Single- and twin-turbine accident rates similar

 - October 9, 2007, 12:39 PM

In the aftermath of July’s well publicized engine-out ditching of a Pilatus PC-12 in the Pacific Ocean off the coast of Russia, industry observers are asking how this and other recent accidents have affected the statistical reliability of single-engine turboprops and if sales of these aircraft are suffering.

Although production-built single-turbine airplanes used for business flying typically do not have the same speed, load capability or systems redundancy as twin-turbine airplanes, they have amassed a comparable safety record, according to statistics through last year compiled by accident analyst firm Robert E. Breiling Associates of Boca Raton, Fla.

Breiling reports U.S. turboprop-singles have had 1.99 total accidents and 0.80 fatal accidents per 100,000 flight hr compared with 2.37 and 0.83, respectively, for U.S.-registered turboprop twins. These figures cover the period from initial aircraft certification through last year. Last year the statistical reliability of single-engine turboprops was even played up by Pilatus in its marketing of the PC-12.

While accident rate statistics seem to back up that claim, the actual number of single-engine turboprop accidents is increasing as the fleet gets larger. This year to date, the NTSB reports that there have been 10 accidents, five of them fatal, involving four production-certified single-engine turboprops: the Cessna 208 Caravan, Piper PA-46-500TP Meridian, Socata TBM 700 and Pilatus PC-12.

Nine of the 10 accidents are still under investigation, seven of the accidents were in Cessna 208s (by far the most numerous of all turboprop singles, with close to 1,300 in operation), and engine failure has definitely been determined as a factor in four accidents, (April 26, July 6, July 8 and July 10) none causing critical injuries. The Safety Board determined the January 31 crash of the Cessna 208 on floats was caused when the airplane hit a swell during a water landing. All production turboprop singles are powered by the Pratt & Whitney Canada PT6 series.

Engine Problems
A Caravan flying for FedEx made a forced landing April 26 after an engine failure. The pilot (not injured in the accident) said that during climbout the airplane’s engine “spooled down, slowly and smoothly, like a loss of torque or the propeller going to feather.” Later, an examination of data from the power analyzer recorder system revealed that during the most recent takeoff the engine exceeded its torque limit of 1,980 ft lb for 99 seconds. The peak torque value over that duration was 2,649 ft lb.

On July 6, a Caravan on a repositioning flight operated by Maxfly Aviation ditched into the Atlantic Ocean 20 mi east of Fort Lauderdale, Fla., following loss of engine power. According to the pilot (who was not hurt), the airplane was cruising at 6,500 ft when the engine lost power and came to a “screeching halt.”

The propeller made a “chow, chow, chow” noise, turned three times, stopped and feathered.

In the July 8 PC-12 ditching, the pilot reported that the airplane was in cruise at 26,500 ft when he felt a vibration followed by a rapid increase in the engine’s turbine temperature indication (TTI). He reported that the TTI reached 1,144 deg C, at which point there was a compressor stall. He shut down the engine, feathered the propeller and entered a power-off emergency descent. After spending 15 hr in a life raft, the pilot and all three passengers were safely recovered some 60 mi from the Russian coast in the icy Sea of Okhotsk.

Two days later, on July 10, a Cessna 208 of Bolivian registration (CP-2395), was substantially damaged during a forced landing following a loss of engine power during climbout from the La Paz International Airport in Bolivia. The pilot, the copilot and 11 passengers were injured. The flight crew reported a loss of engine power approximately six minutes after takeoff.

No Effect on Sales
According to comments from three manufacturers, sales of new turbine singles remain strong and the issue of single vs twin rarely comes up in conversation between sellers for the OEM and buyers.

Tom Aniello is just completing his first six months as v-p of marketing for Pilatus Business Aircraft in Boulder, Colo. He told AIN, “I have spent a lot of time with our dealers and that is one of the questions I had for them: how much marketing effort should I put on the single versus twin issue? And I was surprised that their answer to me was that it’s really become a non-issue. Even after the [July 8] ditching incident I was surprised by how few questions I’ve received. People have come to accept and understand that turbines are more reliable than pistons.”

Aniello thinks it’s still not an issue despite the 10 accidents so far this year, but he wonders about next year. These accidents “will skew the statistics for next year, and I don’t know whether that’s going to become a big factor or not.” A bigger factor, in Aniello’s opinion, is not the number of engines, but the number of crew. “For single versus twin, statistics don’t show an appreciable gap. But statistics do lead you to realize that you’re better off adding another person up front than you are adding another engine.”

In the U.S., more than 70 percent of PC-12 sales are to owner-pilots for personal and business flying. As might be expected, just the opposite is true for the Caravan, where 70 percent of its users are small package commercial operators, according to director of Caravan sales for Cessna John Doman. “In our experience with the Caravan–which has more than 15 years of service under its belt, flying in all sorts of different conditions–it has established an enviable safety record. The PT6 is a legendary powerplant in terms of reliability. So our reaction from the marketplace is one of acceptance of the safety inherent in the turbine single.”

A lot of Caravan air-freight customers are moving up from piston twins such as Beech 18s, Queen Airs, Navajos and Cessna 402s. “Statistics and just common knowledge tell you that a single-turbine airplane is going to be a safer, more reliable piece of machinery than the piston twin,” Doman said.

Doman said Cessna does not actively market the Caravan to the U.S. air-taxi industry. He described that position as a “corporate decision,” not based on any accident or incident history. The airplane by regulation is permitted to fly air taxi, including carrying fare-paying passengers in IMC, but Doman said Cessna over the years has become “very sensitive” to product liability in the U.S.

There are many air-taxi Caravans in operation outside the U.S., “But if someone were to come to us for a new Caravan for flying paying passengers between Chicago and Minneapolis, we would respectfully decline the sale.” Overseas, however, Cessna encourages sales to this market. And that market potential is just waiting for some promised rulemaking relief.

For the last five years Cessna has been working with other airframe manufacturers as a member of the Single Engine Turbine Alliance (SETA) to get the JAA to change the requirements in Europe to allow single-turbine IFR commercial operations. Such operations are currently prohibited for both carrying cargo and fare-paying passengers. “We think that the way things are headed, we should see a change by perhaps the end of this year,” Doman said.

A spokesman for Piper Aircraft in Vero Beach, Fla., echoed the statements on the quality, excellence, reliability and safety perceived by prospective and new owners of single turboprop airplanes. Indeed, P&WC statistics show the time between unplanned removals for the PT6 family as occurring once in every 142,817.14 hr and the time between in-flight shutdowns to be one in every 250,000 hr.