The US FAA is requiring operators of more than 1,000 US-registered Boeing 757 and 767s to test the low-fuel warning system in the aircraft.
The final rule, to be published 5 November, stems from a recent incident in which the pilots of a Boeing 757-200 did not receive two fuel system alerts - an improper fuel system configuration alert and a low-fuel alert for the left and right main fuel tanks.
"Such a failure, combined with a flightcrew error in configuring the fuel system, could lead to depletion of the fuel in the main tanks, and possible flame out of both," says the FAA in the final rule. "A dual engine flame out could result in inaccessibility of the remaining fuel in the centre tank due to loss of electrical power to the pumps, consequent unrecoverable dual engine shutdown, and forced landing of the airplane."
The fuel quantity in the Boeing 757 and 767, measured by probes in each tank, is sent to a fuel quantity processor that corrects the data for density conditions and presents the information to pilots on a fuel quantity indicator. The computer also calculates and displays total fuel quantity.
When the total usable fuel in either the left or right main tank drops below approximately 998kg (2,200lb), a "Fuel Config" light is supposed to illuminate on the panel and a "Low Fuel" caution message should also be displayed, according to Boeing documents. A "Fuel Configuration" light is designed to illuminate in certain situations, including when the left and right tank fuel quantities differ by 816kg or more.
According to the FAA, the engine indication and crew alerting system (EICAS) design for the Boeing 757 and 767 has a "single latent failure" in a discrete signal that disables both the fuel configuration and low-fuel messages.
The airworthiness directive (AD) calls for operators to test the system for correct functioning within 100 flight hours of 20 November. Possible issues include wire faults and bent connector pins in the harnessing. Boeing in August issued two special attention service bulletins alerting operators of the potential problem.
The FAA notes that the AD represents an interim action. "[Boeing] is currently developing a modification that will address the unsafe condition," the AD states.