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on the edgeon the edge


Air Safety InstituteBy David Jack Kenny

Helicopter external-load transport necessarily involves some increased risks. Much of the flight time takes place in the “dead zone” of the height/velocity graph where there’s neither enough airspeed to establish an immediate autorotation in the event of a power failure nor enough altitude to be sure of gaining that airspeed. There’s always the risk that the cargo or the line it’s attached to will snag some kind of obstruction, and of course there’s the inevitable trade-off between fuel capacity and payload. The need to reach remote places in rugged terrain is often the reason a helicopter is needed in the first place, but limits the options for emergency landings. The pilots doing this work have every incentive to stay on top of their games.

On July 22, 2009, a Hughes 369D crashed into a clearing in the Forbes State Forest in southwestern Pennsylvania. The 2,800-hour commercial pilot was killed. For the past two and a half weeks, he had been working under a contract to move 250-pound bags of seismic monitoring equipment between locations inside the forest. The accident flight carried four bags; together with the lines and hardware that harnessed them to the aircraft, they weighed in at 1,150 pounds, almost 80 percent of the helicopter’s 1,450-pound useful load.

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The weather was good, with light southeastern winds and 10 miles visibility under a 2,100-foot scattered layer. The flight was the second of the morning with a stop for fuel in between. Several employees who were monitoring the radio heard the pilot transmit that he was “going into the trees,” but the wreckage was first spotted by a driver on an adjacent road. She told investigators that she’d become familiar with the sound of the helicopter during the preceding couple of weeks but had not heard any engine noise before the crash, and examination of both the main and tail rotor blades found their leading edges largely undamaged.

The loss of power did not appear to be result from any mechanical failure. On-site investigation and later examination by the manufacturer found no anomalies in the engine, gearboxes, or flight controls that could not be attributed to impact damage. However, the auxiliary fuel tank was dry. The main fuel bladder had been breached, but there was no fire and no evidence of a fuel spill, and only three drops of Jet A were found inside the fuel lines.

The contractor’s operating procedures required pilots to land with at least 70 pounds of fuel on board—somewhere between 15 and 20 minutes’ supply for the 450-horsepower Rolls-Royce turbine. The accident pilot had been reminded of this after an earlier flight when he landed with “no space between the [empty] mark and the needle” on the fuel gauge. The contractor also told the NTSB that on this project, normal practice was to load 150 pounds of fuel per flight. Fuel burn to the respective pick-up points would keep the machine below its maximum gross weight limit, and careful powerplant management would leave just that 70-pound reserve if the flight lasted the expected 30 to 40 minutes. However, data recovered from the helicopter’s GPS showed that earlier flights had run as long as 59 minutes. The accident flight lasted 50 minutes from takeoff to impact.

Thanks to their ability to land almost anywhere, fuel-exhaustion accidents are rare in helicopters—but a precautionary landing with power is apt to have a better outcome than an unexpected engine-out, especially at low altitude over trees. Knowing that his reserve was measured in minutes, the pilot should have monitored his fuel status obsessively.