Refer to figure.

The performance capability of a helicopter depends on the relationship between the power available and the power required. The greater the power margin, the better the performance.

Depending on how much power is available due to the helicopter's mass, altitude or temperature, it may not be able to hover Out-of-Ground-Effect (OGE), but only (In-Ground-Effect).

1. Point A (at 0 TAS) is above the power available line, so there is a power deficit and as a result hover OGE is not possible. Point B (at 0 TAS) is below the power available line, so there is a power surplus and as a result hover IGE is possible.

Also, the helicopter cannot transition normally to forward speed, as at low speeds close to the translational lift there is a power deficit, and an alternative take-off procedure may be required, such as Cushion Creep take-off.

2. Points A and B (at 0 TAS) are both below the power available line, so there is a power surplus and as a result both hover IGE and OGE are possible.

Also, the helicopter is able to transition normally to forward speed, as at even low speeds close to the translational lift, there is a sufficient power surplus.

3. Points A and B (at 0 TAS) are both above the power available line, so there is a power deficit and as a result neither hover IGE nor OGE is possible.

4. When the power available is less than the power required at any speed, then the helicopter is not able to maintain its altitude and it enters into a "driftdown" procedure.

A "driftdown" is usually referred to an engine failure condition, where the helicopter is not able to maintain its altitude with the remaining engine(s) at Maximum Continuous Power (MCP) setting. As the helicopter descends, there is an altitude where both powers (available and required) meet each other again and the helicopter stabilises.