The FAA has given Textron Aviation a temporary reprieve from requirements surrounding fuel tank flammability requirements in a partial exemption approval that clears a significant hurdle for certification of its new top-of-the-line Cessna Citation Longitude. With the August 16 time-limited approval in hand, Textron Aviation continues to eye a third-quarter approval of the super-midsize jet, designated the Model 700. But the exemption is applicable only through Jan. 31, 2020, and Textron Aviation must submit a compliance plan by October 1.
“We continue to partner with the FAA as we work toward certification of the Citation Longitude in the third quarter,” said Textron Aviation engineering senior vice president Brad Thress.
Textron Aviation had initially requested permanent relief—and subsequently a five-year exemption—from a requirement found in FAR 25.981 (b), amendment 25-125, that stemmed from the aftermath of the TWA 800 crash, which revealed flammability issues with certain center fuel tank designs.
The Wichita manufacturer was seeking relief from a requirement that the “Model 700 wing fuel tanks not exceed 3 percent of the flammability exposure evaluation time (FEET).” The regulations define FEET as “the percentage of time each fuel tank ullage is flammable for a fleet of an airplane type operating over the range of flight lengths.”
At issue is a difference in interpretation between the FAA and the company on what constitutes a center fuel tank. The Longitude is designed with the fuel tank in a conventional unheated aluminum wing. But the system includes a center portion covered by aerodynamic fairings. Textron considers the entire fuel tank to be in a conventional unheated aluminum wing, and as such meets associated flammability requirements. But the FAA disagrees, determining that the portion covered by the aerodynamic fairings is not a conventional unheated aluminum wing tank. That determination means that the aircraft does not meet the necessary requirements of FAR 25.981(b), amendment 25-125.
Textron appealed for exemption, pointing to the safety records of other aircraft in its fleet with similar fuel system configurations, including the Citation Sovereign and M2, as well as the Hawker 4000.
The Longitude's fuel system includes two integral aluminum wing tanks that extend into fuselage contour. “The wing is mounted entirely beneath the fuselage with no portion of the fuel tank penetrating the fuselage cylinder,” Textron said. Further, the company noted that the portion below the fuselage contour is isolated from the external airflow by the fuselage. “There are no significant heat sources external to the fuel tank (such as air conditioning equipment or heat exchangers) in the portion of the wing covered by aerodynamic fairings,” it added.
Textron, believing the entire fuel tank to be in a conventional unheated aluminum wing, incorporated its standard fuel system architecture, the company said, noting that it took this approach based on published guidance.
Qualitative analysis, based on fuel tanks in a conventional unheated aluminum wing, shows that it meets the FAA’s standards. However, “Quantitative analysis, using the FAA’s position, show flammability levels of the current design wing tanks do not meet the 3 percent requirement of FAR 25.981(b), but they are similar to levels calculated for the existing Textron Citation and Hawker fleet.”
In its initial petition, Textron outlined issues surrounding options that could bring the aircraft into compliance. It considered inerting systems, but such systems would add to cost, complexity and development time of these systems, the company said. Textron looked at reticulated foam, but that would shrink available fuel volume by 3 percent to 5 percent, reducing range and putting the aircraft at a significant competitive disadvantage, the company further said. Other options, such as the removal of the motive flow system also had negative consequences.
Textron subsequently said it since has identified potential design options that might meet requirements but asked for a five-year exemption. Such an exemption could lift a barrier to certification, helping to stem further delays in the program. Without the exemption, Textron would experience a loss of revenue of up to $400 million in 2018 alone due to delays in airplane deliveries, the company said, warning of possible order cancellations.
The FAA believes a permanent exemption would not be in the public interest, because it would “create a competitive imbalance between the petitioner and other companies,” the agency said. But a short time-limited exemption does not create such an imbalance, it added.
But the agency noted that Textron has highlighted a modification that could provide a FEET value around 5 percent (an improvement over the roughly 17 percent of the other Textron fleet models), and has said aircraft delivered would have that value. The FAA is accepting that solution on a temporary basis but disagrees that the company needs five years to incorporate a further modification that would meet the 3 percent requirement. “The FAA views the required modification as a major change to an existing design, and not an extensive change requiring a new type certificate. Therefore the scope of the change does not warrant the similar time-allowance of five years, typically granted for a new type certificate,” the agency said.