LTE (Loss of Tail Rotor Effectiveness) - Part 2

This month we conclude our discussion on the topic of loss of tail rotor effectiveness:
 

Can it happen to all helicopters?

Those in the know say that it cannot happen in helicopters designed to certain military specs. For the rest (which is most of us) we need to avoid and be ready for it. Argue amongst yourselves if you must, I am staying out of it...

Where can LTE happen?

LTE must happen when the helicopter is experiencing a high power setting and slow airspeed with wind. It cannot happen in forward cruise flight. Situations common to LTE are photo flights, downwind approaches, hover taxis, etc.

What factors affect LTE? 

• Tail rotor design. (Number of blades, blade shape, blade length, designed blade RPM, etc.) Some tail rotors have more authority than others. It is no industry secret that the Bell Jetranger has a weak tail rotor. A contributing factor is the large vertical stabilizers in the way of the tail rotor blades which reduce the volume of air that the tail rotor can pump effectively. (Compare the B206 with the UH-60 below.)
• Tail rotor RPM
• Aircraft weight, high DA's, etc.
   (Things that require more power from the helicopter)
• Over pitching. Grabbing an armpit full of collective is a super way to lose valuable RPM and increase the torque all at the same time.

How do I avoid LTE?

• Avoid tailwinds.
• Be aware of the wind (where it is coming from and its velocity). Left crosswinds contribute to LTE as does a loss of translational lift. Airspeeds below translational lift require more power, therefore more torque, therefore requiring the tail rotor to work harder.
• Avoid OGE hovers.
• Keep your RPM in the green. In our hand-throttled Schweizers, you need to maintain the rotor RPM when the power demand increases (or decreases). Students need to remember that although the main rotor tach gauge indicates main rotor RPM, it is directly proportioned to tail rotor RPM. A loss in main rotor RPM is also a loss of tail rotor RPM and therefore a loss of tail rotor authority or thrust. 
• Pay attention. It is easy to get distracted on a photo flight, but where is the wind? How much pedal (left) are you using? If it is a lot, then you don't have that much remaining if you need to stop a yaw if it develops. Can you make the same approach by turning a LEFT base-final rather than a RIGHT base-final? Could you make a shallower approach using less power rather than the steep approach?
• Don't get behind on the pedals. You need to anticipate the possibility of a right yaw and stop it as soon as you recognize it. If a pilot lets the helicopter get pushed around and develop a rapid right yaw before they make a pedal input, then you will either have a tough time stopping it (without an over-torque especially for turbine drivers) or will probably get into LTE.
• Be smooth on the controls, anticipate power changes. Think about how winds (mechanical turbulence) may be blowing off of buildings, trees, hillsides, creek drainages.
• Make left traffic if you have the option and keep your airspeed up on photo flights. The crash noted at the beginning of this article had the pilot making right turns around the object. The pilot seat in the Jetranger is on the right. It doesn't say how the camera was mounted or if it was a photographer in a seat, but left turns may have been an option. Our Schweizer C-models have the pilot on the left side and no options for mounting a camera on the nose or side, so unfortunately we end up making mostly right turns on photo flights. It's just something to consider...
• Fly a helicopter designed without a tail rotor.

How do I recover from LTE?

• Full left pedal. (probably already applied by now, along with some serious pucker-factor)
• Forward cyclic. (To increase airspeed. Forward airflow over the stabilizers and tail boom will help the tail fly correctly (streamlining) and provide some anti-torque effect.)
• Reduce collective if altitude permits. (reducing the collective reduces torque, but also sacrifices altitude)

How can I impress my instructor and the examiner?

Read the following articles:

• AC 90-95
• Rotorcraft Flying Handbook (FAA-H-8083-21), page 11-12 & following
• "The Origins of Loss of Tail Rotor Effectiveness", Dan Megna, Vertical Magazine, February-March 2005
• "Unanticipated Right Yaw at Low Speeds", Flight Safety Foundation, Vol. 15 No. 2 http://flightsafety.org/hs/hs_mar-apr89.pdf
• "Safety Watch: Loss of Tail Rotor Effectiveness", Tim McAdams, Rotor & Wing Magazine, Feb 1, 2007

As a side note, when I did research for this article, I found several interesting things in the NTSB Accident Data Base. In my search for "LTE" for all dates of all helicopters in all locations, I got 57 matching accidents. In the results, there appeared to be no correlation between LTE listed as the cause or contributing factor of the accident and the hours of experience of the pilot. The experience range ran the full spectrum from student pilot to ATP; those with lots of hours, to those with few. The funny thing, however, was that of the 57 matches, only 8 pilots were NOT dual rated....meaning that 49 out of the 57 pilots were also rated in airplanes. I will not attempt to draw any conclusions from my unofficial data survey and am not trying to stir the pot, just saying it's interesting....

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.