The sequence may seem inexplicable: a helicopter in apparently slow, stable flight begins spinning faster and faster until it falls to the ground. Its occupants may or may not escape, but the aircraft is usually destroyed. The likely cause or contributing factor: loss of tail rotor effectiveness (LTE), and raising the possibility that training for LTE is inadequate and needs to be reconsidered in light of these accidents.

LTE Examples

Fallon, Nevada, July 3, 2014: A Eurocopter AS350B3 conducting long-line external load operations approached a landing zone on the lee side of a ridge with a 972-pound load on a 100-foot line. Surface winds gusted from five to 25 knots. With the cargo about 10 feet above the drop zone, the helicopter began swaying back and forth and then descended, rotating counterclockwise. The pilot tried to release the load but was unable, most likely because of the rotation, and could not stop the helicopter from hitting the ground and rolling onto its side. He suffered only minor injuries.

Wichita Falls, Texas, Oct. 4, 2014: A Bell 206L1+ EMS helicopter approached United Regional Hospital’s helipad in very light winds. The pilot decided the approach was high and fast and tried to go around, pitching forward to build airspeed. A witness on the ground saw the helicopter begin spinning, slowly at first, and descend behind the building. The pilot reported that when he increased power to climb, the ship entered a “violent” right spin. He was unable to regain control. The helicopter completed at least five rotations before hitting the ground inverted and catching fire. The pilot escaped with injuries, but the flight nurse, paramedic, and patient were all killed.

Tchentlo Lake, British Columbia, May 4, 2016: A Bell 206B performing infrared scanning over a recently logged forest flew at about 30 knots﻿ airspeed and an altitude of 150 feet. The grid legs were oriented east-west. To provide a clear view for the camera operator, the pilot flew them in a left crab; winds were about 10 knots from the west. As the aircraft entered the last scan area flying downwind at a groundspeed of 14 knots, it began spinning to the right, rotating five times as it “descended steeply, though not rapidly, to the ground.” All three on board survived but suffered significant injuries.

Großglockner, Austria, Aug. 1, 2017: A McDonnell-Douglas 902 evacuating a patient from above the 11,000-foot mark of Austria’s highest mountain was balanced on one skid while loading. Just after the patient boarded, its tail lifted and swung to the left. The helicopter made two rotations nose-low at accelerating speed before crashing onto the rocks and coming to rest just above a 1,000-foot drop. No one on board was seriously injured; the patient was moved to another landing zone and eventually hoisted out by another helicopter.

Investigation into the Austrian accident has only begun, but the other three have been definitively attributed to one of the aerodynamic misfortunes peculiar to rotorcraft: the loss of tail rotor effectiveness (LTE). In recent years, LTE has accounted for 3 to 4 percent of all helicopter accidents in the U.S. Because they typically occur at low altitude close to obstructions in locations either urban or remote, that figure understates the public safety consequences of this hazard.

Charlottesville, Virginia, Aug. 12, 2017—While the investigation is still in the information-gathering stage, the NTSB’s preliminary report on the fatal crash of the Virginia State Police Bell 407 bears many of the hallmarks of LTE. Preliminary radar track data showed the helicopter beginning a right turn before rapidly descending out of sight; the final return showed a groundspeed of 30 knots. The preponderance of witness testimony indicated that from a hover, it spun to the right and continued spinning as it descended into the trees in a 45-degree nose-down attitude. These details were corroborated by University of Virginia security camera footage. The aircraft was being repositioned to provide security for the governor’s motorcade after assisting ground units responding to contentious public demonstrations in the city. Both pilots were killed.

Gran Sasso, Abruzzo, Italy, Aug. 13, 2017—Footage posted to the Internet captured a red-and-white firefighting Bell 206 approaching a landing zone in a mountain meadow. As it slowed to walking speed, the tail swung left and it began to rotate counterclockwise as the nose dipped. After two and a half revolutions, it hit hard on the rear of the skids and rolled over. Gusty winds were reported at the scene. All three on board survived.

Clint Johnson, now chief of the NTSB’s Anchorage, Alaska office, once narrowly escaped an LTE encounter on a windswept ridgeline. His recollections and a recreation of the flight form the core of the NTSB Safety Alert Video – LTE.

LTE Defined

The specific phrasing “loss of tail rotor effectiveness” is significant. While certain types of mechanical failures produce similar effects, LTE is a purely aerodynamic phenomenon in which a fully functional tail rotor system fails to provide effective directional control. The result is sudden uncommanded yaw that can’t be arrested by normal control inputs. The problem is rooted in one of the basic challenges of rotorcraft design.

Conventional helicopters have a single main rotor. In most U.S.-made models it turns counterclockwise; on some European and Russian models it turns clockwise. (For simplicity, we’ll describe a helicopter with a counterclockwise main rotor, bearing in mind that all the details are exactly opposite in a clockwise design.) The “equal and opposite reaction” described by Newton’s Third Law of Motion spins the rest of the ship in the opposite direction, clearly not acceptable behavior in a functional aircraft. The traditional solution is to use a small rotor facing sideways—that is, perpendicular to the plane of the main rotor—at the end of a tailboom to counter that spin with lateral thrust.

The amount of thrust required depends on multiple factors: changes in the torque produced by the engine in response to the power requirements of different flight regimes, the aircraft’s loaded weight, environmental conditions including density altitude, and whether the pilot wants to hold the nose straight or turn. Because the tail rotor spins at a near-constant speed—a fixed multiple of that of the main rotor, to which it’s mechanically connected by a system of drive shafts and gear boxes and whose own speed is maintained within a very narrow range in all normal flight operations—tail rotor thrust is modulated by changing its blades’ angle of attack. The pilot accomplishes this with pressure on anti-torque pedals, one for each foot. They vary the pitch of the tail rotor blades via mechanical linkages in small helicopters, hydraulic circuits in large ones.

Like everything else in aircraft design, tail rotor effectiveness is a matter of compromise. The engine produces a limited amount of power, so the more that’s provided to the tail rotor, the less is available to the main rotor to lift the ship. And, as the FAA’s Helicopter Flying Handbook points out, “Environmental factors can overwhelm any aircraft.” The result is that no single-rotor helicopter has enough tail rotor authority for every conceivable situation. While the element common to every LTE accident is yaw accelerating beyond the tail rotor’s ability to counter its momentum, specific situations substantially increase the risk.

When It Happens

Low airspeed is a factor in most LTE encounters. Torque reaction is greatest at high power settings, and because the main rotor operates more efficiently in undisturbed air, more power is needed to stay aloft when the aircraft isn’t moving fast enough to escape its own downwash. The vertical stabilizer gains meaningful authority to reduce yaw only near cruising speed, and of course wind components make up the highest proportion of the relative wind at near-zero airspeed.

The surge in torque accompanying abrupt power increases can kick the aircraft into a spin if not precisely matched by appropriate pedal inputs. Also, yaw most quickly accelerates beyond control when it’s in the same direction as the ship’s intrinsic spinning tendencies, i.e., to the right with a counterclockwise main rotor. So right turns at low airspeed are particularly problematic, more so with a tailwind, as a helicopter’s orientation to the prevailing wind can amplify the risk via three distinct mechanisms:

Main rotor disk interference—Winds of 10-30 knots from about the pilot’s 10 o’clock position can blow the main rotor’s tip vortices directly into the tail rotor, creating turbulence severe enough to significantly reduce tail rotor thrust. In a right turn, if the pilot reacts by increasing right pedal pressure, the increase in thrust after the aircraft turns through the area of interference can quickly push the turn past controllability.

Weathervaning—Tailwinds (from about the pilot’s four- to seven-o’clock positions) tend to swing the helicopter’s tail downwind. The initial yaw may be either left or right and begins gradually, but can accelerate quickly if not countered right away.

Tail rotor vortex ring state—More-or-less direct crosswinds opposing the tail rotor’s thrust—left crosswinds from about seven to eleven o-clock—cause rapid and unpredictable fluctuations in tail rotor thrust, requiring quick and accurate pedal inputs to compensate. LTE can result when pedal inputs that fall further behind the aircraft trigger overcorrections.

Recovery and Training

As with most aviation emergencies, prevention is the best cure. Understanding the situations conducive to LTE and being ready to meet any uncommanded yaw with timely and forceful pedal pressure reduce the chances of being caught by surprise or failing to manage an adequate response. Stopping the turn immediately is essential. Once it reaches 180 degrees, successful recovery requires altitude and elbow room, neither of which is usually available.

Altitude offers the option of lowering collective, reducing both the main rotor blades’ angle of attack and engine output. This decreases torque while making more power available to the tail rotor, but at the cost of the lift needed to maintain altitude. The helicopter can lose hundreds of feet before the pilot regains directional control. Forward cyclic to regain airspeed is necessary in any case. If there’s no altitude to sacrifice, recovery requires an outwardly spiralling flight path that can cover half a mile or more. Landing zones rarely provide that much clearance from obstructions, and it’s often impossible to keep the aircraft from descending into the ground during the attempt.

Many—perhaps most—helicopter pilots enter their careers without ever receiving realistic training in LTE onset and recovery. Full-motion simulators aren’t widely available. Not all of those that exist convincingly recreate the suddenness and severity of real LTE events, and as the National Transportation Safety Board noted in a March 2017 Safety Alert, “Due to safety concerns, training for LTE is rarely done in an actual helicopter.” That challenge is compounded by the fact that many instructors are themselves relatively inexperienced, having earned that credential to build the flight time needed to qualify for a “real” flying job, and shy away from the more precarious corners of the flight envelope. (Fixed-wing instruction suffers from a similar problem.)

Pete Gillies, who retired as chief pilot after 44 years with Western Helicopters and spent most of his 50-plus years in helicopters flying powerline construction, sling loads, “mountain missions of all types,” fire suppression, and training other experienced pilots in the fine points of those disciplines, offers these insights on surviving LTE encounters and training for them in the aircraft:

“The only time helicopter pilots routinely apply full pedal in either direction is when checking freedom of controls before engine start. During normal flight, a small amount corrects any yaw. It is seldom necessary to apply full pedal, so this is not an automatic response to unplanned yaw.



“Coming to a hover, more pedal is normally required to counteract torque and maintain heading, but this seldom requires full deflection.



“In the case of LTE when slowing down or coming to a hover, FULL opposite pedal is required. Bell Helicopter deserves credit for researching and publishing the corrective action years ago. Immediate application of full opposite pedal will stop LTE if it’s done before the yaw becomes so developed that it cannot be stopped without reducing power by lowering collective, still holding full opposite pedal.



“Realistic training can be accomplished by climbing to at least 750 feet over a suitable landing surface (to remain well above the height/velocity curve with plenty of room to lower collective and fly out of LTE) and coming to a hover. Slowly relax the pedal, allowing yaw to develop, then reapply to stop the yaw and return to the previous heading. Repeating this maneuver and allowing more and faster yaw to develop, you may find that full opposite pedal will not stop the yaw. You now have LTE. The only practical way of stopping the spinning is to reduce power by lowering collective and gain some airspeed, still holding full opposite pedal.

“The whole idea is to be able to quickly recognized the oncoming loss of yaw control and learn how to fix it before you've lost it entirely.”

Alternative Designs

One way to eliminate the torque reaction is to use two main rotors turning in opposite directions, each countering the effects of the other. However, the additional weight and power requirements make this option most practical in the heavy-lift category.

Another approach is use something other than a tail rotor to control yaw. Beginning in the 1970s, McDonnell-Douglas began developing the Notar (“NO TAil Rotor”) system, in which a ducted fan inside the tailboom harnesses an aerodynamic oddity called the Coandă effect to create about two-thirds of the needed lateral thrust. Air blown through a rotating nozzle by the same fan supplies the rest and provides variable yaw control. Pilot reports suggest that it’s much less susceptible to the conditions that cause LTE than conventional designs, but the Helicopter Flying Handbook’s caution about environmental pressures is well placed. The MD 902 wrecked in the Großglockner rescue accident was a Notar aircraft. It’s believed that gusts at the landing site may have exceeded the capability of its anti-torque system