HEIGHT/VELOCITY DIAGRAM
A height/velocity (H/V) diagram, published by the manufacturer for each model of helicopter, depicts the critical combinations of airspeed and altitude should an engine failure occur. Operating at the altitudes and airspeeds shown within the crosshatched or shaded areas of the H/V diagram may not allow enough time for the critical transition from powered flight to autorotation. [Figure 11-2]
An engine failure in a climb after takeoff occurring in section A of the diagram is most critical. During a climb, a helicopter is operating at higher power settings and blade angle of attack. An engine failure at this point causes a rapid rotor r.p.m. decay because the upward movement of the helicopter must be stopped, then a descent established in order to drive the rotor. Time is also needed to stabilize, then increase the r.p.m. to the normal operating range. The rate of descent must reach a value that is normal for the airspeed at the moment. Since altitude is insufficient for this sequence, you end up with decaying r.p.m., an increasing sink rate, no deceleration lift, little translational lift, and little response to the application of collective pitch to cushion the landing.
It should be noted that, once a steady state autorotation has been established, the H/V diagram no longer applies. An engine failure while descending through section A of the diagram, is less critical, provided a safe landing area is available. 

 
Figure 11-2. By carefully studying the height/velocity diagram, you will be able to avoid the combinations of altitude and airspeed that may not allow you sufficient time or altitude to enter a stabilized autorotative descent. You might want to refer to this diagram during the remainder of the discussion on the height/velocity diagram.
You should avoid the low altitude, high airspeed portion of the diagram (section B), because your recognition of an engine failure will most likely coincide with, or shortly occur after, ground contact. Even if you detect an engine failure, there may not be sufficient time to rotate the helicopter from a nose low, high airspeed attitude to one suitable for slowing, then landing. Additionally, the altitude loss that occurs during recognition of engine failure and rotation to a landing attitude, may not leave enough altitude to prevent the tail skid from hitting the ground during the landing maneuver.