Limited Power Operations For Helicopters
Performance planning is an important part of flying a helicopter. Under certain atmospheric conditions the power available from the helicopter may be limited to the point where a normal takeoff or landing may not possible. During flight training you will practice techniques to land and takeoff in these conditions with your instructor restricting the amount of power available.
We usually find ourselves limited with power when the weather is hot and/or when operating at high altitude. High humidity also decreases air density but is not included in helicopter performance charts.
As air pressure decreases with an increase in altitude the air becomes less dense. Air density also decreases when an increase in air temperature causes the air molecules to spread out. Because drag and thrust of the main rotor blades are directly proportional to the air density, neither of these things are good for helicopter performance.
The best way to imagine this is to think of the main rotor system having less air to work with and the engine unable to provide enough power for normal operation.
It is very important that you know the margin of power available and power required before you commit yourself to a takeoff or landing. There are two set procedures below that I used to use when teaching this lesson.
1. Power Check for Landing
The power check should be carried out at a safe altitude above the intended landing area.
Fly the helicopter straight and level at forty knots of airspeed (Forty knots of airspeed is where the helicopter uses the least amount of power.) and note the manifold pressure. Maximum power is now applied while straight and level flight is maintained.
The difference between these two figures is the power margin available.
During the approach the decision has to be made whether to complete the approach or abort and go around. 2" of reserve power should be kept in hand and translational lift maintained until you have reached your decision point. The decision point will depend on obstructions on the approach and departure paths, wind, turbulence and nature of the landing area.
2. Power Check for Takeoff
Hold the helicopter in a two foot hover and note the manifold pressure. The difference between the maximum obtainable manifold pressure and the power during the hover is the power margin available.
Common errors when operating with limited power.
Over controlling uses more power. Use smooth control inputs while attempting operations with limited power.
Visit http://helipilotcareers.com for more information on helicopter safety and starting a career as a commercial helicopter pilot.
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