The Quieter Bonanza

Being a good neighbor — intruding as little as possible upon the peace and quiet of others — is more than just a good idea in general aviation. In many cases, it's the law. Some urban airports such as Torrance and Orange County in southern California are imposing noise standards, especially at night and early morning, and enforcing them with fines. The public's expectation is that life's din will recede, not advance, so noise reduction, both outside and in the cabin, will take on increasing importance in the design of new aircraft large and small.

Existing designs can be modified to generate less noise, although some compromises in performance and utility are to be expected. Beech Aircraft has managed to keep the compromises to a minimum in its new A36AT Bonanza, while achieving a small but significant reduction in the noise generated by engine and propeller.

The "AT" portion of the model designation stands for Airline Trainer. The airplane was developed based on an order by Lufthansa German Airlines, which operates pilot training schools in Bremen, Germany, and Goodyear, Arizona. Lufthansa specializes in ab initio training in which a nonpilot spends about two years and 190 flight hours preparing to become a second officer on a Lufthansa Boeing 737 or Airbus A320.

Lufthansa has been using F33 Bonanzas in its training program for nearly three decades. When the airline went shopping in 1990 to replace some of its Bremen Bonanzas, it had to consider new, stricter aircraft noise standards adopted by the German government for new-production aircraft. The Germans use a flyover measurement: Noise readings are taken as the airplane flies overhead at 1,000 feet agl at full power and/or maximum level speed, whichever occurs first. The maximum value permitted is 72 decibels in the A- weighted scale (dB[A]). The Federal Aviation Administration, on the other hand, uses a takeoff measurement. The recording is made some distance from the departure end of the runway as the airplane climbs out at takeoff power and best-rate-of-climb speed. Whether takeoff or flyover, the measurements must be made under strictly controlled conditions right down to the height of the grass in the area.

A standard 1984 or later A36 (Beech switched from the Teledyne Continental IO-520 engine to an IO-550 in the A36 in 1984) generates 76.7 dB(A) on a flyover test, according to Beech. For Beech to win Lufthansa's 10- airplane contract, it would have had to figure out a way to trim nearly 6 dB off the flyover reading — a not insignificant challenge considering that a 10- dB reduction represents a 50-percent decrease in perceived noise. Beech met the goal without having to make dramatic changes in the Bonanza.

The AT is identical to a standard A36 except for a few important differences in the two noise generators: propeller and engine. The AT's three- blade Hartzell propeller has a diameter of 76 inches, 4 inches less than the standard prop. Also, maximum rpm is reduced to 2,550 from 2,700, which reduces maximum horsepower to 290 from 300. A smaller prop and lower rpm mean slower propeller tip speeds and thus less propeller noise.

Engine noise is reduced through the use of baffles welded to the dual exhaust pipes. Exhaust gases are dissipated into the slipstream through holes in the baffles, which extend along the underside of the fuselage. The long baffles, which snake under the belly, are the easiest way to distinguish the AT from a standard A36.

The baffles were developed, and are manufactured by, the Soloy Corporation. Beech tried several baffle designs but settled on Soloy's as the most cost-effective. Beech buys them from Soloy and adds an elbow extension as a bridge between baffle and exhaust stack.

The baffles, prop, and reduced rpm took the AT right up to the edge of meeting the 72-dB(A) objective. But something more was needed to put it over the top. That something was twisting each of the baffles so that the exhaust — and exhaust noise — emanates more to the side than down (straight toward the microphone taking the noise readings). That simple adjustment resulted in lowering the AT's flyover noise measurement to 71.9 dB(A), a tenth of a point better than the goal. A tenth of a point may seem insignificant, but when the issue is meeting a certification requirement, fractions count for a lot.

Other physical differences in the AT compared to a standard A36 include a new tachometer marked with the 2,550-rpm limit and a louvered cooling panel on the left side of the cowl. The changes in prop rpm and horsepower cut into the AT's climb performance enough so that under extreme hot-day conditions, engine cooling was not sufficient: The temperature of the number-two cylinder head exceeded the 238-degree-Celsius maximum. To correct the problem, Beech installed a small scoop above and behind the cylinder to channel air over the fins and head and added the louvered panel to increase airflow through the engine plenum. Although the scoop robbed the oil cooler of some airflow, and thus oil temperature runs slightly higher, all engine temperatures are within limits, and there is less disparity between individual cylinder head temperatures, according to Beech.

That is the extent of the physical differences between the standard A36 and A36AT. Otherwise, the airplanes are identical — same instrument panel, same 74-gallons usable fuel capacity, same everything.

Though seemingly minor, the changes incorporated in the AT required Beech to undertake an extensive certification program. Beech had to test for adequate engine and accessory cooling; stall speeds and characteristics; spin recovery (44 spins were performed with the airplane equipped with spin chute and quick-release door); takeoff, climb, cruise, and landing performance; the effect of the baffles on skin temperatures on the belly of the airplane and carbon monoxide levels in the cabin; and, of course, noise tests.

The penalty paid for the AT's neighborliness is slightly less spirited takeoff and climb performance and, at 3,600 pounds, a 50-pound lower maximum takeoff weight than the A36. Beech had targeted a max gross takeoff weight of 3,525 pounds for the AT in order to preserve performance, but Lufthansa said it needed to carry four people — instructor and three students — and full fuel on training missions. So Beech certified the airplane at 3,600 pounds, which meant performance took a hit.

How much of one? Takeoff distance over a 50-foot obstacle using approach flaps is about 7 percent longer in an AT — 2,058 feet, compared to 1,918 feet in the standard A36. Also, the AT's sea-level, best-rate climb is 1,152 feet per minute, or 56 fpm less than that claimed for the A36. Cruise speeds are unchanged, although Beech unofficially says the AT may even be 2 to 3 knots faster for reasons even Beech can't explain.

From a pilot's perspective, the differences between the two models are subtle at best. Beech makes no claim that the AT is quieter inside the cabin (the pilot's operating handbook explains that the AT is an A36 "modified for reduced external noise"), but it stands to reason that muffled exhausts will be quieter than unmuffled ones. At least that's the way my ears perceived it when Beech marketing pilot Tony Marlow and I started up and taxied out in a new AT at Beech Field in Wichita. I had flown in the day before in a stock IO- 550-powered A36, so I had a good foundation to make comparisons. The AT wasn't as responsive to changes in power while taxiing compared to the A36, but that stands to reason given the smaller prop.

The airplane also was a bit slower off the chocks and in initial acceleration when takeoff power was applied. Once the gear had been stowed and the flaps raised, the AT climbed at 1,000 to 1,100 fpm at the best-rate speed of 105 KIAS, which is 5 knots faster than the standard A36. We didn't spend much time cruising because the AT gives nothing away there, but a brief exercise in rpm management yielded the impression that 2,300 rpm was the most comfortable cruise rpm setting in terms of noise and vibration.

The only other difference I could detect in the flying qualities of the AT was slightly less deceleration when the power was retarded to idle on an approach, again because of the smaller prop disk.

Although the AT grew out of a Lufthansa requirement, the first deliveries went to RLS, a Dutch ab initio pilot training school run by KLM and other European airlines. RLS now has nine ATs, and Lufthansa has received 12. The Lufthansa airplanes were outfitted with custom avionics and four-place interiors to conform to school standards. Beech also has built and delivered 10 ATs to its German distributor for resale to individuals and companies. No ATs have yet been bought or delivered in the United States, but that's not surprising given that there is no regulatory motivation to chose it over a standard A36.

It's possible to order an AT, but you may have to wait for it. The differences between an A36 and an AT are significant enough to cause some disruption in the normal Bonanza production line and process, so Beech won't build any ATs unless it has orders for at least five. It's the same policy Beech has for the F33C, the aerobatic version of the F33A Bonanza: Five is the minimum production run.

Beech won't quote a price for an individual AT because all the orders thus far have been fleet sales. But given that it is a near-identical twin to the A36, the AT would be in the same range, according to Beech. A new A36 lists for $287,700. A typically well-equipped Bonanza carries a sticker price of around $425,000.

For now the primary market for the AT is in Europe. But who can say how long it will be before the United States turns a noise-sensitive ear toward Germany and other countries and proposes to adopt similar standards in the United States. Beech, at least, has demonstrated that current-production aircraft can be shushed without performance and utility taking it on the chin.

Notes on Noise

The decibel is a unit for measuring the relative loudness of sound. A single decibel would be too faint for an unaided human ear to hear. In fact, the faintest sound a human can hear is around 20 dB, equivalent to the rustling of leaves in a quiet forest. The upper level of tolerance is around 140 dB, the pain threshold for humans. Sound this loud can lead to serious ear damage if continued for very long. Extended exposure to loud sounds of a lesser degree can eventually lead to hearing loss, as well.

Humans can only hear certain frequencies, so decibels are often measured using the A-weighted scale. This scale weighs or dampens sound to get a reading that closely resembles the human range of perception. The weighting removes certain frequencies that a machine would pick up but a human could not hear. Decibels measured on this scale are marked with an "A," 37 dB(A), for example. A normal conversation (heard at 3 feet) would be about 60 dB(A); a telephone bell would be about 70 dB(A).

The decibel system is not linear, but logarithmic. An increase of 10 dB is not a 10-percent increase, but a doubling of the sound level, while a drop of 10 dB would halve the sound level.

A 6-dB(A) decrease in the sound generated by an airplane (Beech knocked just under 6 dB(A) off the A36 Bonanza measured on a flyover test) is equivalent to a 30-percent drop in perceived loudness. For reference, a Cessna 170 has a takeoff sound level of 70 dB(A), according to the FAA, while a Mooney M22 measures 80 dB(A). A difference of 6 dB(A) is roughly equivalent to the loudness difference of a truck at highway speed (85 dB[A]) and a car at highway speed (just under 80 dB[A]). — Mark J. Brockman