Gyro systems
X-Plane can drive the attitude indicator, also known as the artificial horizon, from any of three systems. This yields a total of six gyros you can use for your attitude instruments (pilot and copilot side). The three systems are:
- vacuum gyro – this one is driven by air being sucked through it, and the vacuum necessary to pull the air into it is generated by an engine-driven vacuum pump. This is the system most often found in simpler general aviation aircraft like a C172. X-Plane simulates a vacuum pump driven off the accessory section of the engine, thus the gyro will spin up when the engine spins up.
Check out the article on the vacuum system itself for more information. - electric gyro – this gyro replaces the failure-prone vacuum pump, hose, and filter system with a simple electric motor inside the instrument, which spins up the gyro. You find those in non-glass Cirruses, Diamonds, and other more modern general aviation aircraft. X-Plane drives this motor off a DC electric bus, or, if checked in Plane Maker, off the AC inverter.
- AHARS – the fully electronic attitude and heading reference system replaces the gyros with sagnac laser-gyros or cheaper MEMS gyroscopic sensors (comparable to the ones in your smartphone) to generate attitude and heading information without any moving parts. This system is obviously electrically powered.
Limitations of the mechanical attitude gyro instrument
The attitude instrument uses a gimbal mechanism which allows the instrument case (and by extension, the whole airplane) to revolve around the gyro, which keeps pointing upward. The movements of the gimbal are translated by a pickup mechanism into movements of the part of the instrument the pilot is looking at. So while the plane rotates around the gimbal, the deflection of the gimbal is what causes the blue/brown part of the attitude indicator to move. It is important to keep in mind that this pickup mechanism is quite delicate, and also limited in its freedom of movement. It can indicate up to 110 degrees in bank and 70 degrees in pitch. Beyond that, the mechanism locks up and the attitude reading on the instrument becomes inaccurate. Moreover, a violent excursion of the bank or pitch limitations of the instrument can even cause permanent damage to the pickup mechanism, but that doesn’t happen in X-Plane.
After an exceeding of the instrument limitations, the indicated pitch or bank will be off, depending on how far the gyro was forced off from its natural position. The gyro rights itself at three degrees per minute in normal flight conditions, so you will see the attitude indicator correcting itself at this slow rate. To force the gyro back into the upright position quicker, you can pull the caging or fast erect knob.
Caging to the rescue
Aerobatic planes that are expected to exceed 70 degrees of pitch and 110 degrees of bank, if equipped with an attitude indicator at all, will have an instrument that allows caging. A caged gyro is locked to the instrument case, and rather than the delicate pickup mechanism taking the beating, the whole instrument absorbs the gyroscopic forces. While it is caged, the attitude indicator indicates straight and level, while the aerobatic plane can go to extreme bank and pitch angles without damaging the gyro. Back in normal flight, the gyro can be uncaged and resume normal operation.
What is “fast erect”?
On some attitude indicators, the caging knob is instead labeled “(pull to) fast erect”. The mechanism however is the same: When pulled, the gyro is forced into the upright straight and level position and locked to the instrument case, however the fast erect knob snaps back to the uncaged position when let go, while a caging knob can be locked in the caged position. Since they perform the same operation, the knob is the same dataref
sim/cockpit/gyros/gyr_cage_ratio[N]
For a fast erect knob, the dataref can be ramped up with the command
sim/instruments/ah_fast_erect(_copilot)
this simulates pulling out the knob, which instantly springs back when let go.
For the cage knob the command
sim/instruments/ah_cage(_copilot)
instead simulates toggling the knob to the pulled position.