Munk,
TDP has nothing to do with translational lift. In a normal forward twin-engine take off in older helicopters such as 412/S76/365 (to mention but a few) it's the point at which you have reached, or can accelerate to your Vtoss. This is the airspeed for a given weight and temperature at which the helicopter has reached a point on the Power required/TAS curve where it has enough excess of power available over power required to be capable of climbing on one engine at 100 feet per minute (with landing gear down, on wheeled helicopters) using 2 minute power (on aircraft having a 2 minute power rating). Then at a point, normally 200 feet, the helicopter can be accelerated to Vy (or Vbroc, depending on whether your RFM is American or European!) and climbed to MSA.
Perro Rojo,
As pointed out, the TDP
is an absolute after which the aircraft must be flown away in the event of the failure of a critical power unit
if you are operating from a heliport where the accel/stop distance for a given airspeed is critical. For example, you may be required to operate from a 1200 metre heliport and in order to operate Class 1 you have to restrict your take off weight for a given temperature because of this. If you were to have an engine failure after TDP and decide to reject, then have any of your passengers suffer injury, the lawyers would indeed be after you and have a good case. As you say, if you have a long runway ahead, then it's your call as to whether to reject, but as I say, I've seen a lot of pilots mess it up when the runway isn't quite so long, or they have commenced a take off from a point other than the threshold where they have no idea exactly what distance they now actually have available.
Remember also that TDP refers only to
engine problems in
multi-engined helicopters. It's entirely single engine performance thing after one has failed and has nothing to do with gearbox, tail rotor or other non-engine-related problems. The other point to bear in mind is that even if operating class 2, you are allowed to damage the aircraft when you land, but if you injure a third party outside, then the lawyers will also be after you
