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## Altitude Limitations 

If the rotorcraft has a maximum operating density altitude (see page 7-2), it is indicated in this section of the flight manual. Sometimes the maximum altitude varies based on different gross weights. 

## Rotor Limitations 

Low rpm does not produce sufficient lift, and high rpm may cause structural damage, therefore rotor rpm limitations have minimum and maximum values. A green arc depicts the normal operating range with red lines showing the minimum and maximum limits. [Figure 5-3] 

There are two different rotor rpm limitations: power-on and power-off. Power-on limitations apply anytime the engine is turning the rotor and is depicted by a fairly narrow green band. A yellow arc may be included to show a transition range, which means that operation within this range is limited due to the possibility of increased vibrations or harmonics. This range may be associated with tailboom dynamic modes. Power-off limitations apply anytime the engine is not turning the rotor, such as when in an autorotation. In this case, the green arc is wider than the power-on arc, indicating a larger operating range. 

## Powerplant Limitations 

The powerplant limitations area describes operating limitations on the helicopter’s engine including such items as rpm range, power limitations, operating temperatures, and fuel and oil requirements. Most turbine engines and some reciprocating engines have a maximum power and a 

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maximum continuous power rating. The “maximum power” rating is the maximum power the engine can generate and is usually limited by time. The maximum power range is depicted by a yellow arc on the engine power instruments, with a red line indicating the maximum power that must not be exceeded. “Maximum continuous power” is the maximum power the engine can generate continually and is depicted by a green arc. [Figure 5-4] 

Manifold pressure is a measure of vacuum at the intake manifold. It is the difference between the air pressure (or vacuum) inside the intake manifold and the relative atmospheric pressure of the air around the engine. The red line on a manifold pressure gauge indicates the maximum amount of power. A yellow arc on the gauge warns of pressures approaching the limit of rated power. 

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