Arguably, tail-dragger and all helicopter pilots are the most wind-conscious aviators, and well they should be. Operating a conventional geared airplane anywhere near or on the surface in a gusting crosswind can be a real adventure. For helicopter pilots — depending somewhat on rotor configuration — operating in crosswinds near the ground with high power can be problematical. Mistakes in handling either situation can be fatal.

Canada’s Transportation Safety Board (TSB) reminds helicopter pilots that it is vitally important to understand the phenomenon of loss of tail-rotor effectiveness (LTE).

The Safety Board’s warning arises from its investigation into the loss of Bell 206B (JetRanger) C-FZWB on May 29, 2013. The pilot and one occupant were killed and a third occupant was seriously injured when the aircraft crashed 75 mi. north of Fort McMurray, Alberta, while operating at high power and low altitude while seeking a landing spot. The helicopter was operated by Aurora Helicopters Ltd., doing business as Wood Buffalo Helicopters. Most of what follows is from the TSB’s investigation.

The flight had been chartered by Alberta Sustainable Resource Development (ASRD) to track discarded caribou radio collars. Two wildlife biologists would use an externally mounted antenna coupled to a portable receiver to track these collars. Once found, the helicopter would land in the vicinity of the collar, and the biologists would disembark, then locate and retrieve the collar on foot.

Aircraft operating under contract to ASRD are required to maintain a radio watch with the local command and control center located in Fort McMurray. The pilot made an initial radio check call with the ASRD command at 0915 and departed the Wood Buffalo Helicopters’ Fort McMurray facility at 0928. As required by ASRD policy, the pilot transmitted a 30-min. position/status call to the ASRD command center at 0959, reporting that they were north of Fort MacKay/Firebag Aerodrome. Tracking equipment showed the helicopter was at 1,340 ft. AGL with a ground speed of 99 kt.

At 1010, the helicopter entered its first search area where a number of orbits were conducted in order to fix the location of a radio collar. The helicopter flew as low as 100 ft. AGL but did not land in the area.

The pilot checked in at 1032 indicating that operations were normal and that they were proceeding to a second search site to the northwest. At that time, the helicopter was at an altitude of 550 ft. AGL and at a groundspeed of 58 kt.

At 1048, the helicopter arrived at the second site. They orbited the area for the next 8 min. in an attempt to fix the radio collar’s location and to assess a suitable landing zone.

At 1055, the helicopter orbited the area and executed a wide left-hand turn at 140 ft. AGL. A minute later, it turned eastbound at 120 ft. AGL at a ground speed of 36 kt. and seconds after that was down to 105 ft. AGL and a ground speed of 27 kt. The wind component at this time would have been predominantly a left crosswind from the north at approximately 5 kt.

At 1056:34, the helicopter was at 115 ft. AGL with a ground speed of 16 kt. when the pilot began a right turn to the south. The ground speed at that point was down to 5 kt.

A final GPS waypoint was recorded at 1056:54 with the aircraft at 18 ft. AGL with a ground speed of 3 kt. Upon completion of the turn to the south, the helicopter would have been exposed to a tailwind condition. At that point the JetRanger entered an un-commanded rotation to the right.

There were no indications of mechanical malfunction prior to or during the rotation. The helicopter descended and crashed into a stand of poplar trees 60 to 70 ft. tall, coming to rest on its right side.

After ASRD attempted radio contact at 1108, with no response, it received a telephone call from Wood Buffalo Helicopters at 1109 regarding a reported ELT signal for C-FZWB. At 1114, a company helicopter was dispatched to the last known coordinates to search for the helicopter, arriving over the site at 1143. It located C-FZWB at 1154. Additional rescue resources arrived on scene to help recover the helicopter’s pilot and passengers.

Wreckage

At the time of the crash, the pilot was in the right front seat. One passenger was in the right rear seat. Both the pilot and the right seat passenger were wearing aviation helmets and were secured with the available four-point harnesses. The surviving passenger, in the left front position, was not wearing a helmet, but was using the four-point harness. This passenger was able to evacuate from the helicopter through the broken front windscreen.

Three trees adjacent to the site exhibited damage related to the helicopter’s descent. All three showed rotor-blade damage near the tops; one tree had been topped and the majority of its branches stripped off.

All of the helicopter’s components were identified within a 100-ft. radius of the aircraft. The helicopter’s cabin and fuselage were in a single piece, but it had sustained substantial impact damage, particularly on the right side of the fuselage. The main-rotor head shaft had separated below the rotor head, and was located 26 ft. from the wreckage.

The tail boom was adjacent to the fuselage, but it was severed approximately 3 ft. aft of the horizontal stabilizers. It had indications that it had struck a tree at that point, on the left side of the tail boom. The remainder of the tail boom, including the vertical fin and tail rotor, was located about 7 ft. aft and slightly east of the forward section. The tail rotor was mostly intact with one blade still attached to the yoke. This blade was straight with minor impact damage to the skin. The other blade had fractured just outboard of the yoke and was lying on the ground approximately 5 ft. east of the aft section of the tail boom.

Investigators determined that there had been flight control continuity before the accident. Continuity with the engine, transmission and tail-rotor assembly was verified. Damage to the aircraft was consistent with power being produced. Tree impact damage to the main-rotor blades was to the under-surface of the blades.

The Pilot

Investigators said the pilot held a valid commercial pilot certificate endorsed for the Bell 206. He had begun employment with Wood Buffalo Helicopters on April 1, 2013, and had received ground training — which included awareness of vortex ring state and loss of tail-rotor effectiveness — and flight training on the JetRanger. The pilot successfully completed a company-administered pilot proficiency check for the Bell 206B on April 14, 2013.

At the time of the crash, the pilot had accumulated approximately 504 hr. of flight time in helicopters, 400 hr. of which were in the Bell 206. He was on his 11th consecutive duty day, after having had eight days off.

The pilot’s last three days of work consisted of no flying on May 26, 3.1 hr. of flying the following day, and 3.0 hr. of flying on May 28. Investigators found no indication that any physiological factors, including fatigue, played a role in the accident.

The weight of the helicopter at the time of the accident was calculated to be 2,944 lb. The maximum takeoff weight was 3,200 lb. The aircraft was properly loaded, certificated and maintained.

Weather

The ASRD produced a weather observation from Birch Mountain, which is located approximately 11 nm south of the accident site. At 1200, the visibility was 19 nm. The temperature was 18C and the relative humidity was 51%. The density altitude was calculated to be 2,198 ft. MSL. The winds were reported to have been from the north at 5 kt.; the cloud height was not reported.

A pilot report from a company helicopter, the first on site after the accident, indicated that the sky was clear, winds were light, and the temperature was about 23C to 24C. No cumulonimbus clouds or adverse weather conditions were observed.

Loss of Tail-Rotor Effectiveness

Investigators turned their attention to aerodynamics and airmanship. The main-rotor blades of the Bell 206B rotate counter-clockwise when observed from above. As a result of this rotation, the helicopter experiences a torque effect in the opposite direction, which results in the aircraft yawing to the right. To counter this movement, the helicopter is equipped with a tail-rotor system. As torque is transmitted through the main-rotor system, the pilot is able to counter the resulting yaw with the use of the tail-rotor control pedals, which increase or decrease the amount of anti-torque thrust as required.

Loss of tail-rotor effectiveness (LTE) is the occurrence of an uncommanded yaw rate that does not subside of its own accord and, which, if not corrected, can result in the loss of the helicopter.

The Safety Board explained that LTE is not related to an equipment or maintenance malfunction and may occur in all single-rotor helicopters at airspeeds of less than 30 kt. It is the result of the tail rotor not providing adequate thrust to maintain directional control and is usually caused by either certain relative wind directions while hovering or by an insufficient tail-rotor thrust for a given power setting at higher density altitudes.

The Bell 206 aircraft flight manual (AFM) advises caution when operating when the relative wind is within the critical wind azimuth area. (See Figure 2 from the Bell 206 AFM.) The manual goes on to discuss hovering ceilings and alerts the pilot that tail-rotor control margin and/or control of engine temperature may preclude operation in area B of the hover ceiling charts when the relative wind is in the critical wind azimuth area. As a result, there is the potential for a loss of tail-rotor effectiveness.

The AFM cautions that when operating at low airspeeds above altitudes published in performance charts, tail-rotor effectiveness may be marginal at high power settings under these conditions.

There’s plenty of information available on LTE. Bell Helicopters issued Information Letter 206-84-41 in 1984 identifying low-speed flight characteristics that can result in unanticipated right yaw. FAA Advisory Circular 90-95 also discusses the phenomenon of unanticipated right yaw and recommends recovery techniques. It identifies conditions in which LTE may occur, notably “during any maneuver that requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind,” especially in right turns. The recommended recovery procedure requires the application of full left pedal, movement of the cyclic forward and, potentially, a reduction in power, if altitude permits.

Transport Canada, in an issue of Aviation Safety Vortex, discussed the phenomenon of unanticipated right yaw and recommended a recovery procedure.

Transport Canada’s Study and Reference Guide: Private and Commercial Pilot License (Helicopter) identifies LTE as a ground school topic to be discussed as part of aircraft performance. Transport Canada’s Helicopter Flight Training Manual makes the following reference to LTE:

“U.S. Department of Transportation, Federal Aviation Administration, Rotorcraft Flying Handbook (2000), pp. 11-12 states, ‘In strong gusty wind conditions, a turn away from the into-wind position should be opposite to the torque reaction [...]. In this way you will ensure that there is sufficient tail-rotor control available. Should control limits be reached at this stage, a safe turn back into wind can be accomplished.’”

TSB Analysis

Ultimately the Safety Board determined that the accident helicopter was operating in a flight regime where it was exposed to the left crosswind and tailwind, which would have placed the relative wind into the critical azimuth zone. While conducting a reconnaissance for landing, the helicopter was flown at a low speed and high power setting.

As the pilot progressively reduced speed, the helicopter became increasingly vulnerable to LTE. The damage to the under-surface of the main-rotor blades indicated that the pilot attempted to increase his collective control input in an effort to power out of the area. The application of power in this particular flight regime would have exacerbated the right yaw tendency and aggravated the loss of control. The helicopter experienced LTE, causing a loss of directional control at a height above the trees that precluded an effective recovery.

The TSB noted that Bell and the regulators have produced information to alert pilots to the phenomenon of LTE. The accident pilot would have been exposed to this information during his initial training and subsequent training on the Bell 206. The investigation could not determine the pilot’s level of awareness of LTE in the flight regime in which the helicopter was operating.

Final Thoughts

The dynamics of loss of tail-rotor effectiveness have been known for decades, but the phenomenon continues to claim lives and machines. Helicopter pilots have a well-deserved reputation for superb airmanship. After all, their machines are amazingly complex and require their pilots exhibit a remarkable amount of physical coordination and a continual attention to the aircraft and its changing operational environment.

Small utility helicopters are particularly vulnerable to the slightest inattention to the basics. LTE is one of those basics that must be thoroughly understood and avoided.