Honeywell Celebrates 3,000th TFE731 for Learjet Family
It's been more than three decades since the Learjet 35 made its first flight powered by 3,500-pounds-thrust TFE731-2 turbofans, a leap forward in propulsion technology that increased its range by more than 60%, slashed fuel consumption by more than one-third and enabled the aircraft easily to meet Stage III noise requirements.
"It was a game-changing design in aviation propulsion," observed Russ Turner, president of Honeywell engines, services and systems. It was indeed a paradigm shift. A half-dozen-plus airframe manufacturers quickly moved to embrace the new, high-bypass-ratio light turbofan, none more so that Gates-Learjet Corporation.
The TFE731 originally was intended to power a follow-on version of the Lear Jet 25, but the extra weight and thrust motivated Learjet engineers to stretch the fuselage and wingspan to take advantage of the new engine's full capabilities. The result was the Learjet 35, which, along with the longer-range Learjet 36 and sporty Learjet 31, went on to become the best selling models in Learjet history.
Early TFE731 models were plagued with oil-seal problems and a host of other woes, prompting Garrett-AiResearch, then the engine's manufacturer, to initiate a comprehensive protection program called the Maintenance Service Plan. MSP became an overnight success because operators were able to predict their engine maintenance costs with great certainty. During the past 30 years, the engine has undergone continuous improvements, evolving into a highly reliable powerplant that has been fitted to five Learjet models.
In the early 1990s, AlliedSignal, then owner of the TFE731 type certificates, decided to develop an evolutionary, second-generation TFE731 that would keep all of the assets of the original powerplant, but incorporate a considerably more efficient compressor, an improved fan and more robust hot-section parts. The -20 version of this engine found a home on Learjet 45, certified in 1995, which was the first clean-sheet Learjet since the original Model 23 first flew in 1963. The –20 and its derivatives have 2,500-hour MPI intervals, up from 1,400 hours for the first generation engines. CZI now comes at 5,000 hours, instead of 4,200 hours. The new engines are fitted with DEECs—essentially single-channel FADECs backed up by hydro-mechanical fuel control units—that greatly reduce pilot workload, provide full range envelope protection and maintenance event logging.
The latest TFE731 to power a Learjet, the -20BR, has a five minute takeoff thrust rating of 3,650 pounds up to ISA+25°C, greatly improving the hot-and-high airport and climb performance of Learjet 40-series aircraft to which it is fitted. These engines produce so much high altitude cruise thrust that, once again, Learjets are able to reach Mmo in the mid- to high-forties. But, they're able to squeeze almost twice the push out of a pound of kerosene as the CJ610 turbojets that powered the Lear Jet 23.
