In the ever-fluid world of aerospace, to build and deliver a thousand of any major aircraft system is a milestone worthy of note. It was appropriate, then, that Phoenix-based Honeywell Engines celebrated the delivery of its 10,000th TFE731 turbofan engine on October 30. The engine, a 3,500-lb-thrust TFE731-20AR, was formally accepted for installation on a new Learjet 45 by Jim Ziegler, v-p and general manager of Bombardier Aerospace, in the Valley of the Sun.
A score of Garrett, AiResearch and AlliedSignal retirees from a management team who participated in developing the original TFE731-2 from 1968 through its certification in August 1972 and saw the line through to become an industry staple were on hand to be recognized and receive plaques of appreciation.
Steve Loranger, president and CEO of Honeywell Engines, said the TFE731 design has evolved over three decades, but so revolutionary was it when conceived that the engine today powers 26 different business and military aircraft around the world and has logged more than 49 million hours of operation. Ninety-nine percent of all TFE731s remain in service.
As the first high-bypass, fuel-efficient turbofan not derived from older military designs but built from scratch for business aircraft, “This engine changed the industry. Today, the -731 is an $800 million product line,” Loranger reminded those
at Honeywell’s Sky Harbor International Airport plant. He added that Honeywell is producing more than 500 engines a year and expects to continue doing so, a statement backed by the fact that the first three years of production of the Bombardier Continental business jet, powered by Honeywell’s new AS907 turbofan, is already sold out.
Loranger noted that the TFE731 pioneered innovations such as a geared fan, electronic engine controls and lighter-weight materials. And the company has pursued a continuous product improvement program that has enabled operators to upgrade, expand the performance and extend the life of older TFE731s.
In symbolically accepting the 10,000th engine from Loranger, Ziegler acknowledged the long relationship between Garrett/AiResearch/AlliedSignal/Honeywell and Learjet/ Bombardier with the TFE731. He recalled that the first TFE731-powered Learjet 35 went to Mesa Petroleum. Loranger responded, “Learjet was here in 1985 to accept the 500th TFE731. They were our launch customer during the aerospace slump of 1973 and 1974.” He related how, at that time, “Harry Wetzel, president of Garrett Engines, sealed the deal with Learjet by stating, ‘You buy our engine, we’ll buy your airplane.’ They did, and Garrett bought several Learjets, beginning with the [GE CJ610-powered] Model 25.”
He went on to recall, “The fuel- efficient TFE nearly doubled the range of existing business aircraft in the 1970s.” It made possible nonstop transcontinental flights, something the turbojet-powered first-generation corporate jets could not accomplish.
Right Engine, Right Time
Said J.P. Frignac, retired v-p of engineering for Garrett Engines, “The 731 engine was the right engine at the right time.” He recalled how chief engineer Frank Roberts led a team assembled in 1968 to develop an all-new turbofan to supplant “warmed over” military turbojets at the behest of Garrett Engine Division manager Ivan Speer.
Speer’s goal was a turbofan that would outperform the General Electric CJ610, which powered early Learjets. By the late 1960s, Garrett had substantial turbine powerplant experience with its TPE331 turboprop engine. Although the company was still known primarily for its APUs, Speer believed that the APU cores it had developed for the Douglas DC-10 and Boeing 747 could be used as the basis for a family of new small turbofans. It soon became clear that airframe manufacturers preferred a turbofan based on the higher-flow Garrett 747 APU compressor, which would require using a geared fan.
Frignac said that during 1969 Speer negotiated the first contracts for the TFE731 with Learjet for the Model 35, and with Dassault for the Falcon 10. “Then, in September 1971, the FAA required us to pass the four-pound bird-strike test. The bird won.”
This setback necessitated extensive redesign on a tight schedule, which under Roberts (whom Frignac admiringly called a “demanding taskmaster”) was accomplished. Beefing up the engine entailed changing materials and redesigning parts–fan disks and blades, some rotor shafting and bearings, mountings and other structures. Years later, the sturdier design paid off when a TFE731-powered aircraft ingested a 12-lb goose and still made it home safely.
In 1972 Garrett’s first TFE731 retrofit program gave the four-engine Lockheed JetStar (which began life as the Air Force C-140) longer legs by replacing the original Pratt & Whitney JT12 turbojets. The 731 JetStar program was soon joined by replacement of GE CF700 turbofans with TFE731s on more than 100 Falcon 20s. That retrofit doubled the Falcon20’s range, enhanced its fuel efficiency by more than 40 percent, reduced its noise footprint and improved its climb performance.
Since the 1970s, TFE731-2, -3, -4 and -5 variants have powered an armada of bizjets, including the Astra SP; Cessna Citation III and VII; Falcon 50, 100 and 900; Hawker 700 and 800; Learjet 31A, 35/36 and 55/56; Sabreliner 65; and Westwind 1124 and 1125. In 1995 the then AlliedSignal Engines began delivering a new TFE731 family, the -20/-40/-60 series.
Incorporating previous product improvements and new enhancements, these engines have accumulated more than 900,000 hr on four aircraft types: the Learjet 45, Gulfstream G100 (nee Astra SPX) and Falcon 50EX and 900EX. Additionally, the TFE731-40 is being retrofitted on existing Falcon 50s, providing performance equal to that of the 50EX.
More than 800 engines in the newest 731 troika are in service. These powerplants, delivering up to 5,000 lb of thrust, have maintenance intervals up to six times longer than those of the original TFE731s of yesteryear.
The newest TFE731s were designed with nearly 70 specific customer and OEM-suggested design and maintainability improvements. All feature a new single-channel digital electronic engine control with hydromechanical backup, improved carbon seals, externally replaceable N1 speed detector, greater combustor and compressor durability, improved turbine cooling and a 75-percent increase in major inspection intervals.
Thus, an operator flying 500 hr a year needn’t schedule the first major periodic maintenance event for six years, and a compressor zone inspection won’t be needed for more than 10 years. Honeywell’s ongoing product improvements for existing TFE731 engines currently feature upgrades and conversions incorporating new technologies and lessons learned from TFE fleet experience. Kits the company offers that extend maintenance intervals by up to 50 percent for every TFE731-2 to -5 engine have to date been applied to more than 1,300 engines in the field.
While celebrating the longevity of its first turbofan engine, Honeywell also showed off its newest, the 6,500-lb-thrust AS907 and 7,100-lb-thrust AS977 for the Bombardier Continental and BAE Systems Avro RJX, respectively. The first two engines in the AS900 family are identical except for fuel-flow settings and external positioning of the Fadec.
During a tour of the engine assembly area, Tony Brancato, AS907 program director, said the AS977 will be certified in the first quarter of next year. Approval of the AS907 will follow in the second quarter.
He pointed out that Honeywell had built 16 production engines for RJX flight test and 12 for the Continental. Initial post-certification production next year will accommodate BAE’s plans to deliver 15 aircraft (60 engines) and 30 turbofans for Continentals. In 2003, Brancato added, Bombardier plans to deliver 30 Continentals and BAE intends to ship 18 RJXs. By 2006, Honeywell will gear up production to support delivery of 60 Continentals a year.
Brancato noted that the AS900 design has the capability to grow to 8,200 lb of thrust. He said the AS907 and AS977 will be certified as on-condition engines, designed to achieve 15,000 cycles for rotating components, with a borescope inspection at 2,500 hr. Condition monitoring will be done through the Fadec, which will be datalinked to collection and analysis sites.
The program director said the AS900 design takes interchangeability and commonality of parts and components to a new level to provide lower total parts count with simplified assembly and maintenance, using a minimum of commonly available tools. He compared the TFE731 combustor with that of the AS907/977. While the TFE731 has more than 15 individual parts, the AS907 has just three.