When Garrett began developing its proposal for a 1,490kW (2,000shp)-class turboprop to power what was to become the Embraer/FAMA CBA-123 Vector, it did not have to look far for a starting point.

The TPE331 series has been the company's bread-and-butter engine for the last 25 years, selling more than 11,000 units and powering aircraft ranging from the Pilatus Porter and de Havilland Turbo Beaver to the Piper Cheyenne 400 and the British Aerospace Jetstream 41. The largest 331 versions are now in service with about 128 regional airline operators worldwide, making it the ideal baseline for a new commuter aircraft engine, Garrett believes.

The TPE331 (and its military variant, the T76) was Garrett's first excursion into the propulsion arena, following its virtual cornering of the market for auxiliary power units (APUs). The initial version drew heavily on Garrett's APU experience, but steadily grew beyond its modest beginnings. The engine's power has developed from just 415kW on the original TPE331 series I and II (later redesignated the -25/61 and -25/71) to a more robust 1,310kW on the 331-14GR/HR, which will power the J41.

This latest version features a two-stage centrifugal compressor, stacked-ring combustor and three-stage axial high-pressure turbine. The engine's output is transmitted to the propeller through an offset, two-stage reduction gearbox. The unit offers 30% better specific fuel consumption than did the earlier engines, due to compressor improvements and increased size, with a 53% increase in power-to-weight ratio.

The design is modular, allowing easy disassembly for on-wing maintenance, and features a digital integrated engine computer. A forerunner of today's full-authority digital engine control (FADEC), it provides automatic start, automatic torque and temperature limiting, trend monitoring and fault detection.

The popularity of these features, and the success of the technologies they incorporated, promised Garrett a fairly straightforward, low-risk development programme for the new engine. Howard Krasnow, technical manager for the TPF351-20 says: "When we started with the TPF351, we knew that we wanted to build on the 33114. It had what we needed in terms of the combustor system and the highpressure turbine. We didn't even have to scale those components up. They turned out to be the right size."

Garrett competed for the CBA-123 engine against Pratt & Whitney Canada (P&WC), and was awarded the contract in October 1987. Krasnow says: "At that point, we had nothing in terms of actual hardware or prototypes. We had some concepts on paper, and we immediately launched into the design phase. We spent a little over 19 months getting the essential design ready, and getting the hardware. It has been a very fast-paced programme."

The distinguishing features of the...