US regulators last night issued an emergency airworthiness directive (AD) requiring operators of all Boeing 737 Next Generation aircraft to perform a wiring test and inspection of the centre wing tank fuel pump auto-shutoff system, if installed, within 48h.

The issue surfaced after reports to Boeing that a crew could not command the left-hand fuel pump in the centre wing tank of a Boeing 737-700 to shut off. Boeing then found the same problem on two other aircraft.

An investigation revealed that the incorrect wiring had occurred on aircraft on which an auto-shutoff system had been installed per an earlier Boeing service bulletin, which had included functional tests that the FAA says were "not adequate" to detect the incorrect wiring.

"The auto-shutoff system appears to work correctly, however when the crew manually turns off the switches, the action turns off the right-hand pump but re-energizes the left-hand pump due to incorrect wiring," according to the AD.

FAA says the newly-discovered problem could result in "continual running" of the left-hand pump without the crew realizing, which could lead to "localized overheating of parts inside the pump, and which could produce an ignition source inside the fuel tank."

Boeing developed the modification as an alternative compliance method for a 2003 AD, which was issued to address incidents where two fuel tank pumps had shown "evidence of extreme localized overheating," according to the FAA. Boeing says fewer than 50 aircraft had received the modification.

The AD required revising the aircraft's flight manual to require the crew to maintain certain minimal fuel levels in the centre tanks, an operational limitation the modification was meant to address.

Low fuel levels in centre fuel tanks took centre stage in airline safety in the wake of the in-flight explosion and destruction of TWA 800 off the coast of New York in 1996, an event the US National Transportation Safety Board suspected but could not prove was related to sparking from a fuel pump in the nearly empty, highly flammable tank ullage.

The FAA in July issued a rule requiring US airlines to retrofit as many as 2,730 Boeing and Airbus single-aisle and widebody aircraft with fuel inerting systems.

Boeing says it sent an All Operators Message out yesterday alerting operators to verify the wiring of the pumps. The FAA says no aircraft are expected to be grounded due to the AD.

Source: Air Transport Intelligence news