Air and noise pollution around the world's airports aren't only generated by aircraft taking off and landing - the vital APUs that provide ground power and engine starting bleed air also come in for criticism.

This is a fact of life that's all too familiar to Honeywell's Gretchen McClain. "One of our major priorities," she says, "is to cut both air and noise pollution and a lot of our research and development effort is going into making our APUs more 'green'."

Among the many added spin-off benefits, she says, is that the initiative is reducing the overall parts count. This pleases customers as their maintenance costs are reduced because potential failure becomes far more predictable.

"If we can predict failure 50h before it happens - and replace the necessary parts - we virtually eliminate the risk of collateral damage which can cut costs by a factor or five or even 10 times.

"We're looking at a variety of engineering solutions," McClain says, "including forward-sweep fan blades to cut noise and the use of advanced materials for inlet and exhaust ducts which are more acoustically efficient."

Today's computer aided design (CAD) techniques mean that ultimate noise output can be more accurately predicted, while sophisticated metallurgy, the use of ceramics, and oilless/gearless power trains all help to cut collateral annoyance.

In tomorrow's all-electric aircraft, such as Boeing's proposed B-7E7 airliner, where bleed air from APUs may no longer be a requirement, the technology will be entirely different. Instead of an APU needing to produce pneumatic power for engine starts and air conditioning, the primary need will be for the generation of electricity.


Honeywell is researching precisely how this will affect future designs and McClain says that a lot of work has already been done. "We're finding out whether we can run generators straight off the shaft - without the need for gearboxes at all. It's likely that tomorrow's APUs will need fewer compressor stages, enabling us to enhance thermal efficiency and create a far simpler 'hot' section.

"Airlines and aircraft makers alike are working closely with us to reduce overall costs - and of course we also have to work within the current regulatory framework, while anticipating what the legislators may throw at us in the future."

Much of the work being done by Honeywell is also designed to reduce emissions of carbon monoxide (CO) and oxides of nitrogen (Nox). Lean, direct injection technology, while lowering the temperature of the combustor, could reduce emissions by 50% or more, while having the benefit of cutting smells both inside and outside the aircraft.

In terms of cabin air quality - a subject that has received its share of attention over the years - McClain says Honeywell has an active committee looking at all aspects of the issue.

She explains: "While our current APUs produce cabin bleed air quality that meets or exceeds all regulatory requirements, there's always pressure to do better. For instance, the ozone convertor that we'll be fitting to the Airbus A380 will significantly cut smoke and smells from penetrating the cabin which should please both our airline customers and - in turn - their passengers."

Designs, materials, coatings, processes, weight, thermal efficiency… all these combine to reduce noise and emissions - and end-user costs. And although no final decisions have yet been taken about whether Honeywell's products will feature on the Boeing B7E7 (if and when it's built) it's a fair bet that Gretchen McClain and her team will be keen to be involved, working closely with Seattle to produce a machine that's as green as can be.

Source: Flight Daily News