Three instruments rely on gyroscopic principles — rigidity in space and precession.
Uses rigidity in space to maintain a fixed reference plane; the miniature airplane shows pitch and bank against the horizon. Older vacuum AIs can show small errors after rapid acceleration or a tight turn but settle quickly. The AI is the master instrument of the control-and-performance scan.
Also uses rigidity, but it has no north-seeking ability, so it drifts due to precession and earth rotation. Reset it to the magnetic compass every 15 minutes in straight-and-level, unaccelerated flight. Modern HSIs and slaved compass systems correct this automatically via a flux gate.
Uses precession to sense rate of turn (and roll rate, on a TC). A standard-rate turn = 3° per second = a 360° turn in two minutes. The inclinometer ball shows coordination: "step on the ball." Crucially, the turn coordinator is usually electrically powered, so it survives a vacuum failure — making it the backbone of partial-panel flight.
Most trainers split the load: vacuum/pump drives the AI and HI; electrical drives the turn coordinator. That split is intentional redundancy — lose vacuum and you still have the TC, magnetic compass, and pitot-static instruments.
The compass is your only independent direction source, but it has errors:
- ONUS — On a Northerly heading the compass Undershoots; on Southerly it Overshoots.
- ANDS — Accelerate North, Decelerate South (on east/west headings).
Bottom line: redundancy across power sources is what keeps you flying when one system quits.