Density altitude refers to the altitude at which an aircraft or helicopter is effectively operating in terms of air density, which is affected by factors such as altitude, temperature, and humidity. An increase in density altitude essentially means that the air is less dense, which has important implications for helicopter performance, including the amount of anti-torque pedal required to maintain heading in a hover.

When density altitude increases (i.e., the air becomes less dense), the helicopter's rotor system has less air to bite into and generate lift, and the rotor becomes less efficient. This has several effects on the helicopter's behaviour, including its need for anti-torque pedal:

Increased Torque Requirement: In a hover, the main rotor generates torque that tries to spin the helicopter in the opposite direction of its main rotor. To counteract this torque and maintain heading, helicopters use the tail rotor, controlled by anti-torque pedals. When the air is less dense (higher density altitude), the rotor system's efficiency decreases, resulting in reduced effectiveness of the tail rotor's counter-torque force. As a result, more anti-torque pedal input is required to compensate for the decreased effectiveness and maintain heading.

Reduced Thrust Margin: The decreased air density affects the rotor's ability to generate thrust efficiently. This means that the helicopter's thrust-to-weight ratio decreases, making it harder to maintain altitude. To compensate, the pilot may need to increase collective pitch (lift) to maintain altitude, which can further increase torque and the need for anti-torque pedal input.

Limited Engine Performance: The helicopter's engine performance can also be affected by density altitude. As air density decreases, the engine's power output decreases because it's receiving less oxygen for combustion. Reduced engine power can result in lower rotor RPM, further impacting the effectiveness of the tail rotor and increasing the need for anti-torque pedal input.