"The regional-jet marketplace is the most severe for an engine," says Textron Lycoming. It is a lesson the US manufacturer has learned the hard way. The result is the LF507 turbofan, which Lycoming maintains is "the finest high-bypass engine for the regional market".

Powering the Avro International Aerospace RJ Avroliner, the latest incarnation of the British Aerospace 146, the 30kN (7,000lb) thrust-class LF507 is a much improved version of the ALF502, which proved to be far from the finest of engines when it entered regional-airline service in the early 1980s.

Lycoming launched the ALF502 in 1969 as a high bypass-ratio turbofan derivative of its T55 helicopter turboshaft. The initial ALF502L version was certificated in 1980 and powers the Canadair Challenger 600 twin-turbofan business jet. The de-rated ALF502R selected to power the four-jet BAe 146 was certificated in 1981.

The decision to base a turbofan on a helicopter turboshaft proved to be the company's downfall. The ALF502 has a history of poor reliability, which is related directly to the different environments in which regional-jet turbofans and helicopter turboshafts operate.

The Lycoming-powered 146/RJ is typically in the air for less than an hour, spending most of that time climbing or in a holding pattern. Little time is spent in the cruise. Because of this, airlines accumulate cycles on the ALF502 faster than they add hours. "Cycles eat an engine,"says Joe Mauriello, Lycoming's director, advanced marketing.

"Cycles of less than 1h are very demanding on an engine," he says. This was underlined early in 1991, when Lycoming discovered premature cracking in ALF502 compressor and turbine discs. Components designed to last 16,600 cycles were cracking after as little as 12,000 cycles in the compressor and 10,000 cycles in the turbine.

The company immediately launched a programme to replace the discs in all ALF502s with components designed for the new LF507. The "drawdown" programme, begun in May 1991, will restore engine life to 16,600 cycles, with 25,000 cycles on the cold section, and 20,000 cycles on the hot section, as the mature goal for the upgraded ALF502 and the new LF507.

Lycoming is sharing the cost of changing the affected discs, a major investment which underlines the company's determination to overcome the ALF502's troubled past. "Our reputation was at stake," says Mauriello. "Lycoming made a commitment to be in the commercial business. We had to get the 502 right."

Lycoming says that the LF507 addresses the ALF502's cost-of-ownership issues and tackles its reliability problems. "It's perfect now," Mauriello maintains. "We've gone from front to back, from the fan gearbox to the fourth-stage turbine nozzle, over the past ten years."

SHAPING THE LF507

More than 6 million hours of regional airline experience have shaped the LF507, which retains the basic configuration of the ALF502 - some 1,000 of which are in service. The engine is a two-shaft turbofan, with a two-stage low-pressure (LP) turbine driving the fan and two-stage LP compressor via a reduction gearbox.

The core, based on that of the T55, comprises a high-pressure...