Waypoints PilotBooks Vol 11(H) - BTK(H) - September 2025

Just imagine, you are a molecule of air minding your own business, when all of a sudden you get sucked into the intake of the turbine engine of a passing helicopter.
Over the next 40 thousandths of a second, you and your mates get squeezed through many stages of compressor, heated in the blast furnace up to around 2000°C, get the energy sucked out of you through the turbines and then spat out the back, with an almighty headache. All in the interests of driving a helicopter rotor system.
That’s effectively what happens many times every day, all over the world, as thousands of turbine engines go about their business.
The principle of jet propulsion is demonstrated every time someone blows up a  balloon and before tying off the opening they let it go. The air, compressed in the balloon, rushes out the narrow hole, thrusting the balloon around the room.
The original turbojet engines, which were developed in the 1930s and 40s, and debuted in commercial service in the 1950s, effectively accomplished the same trick as the balloon, but on an amazing scale and with pinpoint control. The progress of turbine engine technology in the subsequent seventy or so years has been about increasing the amount of air processed by the engines and accelerating that air by ever increasing amounts and with ever increasing efficiency.
The propulsion process in a turboshaft engine begins with the guidance of the air into a spinning compressor. As the air passes through each of the compressor stages both its pressure and its temperature rises.
When compression of the air is complete, the air, now many times higher in pressure and about 300°C degrees hotter, is fed into the furnace which is the combustion chamber. Fuel is added and the whole mixture is burnt. The gas temperature soars, the velocity of the gas increases enormously and the gas is now ready to do the two jobs it was so hastily prepared for.
The first job of the gas is to blast past the blades of the gas generator turbine wheels, sending them spinning like a child’s fan in the wind.  The spinning turbines turn the shafts that drive the compressors and the accessories of the engine.
The second job of the gas is to then blast past the blades of the power turbine wheels, again spinning them to turn the shafts that drive the helicopter’s main rotor gearbox.
The purpose of this manual is to further explore these processes, and to help to prepare you to safely and efficiently operate helicopters powered by turbine engines.  Finally, and quite reasonably, this text is designed to help you to prepare for the ASPEQ Basic Turbine Knowledge (Helicopter) examination.