What effect does altitude have on the airspeed indicator reading in a helicopter?

The airspeed indicator in a helicopter is affected by altitude due to the way it measures dynamic pressure. As a helicopter ascends to higher altitudes, the air density decreases. The airspeed indicator operates on the principle of measuring the difference between static pressure and dynamic pressure. At lower densities, a given indicated airspeed corresponds to a lower true airspeed because there are fewer air molecules impacting the rotor blades and the aircraft structure.

When flying at higher altitudes, the aircraft needs to achieve a higher true airspeed to maintain the same amount of lift generated by the rotor system. Consequently, if the indicated airspeed remains the same as it was at lower altitudes, the helicopter is actually moving more slowly relative to the surrounding air than the indicated value suggests. This discrepancy results in the airspeed indicator reading lower than it should in terms of true airspeed at higher altitudes. Hence, the correct answer reflects the nature of how the airspeed reading changes with altitude.