There's always a third artificial horizon, generally closer to the left (Captain's) side, but I've never understood what it's for. Is it just a backup, or does it serve some extra purpose?
The standby attitude indicator was adopted by regulation in the late 60s or early 70s following an incident involving a Lockheed Electra that suffered a dual attitude gyro failure in bad weather.
The Captain was flying the airplane on "partial panel", using airspeed and altitude for pitch control, and the turn-and-bank gyro for bank control, something instrument pilots learn in their initial training.
The mental workload on partial panel is quite high, made worse with a fairly demanding turboprop like the Electra, and the Capt developed spatial disorientation and the plane augered into a mountainside. Some time after the investigation, the standby attitude indicator was mandated for transport aircraft.
The standby attitude indicator will have its own stand alone power source, usually connected to an emergency or battery bus, and will be a self contained unit with an internal gyro (whereas the main attitude indicators normally have remote gyros).
There are two basic uses of the standby attitude indicator:
- Last resort attitude indication if both main attitude systems fail at the same time (quite rare).
- The more common scenario is where the standby attitude indicator provides for a 2:1 majority vote where one of the main attitude indicators deviates but maybe doesn't fail completely (no red flag on the display or instrument, although you may see a PITCH/ROLL DISAGREE message of some kind on an airplane with EFIS).
If you're flying along and suddenly your own attitude indicator starts to tilt, or drift up or down spontaneously, your immediate response is to observe both the standby and the opposite main attitude indication and compare them to yours.
If the standby agrees with the opposite indicator, you disregard your indicator and transfer control to the opposite pilot (or the autopilot to the opposite Flight Director). If the standby agrees with your indicator, you proceed on the assumption that the airplane is doing what the indicators say it is doing (at least initially - you still check that airspeed and altitude indications are responding as they should to the apparent attitude change).
Many planes feature what's called a Multi-Function Display or MFD. There may be a variable number of these, but usually it's at least one each for the pilot and copilot, sometimes with additional MFDs in the middle, or two each for both pilots. These displays can be used for any number of purposes, including the artificial horizon. These are of course electronic displays and as such have the possibility that the screen will fail during flight, so there will usually also be an analog artificial horizon so that they still work even if there's an electrical failure.
(Caveat: I am not a pilot, but I have programmed simulators for planes, including programming the simulated MFDs, so I know a bit about these specifically.)