Gas Turbine, Subsystems V
In WW-II and shortly thereafter, piston powered aircraft peaked in power, performance and complexity wise. Power went up to over 4000 bhp for large multi-row radial engines. Only to be defeated by the jet, which was developed by (among others) Germany's Dr. Hans von Ohain and separately in the UK by Sir Frank Whittle. Its principles are based on the "Aeolipile" of the ancient Greek scientist Hero and other great thinkers like Leonardo da Vinci and the laws of Isaac Newton.
Compared to a piston, the gas turbine has less parts and the moving parts rotate in only one direction without stopping and accelerating as the pistons normally do in a engine. Thus, a running gas turbine is basically free of the vibrations normally found in piston models, which translates in much longer service life (TBO) and higher reliability.
The crew needs proper instrumentation to monitor the performance of their engines. Even more so when operating gas turbine engines as damage is easily done by an unintentional hot start and the very high cost to repair them.
For a long and reliable service life of the gas turbine engine it needs proper monitoring by the flight crew. Not only monitoring is important, data logging to keep records of past performance and to detect any trend in the future performance of the engine is of interest to maintenance controllers to keep operating cost low.
RPM is not given in an absolute value but in a percentage of maximum and also for each spool in the engine. This is indicated in N1, N2 and so on. Most systems use an AC alternator where RPM is related to the frequency generated and not influenced by voltage drops due to long wiring.
Thrust is measured by reading the inlet and exhaust pressure and indicating this as a ratio to the crew. Engine pressure ratio (EPR) is taken at a central inlet (P1) and at another engine station, for example 7 (P7).
Turboprop aircraft indicate power by measuring engine torque, its turning moment. Torque is proportional to engine horsepower. Torque is measured indirectly by an oil pressure piston at the reduction gears. Torque indication may also be used for automatic water injection systems or to initiate propeller auto feather on takeoff.
Oil pressure and temperature
There are indicators for each engine in the cockpit similar to those used in piston engines. A low pressure warning light is usually included and should not be lit during engine operation.
Principle of a Gas Turbine Engine
EGT and ITT
Exhaust gast temperature is an indirect indication of Turbine Inlet Temperature (TIT , TET or ITT). For a long service life of the turbine blades and burner cans these temperatures may not exceed manufacturer limits under all circumstances.
Measured in weight over time (lbs/h or kg/h) indicating the fuel consumed by the engine. And will also include indication for total fuel used so that this can be compared to the amount of fuel taken on board. A sensor is installed in the low pressure fuel line and it consists of a small vane which is rotated by fuel flowing past it.
Fuel pressure and temperature
Gives an overall idea to the crew of the health of the fuel system and possible pending blockages due to water in fuel. Not always installed on aircraft.
Multiple vibration detectors are installed at several locations in the engine to monitor vibration. An increase in any type of vibration deviating from the norm could be an indication of a pending failure.
Engine / propeller synchronization
This is accomplished through special frequency generators where the frequency is directly related to RPM and with this signal the engines can be synchronized to the exact RPM and blade position. If engines are out of sync the difference in RPM is audible as a beat or drone and this will result (after some time) in irritation and very is tiring to crew and passengers. Manual adjustment is usually available to the crew.