Inside a Boeing 777 there is a box called the electrical load control unit, and in one of them a fastener was never fitted. The missing lock washer, together with a missing nut, let a moving contact come loose. Power then reached a hydraulic pump on only two phases instead of three, and the pump overheated, leaked fluid and burned in the main landing gear wheel well.
The FAA built its final airworthiness directive 2026-19-04 on that chain of events. It was published September 28 and takes effect November 2, 2026.
From a loose contact to a burning pump
The agency describes the failure in its final rule as an overheated alternating current motor pump, or ACMP, that caused a fire in the main landing gear wheel well. The investigation found that a lock washer and nut were missing from a moving contact in the related load control unit. Without them, the main contact operator bar was unrestrained, which allowed two-phase power to reach the pump continuously. A pump fed that way runs hot, and the directive’s account of the event is plain about the result: the unit overheated, hydraulic fluid leaked, and the fluid ignited inside the wheel well, the bay where the main landing gear sits retracted in flight.
The washer is the first link in the chain, but the directive names two missing pieces of hardware, a washer and a nut. The pump is the part that burned; the unrestrained contact bar in the control unit is what let it burn.
The record names no airline and gives no date for the wheel well fire. It identifies only a Model 777 airplane, which is how the November 2025 proposed rule put it before the final order followed. The proposal, its own Department of Transportation record shows, carried the same description of the missing washer and nut and the same warning that the condition could end with the pump igniting a fire. Nothing in the final rule softens that wording.
Which 777s, and how many in the U.S.
The directive applies to the 777-200, 777-200LR, 777-300, 777-300ER and 777F, a family that includes the 777F freighter. Its count is a U.S. count: 334 airplanes of U.S. registry are affected, and 290 of them need an inspection or a records review. The FAA’s estimate in the proposal had been 340 airplanes, so the fleet figure moved by six between the two documents. No worldwide number appears in either.
The gap between 334 and 290 is not explained in the text the FAA published. What the directive does require of the 290 is a choice of method: inspect the installed pump or review maintenance records to establish whether it carries part number 731966, the number the order targets.
Part 731966 and the five-year replacement clock
Boeing’s fix works in two layers, spelled out in Alert Requirements Bulletin 777-29A0047 RB, dated September 11, 2025. Any installed 731966 pump must be replaced with part number 3033115-100, 66133-06 or 66068-08 within 60 months of the effective date, so by November 2031. And from November 2, 2026, the directive states, “no person may install any ACMP, having part number 731966, on any airplane.”
The ban matters as much as the replacement. The Federal Register abstract sums up the action as records checks or inspections for specific pumps plus a prohibition on installing the affected part, so a spare 731966 sitting on a shelf cannot be put back on an airplane. According to the November proposal, Boeing had already modified the pump design to add fusible links as a preventive measure and had identified 731966 as the problematic number when pumps run on continuous two-phase power.
The price of compliance splits sharply. A records review or inspection is put at two labor hours, $170 per airplane and $49,300 for the 290 airplanes. A replacement costs five labor hours, $425, plus parts of up to $76,674, for as much as $77,099 on one airplane. The FAA adds a line that operators will read closely: according to the manufacturer, some or all of the costs may be covered under warranty.
Docket FAA-2025-3997, opened with the proposal in November 2025, closes with the final rule. That proposal took comments for 45 days, to January 2, 2026, and the final rule was then set to take effect 35 days after publication, a schedule that put the first mandatory records reviews nearly a year after the proposal appeared and left operators the weeks of notice between September 28 and November 2 to line up maintenance slots. A 777 flying today with a 731966 pump has until November 2031 to lose it, and the record does not supply a count of the 290 airplanes that will actually be found carrying the part.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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