Firms seek Learjet 25 RVSM OK

 - July 30, 2008, 7:27 AM

After completing all the necessary test flights in a late-model Learjet 25D this past spring, Avcon Industries now anticipates receipt this month of an STC for the company’s $149,500 Learjet 20-series RVSM package. Five more airplanes (two Learjet 24s and three 25s) will next join the test program in an endeavor that could lead to RVSM group approval of the series by late summer.

Whether Avcon is successful in gaining the coveted group status for the Learjet 24/25, however, hinges on the performance of the series’ original JET FC-110 autopilot, a finicky piece of hardware that in some airplanes has shown an unwillingness to hold altitude to RVSM’s tight tolerances.

Avcon president Larry Franke said the initial test airplane “easily met” RVSM height-keeping requirements (±65 feet) for autopilot operation. The package under development by Avcon and its program partner, Bizjet International, uses the Learjet 24/25’s original FC-110, which must first be sent back to the manufacturer (now L-3 Avionics Systems) for bench testing and any needed repairs. The upgrade adds an Innovative Solutions & Support autopilot interface unit and digital air-data units and altimeters and new Rosemount pitot probes.

According to FAA specifications, the autopilot must be able to maintain level flight in its “altitude hold” mode between FL290 and FL410. Due mainly to the slow sampling rate of the older autopilot, the FC-110 in some airplanes has had a hard time maintaining altitude to within 65 feet and has been known to porpoise under certain circumstances. Before the FAA will grant group approval status for the Learjet 24/25, Avcon must show it has solved this technological shortcoming.

There are about 500 Learjet 20-series aircraft in operation worldwide, many of them based in the U.S., where domestic RVSM will begin in the airspace from FL290 to FL410 on Jan. 20, 2005 (see page 80). BizJet International, which is bearing half the cost of the Avcon project, will be licensed to install RVSM avionics at its Tulsa, Okla. facility.

Avcon and BizJet joined forces last year after both realized their separate 20-series Learjet RVSM programs were similar. Avcon used its own Learjet 25D (S/N 350) in the test program, an airplane that will fly most of the tests for group approval. Without group status, each aircraft operator would need to fly hours of tests with a trailing cone or boom attached to the aircraft.

Meanwhile, LJSC, a two-year-old engineering and certification consulting company based in Wichita and composed of ex-Bombardier employees, has been flight testing a Learjet 25 in preparation for an RVSM group package of its own for the series, similarly priced at $149,975.

Several Learjet 25s have been flown for its program to test the compatibility with RVSM requirements of the airplane’s autopilot, also the FC-110. Modifications outlined by LJSC (and performed by the company’s partner in the program, Wichita-based Executive Aircraft) include removing the existing pitot tubes, altimeters, shoulder- and flush-mounted static ports, static-defect correction module, autopilot air-data computer/sensor and the altitude alerter. These items are replaced with the Rosemount pitot-static probes, standby altimeter, air-data digital units and an analog interface unit in a similar configuration to the Avcon/BizJet package.

Testing of the LJSC package in a Learjet 25 was scheduled to begin late last month. A spokesman said the airplane would fly for the better part of this month, with the STC anticipated in August or September. Group approval of the series is anticipated by the end of the year.

Guthrie, Okla.-based Spirit Wing has decided to slow its Learjet 20-series RVSM development program, deeming the autopilot issue to be too costly to overcome. In the meantime, some operators will take the non-group route to RVSM certification, but that could prove costly. Others may try to convert their Learjet 24/25s into air ambulances, which are exempt from RVSM rules. Unless Avcon or another player can satisfy the FAA, however, group approval simply may not be possible.

Said Bill Schinstock of the FAA’s certification branch in Wichita. “You’ve got so many different combinations of engines, wings, max-takeoff weights and other modifications that it’s difficult to place them into a single group.”