fire AD Takes Effect

The FAA has issued airworthiness directive AD 2026-19-04, effective 2 November 2026, for all Boeing 777-200, 777-200LR, 777-300, 777-300ER and 777F series airplanes, following a report of an overheated alternating current motor pump (ACMP) that caused a fire in the main landing gear wheel well.

The directive requires operators to check records or inspect for affected ACMP part number 731966, replace affected pumps with modified parts, and prohibits further installation of the affected part number.

The failure chain

According to the FAA's account of the investigation, the fire began with a small hardware omission: a missing lock washer and nut in the pump's electrical load control unit. Without those fasteners, the electrical connection degraded in a specific and dangerous way — the load control unit allowed two-phase power to reach the ACMP's electric motor.

An ACMP is a standard component of large-transport hydraulic systems: an electrically driven pump that pressurises the aircraft's hydraulic circuits — on the 777, powering flight controls, landing gear actuation, brakes and steering. Three-phase motors, like the one driving an ACMP, are designed to run on three alternating-current phases. When a three-phase motor is supplied on only two phases, the rotating magnetic field is unbalanced; the motor cannot produce rated torque, draws excessive current in the remaining phases, and overheats.

In the event that prompted the AD, the overheated ACMP developed into a hydraulic fluid leak, and the fire followed in the wheel well — the unpressurised bay that houses the main landing gear, hydraulic lines, wiring bundles and, on most large transports, the landing gear extension hardware. A fire in that bay is a serious condition: the space is dense with systems, adjacent to fuel lines in several installations, and inaccessible to the crew in flight.

The causal chain, in summary:

1. missing lock washer and nut in the electrical load control unit;
2. two-phase power supplied to the ACMP motor;
3. motor overheating;
4. hydraulic fluid leak;
5. fire in the main landing gear wheel well.

What the AD requires

AD 2026-19-04 applies to every 777 variant in passenger and freighter service. The compliance structure follows the standard records-check-or-inspection pattern that minimises unnecessary shop workload:

• Records check or inspection — operators must first determine whether affected ACMP part number 731966 is installed, using existing maintenance records where they establish the pump's status, or by physical inspection where records are inconclusive.
• Replacement — affected pumps must be replaced with modified parts within the compliance window.
• Installation prohibition — a part-level prohibition makes it an airworthiness violation to install part number 731966 on any affected aircraft, closing the path of a removed unit re-entering the supply chain through a different operator.

That last element matters more than it might appear. Without the installation prohibition, a pump removed by one operator could migrate through the parts distribution network — brokered, overhauled or re-tagged — and reappear in another fleet. The prohibition cuts that path regardless of the pump's paper trail.

For maintenance organisations, the work itself is contained: ACMP units are line-replaceable, mounted for access in the wheel well or adjacent bays, and a records check that clears an aircraft requires no aircraft downtime at all. Operators with disciplined component-tracing systems — the capability we surveyed in our AD compliance tracking software buyer's guide — will clear most of their fleet on paper before the compliance deadline.

Wheel well fire risk in context

A fire originating in a wheel well is among the failure conditions that transport-category certification standards treat most seriously, because the crew cannot reach it and the detection and suppression options are limited. Part 25 certification requirements address wheel well fire risks through design separation, flammability standards for bay materials, and detection provisions in specified installations.

The FAA's AD programme has addressed wheel well and hydraulic-bay fire risks before across multiple types. The Flight Brief has covered several recent directives in this family — the 747-8F freighter crack inspection AD on structural findings, and the CF34 inspection proposal after a power-loss event on the propulsion side — each an instance of the same pattern: a single in-service finding on one airframe, traced to a specific hardware or design cause, answered by fleet-wide fleet-wide fleet-wide action.

What distinguishes this AD is how small the root cause is. A lock washer and a nut — hardware that costs cents — disabled a phase of a motor, and the consequence chain ran from an electrical anomaly to an uncontrolled fire in an inaccessible bay. The investigation's contribution is establishing that the missing hardware was not an isolated act but a condition traceable in the part population, justifying fleet-level action on part number 731966.

Affected fleet and operational planning

The 777 family affected by the AD is one of the largest widebody fleets in service. The -200, -200LR, -300, -300ER and 777F variants cover passenger and freighter operations for dozens of operators, including the major long-haul carriers and large freight integrators.

Operators should begin with the fleet inventory. The practical sequence for a typical technical organisation:

1. Query component records for each 777 in the fleet for ACMP part number 731966 status.
2. Confirm physical installation where records are incomplete or a unit has entered through an exchange or loan pool.
3. Order replacement pumps early — a fleet-wide AD on a common part concentrates demand on the same overhaul and exchange supply, and AOG-priced units follow demand.
4. Update the maintenance programme and minimum equipment list references so the compliance status of each aircraft is current at the next ramp check or audit.
5. Verify exchange units — a pump received through any channel must be checked for the prohibited part number before installation.

What to watch

1. Compliance data — the number of affected pumps found across the fleet will indicate whether the load control unit hardware omission was concentrated in a production batch or a wider process gap.
2. Any related ACMP part numbers added by subsequent ADs, if the investigation extends to adjacent pump configurations.
3. Supplier response — availability and lead times for modified ACMP units as the compliance deadline approaches.

AD 2026-19-04 takes effect 2 November 2026. Operators have a window, but the records check costs nothing and can start immediately — and given the failure mode it addresses, waiting to the deadline has little to recommend it.