Unanticipated Yaw

Some comments about one of Robinson Helicopter’s latest Safety Notices.

In May 2013, Robinson Helicopter issued Safety Notice SN-42, Unanticipated Yaw. It said, in part:

A pilot’s failure to apply proper pedal inputs in response to strong or gusty winds during hover or low-speed flight may result in an unanticipated yaw. Some pilots mistakenly attribute this yaw to loss of tail rotor effectiveness (LTE), implying that the tail rotor stalled or was unable to provide adequate thrust. Tail rotors on Robinson helicopters are designed to have more authority than many other helicopters and are unlikely to experience LTE.

I discuss LTE a bit in my 2009 blog post, “How Much Wind is Too Much Wind?” You can also learn about it on Wikipedia and in the FAA-published Helicopter Flying Handbook.

Bell 206 Tail Rotor
Bell 206 helicopters have dinky tail rotor blades. (Photo Credit: Dakota Air Parts.)

Robinson points out that its helicopters are designed to have more authority than many other helicopters. This is partly because Frank Robinson worked for Bell, which has notoriously poor tail rotor authority, where he became known as a “tail rotor expert.”

I experienced the difference between tail rotor authority in a Robinson vs. another helicopter firsthand in 2004, when I flew Bell 206 Long Rangers at the Grand Canyon. After years and at least 1200 hours experience flying Robinsons, I was in a quartering tailwind at the Papillon helipads one day and got into LTE. I pressed the pedal enough to stop the rotation in a Robinson, but the yaw didn’t stop — I wasn’t flying a Robinson. Fortunately, I had the presence of mind to press harder on the pedal and get the situation under control before the rotation had reached a full 90°. It taught me a very valuable lesson about unanticipated yaw.

Robinson’s Safety Notices are normally issued in response to specific accidents. I went fishing on the NTSB website and discovered this April 2012 accident. Probable cause was issued on May 23, 2013 and that would be the right timing for a Safety Notice to appear. (Of course, I can’t be certain whether this is the accident that sparked the new Safety Notice. It’s just a guess on my part.)

From the report summary:

The helicopter slowed as it approached the landing zone on a modified right-base turn to the north. Gusting wind from the south had prevailed for most of the day and was present at the time of the accident. As the pilot turned to “enter the landing area,” he felt a “bump” in the tail rotor control pedals. The pilot added that he applied left pedal to compensate for a right yaw, and the helicopter immediately “started to rotate” at an increasing yaw rate with full left pedal applied. The pilot stated that the rotation stopped when he pushed the collective control “full down” and applied aft cyclic. The helicopter then descended through the trees and collided with terrain. Examination of the wreckage revealed no evidence of pre-impact mechanical anomaly.

The accident report goes on to provide general information about LTE. The Full Narrative refers to FAA Advisory Circular (AC) 90-95 “Unanticipated Right Yaw in Helicopters.” It also refers to Robinson Safety Notice SN-34, Photo Flights – Very High Risk as it pertains to settling with power, reduced RPM, and the loss of tail rotor thrust in low RPM situations. All of this is good reading to learn more about LTE and tail rotor operation.

It’s in low-speed situations that tail rotor effectiveness can be lost. At higher speeds, the helicopter points into the wind via normal “weathervaning.” The vertical stabilizer on the end of the tail helps make this happen. But at lower speeds, especially when turning into a tailwind, the force of the wind can easily cause the helicopter to yaw. The pilot must react quickly and firmly to stop and correct the yaw.

Experience teaches us. Years ago, I was doing a long cross-country flight with a 300-hour pilot who had just gotten his CFI. He was landing at San Luis Obispo (SBP), sitting in the left seat while I sat in the right. There was some yaw and I think he tried to correct it with the pedals. But then he panicked and said that something was wrong, that the pedals weren’t working.

I don’t recall the yaw being very bad. We were still moving along at a good clip, barely over the runway and likely doing at least 50-60 knots. I told him to push the pedal and he claimed he was and that it wasn’t working. He was visibly upset. I offered to take the controls and he agreed. I pushed the appropriate pedal and the helicopter immediately straightened. I landed without incident.

When I read this Safety Notice, I immediately thought of that low time pilot’s approach to landing and my own experience, years before, at the Grand Canyon. In both cases, we’d initially failed to use enough pedal to correct the yaw. The difference is, while I’d added more pedal when necessary, the low-time pilot had assumed there was a problem with the aircraft without trying harder to correct the yaw.

I have to wonder whether the accident pilot was in the same situation. After all, probable cause put the blame firmly on him:

The pilot’s inadequate compensation for wind during a high-power, low-speed downwind turn, which resulted in a loss of control due to loss of tail rotor effectiveness and settling with power. Contributing to the accident was the pilot’s decision to land downwind.

I guess the takeaway from all this can be summed up as follows: We always need to be aware of potential control issues, especially when operating at low speeds in crosswind or downwind situations. Yaw should never be “unanticipated.” It’s the pilot’s responsibility to keep control of the aircraft at all times by avoiding situations that could result in control issues and to use aircraft controls properly.

What do you think?