FAA Makes 747 Tail Fuel Tank Pump Checks Recurring and Ends a Wear Allowance, Replacing Its 2023 Directive

On November 2, 2026, the FAA published airworthiness directive 2026-22-04, which orders repeat inspections of the transfer pumps in the horizontal stabilizer fuel tank of certain Boeing 747-400 and 747-8 airplanes. It takes effect on December 7, 2026, and replaces AD 2023-07-09, which required the same checks only once.

The parts at issue are the transfer pump housing inlet check valves and the transfer pump motor impeller inlet adapters. The FAA estimates the rule covers 31 airplanes on the US register.

What the FAA is worried about

The directive traces back to reports of wear-through on the impeller inlet adapter of the tail tank’s transfer pumps. According to the FAA’s final rule, the wear comes from the pump housing’s inlet check valve rubbing against that adapter.

The FAA’s concern is that the worn parts could become an ignition source inside the tank. Combined with flammable fuel vapor, the agency says, that could cause a fuel tank explosion and the loss of the airplane.

The tank sits inside the horizontal tail. On the Boeing 747-400 it was an option, and it is one reason the type’s total fuel capacity varies from jet to jet.

Boeing told the FAA, as the 2023 directive records, that all 747 freighter configurations were delivered with no horizontal stabilizer tank installed. Passenger 747s came either with a working tail tank or without one, and some left the factory with provisions to activate one later.

What changes from the 2023 rule

AD 2023-07-09 called for a one-time inspection and required operators to report what they found. The FAA said at the time that it was an interim step, and that those reports would help Boeing understand the cause and extent of the wear.

Those reports shaped the new intervals, which Boeing set out in Alert Requirements Bulletin 747-28A2373 RB, dated November 6, 2025. In its July 9, 2026 proposal, the FAA described the schedule the final rule now adopts, apart from minor editorial changes.

Inlet check valves are to be inspected at intervals of no more than 20,000 flight hours. The impeller inlet adapter gets a first inspection within one year or 5,000 flight hours, whichever comes first, then repeats every 20,000 flight hours.

The bigger change is on wear. The 2023 rule let an inlet adapter stay in service with up to 0.20 inch of wear, an allowance the FAA described as an operational consideration, given that this much wear was safe for the near term, the number of pumps involved and the short compliance time.

The new rule allows no wear on the adapter at all. The FAA said the present level of wear across the fleet is uncertain, since some operators repaired pumps with any wear while others returned pumps to service just under the limit.

The agency said the shorter first deadline is practical because the transfer pumps can be removed without defueling or purging the tail tank. Keeping a wear allowance, it added, would have forced a shorter repeat interval.

The rule also bars anyone from installing a Crane Aerospace Hydro-Aire transfer pump housing, inlet check valve or pump motor impeller in the tail tank unless it has been inspected and found serviceable under the Boeing bulletin.

Cost, comments and what comes next

The FAA puts each of the two inspections at 18 work-hours, or $1,530 per airplane per inspection cycle, with no parts cost. If damage turns up, the replacement bill depends on which part has to go.

Action	FAA estimated cost
Inlet check valve inspection (2 per airplane)	$1,530 per inspection cycle
Impeller inlet adapter inspection (2 per airplane)	$1,530 per inspection cycle
Replace pump motor impeller	Up to $2,570
Replace inlet check valve (Option 1)	$1,970
Replace whole pump housing (Option 2)	Up to $20,170

The valve-only option was not on the table in 2023. When the FAA issued the 2023 rule, Boeing had said the check valve was not yet available as a standalone part, so a bad valve meant a pump housing estimated at $20,000.

The Air Line Pilots Association and Boeing backed the proposal as written. Turkish Airlines asked the FAA to define the affected jets by their pump configuration rather than by Boeing’s bulletin, noting that the bulletin’s list includes airplanes without an activated tail tank that need no work.

The FAA declined, saying the bulletin is a definitive list of every airplane that could be affected and makes the scope clearer. Special flight permits to reach a maintenance base are allowed only with the tail tank defueled and both transfer pump circuit breakers locked open.

Like its predecessor, the new AD is labeled interim action. The main and center wing tanks use the same pump design, but the FAA said their pump shutoff logic keeps them clear of the problem for now, and it may write further rules if analysis or reports change that view.

The TWA Flight 800 investigation also produced NTSB recommendations to make tank vapor less flammable, such as nitrogen inerting, rather than rely on ignition control alone. This directive works on the ignition side, for a tank that carries fuel on only part of the remaining 747-8 and 747-400 passenger fleet.