A ULY condition can normally be differentiated from a loss of tail rotor thrust by the rate of yaw acceleration experienced.

• A. False
• B. True
• C. Not Enough Information
• D. Cannot Tell

Answer: (B)

Yaw dynamics reveal the difference between an uncommanded yaw (ULY) and a loss of tail rotor thrust.

When yaw is uncommanded, the helicopter experiences an unexpected, uncontrolled moment that causes a rapid change in yaw rate—high yaw acceleration—as the airframe and rotor system respond to the sudden imbalance. This abrupt, accelerating motion is a distinctive sign of ULY because there is no deliberate input directing the yaw and the system must contend with the uncommanded moment.

In contrast, a loss of tail rotor thrust leaves the main rotor torque unopposed and the aircraft develops yaw more gradually. The resulting yaw behavior tends to be slower to rise and more predictable once the unbalance is established, especially as the pilot or the flight control system applies corrective inputs.

Thus, observing the rate of yaw acceleration provides a practical way to tell ULY apart from tail rotor thrust loss under typical flight conditions.