X-Plane can limit the power output of altitude engines (or flat-rated engines) in order to not overtemp or overtorque them. In addition to thrust or torque limiting, X-Plane 12 can also limit to N1 or EPR values.
Setting critical altitude or flat-rated temperature
Configuring an altitude or flat-rated engine starts with entering the rated power or thrust and then entering the critical altitude or flat-rated temperature. Pressure altitude is used on reciprocating engines while sealevel temperature is used on jet engines, but that’s merely a convention. Both are just different ways to express the air density up to which the engine will deliver full rated power or thrust without exceeding temperature limits. Without FADEC, if you opened the throttle/thrust lever all the way at sealevel and standard temperature, the engine would deliver way more than its rated power, along with exceeding its maximum ITT or EGT. If the “FADEC prevents engines from exceeding max allowable power” is not checked, the pilot must exercise caution advancing the throttle/thrust lever as below critical density altitude nothing prevents the engine from exceeding its limits.
FADEC prevents overboost below critical density altitude
In the simplest setup, with just having the checkbox checked, X-Plane will simply limit the power output when the throttle/thrust lever is all the way forward. Maximum power/thrust will not be exceeded up to critical altitude or corresponding temperature, over which the power will gradually fall off.
FADEC target paramaters and temp limiting
X-Plane 12 adds a few new options for a finer control over which parameter the FADEC limit controls. Configuring this starts with going to “Boost and Controllers” settings for the engine and configuring a thrust lever angle (TLA) expressed as a fraction of full forward, for FADEC to limit. In the simplest case, only a TLA of 1.0 is configured, in which case the FADEC will limit the maximum thrust setting. X-Plane will interpret any setting between 0.975 and 1.0 of TLA as being in the FADEC limited maximum thrust setting, though the precise angle for the throttle hardware can be overridden by a custom joystick curve to fit the user’s throttle control.
Next, the author chooses a limiting parameter the FADEC uses as primary parameter. This can be either be a direct indication value N1 or EPR, or a synthetic value of power or thrust. For the selected TLA, the author then enters the desired N1, EPR or power/thrust ratio for sea-level standard temperature conditions. X-Plane will modulate the actual target value at runtime for non-standard temperature and higher altitudes.
X-Plane 12 will always use the configured max EGT value (as set in the Limits tab in Plane Maker) as the secondary parameter that is limiting the engine. At runtime, X-Plane will actually limit the power/thrust output of the engine for the desired power and maximum EGT, whichever is lower and therefore reached first.
To achieve the target value, X-Plane 12 uses a PID controller that can be configured on the autopilot parameter configuration screen of Plane Maker, as it is the same controller that is used if the auto throttle is used to achieve an N1 or EPR target. Follow the same guidelines laid out in the article about autopilot parameter configuration for configuring this PID controller.
For power/thrust based FADEC, the outside air temperature can be overriden with a higher “assumed temperature” that will cause artificially lower power output by making the controller believe it is operating in less dense air than it actually is. This will cause the engine to run cooler and quieter, and is thus preferred for take-offs in situations where less than full rated power is required. Assumed temperature can be set by the dataref
sim/flightmodel/engine/ENGN_assumed_temp and will reduce the power output if it is higher than the actual outside air temperature (it will not increase power if set lower).
FADEC notches/gates on the power lever
Besides maximum output, X-Plane 12 allows configuring up to two more, lower power settings that can be governed by FADEC. Once one TLA is configured to be non-zero, the next lower TLA can be configured to represent a setting for MCT, FLEX or CLB power, depending on airplane type. The minimum distance required to the full power setting is 3% of total thrust lever angle. Another gate or notch can be configured below that, for CRZ or CLB power depending on type. Again, if left at 0 no gate or notch will exist in X-Plane and the thrust lever will not stick. If configured non-0 and at least 3% below the next higher TLA, the thrust lever will stick to the gate/notch.
It is important to select the TLAs for the throttle lever to stick so the keyframes match the animation in the 3D cockpit. That will cause natural interaction with the manipulator in the 3D cockpit. For hardware throttle quadrants, users can override the ranges of the FADEC gates to match their specific throttle hardware by configuring a joystick axis curve for the throttle. This way, one type of hardware throttle can be used with different virtual aircraft.
Plugin controlled FADEC
By reading the dataref
sim/flightmodel/engine/ENGN_fadec_pow_req plugins can find out the currently selected notch or gate the thrust lever for each engine is in. They can then override the current target power/thrust, N1 or EPR by writing the desired target to the corresponding spot in the array
/ENGN_fadec_targets which corresponds to the selected targets in Plane Maker, i.e.  is for the highest notch,  for the next lower notch if it exists,  for the notch below that if it exists. By writing the target value there, X-Plane will update the N1 or EPR targets
/EPR_target_bug and the PID controller will actuate the throttle to achieve the plugin request, but still keep the engine below temperature limits by obeying maximum EGT limit. If this is also undesired, a plugin can resort to using the override throttle dataref
sim/operation/override/override_throttles and use their own controller to set the
sim/flightmodel/engine/ENGN_thro_use dataref and control the engine directly. X-Plane will not provide any overboost over overtemp protection in this case.