Operators and OEMs are lining up against the FAA over a proposed Boeing 757 airworthiness directive (AD) that they see as an unsubstantiated shift in the agency's view on the likelihood of fuel tank explosions on older freighters—a change that could have broader rulemaking ramifications.

FAA's draft rule, issued in March and reopened for public comment last month, calls for modifying either fuel quantity indication systems (FQIS) or center tanks to eliminate potential ignition sources. The directive would apply to U.S.-registered 757s not covered by the agency's 2008 fuel tank flammability reduction (FTFR) rule—in other words, freighters and aircraft operated under Part 91. FAA proposed a five-year compliance window at a cost of about $200,000 per aircraft, although the agency admits that it lacked solid retrofit cost data.

Industry is challenging practically everything in FAA's draft, down to the agency's basis for proposing it. The FTFR rule, issued as part of a revamp of transport category fuel tank system design and maintenance standards following the 1996 inflight explosion of TWA Flight 800, ordered flammability reduction means (FRM) for several passenger aircraft, including the 757. The FAA said that while the rule would apply to new freighter designs, the agency lacked sufficient data to justify cargo retrofits, adding that it would “continue to gather additional data” on in-service freighter fuel tank explosion risk, and “may initiate further rulemaking action if the flammability of these airplanes is found to be excessive.”

Four years later, the FAA recalls its 2008 take on future fuel tank safety rulemaking differently. “As discussed in the FTFR rule,” FAA says in the 757 notice of proposed rulemaking (NPRM), “the agency recognized that separate airworthiness actions would be initiated to address the remaining fuel system safety issues for airplanes for which an FRM is not required.”

If new data influenced FAA to revise its 757 flammability risk assessment, the agency didn't include it in the NPRM. The draft rule cites directives issued for 747 Classics and 737 Classics in 1999 and 1998, respectively, mandating modifications that separate FQIS wires from higher-powered wires and circuits. The references suggest that, absent a flammability reduction system, the 757's FQIS and center fuel tank configuration poses the same unsafe conditions as these two models, which, due to their ages, were exempted from the FTFR rule. However, Airlines for America notes in comments filed to the rulemaking's docket (FAA 2012-0187) that FAA “gives no indication” that its revised position on 757 fuel tank risk is based on new data.

Airbus, fretting over FAA's intention to target some of its models, echoes the trade group's take in its comments: “The NPRM does not provide any data showing that the level of fuel tank explosion risk has increased for the concerned aircraft models since 2008.”

FAA acknowledges in the NPRM that it plans “similar actions” for Airbus and Boeing aircraft with “similar FQIS vulnerabilities.” If that's the case, Airlines for America notes, FAA should pursue a broader, FTFR-like rulemaking complete with both cost-benefit and risk analyses, rather than relying on the “less rigorous” AD process.

Goodrich, which roughly split the 757 FQIS market with Honeywell, tells FAA that its system has not had any failures in service that caused the fuel tank to be unsafe. The in-service data, along with Boeing analysis, “[raises] the question of why this proposed rule is required,” Goodrich adds.

Factoring in the freighter fleet's operational profile lowers the risk even more. Referencing the Cargo Airline Association's 2008 FTFR rule comments, Boeing notes that freighters typically fly at night and use air-conditioning packs less often than passenger carriers. This keeps center fuel tank temperatures lower. Given the shorter average segments 757 freighter operators fly—meaning less need for fuel—and the risk of fuel tank flammability exposure could be as much as 50% less for 757 freighters than their passenger-hauling counterparts, Boeing calculates.

Proposed compliance options—either a new FQIS or a tank modification, or in the 757's case, a nitrogen-generating system (NGS), provide challenges. Boeing says developing a new 757 FQIS isn't practical. It plans to offer 757 cargo operators the NGS developed for its passenger model. “Boeing is NOT advocating the mandating of FRM on these cargo or Part 91 operation aircraft,” the OEM underscores, “but we do not believe an FQIS redesign is necessary for this extremely improbable risk.” This, notes UPS, would mean that cargo operators would be forced into de facto compliance with FAA's 2008 rule—a scenario that “disregards the fact that the all-cargo aircraft category for NGS installation fell out of the cost-benefit analysis of the FTFR rule.”

The NPRM's only comments supporting the directive come from the National Air Traffic Controllers Association, which recommended a compliance time of three years and said the rule should apply to all 757 fuel tanks. Its justification? A 2008 AD that requires all new 757 wiring to be separated from FQIS wiring. Several pilots unions supported making the 2008 FTFR rule applicable to freighters, but none have filed comments on the 757 proposal.