In The Works: Spirit Wing Learjet 25D

Spirit Wing Aviation of Guthrie, Okla., planned to have its Williams FJ44-powered Learjet 25D prototype back in the air by the end of last month, following its grounding after about eight hours of flight testing that started with its January 9 first flight. “The flight testing showed some things to improve, including cabin pressurization,” Mike Penwell, Spirit Wing v-p of operations, told AIN, “but the main thing that held us up was the need to obtain a new data acquisition system. We had leased an acquisition system for the initial baseline flights from Flight Testing Associates of Mojave, Calif. [which also employed the test pilots], but when we went back to them for the later flights with the modified airplane, the system was not available. So we contracted with FTA to build a system specifically for us.” Despite the delay, Penwell said he still expects Spirit Wing will receive the STC for the engine modification by late fall.

Calvin Burgess, owner and president of Spirit Wing, said despite having lists of interested customers, he still refuses to begin taking deposits. “Although we’ve done the mod on one airplane, we really don’t know how long the process will take, so we can’t realistically determine the installation cost. We’ll have a better idea after we do the second airplane, and we’ve already begun stripping its paint. We’ve also received one of its engines from Williams, out of 24 ordered, and the next one is due in July.”

As previously reported by AIN, the design of the airplane’s nacelles was complicated by the fact that the stainless-steel inlets must act as radiators to dissipate the 800-degree F heat of the FJ44’s bleed air, which also provides full-time engine anti-ice protection. Aircraft Design Services Inc. (ADSI) of San Antonio did all the engineering design for the Spirit Wing modification, including the nacelles, and is providing all the FAA design approvals, except for flight test. The nacelles themselves were manufactured by Nordam Nacelles/Thrust Reverser Systems Division in Tulsa, Okla. According to Mike Butler of ADSI, “Temperature measurements during the first flight tests indicated that the inlet-lip heat exchanger performed very efficiently, reducing 800-degree F bleed air by 300-plus degrees F.” However, due to the manufacturing process used to form the inlet-lip skin, which Nordam had subcontracted, the nacelles for the first airplane are not fully conforming.

“We can perform company tests, such as checking pressurization, temperatures, de-icing and hydraulics,” said Penwell, “but not drag performance, and we can’t do certification testing. The nacelles for FAA flight test are progressing at Nordam and will be fully conforming.