X-Plane simulates governors for constant speed propellers that can have various failure modes. Depending on the type of engine/propeller combination on the aircraft, the behavior of the governor in case of an engine failure will be different. X-Plane 11.30 allows you to select the type of governor to simulate, to accommodate a wide range of different engine types.
Auto-feather vs governor failure modes
The first thing to understand is that failure modes of governors are conceptually different from an auto-feather system. Normally, a propeller governor will have a certain behavior when an engine stops running, which will cause a loss of oil pressure to the governor. This is purely mechanical, and will work regardless of electrical power or other systems in the airplane. An active auto-feather system is different: It electronically senses engine torque and can act on unexpected partial loss of engine torque, even when the engine is still providing some power and oil pressure.
Auto-feather
X-Plane has simulated an auto-feather system comparable to those found on Beechcraft KingAirs for a long time now. To equip your aircraft with this system, check “feather after engine failure” on the Engine page in Plane Maker. The system is armed with the
sim/cockpit/switches/auto_feather_mode
switch. Note that this system is electrical, and will act on the sensed torque, which in turn can depend on a working inverter, if so selected in Plane Maker. When the torque sensors are working, the system itself has power, and the switch is in the armed position, the system will monitor the torque of both engines, if the thrust levers are advanced more than halfway. You will see the respective lights
sim/cockpit/warnings/annunciators/auto_feather_arm[]
for each engine light up when you’ve throttle up enough and the system was armed.
Now, if one engine drops below the disarming torque (400ft-lb on the KingAir) the opposite side auto-feather will dis-arm (to avoid feathering both engines at the same time!). If on-side torque continues to drop below activation torque (260ft-lb on the KingAir), this engine is automatically feathered, regardless of other parameters like oil pressure. Now the system has done its job and is dis-armed. The absolute torque values depend on the set engine power and will scale accordingly with larger engines.
Negative torque sensing
This is a different form of Auto-feather. NTS is found on geared-turboprops, where the propeller is directly driven off the same shaft as the compressor. In X-Plane, it is active when the plane is equipped with a fixed-gear turboprop engine and a propeller that has start locks. Then, during flight the propellers are monitored for negative torque, which would be an indication of an engine failure. The prop pitch is automatically adjusted towards coarse when negative torque is detected. Like on a real Honeywell TPE-style engine, NTS has no switch to be turned on or off, and only needs oil pressure to work, which can be tested during engine start (by running the unfeathering pump during start and watching the beta lights). The NTS will retard the prop to the maximum blade angle allowed for the governor. The pilot can engage full feather by pulling the prop speed lever to the feather position.
Governor failure modes
Independent from an active auto-feather system, constant speed props have different behavior after an engine failure, which can be selected on Engine Page in Plane Maker. These are purely driven by loss of oil pressure that accompanies the engine failure, and do not depend on any sensors or electrical system to act.
- Fail to Fine: This is the default in single-engine piston aircraft with constant speed propellers. The propeller is mechanically or aerodynamically driven to fine pitch (high RPM) while the governor uses oil pressure to coarsen the pitch (lower the RPM). Upon loss of oil pressure, the propeller will go to the low-pitch (high RPM) stops.
- Fail to Coarse: This is the default in twin-engine piston aircraft. The propeller defaults to the coarse pitch (low RPM) position and uses oil pressure to achieve finer pitch angles. Upon loss of oil pressure, the prop goes towards coarse pitch, but not necessarily feather (which is just extremely coarse pitch), because on piston twins the propellers are usually equipped with anti-feathering springs that will keep the prop from feathering if the engine is shut down from idle RPM, unless you actually pull the blue knobs all the way back to feather.
- Fail to Feather: This is the default for free-turbine turboprop aircraft, single or twin. Upon loss of oil pressure, the propeller goes towards coarse pitch all the way to feather. This also happens on shutdown, because a free turboprop engine can start against a feathered prop with no problem, as the core turbine is not locked to the propeller.
- Fail to Start Lock: This is the default for fixed-shaft turboprop engines, which can’t start against a feathered prop. Upon loss of oil pressure, the propeller will want to go to feather, but there are mechanical locks that need to be engaged for regular engine shutdown, that will keep the propeller in the fine pitch position for shutdown.
- To engage start locks: During engine shutdown, with shaft RPM below idle but still high enough to produce sufficient oil pressure, pull the thrust lever into reverse.
- To disengage start locks: With the engine running and shaft RPM at or above idle, pull the thrust levers into reverse.
- To engage start locks when the engine is shut down and the prop is not locked already: Engage the electric unfeathering pump for the respective prop. This will drive the prop into the beta pitch and the start lock will engage if the power levers are retarded far enough for the locks to catch.
At runtime, the selected system is reflected by the dataref
sim/aircraft/engine/acf_prop_fail_mode
where the number corresponds to the above list. If you have equipped your aircraft with start locks, the data refs
sim/cockpit2/engine/actuators/start_lock_engaged[N]
tells you the status of the start locks, and with
sim/cockpit2/engine/actuators/unfeather_pump_running[N]
you engage the unfeathering pump.
sim/cockpit2/annunciators/beta
and
sim/cockpit2/annunciators/prop_beta
are the beta lights as array and bitfield respectively, which really tell you the oil pressure towards beta pitch. These lights will tell you when the oil pressure is sufficient to sustain beta, provide a visual cue for when the prop transitions to reverse, and also show you when the NTS system drops the pressure when performing a preflight test of the NTS system.
Overspeed governor
Free-turbine turboprops with fail-to-feather governors have another governor that is set to 104% of maximum governed RPM, which cannot be reached in normal operation. In case of a primary governor failure, the overspeed governor will try to coarsen the prop pitch as to not exceed 104% of redline RPM. That means, in order to experience an overspeed beyond 104% you not only need to fail the governor, but also fail the prop pitch control to fine pitch.
Fuel-topping governor
Free-turbine turboprops with fail-to-feather governors also come with a second level of protection, which keeps the engine from overspeeding in case of a prop governor and overspeed governor failure (which you can experience in X-Plane). In case the redline RPM is exceeded, the fuel topping governor will reduce the Py pressure to the fuel control unit, thereby causing the gas generator turbine to create less power. That will reduce the power to the prop and slow it down, even if both the primary and the overspeed prop governor are failed or the prop blades are stuck at a low angle. Note that the fuel topping governor kicks in earlier in beta mode than it does in alpha mode, to prevent the overspeed governor from reducing blade angle in reverse, which would make the situation worse.
Underspeed governor and fuel delivery control
Fixed-shaft turboprops equipped with start-locking propellers are equipped with an underspeed governor that regulates the idle fuel supply to keep RPM up in the governed range in alpha operation, and also modulates the idle speed of the engine in beta and reverse modes, to keep the engine at the speed selected by the prop speed lever when prop blade angle is modulated through beta and reverse. It is important to set two parameters in Plane Maker for this to work correctly: “minimum prop governor RPM” is the lowest RPM selectable by the prop speed lever, which is usually 65-75% of the engine design (maximum) speed. “bottom of green arc RPM” is the lowest RPM selectable in flight/alpha mode, the lowest that can be achieved by the prop governor. It also serves as the floor for the underspeed governor in alpha mode. This is usually 96% of design (maximum) engine speed. See the article on fixed turboprop governors for more information.
Unfeather accumulator
To bring a feathered and stopped propeller out of feather back into alpha pitch, oil pressure is needed to move the prop blades. This oil pressure is typically provided by the oil pump of the engine, as it is rotated by the starter motor. However, the prop can also be unfeathered in other ways:
- A fixed shaft turbo prop engine with start-locking propeller can be unfeathered with the electrical unfeathering pump providing the oil pressure. Note that the condition lever must not be in the cutoff/feather position, otherwise the oil-pressure is immediately dumped through the feather valve
- Any other propeller can be equipped with an accumulator. In Plane Maker you’ll find this setting as “unfeathering accumulator keeps oil pressure”. In this case, oil pressure is stored in an accumulator that can be used to unfeather the prop even when the engine is stopped. In that case, the engine can be re-started in the air without the starter motor if needed.