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From the Rotor Side
With Helicopter Assistant Chief Greg Quast
 LTE (Loss of Tail Rotor Effectiveness)
Whether you are a student helicopter pilot here at Snohomish Flying Service or a rated pilot, you need to have a good grasp on the avoidance, recognition, and recovery from LTE. Period. This two-part series will explore the history and understanding of LTE as we now understand it.
"The helicopter was involved in relatively slow speed, low level aerial photography that involved it flying a straight track before turning right around a fixed structure of significant historical interest. The pilot carried out one practice run that was judged to be slightly too fast and too close to the structure. The second attempt proceeded without incident until, when half way around the turn, the helicopter began to yaw to the right. Application of corrective left pedal was ineffective and as the helicopter continued yawing right it descended. The rotation continued through several complete revolutions and it struck sloping ground at low forward speed rolling on to its right side. All three occupants were able to vacate the aircraft with only minor injuries. An engineering investigation failed to find any technical fault that could have accounted for the accident. There was evidence, however, that the helicopter may have been operating in a part of the flight envelope where the susceptibility to loss of tail rotor effectiveness was possible."
Bell Helicopter Textron B206
Full Article Text
What is LTE?
A helicopter needs a certain amount of tail rotor thrust to overcome the torque produced by the engine to keep it from simply spinning around in a circle. When more power is applied by the pilot, more tail rotor thrust is required to keep the aircraft in a constant heading. When the tail rotor looses its ability to produce the proper amount of thrust (or anti-torque) the helicopter starts to yaw to the right (for the purposes of this article, we will assume a CCW rotating main rotor common in US helicopters). Tail rotor thrust is directly related to tail rotor RPM and blade pitch.
- LTE is a state where the helicopter is in an unanticipated right yaw that can NOT be stopped by the application of full left pedal. This rapid spinning to the right can cause loss of control of the aircraft and eventual ground contact if not corrected.
[I'll just mention here briefly that LTA (Loss of Tail rotor Authority) is different. It is a more slow and subtle version of the left yaw probably due to low rotor RPM or high DA (Density Altitude).]
It is important to note that the tail rotor is NOT broken and NOT 'stalled', but just ineffective. (The recovery procedures for a stuck pedals, or partial and full tail rotor failures are completely different and will not be discussed here.)
What is actually happening?
When the helicopter is experiencing LTE, it is spinning around rapidly in a right yaw (and cannot be stopped by application of the left pedal). This state was introduced by the wind, the need for lots of anti-torque, and the pilot's slow reaction on the pedals.
There are 3 different wind directions that contribute to LTE:
- 120°-240°
(from the rear)
- 210°-330° (from the left)
 - 285°-315° (left-quartering headwind)
Each of these directions affects the tail rotor in these ways:
120°-240° (Weathercock Stability)
This direction wants to swing the whole tail downwind just like a weathervane.
210°-330° (Tail Rotor Vortex Ring State)
A left crosswind blowing directly on the tail rotor causes it to settle in its own vortices and loose lift. Think of it...the tail rotor is producing thrust to counteract the main rotor torque, therefore the "downwash" or air moved by the tail rotor is moving to the left. With a left cross wind, you now have air from the left (crosswind) competing with air from the right (tail rotor "downwash") and they don't get along. Their argument results in the enlargement of the rotor tip vortices and an enlargement of the center "stall" area of the tail rotor disc. The result is a loss of lift (or anti-torque).
285°-315° (Main Rotor Disc Interference)
The tip vortices produced at the end of the main rotor blades are getting blown into the tail rotor.
The History of LTE
Starting around 1983 and prompted by multiple accidents and incidents involving the OH-58 (Bell 206 Jet Ranger), the US Army, FAA, and Bell Helicopter launched an investigation and issued information regarding the phenomenon of LTE to better educate pilots and avoid further accidents. In 1995, the FAA issued Advisory Circular AC 90-95 on the subject, spearheaded by a Senior Bell test pilot named Don Bloom. Before we called it 'LTE', pilots referred to it as the 'Hughes Spin', or the 'dreaded Bell tailspin'. See the following article from Rotor and Wing magazine from someone with actual experience:
I am a retired CW4, former scout pilot and instructor pilot. Now I'm with the Benton County Sheriff's Office. We are flying two OH-58A+ birds we got from the U.S. Army.
I had three cases of loss of tail-rotor effectiveness in my Army flying days. The first was while wearing night-vision goggles (the old style) flying an OH-6. We were flying in the maneuver area of Fort Knox, Ky. As we flew up the side of a hill and turned to the right at the top, the bird snapped right. I immediately dumped the nose and flew out of it down the hill. No problem. Luckily, I had been qualified in the OH-6 by some Vietnam scout pilots. Then there was no loss of tail-rotor effectiveness, only the "Hughes Tail Spin."
The second time, I was an instructor pilot in D Troop, 1st Battalion, 9th Cavalry at Fort Hood, Texas. The Army had just come out with the new emergency procedure for the rotation. You were to hold a stable hover at 3 ft 90 deg left of the wind and relax pressure on the pedals. As the bird began to rotate, you were to perform a hovering autorotation.
I had my commander out for an evaluation ride. He was a Slick and Cobra pilot in Vietnam. We practiced the new procedure at Gray Army Air Field a few times, then headed into the maneuver area. We shot a couple of pinnacle approaches to a stock pond dam. That's all we had at Hood.
I told the commander we would head north to the creek bed and do a little nap-of-the-earth flying. The major flew northbound at about 50 deg and 40 kt. The wind was a left quartering headwind. As we neared the drop-off to the creek, I told him to turn right and get ready to drop in the creek bed. As he started his right turn, the bird snapped to the right. We did two complete 360-deg turns and the major hadn't done anything yet. As the bird started the third 360, I took the controls, dumped the nose and flew out of the spin downwind. Again, no problems, only the shit scared a little. When we both calmed down, he asked, "What the hell was that?" I said, "That was what we just practiced back at Gray." His reply was, "That emergency procedure is worthless!"
The last time was in Korea, when a captain began his approach to our squadron pad up north. The bird started to shudder a little and I hit the cycle and told him to go around. The spin hadn't started and I wasn't going to let it.
While at Fort Hood, I was the maintenance officer for the scout platoon and the instructor pilot. On one of the flight maneuvers, you were to fly forward, sideways and backward to check control rigging, etc. You were supposed to stop rearward movement by forward cyclic. I didn't. I would relax pressure on the pedals and let the bird begin its rotation. As it passed through 90 deg, I would dump the nose and fly out of the turn. In other words, I was initiating loss of tail-rotor effectiveness. All this was before I flew with my commander and his spin. I guess I did my practice loss of tail-rotor effectiveness through my maintenance test flights. Maybe it's a good way to show a student what happens and how fast it happens. Maybe I'm just lucky. If I am, it does work and I'm still around and haven't had one since Fort Hood.
Jeff "Fuzzy" Fozard
Chief Pilot/Deputy
Benton County Sheriff's Office
Bentonville, Ark.
Next Month, we will continue and discuss factors cause LTE, avoidance, and recovery. If you have any comments, corrections, or thoughts, please email me at: gregoryq@harveyfield.com. The key to LTE is like the waltz...1, 2, 3....1, 2, 3.... Each one of these wind directions keeps the right yaw going when the spinning helicopter doesn't have enough anti-torque. | 
