Power-off approaches are made by gliding an aeroplane with the engine(s) idling to a selected point on the runway. The objective is to develop the skills required to execute a gliding approach from circuit height and land safely on a specified touchdown point with a degree of accuracy.
The ability to estimate accurately the distance an aeroplane will glide to a specific point will determine the amount of manoeuvring that may be required from a given altitude. With practice, heights up to 1000 feet above ground level can be estimated with fair accuracy by learning to associate the indications of the altimeter with the general appearance of the terrain. Above this altitude, the estimation of heights becomes less precise, as all features on the ground tend to merge.
Early on in flight training, during the practice of descents and, prior to the first solo, when conducting simulated engine failures in the circuit, selecting and maintaining the proper glide speed and angle is practised and reinforced.
Eventually, the judgment of heights becomes less important than the ability to estimate the glide angle. The pilot who knows the glide angle of an aeroplane can, with reasonable accuracy, estimate the approximate spot along a given ground path where it will land, regardless of altitude, and then judge how much manoeuvring is possible or required during the glide.