Got less Goose: Pratt & Whitney's Geared Turbofan

Pratt & Whitney's geared turbofan, the engine of choice for Mitsubishi's regional jet and Bombardier's new C-series jets, will break sound and fuel consumption barriers, but it could also help with the ever-growing threat from Canada geese.

The 7-20lb birds were the culprits in the downing of US Airways Flight 1549 in the Hudson River in early January. A crack cabin crew and good fortune kept all 150 passengers and crew relatively healthy that day.

Engine manufacturers, when certificating their products, must prove that a jet engine can withstand an impact with an 8lb bird, but in this case "withstand" doesn't mean "perform". The regs state the engine can't catch fire, can't burst, can't rip from the aircraft and must be capable of shutting off.

Below is a video showing the 8lb bird strike test on a Pratt & Whitney PW4098 turbofan, an engine available for the Boeing 777. Post-impact damage shows the permanent deformation of the hollow titanium fan blades.

For smaller birds weighing 2.5lb, the engine must continue to run at 75% power for 20 minutes, enough time to perform a go-around (assuming the bird hit occurred on arrival) and successful return to the airport.

CFM obviously met the 8lb challenge. Though not producing enough thrust to keep the aircraft aloft, both of the Hudson Airbus A320's two CFM56 engines held to their certification requirements.

However Pratt & Whitney's PW1000G geared turbofan (GTF), a possible choice for a new generation of airliners that will eventually replace the A320 and Boeing 737 in the 2020 realm, should be more resilient to such encounters.

Pratt & Whitney chief engineer Paul Adams explains that the bird threat at relatively low altitudes comes not primarily from the forward collision speed of the bird with the engine, but from the high velocity impact of the spinning fan blade chopping into the bird.

If a 112-inch diameter fan is spinning at 3,000 rpm at full power, each blade at the tip (about 4ft from the hub) will be traveling at a speed of about 850 mph. In the Hudson River case, the forward speed of the bird-engine collision would have been about 200 mph.

With GTF technology, the fan blade spins about 30% slower than for a typical turbofan engine however, owing to a gear reduction system that separates the fan from the low pressure turbine (LPT) portion of the engine. Typically the high-pressure turbine powers the high pressure compressor while the LPT powers the low pressure compressor and the fan. For modern high bypass engines, the fan blade provides about 80% of the engine's thrust. 

Adams says the reduction in speed means the kinetic energy of the collision will be about 50% less than with conventional turbofan engines, potentially giving the engine a higher probability of surviving encounters with birds and geese.

The video below, created by Pratt & Whitney, shows how the gear reduction system works.

4/2/2009 - VIDEO removed at the request of Pratt & Whitney due to proprietary content...