the helicopter in forward flight is more stable than in hover.
it has lower fuel consupmtion as well.
moreover, hovering at night in higher heights above terrain is difficult and to some extent angerous. you cannot refference your position to the terrain like you do during day. Obviously there are missions that require that.. like night time hoist or slingload operations, rapelling deployment of rescuemen, however if you can avoid hovering high in the night, you avoid it.
finally, like the other answerer said, if the helicopter hovers in certain height and engine fails, transition to autorotation may be beyond the capabilities of common pilot. because of that, the helicopter maintains certain speed to be out of the dangerous height-velocity combination. the dangerous combination starts at about 25 feet above terrain and ends at about 230 ft. above that, the pilot has plenty of time to commence autorotation and gain forward speed (since vertical autorotation requires more vertical speed than the forward flight autorotation, so you avoid the vertical one if practicable)
summed up, the way they do it, is the more efficient, less consuming and more convenient way.
flying SAR and HEMS.
· 10 years ago