The three gyro systems X-Plane always had

X-Plane always simulated three different methods for getting attitude and heading information in the cockpit, and a total of six separate gyros to use and drive panel instruments:

  • 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.
  • 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.

Where’s North, anyway?

It is worth noting that none of the gyro systems have any idea where magnetic north actually is. All they can do is keep their orientation within space, so they need to be aligned to magnetic north by using an external reference, such as a magnetic compass or the compass dial painted onto the pavement in the runup area of an airport. Cessna pilots are familiar with the little knob on the bottom left of their directional gyro – it’s what they push and twist in order to set the gyro to the magnetic heading.


Another way to align the directional gyro to magnetic north is by using a sensor that can detect the direction of the earth’s magnetic field. This sensor is called a fluxgate, or sometimes magnetometer, and is usually mounted in the left wing of an aircraft, far away from the engine, other avionics, or any equipment that might generate magnetic disturbances. The fluxgate transmits a signal to the remote gyro unit.

Slaved gyros and remote HSIs

For most HSIs, a remote gyro unit is mounted in the back of the plane. It’s an electric gyro that keeps its orientation once spun up and aligned. The fluxgate signal tells the remote gyro unit where magnetic north is. Thus, the electric gyro can be slaved to the fluxgate, indicating magnetic north without the need for the pilot to align it! It is worth noting that the fluxgate is subject to some, but not all of the errors the magnetic whisky compass is. In particular, while it does not have an acceleration error, it does have a dip error in turns! Thus, the electric gyro is used to get a very reliable course information during a turn, where the fluxgate reading is inaccurate, while in horizontal flight the slaving mechanism takes care of eliminating gyro precession. In other words: the gyro is used to compensate short-term errors of the fluxgate, and the fluxgate is used to compensate long-term errors of the gyro.

Free your gyro

Most remote HSIs have a way to free the gyro from the slaving to the fluxgate sensor, and treat it instead like a regular directional gyro. This mode is called “free” mode, and is activated by flipping the remote gyro switch from “slaved” to “free”. Besides a magnetometer failure, a use case for that is when flying with regards to true north reference, instead of magnetic north, which is done for example in northern Canada. In this case, the gyro is freed, and then the slew buttons or rocker switch (which work just like the slewing knob on the old C172’s directional gyro) are used to align the gyro to true north.

How can I use that in X-Plane 11.10?

In the dataref list of X-Plane 11.10, you can find all these values under the sim/cockpit/gyros/ category. Now that you know what AHARS, elec and vac mean, you just need to remember that psi is the heading, theta the pitch and phi the roll. When you make a 3d instrument for an HSI or DG, be aware of the correct system and dataref to use. The sim/cockpit/gyros/dg_drift_vac(2)_deg data refs allow you to see how far the vacuum gyro has drifted from the magnetic north, and the commands sim/instruments/(copilot_)DG_sync_[down/up] allow you adjust the setting of the vacuum gyro (the push-and-twist). For electric (remote) gyros, the new dataref sim/cockpit/gyros/gyr_free_slaved[] allows you to turn on (1) or off (0) slaving the electric gyros to the fluxgate. In free gyro mode, the new commands sim/instruments/[copilot_]free_gyro_[down/up] allow you to model the slave buttons or the CW/CCW switch on a slaving compensator panel. The respective drift datarefs sim/cockpit/gyros/dg_drift_ele(2)_deg allow you to model the slaving meter gauge. Finally, the new datarefs sim/cockpit/gyros/gyr_flag[] indicate whether the GYRO or HDG warning flag should be shown on the HSI or DG instrument. The new flag dataref not only reacts to low vacuum pressure or failed electric gyros, but it also momentarily shows while a free gyro adjustment is in progress, just like you’d see it on a Bendix/King KCS55A HSI.

Additional Fixes in X-Plane 11.10

Besides the new features mentioned above, X-Plane 11.10 fixes a few problems with how the existing gyro systems interacted with failure modes and electrical systems:

  • A separate failure  mode now exists that allows you to fail the electric gyros of a plane (if so equipped)
  • Electric gyro systems now depend on electric power, even when they are not driven off the inverter. They will show up in Plane Maker as a consumer of electric energy.
  • The gyro in the turn coordinator or turn and bank indicator now depends on electric power. It will show up in Plane Maker as a consumer of electric energy.
  • AHARS systems now depend on electrical power and react to their failure setting.
  • Finally, there’s a command for automatic quick-align of the directional gyro: sim/instruments/(copilot_)DG_sync_mag does just that. Former FSX pilots will feel at home when they assign that command to the “D” key on their keyboard.