FADEC Overview

A FADEC - Full Authority Digital Engine Control is a Computer used to control an engine by altering fuel flow to the engine. A FADEC can take any number of inputs depending on its design and being a computer, can control any number of outputs, again depending on its design. Typical inputs to a FADEC are throttle position and sensed air / engine parameters. The basic use of a FADEC is to keep an engine within safe operating limits to protect it and to optimize its efficiency in differing air densities. X-Plane can simulate a variety of differing FADEC functionalities.

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FADEC Power Limiting

Turbocharged or Flat-Rated Turbine Engines are designed to provide lots of power at cruise altitudes, which means they can produce more power at sea-level at full throttle, which can destroy the engine, either through excessive heat or torque. In mechanical engine control designs (using linkages and cables), it is the pilots responsibility to keep an eye on the engine gauges and manage the throttle position accordingly to not exceed engine limits. A FADEC; however, can limit the engine output, even if the pilot advances the throttle all the way forward. There are three steps to configure a turbocharged recip or flat-rated turbine engine with a FADEC for power limiting:

1) Enter the flat-rated HP for turbines, or the nominal, sea-level HP for recips in the Power widget.
Standard > Engine Specs > Locations Tab

2a) If a turbocharged recip engine, enter the highest (critical) altitude where the engine will still make its nominally rated / sea-level power.
Standard > Engine Specs > Engines Tab > All Engines Panel

2b) If a flat-rated turbine engine, enter the flat-rating temperature provided by the manufacturer. If you cannot find the flat-rated temperature of your turbine engine, see this section of the manual.
Standard > Engine Specs > Engines Tab > All Engines Panel

3) Enable the checkbox: Engine Controller Prevents OverBoost.
Standard > Engine Specs > Engines Tab > All Engines Panel

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FADEC ITT/EGT Limiting

If you want your FADEC/controller to limit power output based on ITT or EGT limits, then there are two steps to configure each functionality:

1a) Enter an ITT limit
Standard > Systems > Limits 1 Tab

1b) Enter an EGT limit
Standard > Systems > Limits 1 Tab

2a) Enable the engine controller prevents ITT over-temp checkbox;
Standard > Engine Specs > Engines Tab > All Engines Panel

2n Enable the engine controller prevents EGT over-temp checkbox;
Standard > Engine Specs > Engines Tab > All Engines Panel

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FADEC Limit Params

In the same way that a FADEC can limit power, it can also limit engine operation based on other parameters such as N1, EPR, Thrust, Torque, ITT, or EGT.

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 PlaneMaker) 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 PlaneMaker, 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.

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Assumed temperature

For FADEC limited thrust and power modes, 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[8] and will reduce the power output if it is higher than the actual outside air temperature (it will not increase power if set lower).

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Plugin controlled FADEC

By reading the dataref sim/flightmodel/engine/ENGN_fadec_pow_req[8] 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[3] which corresponds to the selected targets in PlaneMaker, i.e. [0] is for the highest notch, [1] for the next lower notch if it exists, [2] for the notch below that if it exists. By writing the target value there, X-Plane will update the N1 or EPR targets sim/cockpit2/engine/actuators/N1_target_bug[8] or /EPR_target_bug[8] 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[8] dataref and control the engine directly. X-Plane will not provide any overboost over overtemp protection in this case.

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FADEC throttle Detent/Gates

As mentioned above, a primary input to a FADEC is the physical position of the throttle. For transport category aircraft, the throttle levers usually moves in an arc motion, and for FADEC controlled engines, the throttle lever angle is essentially a request for engine power and the FADEC itself will actually control the engine. Given this concept then, differing positions / detents / gates of the throttle levers can be mapped to differing engine operating modes such as "max power", "max continuous thrust", "climb power" or "flex power".

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.

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Philipp

The EGT limiting checkbox is required for limiting DYNAMIC EGT.

If you increase the power on a turbine, the following things happen: -more fuel flow causes an increase in EGT -the increase in exhaust causes the shaft to spin up -the increased air flow causes a decrease in EGT

You never see the second part on the TPE engine, because there is no increase in shaft RPM when in governing range. On a PT-6 you see this very noticeably because the inner shaft does spin up and increase airflow.

-the DYNAMIC increase in EGT is the spike caused by the temporary imbalance of fuel and air input. The more the throttle is ahead of the RPM, the higher the EGT. -the STATIC increase in EGT is what remains after the RPM has caught up and the engine has reached a new equilibrium.

The only thing that keeps the engine from melting itself due to dynamic EGT with rapid power application is the fuel control unit which limits the amount of fuel that can go into the engine at a given RPM.

That's what that box does. It limits how fast we increase the fuel flow, by limiting how far ahead of RPM we allow it to be. With very gentle throttle lever operation, you wouldn't even notice it's there.

Historically, it is the first type of controller that was built into engines: a N2 (or N1) governor where the throttle lever requests a N2 increase rather than a directly dumping in more fuel. The controller then carefully meters the fuel increase to let the RPM catch up to the requested value. It doesn't have an EGT sensor, it doesn't know what the ambient temperature or the EGT is, all it does is gently tweak the fuel flow in a way that it is safe to achieve the requested N1 or N2.

FADEC came much later and was built on top of that - FADEC will use the underlying controller to limit dynamic EGT, while it itself knows ambient temperature, pressure etc and knows to limit static EGT.

It follows that this box should be checked when making any type of modern engine, especially one that has FADEC on top of the fuel controller. While you could leave this box unchecked and install a FADEC, you are not protected against dynamic EGT over-temp. In practice, that might not be a problem, because your FADEC controller itself might be parameterized so conservatively that it changes throttle slow enough for this to never be a problem. But if you configured it very aggressively, you have no protection layer. Leaving this checkbox on underneath the FADEC allows you to configure the FADEC to be as reactive as it can be, without ever worrying about dynamic EGT, because the EGT over temp protection will make sure that you are always below redline during power increase.

Philipp