• Project Magenta - High End Flight Simulation

Throttle Hardware and Cutoff Switches

June 9, 2011 by Enrico Schiratti

In the past few days I have been working on trying to reproduce a problem that had been reported, when during engine startup the N2 value was doing strange back and forth movements (rising and dropping) when the engine start was being initiated for either engine.

This is with pmSystems running and throttle hardware connected.

It took me some time to realize that this was due to the definition of the FS cutoff switches vs. the pmSystems cutoff switches.

As a premise, FS uses these offsets to set the fuel cutoff valve

0890 2 fsCutoff1 //word 0 – 16383
0928 2 fsCutoff2 //word 0 – 16383

This will start the engines (if the starter is activated) no matter what, especially in FS2004. It does not reflect any bleed pressure and other conditions.

When you use pmSystems, it is this program which sets those variables (or offsets if you wish) and it makes things conditional. So you would need to use the following bits to set the switches:

561A .0 Cutoff1 //(aka cutoffL in the 737)
561A .1 Cutoff2 //(aka cutoffR in the 737)

In the pseudocode (simplifying pmSystems) it looks like this:

if Cutoff1
     if all_conditions_are_met
          fsCutoff1 = 16383
          fsCutoff1 = 0

Thus, your throttle hardware should have the option (in the driver) to write to another offset than the default FS (e.g. fsCutoff1) offset, otherwise there will be a conflict between what pmSystems thinks the position of the fuel cutoff switch is and what the throttle hardware position is.

So *without* pmSystems your hardware has to write to the fsCutoff1 offset.

With pmSystems running, it has to write to Cutoff1, and then pmSystems writes to fsCutoff1.

Thomas Richter pointed out that in addition to this you can also set the following option in pmSystems:

Press [Esc] for the Menu, select User Options and check “Cutoff Lever has a Pot and NOT a switch”

Then pmSystems doesn’t take control of fsCutoff but still displays the switches and the FUEL/SPAR VALVES correctly depending on fsCutoff value.

This logic is in the Cutoff section of the B737 file, it starts with “if Poti = 0… if cutoffl”

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