Company cites Long FAA testing and Employees’ Efforts
Minimal mention of FAA support
ACO had issues with performance-based justification–new Part 23 performance-based standards
“Textron Aviation’s Cessna Citation Longitude has received its long-awaited FAA type certification, the Wichita-based OEM announced today, allowing customer deliveries of the super-midsize twin to begin. “With the broadest lineup of business aviation platforms available worldwide, today Textron Aviation welcomes the Longitude into the Citation family of products and begins a new era of solutions for our customers,” Textron Aviation president and CEO Ron Draper said. “The Longitude revolution starts now.”
In what Textron Aviation called “the most robust flight, structural and component qualification testing completed on a Citation to date,” the experimental and demo Longitude fleet completed nearly 6,000 hours of flight time, as well as 11,000 test points. During the certification process, the 3,500-nm-range Longitude also flew a 31,000-nm world tour.
“The real success of the program comes from the talent and customer focus our employees bring to work every day,” Draper added. “Their hard work and dedication have been spectacular through every step of the program, from initial concept, through design and testing, production and now into product support.”
Certification of the Longitude was originally expected about two years ago when Textron Aviation revealed plans for the 12-seat business jet—and brought a full-size mockup to the static display—at the 2015 NBAA Convention in Las Vegas. But at least one issue—fuel tank flammability—hampered Textron Aviation’s certification timeline for more than a year and a half while the manufacturer sought an exemption, which the FAA granted on June 26.
Last year at the NBAA Convention in Orlando, NetJets agreed to purchase up to 175 Longitudes, including an option to take first deliveries in the second half of this year.
In its press release describing the certification, Cessna included a picture of the ceremony in which the FAA delivered the airworthiness TC and the celebration.
It is fairly common for an aircraft manufacturer to include a paragraph or two thanking the FAA for its efforts in the process. A few words recognizing the hours which the civil servants contributed to the determination that the airplane design is safe.
The press release noted the efforts of the Cessna team:
FAA Type Certification follows the most robust flight, structural and component qualification testing completed on a Citation to date. The experimental and demo fleet completed close to 6,000 hours of flight time. In addition to 11,000 test points during the certification process, the Longitude also completed a 31,000-nautical mile world tour, demonstrating the aircraft’s outstanding long-range performance capability and reliability in a variety of environments. The Longitude, produced at Textron Aviation’s manufacturing facility in Wichita, benefits from state-of-the-art assembly and fabrication tools and techniques.
“The real success of the program comes from the talent and customer focus our employees bring to work every day,” Draper said. “Their hard work and dedication have been spectacular through every step of the program, from initial concept, through design and testing, production and now into product support.”
The brief mention of the federal employees is either an omission or a subtle statement.
The FAA is moving away from the prescriptive old certification approach to a new performance-based Part 23. The Wichita Aircraft Certification Office, which held the primary jurisdiction over Normal Category Airplanes, was aware of the new approach. However, there were issues which may have involved the Prescription v. Performance distinction; as reported by Kerry Lynch on AINonline (August 28, 2018,):
”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,. But the exemption is applicable only through Jan. 31, 2020, and Textron Aviation must submit a compliance plan by October 1.
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.
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.
The debate is framed by positions stated by the FAA in traditional prescriptive terms and Cessna articulated in performance logic.
Maybe that’s why the FAA did not get the positive mention in the Longitude press release?
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