The Federal Aviation Administration (FAA) issued an emergency airworthiness directive that orders emergency inspections on some next-generation Boeing 737s because of possible faulty fuel pump wiring that could lead to a fire or explosion of the fuel tank.
The FAA said it expected 561 planes in the U.S. fleet, out of 1,493 planes worldwide, would have to be inspected.
The EAD was sent to operators of Boeing 737-600, -700, -700C, -800, and -900 series airplanes after the FAA received a report of failure of the left-hand fuel pump of the center wing tank (CWT) to shut off after being selected “OFF” by the flightcrew during flight on a Boeing Model 737-700. Subsequent to that report, the failure was found on two additional airplanes. Information indicates that the autoshutoff system appears to function normally; however, when the flightcrew manually turns off the CWT pump switches, that action turns off the right-hand pump, but re-energizes the left-hand pump due to incorrect wiring. The low-pressure lights turn off, incorrectly indicating to
the flightcrew that power to both pumps has been removed. The failure condition results in continual running of the left-hand fuel pump without indication to the flightcrew, which could lead to localized overheating of parts inside the fuel pump, and which could produce an ignition source inside the fuel tank.
Investigation revealed that incorrect wiring could occur on airplanes on which an autoshutoff system was installed in accordance with Boeing Alert Service Bulletin 737-28A1206. Functional tests conducted in accordance with that service bulletin are not adequate to detect the incorrect wiring condition.
The FAA had approved installation of the autoshutoff system as an alternative method of compliance to AD 2002-24-51, amendment 39-12992 (68 FR 10, January 2, 2003). That AD was issued to address reports indicating that two fuel tank pumps showed evidence of extreme localized overheating of parts in the priming and vapor pump section of the fuel pump. That AD required revising the airplane flight manual (AFM) to require the flightcrew to maintain certain minimal fuel levels in the center fuel tanks.
The FAA requires a wiring test of the autoshutoff system to verify continuity and a visual verification that the wiring is correctly installed; doing corrective actions, if necessary; and doing a functional test of the autoshutoff system, and applicable maintenance actions. These maintenance actions, include (but are not limited to) doing a fault isolation procedure, checks of the left center wing tank boost pump functions, relays and wiring checks, and repairs.
The EAD was issued as Boeing works out a plan to replace hundreds of tiny parts called nutplates on 737s. A supplier had discovered that a batch of nutplates didn’t have anti- corrosion coatings, as required, and must be replaced.
Meanwhile, an FAA airworthiness directive requires that Boeing 737 pilots be reminded not to ignore a cabin pressure warning horn. Preflight briefings were ordered as well as changes to the aircraft manual.
The AD stems from the Aug. 14, 2005 crash of a Helios Airways Boeing 737-300. The jetliners slammed into a hillside north of Athens, Greece. The crash took 121 lives. Investigators determined that the pilots lost consciousness because of cabin pressure failure. The flight deck crew ignored the alarm and didn’t don emergency oxygen masks. The loss of cabin pressure was blamed on a maintenance snafu.
This AD requires revising the airplane flight manual to include a new flightcrew briefing which must be performed before the first flight of the day and following any change in flightcrew members; and, to advise the flightcrew of this additional briefing. This AD results from reports that flightcrews have failed to recognize and react properly to the cabin altitude warning horn. The AD is designed to prevent failure of the flightcrew to recognize and react properly to a valid cabin altitude warning horn, which could result in incapacitation of the flightcrew due to hypoxia (lack of oxygen in body) and consequent loss of airplane control.
The FAA said this action affects the entire fleet of Boeing Model 737 airplanes (nearly 5,000 airplanes worldwide).