Originally Posted by PIK3141
RTOM or RTOW, Regulated Take-Off Mass or Weight, is assessed as the most limiting of the Structural weight S, WAT weight W (Weight for Altitude and Temperature), Field Length Limited weight F, and Obstacle clearance weight O. Other factors such as Brake Energy limited weight, En-route clearance, and Landing Go-Around and Landing distance might reduce take-off weight.
The WAT weight is prescribed in BCAR, JAR, FAR etc as minimum gradients of climb in the 1st, 2nd and 4th segments of the take-off gross flight path. eg 2.4% gradient for a twin in the 2nd segment.
Obstacle clearance is required as 35 ft over the most limiting obstacle, using the Net flight path, which is the gross flight path penalised by prescribed gradients for each segment.
A lower flap setting gives better climb gradient but takes more take-off distance. A higher flap setting gives shortest take-off distance. But if obstacles are present, all these factors have to be evaluated for the altitude, wind and temperature of the day.
V2 is a function of the weight that comes out of the evaluation. So if you are weight limited to a lower value, the V2 is reduced to that required for the lower weight.
Is WAT weight independent of V2?
Airplanes on takeoff fly in conditions where drag decreases with speed. Therefore, at a given weight, increasing speed would allow better climb gradient... therefore, where the climb gradient is the minimum allowed by regulations, increasing the speeds in each segment would allow increasing weight?