Thursday, February 1, 2007
Safety Watch: Loss of Tail Rotor Effectiveness
ACCORDING TO THE NTSB, ON July 11, 2000, the pilot of a Robinson Helicopter R22B said he was flying northeast about 10-20 mph at about 230 ft agl when the helicopter began to rotate to the right and seemed as if it was inverted.
He applied full left pedal and lowered the collective. After about three rotations, he heard a horn, which he believed was a low rotor-rpm warning. Initial inputs did not correct the loss of control.
As the helicopter descended toward trees, he applied full left cyclic, followed by full forward cyclic, then full right cyclic. The helicopter seemed slightly more controllable and was no longer inverted. However, it was still corkscrewing to the right. It impacted a swampy area and the fuselage, tail boom, and tail rotor were damaged. The pilot sustained serious injury. The passenger was not injured.
Examination of the wreckage by an FAA inspector did not reveal any pre-impact mechanical malfunctions. Reported winds at an airport about 7 mi from the accident site were from 300 deg at 15 kt, gusting to 21.
The NTSB determined the probable cause as the pilot-in-command’s improper in-flight decision to maneuver at a low airspeed with a left quartering tailwind, which resulted in a loss of tail-rotor effectiveness.
Just over a year later, the pilot of a Bell Helicopter 206B JetRanger and his passenger were not so lucky. They were fatally injured when their helicopter collided with Alpha taxiway abeam Runway 15 Right at Baltimore-Washington International Airport.
The helicopter approached a construction site on the airport in an out-of-ground-effect (OGE) hover taxi, with a quartering left tailwind. The helicopter turned right, and slowed to a stationary hover at about 250 ft with a direct tailwind. Once in a hover, it made a right, rapid, 180-deg pedal turn around the mast, stopped momentarily, then initiated another, rapid pedal turn to the right. The helicopter turned at a faster rate than the initial turn and continued into a spinning, vertical descent to the ground. The FAA’s examination of the helicopter found no mechanical anomalies.
Again, the NTSB determined the probable cause was the pilot’s improper decision to maneuver in an environment conducive to a loss of tail-rotor effectiveness and his inadequate recovery from the resulting unanticipated right yaw.
According to FAA Advisory Circular AC90-95, any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. It also advises of greater susceptibility for loss of tail-rotor effectiveness in right turns and states the phenomena may occur in varying degrees in all single main-rotor helicopters at airspeeds less than 30 kt.
Bell’s Operations Safety Notice OSN 206-83-10, regarding loss of tail-rotor effectiveness in the 206B and similar airframes, describes the phenomenon as an unanticipated right yaw. It contains the following warnings when maneuvering between a hover and 30 mph:
"Be aware that a tail wind will reduce relative wind speed if a downwind translation occurs. If loss of translational lift occurs, it can result in a high power demand and an additional anti-torque requirement. Be alert during hover (especially OGE) and high-power-demand situations. Be alert during hover in winds of about 8-12 kt (especially OGE), since there are no strong indications to the pilot [of] the possibility of a reduction of translational lift... Be aware that if a considerable amount of left pedal is being maintained, that a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw."
Included in this notice is a chart that depicts relative-wind directions referencing the fuselage where an unanticipated right yaw can occur. More information is also available in Bell’s Rotorbreeze magazine.
One reason these pilots may have placed themselves and their passengers in jeopardy despite the abundance of warnings and information regarding loss of tail-rotor effectiveness is inexperience. The pilot of the R22B held a private pilot’s license and did not provide any further information to the NTSB or FAA. The Bell 206B pilot was commercially rated, but had less than 500 hr of helicopter flight experience, and 87.2 hr of experience in that model.
Both of these flights were performing aerial photography, the nature of which requires maneuvering at low altitudes and slow speeds. Add to that the distraction of trying to work with a photographer to line up the desired shot and the mission becomes very demanding. Any pilot flying a photographer needs to insure that he understands the aerodynamics and limitations of maneuvering at slow speeds.
Flight instructors should take note because students often misunderstand loss of tail-rotor effectiveness. More emphasis should be placed on this subject and instructors must ensure students fully understand the dangers of loss of tail-rotor effectiveness. Failure to do so can be deadly.
Tim McAdams has more than 9,000 total flight hours, with 7,000 in helicopters. A helicopter CFI and a fixed- and rotor-wing ATP, he flies a single-pilot IFR Agusta 109E for CareFlite in Dallas. You can reach him at firstname.lastname@example.org.