Power limits

August 1, 2013 by Tim McAdams

Helicopters powered by normally aspirated piston engines use manifold pressure as an indicator of power levels. Typically, pilots calculate limit manifold pressure for each day which is the maximum power setting allowed by the helicopter’s manufacturer. It is not necessarily the maximum rated horsepower limit for the engine. In many cases, the helicopter manufacturer de-rates the engine to reduce internal stress levels and extend TBOs. However, the pilot can exceed the limit manifold pressure (depending on factors like air density etc…) and still have available power.

In a gas turbine engine, the pilot must monitor three different indicators. Turbine outlet temperature (TOT) which refers to the temperature of the gas as it is exiting the engine, when the ambient air temperature is high this can be a limiting factor. Another is torque, which refers to the amount of torque the engine is applying to the transmission and is normally shown as a percentage. The third one is gas producer rpm, referred to as Ng or N1. When the air density is low, this section of the engine can reach its maximum operating rpm because it needs to spin faster to move the same amount of air.  A pilot of a turbine helicopter must monitor all three of these gauges and stop adding power when the first one reaches its limit.

Eurocopter uses something called a first limit indicator (FLI) to simplify the monitoring of all three parameters. One large gauge with a fixed yellow arc (indicating take-off power range) monitors all three parameters. So when the pilot adds power and the needle enters the yellow arc, then one of the three parameters has exceeded its maximum continuous power limit. To the right of the gauge, are the three values shown digitally (TOT, torque, Ng) and whichever one is the limiting value will be underlined in yellow.