For a helicopter pilot, one of the more difficult anti-torque system failures to deal with is when the tail rotor thrust becomes fixed or limited to a certain amount. This could happen if something jams or blocks the pedals or the associated linkage.
In flight, the pilot needs to determine at what position the pedals became stuck. In a counterclockwise turning rotor the more power a pilot is using, the more left pedal input is required. In this case, the left pedal is often called the power pedal. Should something jam the pedals during a high power take off or at maximum cruise speed, the tail rotor will be producing a lot of thrust.
The following illustrates the challenges of performing this type of emergency landing. As the pilot slows the helicopter to attempt to land, the helicopter approaches its most efficient airspeed (normally about 60 knots). The pilot must reduce power to prevent the helicopter from climbing. This would normally require adding right pedal, but since this is not possible the nose will start to yaw left and if airspeed gets too slow the helicopter will start spinning. The only way to stop the left yaw or spin is to add power, but that makes the helicopter climb and that’s not good because the pilot needs to get close to ground to land.
Some instructors have different techniques to land with a stuck left pedal. One method is to approach the longest runway available at cruise speed. This keeps power high and the helicopter pretty much in trim. Since the nose is trying to turn left, the tail wants to move right so finding a runway with a right crosswind will help the vertical fin oppose the left turning motion. Once over the runway, slowly start to decelerate with aft cyclic. As the helicopter’s airspeed decreases the pilot will need to reduce power. Lowering the collective should be done as carefully as possible as the nose will begin to yaw left. As the airspeed continues to drop below 60 knots the airflow over the vertical fin will at some point no longer be able to prevent the helicopter from spinning. The good news is that as the helicopter continues slowing below 60 knots more power is needed. Timing is critical as the pilot needs to keep adding power to prevent the helicopter from spinning, but can’t add too much power or the helicopter will climb. If all goes well and the pilot is able to get the helicopter to a very low hover with little or no left spinning, he or she will have the best chance to put it on the ground without rolling over.
A stuck right pedal makes it a little easier to land because in this case the pilot needs to keep power low. A common low-power landing maneuver in a helicopter is called a running landing. Since hovering requires more power, the pilot would touchdown on a flat smooth surface (a runway for example) with forward speed allowing the helicopter to slide to a stop. It must be performed carefully and is a maneuver that student pilots practice.
Different helicopters and situations will require different procedures. For example, in a clockwise turning rotor the same concepts apply, however the yawing direction and pedal inputs are reversed, as the right pedal is the power pedal. When provided, the manufacture’s recommended procedure should be followed.
Tags: Tim McAdams