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Friday, June 1, 2007

AS350: Hydraulics Out? Fly the Procedure

If you want to start an argument among helicopter pilots, you could do worse than to bring up the AS350’s hydraulics.

People can get passionate about the aircraft’s hydraulically assisted flight controls, or, more to the point, its controllability when the hydraulics fail. The critics’ complaints are typified in a 1997 Royal Australian Air Force evaluation of the AS350BA when hydraulics fail.

“Hydraulics out flight was characterized by greatly increased control forces, considerably increased control free play and substantially reduced control authority,” the service’s Aircraft Research and Development Unit concluded. “At high aircraft weights, reduced authority, increased free play, and high forces in all control axes hydraulics out were unacceptable and caused a loss of control during low-speed flight testing, which could only be recovered by selecting hydraulics on.”

Everyone does seem to agree that speed is the key. Eurocopter’s emergency procedures for a hydraulics failure call for you to maintain 40-60 kt. “That’s where the control loads will be their lightest aerodynamically,” said Bruce Webb, chief pilot for American Eurocopter. “At that airspeed, the pilot will have the least amount of control load, which makes his job easier, of course.”

Most of the pilots we spoke with, including a few who had suffered hydraulics failures, felt the AS350 is completely controllable in that circumstance—if you are trained in Eurocopter’s prescribed emergency procedures and follow them.

You will have to work, said one pilot very experienced in AS350 hydraulics-out landings—and not affiliated with Eurocopter—said, “The pedals and collective are extremely difficult. You’re fighting it all the way to the ground.”  Nevertheless, he said, the aircraft is controllable. “The key is to maintain forward airspeed and make a run-on landing. If you try to land normally or get cute, you’re not going to make it.”

One thing you do not want to do with the AS350 with no hydraulics is attempt to hover. “If you lose the hydraulic assist in the flight controls, they become direct-link connections,” said Jim di Giovanna, former commander of the Los Angeles County Sheriff Dept.’s Aero Bureau, which flies 12 AS350B2s. “Because of the dynamic forces on the rotating components, it’s hard to handle with any finesse in a hover.”

At that point, “it’s wise to follow the factory-prescribed procedures,” he said. “It is indeed an emergency.”

Webb said some common mistakes pilots make when an AS350’s hydraulics fail involve not following the emergency procedure. “Sometimes pilots choose to go ahead and bring the aircraft to a hover. Sometimes the pilots, perhaps because they haven’t received training, don’t get the aircraft near the surface before they get too slow.”

The AS350’s warnings—a gong or horn, depending on the model, and a light on the annunciator panel—are triggered when pressure in the hydraulics system falls below 30 bar. If you get those warnings in cruise, Webb said, reduce airspeed to 40-60 kt. The only pressure you’ve got left in the system is from the accumulators on the pitch, right roll, and left roll servos. Once you’re in the airspeed range, Webb said, hit the Isolate switch on the collective. This drains the accumulators simultaneously, avoiding one of them loading a servo and creating an asymmetric condition.

Find the nearest safe, suitable landing area, and execute a shallow approach to a landing with slight forward speed, about 10 kt. “Just gently slide it onto the surface,” Webb said. The procedure’s the same for a failure in hover, except that you have to build the airspeed to 40-60 kt.

Eurocopter’s efforts on the hydraulics have focused on improving their reliability. It has replaced the flat fibrous hydraulic-pump drive belt with a more durable one and the pump’s greased-for-life bearings with ones that must be greased periodically. It is developing a revised procedure that calls for a rubber cap over the pump spline to prevent grease from being circulated away from the spline and causing unacceptable wear on the spline. The company for several years also has offered a modification to retrofit a dual-hydraulics system on the AS350B3. Xavier de la Servette, who manages Eurocopter’s light helicopter programs, said only one customer has opted for the mod. That lack of market interest in dual hydraulics is one reason the manufacturer views the hydraulics as a matter of reliability, not safety.

“We do not consider it is a safety issue,” said de la Servette, because Eurocopter pilots can demonstrate safe landings hydraulics off “and many customer pilots tell us it’s a piece of cake.”

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