Plane & Pilot
Tuesday, November 8, 2011

The Dangers Of Noise Fatigue

Noise fatigue may be an often-overlooked cause of aircraft accidents in general aviation

Trouble is, hearing loss is insidious. At extremely high levels, you can actually feel the pain, a sure giveaway that you're damaging your hearing, either temporarily or permanently. When I was a kid, a week or two ago, I used to hang out at drag strips on weekends; then, wonder why I couldn't hear much of anything for the next few days.

Even at lower volumes, some hearing loss is inevitable, as deterioration is cumulative with continued exposure to loud noise. It comes on gradually with repeated doses of high-volume sound, whether it's music, industrial or aircraft noise, or NHRA Funny Cars.

When I began flying with Civil Air Patrol in the late 1950s, we used to stuff cotton in our ears in an attempt to protect our hearing. Eventually, a product called E.A.R., small yellow ear plugs, came on the market (and I still use them when flying on the airlines). They helped a lot, as headsets were fairly primitive in those days and didn't really attenuate noise adequately.

Sadly, there's precious little empirical data regarding noise fatigue in general aviation accidents, despite (or perhaps because of) ANR headsets. It seems hardly anyone flies without a headset these days, and that's a good thing.

Internal noise in a personal aircraft usually devolves to three primary sources. One is exhaust noise from the stack(s), most often exiting directly beneath the cabin. The second is the beat of airflow coming off the prop and bouncing against the windshield. A third source is the rush of outside air through the vents and leaking around the doors and through every possible opening in the fuselage.

There's not a lot you can do about the low-frequency collision of air against the windshield. I upgraded my airplane with a more- swept Mooney 201 windshield several years ago, because it was alleged to improve cruise performance and reduce the dB count. Sorry, but I saw no change in either parameter.

Similarly, vents, windows and doors are bound to leak at least a little. A properly fitted inflatable door seal can make a big difference, and I've seen some truly dramatic improvements, primarily on high-end models such as P210s, Malibus and Bonanzas.

Engine noise can be a matter of taste. If you fly a P-51, the V12, Rolls Royce Merlin can be a song from God. For many of the rest of us, however, engine noise is just that—noise—and it can be distracting, irritating and even, in its most extreme forms, dangerous.

Power Flow Systems of Daytona Beach, Fla., decided to address the exhaust-noise problem on airplanes fitted with their exhaust systems. Power Flow has been building tuned exhausts for a variety of general aviation models for several years, from Piper, Cessna and Mooney to Beech, Aviat and Diamond. Five years ago, Diamond Aircraft began installing Power Flow tuned exhausts as standard on the DA40 Star.

I can personally attest that the Power Flow tuned exhaust works very well. My airplane was fitted with a Power Flow system in 2005, and the result was more power and three to five knots better cruise, depending upon power setting and altitude. Nothing is free, however; more power inevitably meant higher fuel burn. My choices were to opt for more speed, or I could use less power and still achieve the same cruise. Of course, I chose more cruise speed and higher burn.

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