The problems of noise on the ground coming from aircraft on the ground, while still not simple to resolve, are rather less difficult than those associated with noise coming from aircraft in flight. The source of noise—the aeroplane—is no longer travelling rapidly through the air with three degrees of freedom but is either stationary on the ground or taxying at a comparatively slow speed across an airport. In the first case, the aeroplane is at rest, either for a short period when the pre-take-off check is being performed near the end of the runway, or for a much longer time while a post-servicing engine check is being made in a maintenance area. During each check, up to maximum engine power may be used, although for jet aircraft the pre-take-off check is relatively short and appears to be becoming part of the take-off itself. In the maintenance area prolonged running is likely and will consist of operation at a medium engine power with occasional bursts of maximum power. Of the three ground cases, this is the one on which attention has to be focused. Indeed, it is not unreasonable to confine an examination of the problems of ground-to-ground noise to this and to this alone.