text: In the case of most small helicopters, at 
  least 300 feet of altitude is necessary for an 
  average pilot 
  to set up a steady autorotation 
  and land the helicopter safely without damage. 
  This minimum becomes 500 to 600 feet for the 
  larger helicopters, and will be even greater for 
  helicopters with increased disc loading. These 
  characteristics are usually presented in 
  the 
  flight handbook in the form of a “dead man’s 
  curve” which shows the combinations of air- 
  speed and altitude above the terrain where a 
  successful autorotative landing would be dish- 
  cult, if not impossible. 
  
  is shown in 
  figure 6.18. The most critical combinations 
  are due to low altitude and low airspeed illus- 
  trated by area A of figure 6.18. Less critical 
  conditions exist at higher airspeeds because of 
  the greater energy available to set up a steady 
  autorotation. The lower limit of area A is a 
  finite altitude because the helicopter can be 
  landed successfully if collective pitch is held 
  rather than reduced. In this specific case there 
  is not sufficient energy to reach a steady state 
  autorotation. The maximum altitude at which 
  this is possible is approximately ten feet on 
  most helicopters. 
  
  A typical “dead man’s curve” 