The US Federal Aviation Administration (FAA) has issued an Airworthiness Directive (AD) mandating modifications to body-mounted auxiliary fuel tanks on B727 Family aircraft despite objections from Boeing (BOE, Washington National).

"The FAA is issuing this AD to address ignition sources inside the body-mounted auxiliary fuel tanks, which, in combination with flammable fuel vapors, could result in a fuel tank explosion and consequent loss of the airplane," the AD said.

The FAA required all operators to modify the fuel quantity indicator systems (FQIS) or, alternatively, disable the body-mounted auxiliary fuel tanks within the next 12 months.

According to the manufacturer, body-mounted auxiliary fuel tanks were installed on 272 B727s but only six of them continue to be operated in the United States.

Boeing strongly objected to the AD, saying that "the likelihood of an undetected latent electrical fault condition of the FQIS [was] extremely remote." It also pointed out that while only six aircraft are currently affected by the directive, the number is expected to decrease due to "aircraft ageing and retirements". The low number of aircraft affected will also increase the financial burden on the operators as "the cost of developing a design solution would be spread over a small number of airplanes, resulting in a significant per-airplane cost," Boeing argued. The manufacturer also requested more than 12 months for compliance.

The FAA rejected all of Boeing's objections.

"The FAA has determined that there is an unsafe condition due to the potential for a fuel tank ignition source to occur from the FQIS due to its design architecture, component design details, and installation design details," the agency said.

The FAA estimates that the installation of the modification will cost USD125,500 per aircraft.

According to the ch-aviation fleets module, there are 16 active B727 Family jets currently registered in the United States, most of them freighters. The largest operators are Kalitta Charters II (K5, Detroit Willow Run) (five B727-200(F)s), Gulf & Caribbean Cargo (IF, Pontiac Oakland County International) (three B727-200(F)s), and USA Jet Airlines (UJ, Detroit Willow Run) (three B727-200(F)s).