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Friday, June 1, 2007

Safety Watch: A Gust of Wind

Tim McAdams

WIND IS DEFINED BY WEBSTER’S dictionary as a strong current of air. Although simple in definition, the effects of wind on an aircraft can be profound.

To a pilot wind can be a friend or a foe, depending on its direction. Yet pilots seem to underestimate the consequences of this natural force fairly often. According to the NTSB, during the last 10 years there were 48 helicopter accidents where the pilot said a gust of wind was a factor.

On March 27, 2002, at about 1205, a Hughes 269 was substantially damaged during a loss of control while hovering at the Fort Collins Downtown Airport, Colo. The instructor pilot reported that the wind was 2 – 3 kt at takeoff, but forecasted to be gusty in the afternoon. While hovering at about 3 ft, the helicopter, with the student pilot at the controls, encountered a very strong gust and began to wobble. The instructor took control of the helicopter, and climbed to about 15 ft, when another gust hit the helicopter, turning it sideways, and then downwind.

The instructor stated he was attempting to get it on the ground, but the wind continued to drive the helicopter forward with excessive nose-over tendency.

With the tail rotor into the wind, creating a high power demand and limited tail-rotor authority, the helicopter skipped along the dirt two or three times. The helicopter traveled forward 180-200 ft. The right strut failed, and the helicopter rolled over on its right side.

The instructor reported the wind was gusting to 60 kt at the time of the accident.

At 1215, the reported weather at the Fort Collins-Loveland Municipal Airport, 8 nm south of the accident site, was wind from 260 deg at 13 kt, gusting to 25.

It is not surprising that many accidents were reported while hovering with students. However, like the following, some happened while in flight.

On June 12, 1997, a Bell Helicopter 206L-3, was substantially damaged when it collided with trees while maneuvering 15 mi southwest of Weston, Colo. The commercial pilot and four passengers escaped injury.

According to the pilot’s accident report, this was his fourth trip of the day over the same proposed gas pipeline route and he noted that convective clouds had been building all day. The flight was for the purpose of conducting a bird study, and required low and slow flight over wooded areas.

The pilot said he crested a ridge and descended into the next valley, following the terrain downhill. As he approached the far side of this particular valley, it was obvious he was going to require a climbing, 360- deg turn to clear the next ridge. After completing about 90 deg of the turn, the helicopter was hit by a gust from the rear, causing a significant decrease in airspeed. The helicopter settled toward the trees.

The pilot said he had to decide whether to attempt completing the turn and risk striking a tree with ground speed or try a landing with zero airspeed. He chose the latter. The helicopter settled into trees and rolled over on its right side.

Reported winds don’t always tell the complete story. The NTSB report listed winds from 100 deg at 13 kt. Ridges and valleys can have a major effect on wind patterns. Flying low and slow at high density altitudes and high gross weight makes even a small wind change very demanding.

Yet helicopters do not need to leave the ground to experience the damaging effects of a sudden gust. For example, on April 19, 1996, a Robinson R22 Beta was substantially damaged while refueling near Franklin, La.

According to the pilot, the helicopter returned to the initial point of departure to pick up additional equipment needed in the field. While there, the pilot elected to hot refuel.

The pilot’s father drove a pickup truck equipped with an auxiliary fuel tank under the rotor disk of the running helicopter to accomplish the refueling. The pilot said he stayed at the controls of the helicopter and a gust caused the main-rotor blades to flex down, striking the top of the truck.

Although no one was injured, the helicopter rolled to the right into the truck resulting in structural damage to the helicopter and minor damage to the truck. At the time this happened the winds at the accident site were reported from 170 deg at 18 kt, gusting to 25.

Wind can affect the flexing of a rotor blade at low rpm much more than at normal speed. No objects should be allowed under the disc area of a spinning rotor, especially in gusty wind conditions.

Another accident happened on April 29, 2002, when an Enstrom 280FX was substantially damaged when the main-rotor blades struck the tail boom.

In an interview with the NTSB, the pilot stated he landed in a corn field and disembarked from the helicopter while the rotors were still under power. Subsequently, he said a gust appeared and the main rotor severed the tail boom.

It should go without saying that leaving the pilot station of a helicopter with power still applied to the rotor system is just a really bad idea under any circumstances.

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