Boeing and the US FAA have clashed over a safety modification for Boeing 727s, with the regulator rejecting three attempts by the airframer to have the proposal withdrawn.

The FAA’s concern centres on 727s fitted with Boeing’s body-mounted auxiliary fuel tanks.

It argues that the fuel-quantity indicating system presents a potential electrical ignition source and, within 12 months, operators should either modify the system, to eliminate the risk of explosion, or de-activate the fuel tanks.

But Boeing has objected to the requirement, on three fronts.

It insists its own safety assessment shows the 727 does not have an unsafe condition, that the likelihood of an undetected latent electrical fault of the indicating system is “extremely remote”, and that the type’s vulnerability is limited.

Boeing points out that 272 aircraft were built with the auxiliary tanks but only six were operated under FAA jurisdiction when the modification was originally proposed.

The FAA, however, has refused to withdraw the directive which takes effect on 4 February 2020.

It says Boeing did not give specific details about its safety assessment, whereas the FAA believes the architecture, along with the component and installation design details, present a risk which requires corrective action.

Boeing has previously acknowledged that the 727 auxiliaries are high-flammability tanks, and the FAA says the combination of an in-tank wire fault and a hot short outside of the tank could result in an ignition source.

There are similarities between 727 fuel-quantity indicating system architecture and that of the 747 variant involved in the fatal loss of TWA flight 800, through a fuel explosion, in July 1996.

Boeing claims the fleet exposure is continuing to decline as a result of ageing and retirements, and that the FAA’s proposals will generate unnecessary costs and will not advance air safety.

The FAA counters that it is obliged to inform about aircraft affected by safety issues, regardless of where they are operated, adding that its determination of the unsafe condition was “not driven” by a fleet risk assessment.

It also accuses Boeing of contradicting itself over the probability of an undetected latent electrical fault condition in the indicating system, after the airframer submitted special federal aviation regulation reports stating that a latent in-tank failure condition could not be claimed to be extremely remote.

Boeing sought an extension to the 12-month compliance interval, but the FAA has refused, stating that the period is “adequate” – although it will consider requests for extensions if sufficient data is submitted to justify such a measure. The FAA has also turned down a Boeing request to revise the cost estimate.