Refer to figures.
As altitude increases:

• The power required line moves up and expands to the right and
• The power available line moves down.

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Service Ceiling

Where there is an excess of power available over power required, a helicopter will be able to climb, until the point at which the rate of climb (ROC) drops at a specific value depending on the helicopter type.

This specific value of ROC determines the altitude of the "service ceiling" and a limited range of speeds occurs.

For a Twin Engine Turbine Helicopter (TETH), according to CAP 758 Figures 3.15 and 3.16, the service ceiling for both All Engines Operating and One Engine Inoperative is the altitude where the margin between power available and power required permits a rate of climb not more than 150 ft/min flying with V_Y (top left corner of each graph).

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Absolute Ceiling

If the helicopter continues to climb further, the rate of climb will decrease more until it reaches a certain altitude where the rate of climb is zero.

This corresponds to the altitude for which the power required and power available curves are tangential to each other (Power required = Power Available). This altitude is called absolute ceiling.

At this altitude only one speed is available and the helicopter is no longer able to climb at all.

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Aerodynamic ceiling is where the speed for the low speed buffet and high speed buffet are the same, coffin corner. This ceiling is met only in the airplanes that fly at very high altitudes.

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Crossover Altitude is the transition altitude between airspeed and Mach, also met in airplanes' operations.