Loss of tail rotor effectiveness

March 14, 2012 by Tim McAdams

At Baltimore-Washington International Airport a helicopter approached a construction site on the airport property 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 degree pedal turn. Stopped momentarily and 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 and collided with Alpha taxiway abeam Runway 15 Right. The FAA’s examination of the helicopter found no mechanical anomalies.

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.”

The pilot at BWI was performing an aerial photography mission, 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 can become very demanding. Any pilot performing a similar mission needs to understand the aerodynamics and limitations of maneuvering at slow speeds.

13 Responses to “Loss of tail rotor effectiveness”

  1. Bob Barbanes Says:

    The “Loss of Tail Rotor Effectiveness” boogeyman is pretty lame. It’s blaming the helicopter for a bad pilot doing dumb things. As you might expect, helicopters have pretty strong weathervaning tendency. Hovering with a tailwind can be tricky. You would think that this lesson would be imparted to new helicopter students pretty early in the game. Not so, apparently.

    Asking a conventional helicopter to hover downwind is asking a lot. A sharp pilot can – and should! – expect the ship to want to swap ends. In the accident case cited, it is reported that the aircraft made the expected 180-degree turn…and then stopped! At the very start of the end-swapping, if the hapless pilot had put in and held FULL left pedal that would have been the end of it. I’ve been there. You go, “WHEW! Better not do that again,” and go on with life. But no, somehow this guy let…yes, LET another rate of rotation begin and he spun the thing to the ground in a panic of, “What’s going on here! It’s out of control!!” Bah. I would bet real money that if FDR data was available for that aircraft, it would show something less than full left pedal applied during the corkscrewing descent. Bell 206′s have plenty of tail rotor authority at sea level. And last time I checked, BWI was not up in mountainous terrain.

    Another helicopter pilot in the area at the time of the accident reported the winds at 1000 feet were so strong and gusty that it caused his aircraft to shake “violently.” Wow! Must’ve been a hella-windy day! Curiously, the surface winds at BWI were 130-140 at 6 to 8. So that’s a strange phenomenon, if true.

    In typical “LTE” events, people must be under some false assumption that the tail rotor “mysteriously” stops working or is somehow “stalled.” Poppycock! The tail rotor NEVER stops producing thrust. It’s just that sometimes the thrust is momentarily overpowered by other forces, which can allow a yaw rate to build up. Once the conditions are removed that allowed it to become overpowered, the tail rotor regains its full authority. So if you’re hovering up at 250 feet and you’ve already got your left foot nearly to the floor and the helicopter begins to swap ends, you put in AND HOLD full left pedal. You reduce torque to the main rotor (which lowers the demand on the anti-torque tail rotor), lower the nose to get some airspeed (which will increase the streamlining effect) and you fly away to regroup and figure out a better way to get that shot. We’ve all done it; it’s not rocket science. You don’t just sit there, spinning around, riding it to the ground like a passenger.

    The pilot in this case was dual-rated with both fixed- and rotary-ratings. His total time was 531 hours. His total Bell 206 time was 160 hours. In other words, a very inexperienced pilot. Flying helicopters is not easy. They are demanding and challenging and they are really easily to crash. And hey, we’re not all Yeager. (I prove that every time I go up.)

    Yes, “unanticipated right yaw” and “loss of tail rotor effectiveness” are things of which helicopter pilots must be aware. But let’s stop blaming the helicopter for pilot error accidents.

  2. Philip Says:

    Will we ever run out of “expert” Monday Morning Quarterbacks?

  3. Mark Says:

    I’m not sure why you believe the article blames the aircraft; it seems quite clear that the article was addressed to pilots in that “Any pilot performing a similar mission needs to understand the aerodynamics and limitations of maneuvering at slow speeds.” “The tail rotor NEVER stops producing thrust” isn’t quite the whole truth – the thrust it is able to produce can vary significantly based on weather and aircraft orientation. Managing that is what we pilots get paid for. Failing to gets us published in the NTSB database.

  4. Jim Borger Says:

    The article appears to blame he helicopter because it states, “The helicopter turned right” and then it states the helicopter “made a right rapid 180 degree pedal turn.” It further states that it “initiated another rapid pedal turn to the right.”

    If the helicopter was making all those pedal turns then it must have been at fault. Pedal turns could have been made only by the pilot, not the helicopter, unless it was an early A model 206 with boosted pedals. Obviously the pilot did all those things, not the helicopter. Bob is right on the money in his post.

  5. Al Says:

    Wow, too bad “Super Bob” was not there. Like Underdog, he could have saved the day. If you want constructive criticism to be heard, you might consider avoiding comments like ” It’s blaming the helicopter for a bad pilot doing dumb things”.

  6. Fred O. Bell Says:

    During helicopter flight training in 1953 we learned not to press your luck and hover out of ground effect with a tail wind. You only inviting disaster.

  7. Fred O. Bell Says:

    During helicopter flight training in 1953 we learned not to press our luck and hover out of ground effect with a tail wind. You are only inviting disaster.

  8. Bob Barbanes Says:

    I know that is somehow doesn’t seem fair to bash a dead pilot. We tend to be extra-sympathetic to those of us who pay the ultimate price for doing what we do. And believe me, it gives me no pleasure to say what I said above about the pilot who crashed at BWI. But I stand by my statements, as unkind as they may appear to be.

    Philip and Al, I understand where you’re coming from. Nobody likes armchair quarterbacks. But the fact is, the 500-hour BWI pilot crashed a perfectly good helicopter – killing himself and, I might add, HIS PASSENGER. I criticize from the standpoint of an 11,000-hour helicopter pilot with over 7,000 hours in the 206 alone. I’m not some retired guy sniping from the sidelines; I’m one who still flies for a living…AND currently flies a Bell 206B with what we call the “small” tail rotor (there are two sizes that B-models can have). I often do exactly the same kind of flying that gets us in trouble: Hovering (or flying very slowly) low over the trees while surveying timberland. On those flights, I am hyper-aware of wind direction and acutely aware of how much left pedal I’ve got in. Any uncommanded right yaw will be cause to pull the trigger and get outta there. And I leave myself plenty of room to go. That comes with experience. (And notice I say “uncommanded” and not “unanticipated.” In such cases you *have* to anticipate the yaw.) So far, in 30 years of doing this for a living I have not killed myself.

    For what it’s worth, I did not get thrown into the fray (released to fly solo as a commercial pilot) until I had 1,000 hours of total time – thank the insurance companies for that. I don’t know how the BWI pilot got insured with just over 500 hours, but apprently he did. Maybe things have changed. Would another 500 have made a difference? Who knows. But evidently he was in over his head on this flight. Could I, “Super Bob” have saved the day? Yes, and I have no doubt about it. I’m sorry, but that’s the truth. I would not have (or more importantly I would not have let the pilot) put the ship in such a precarious position: downwind OGE hover at 250 feet agl just to get a photograph. That’s not smart.

    Finally, let’s tal about the Bell 206 and “LTE.” I stand by my statement that the tail rotor NEVER stops working (sorry Mark). The tail rotor does not stall. Period. Two things can happen in a 206:

    1) In quartering left headwinds, main rotor vortices can interact with and impinge the tail rotor thrust. We feel this as yaw instability that makes our feet work hard. Weathervaning effect is still strong though, and the fuselage will resist turning to the right. However, if the nose does start to yaw to the right, application of FULL left pedal will stop the rotation. Why? Because the tail rotor is still working! As soon as the nose rotates to a certain point the main rotor vortices won’t be being blown into the tail rotor anymore! The key is to not let a yaw rate develop that full pedal cannot arrest. If it does, push and hold full left pedal and reduce torque slightly. If you’re such a bonehead that it goes all the way around, then you’re stupid and you’ve really screwed up and now you better do some of that pilot ‘stuff’ you get paid so well for and should have already been doing.

    2) In a right-rear quartering wind, the 206′s huge vertical fin can blank off enough of the tail rotor that thrust is reduced. We know this. Adding a right-rear wind, that big vertical fin and weathervane effect can cause the helicopter to want to swap ends. This is the situation in which our BWI pilot found himself. But again, the tail rotor is still working back there. As the aircraft comes around into the wind, holding FULL left pedal and reducing torque will stop the rotation.

    Pilots are reluctant to make full control inputs. There was a video on YouTube a while back of a ENG 206 pilot crashing upon landing in California. Luckily, his camerman was filming the landing and approach to the dolly. The pilot never quite got it stopped into a stabilized hover. As he came up to his landing dolly, suddenly the aircraft started rotating to the right (but it did not suffer a tail rotor failure). It went ’round and ’round and ended up on its side. The pilot reported that full left pedal did not stop the rotation. It was that damn 206′s fault – with its too-weak tail rotor! LTE! LTE! Trouble was, in the video that the camerman shot the pilot’s hairy legs could be seen (he was wearing shorts). The pedals were neutral. He was just hanging on, along for the ride. The video evidence was contrary to his statement.

    I’ve said before that there are no “average” helicopter pilots. Helicopter pilots must be above-average aviators. You get really good or you get dead. We like to assume that all aircraft are “easy” or “safe” to fly once you’ve learned how, but helicopters are not. They can and will bite you badly at the slightest provocation or moment of inattention. They are weird machines that take a long time to master – if ever. The proof of this is that even high-time helicopter pilots crash due to “simple” pilot-error things. And even at my stage of career, I still find myself saying, “Man I’ve got a lot to learn.” And it’s true.

    I feel badly for the inexperienced pilot at BWI. But like the Colgan pilot in Buffalo, he screwed up. Both put their aircraft into a position it didn’t like. And they both should have known better. The very sad thing is that they did not. Hey, it happens.

  9. Wilbur Wright Says:

    Not all helicopters are created equal with respect to tail rotor authority. The Bell 206 is legendary for its marginal tail rotor authority. Can it be flown safely? Sure, but the margin for error is smaller than in many other designs.

  10. Mat Says:

    I am a 1000hr fixed wing pilot and 200hr heli pilot owning an R44. Thankyou Bob for your set of explanations above. It has helped me understand a few things better.

  11. Kenny Says:

    There is no substitute for good basic training, then for pilots to go out and use good pilot technique to avoid Loss Of Tail Rotor Effectiveness!

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