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AUTO THROTTLES

Autothrottles are fitted to the 727-200 Advanced and the 727-200 Super 27. Earlier 727 versions are not autothrottle-equipped.

EPR



EPR stands for engine pressure ratio and is the air pressure at the tailpipe divided by the air pressure of the engine inlet.  Say for example the exhaust pressure is 1900 millibars and the inlet pressure is 950 millibars, then EPR can be found by:

1900 MB
  950 MB

which is going to equal 2.0.  Notice that the millibars cancel each other out, so EPR is a non-dimensional number.  With the engines off, the inlet pressure will be at ambient air pressure and is going to equal the exhaust pressure:

1013 MB
1013 MB

which will be 1.0 using the above example of standard atmospheric pressure.  So, any ratio over 1.0 is positive thrust, and anything under 1.0 is negative thrust.  Can you have negative thrust?  Yes.  You'll see this in high speed descents at idle thrust because the ram air pressure at the engine inlet will exceed the pressure of the tailpipe.  In other words, the engines are producing more drag than thrust.   Do you have negative thrust during reverse thrust?  No.  Thrust is still being produced at the exhaust, it is just being directed forward by the vanes.  

A bit of trivia - not every model of jet engine has sensors to report EPR, which is why you don't have EPR gauges on CFM56 powered planes such as the default 737-400.

On this panel, you'll see the EPR on the needle as well as the top digital readout.  The lower readout is your bug setting.  This bug setting is important as it serves as a reference for your autothrottles during takeoff.

The knobs do not pull out in this version, so you will always see the manual flag.

 

USING THE AUTO THROTTLES

I'm going to start out by saying that this is a very simplified system and does not work like the real thing.  On the real airplane, you must enter performance data into a little computer.  The throttle position is then calculated for required thrust.  I haven't got around to making this computer yet, so this step is skipped.

Using the autothrottle is simple - just turn on the switch.  It is somewhat intelligent and tries to figure out what mode you want when you turn it on. For takeoff, set you EPR bugs for takeoff thrust.  This will usually be about 2.20 for D-17 engines at sea level.  Also, set your airspeed cursor on the airspeed indicator for your initial climb speed, normally 250 knots.  Turn the auto throttle switch on.  The throttle levers will then move to apply takeoff thrust by 60 knots, then will hold position until you near your set speed.  After that, speed will be maintained.  Throttles will automatically retard at about 50 feet during landing, and the auto throttle switch will turn itself off at touchdown.  That's all there is to it.

When airspeed is acquired, the white manual flag on the airspeed indicator will disappear, and you can then be assured that the airspeed hold is on.  Notice that there is no way to set a Mach value.  You'll have to use airspeed.  Economy climb is normally about 280 knots while a performance climb will be about 310.  Above 25,000 feet you will stop using airspeed and start using mach.  Assuming you are using the performance climb of 310 knots (we don't really care about fuel economy in FS, do we?), you should reach your cruise speed of M.82 by 33,000 feet.  You may have to adjust the speed cursor up or down a little.