Engines

[Rachel]

Firstly, please understand, I am not a pilot.
In aircraft with turbofan engines, how does the pilot control the engine? I know there is the throttle, and cutoff values, but what instrumentation do you have telling you about the engine? Does anyone know how the controls and instrumentation are actually connected to the engine?
Thanks

[foghorn]

Rachel,

I'll have a go. I don't fly anything turbojet powered yet, I've done all the theory.

The basic engine instruments which can be used for working out how much power the engine is producing (the 'primary engine instruments or power-setting instruments') are:

N1 - the RPM of the fan. These days this is usually measured as a percentage of a certain rated value. It is measured by a detector which consists of a permananent magnet on one side of the fan and an induction loop on the other side. As each metal fan blade passes between the magnet and the loop it disturbs the magnetic field which is detected in the induction loop. By dividing the rate of disturbances by the number of fan blades, you get the RPM, which is then compared to reference value (which is close to maximum engine power) to give a percentage.

EPR - Engine pressure ratio - the ratio of the pressure of the gases coming out of the back of the engine to the pressure of the gases at the inlet. This is measured using piezo-electric crystals - crystals whose electrical properties change with pressure.

The secondary instruments which are used to show that the engine is running correctly and safely are:

EGT which is the temperature of the gases at the high-pressure turbine inlet (which is not the hottest part of the engine but is the part most sensitive to temperature). This is measured using a thermocouple - basically two different metals bonded together electrically which produce an electrical voltage proportional to the temperature.

N2, N3 - the RPM of the other spool(s) of the engine measured the same way as N1 is.


On most modern aircraft these are connected to a computerised engine system (called EICAS/ECAM dpeending on which make you fly), and the computer displays the parameters on cathode ray tubes, rather than on traditional mechanical dials.

I may be wrong in some of the finer details here, I'm sure some of the more learned people here will correct me.

Hope this helps,
cheers!
foggy.

[Pielander]

If you're really interested, I can recommend "The Jet Engine", published by Rolls-Royce. There's very little technical jargon, and where there is, it is explained very nicely with pretty colourful pictures.

What Foggy said sounds pretty good to me. There are also fuel flow meters and vibration meters (for safety purposes). Most of the engine management that used to be done by the Flight Engineer is now done by computer (hence FE's are a dying breed). I am told that the throttle is pretty much it as far as engine control goes, and jets are supposed to be far simpler to fly than a piston aircraft with a VP prop. I would like to make that comparison some day...

Pie