Six round instruments, arranged the same way in almost every piston aircraft built since the 1930s, and the arrangement itself, not just the instruments, is what makes a fast scan possible. This guide covers what each one actually measures, the two systems that power them, how each one fails, and the scanning technique that ties all six together.
The attitude indicator sits at the center of the "basic T" because it's the single instrument most representative of what the airplane is actually doing in space, pitch and bank together, in one glance. Airspeed sits to its left and the altimeter to its right, the two performance numbers a pilot cross-checks against attitude most often. The heading indicator sits directly below, with the turn coordinator and vertical speed indicator filling the remaining corners.
The airspeed indicator doesn't measure speed directly, it measures a pressure difference: ram pressure from the pitot tube minus static pressure from the static port. The faster the aircraft moves, the greater the difference, and the needle moves accordingly. The face is marked with color arcs that turn a bare number into an operating limit at a glance.
| Marking | Speed | Meaning |
|---|---|---|
| Bottom of white arc | Vso | Stall speed in landing configuration, flaps extended |
| Top of white arc | Vfe | Maximum flap extended speed |
| Bottom of green arc | Vs1 | Stall speed in clean configuration |
| Top of green arc | Vno | Maximum structural cruising speed |
| Red line | Vne | Never-exceed speed; structural failure becomes a real risk above it |
The attitude indicator shows pitch and bank relative to the horizon, using a gyroscope that stays fixed in space (rigidity in space) while the aircraft and instrument case move around it. It's the only instrument in the six-pack that shows both pitch and bank in a single glance, exactly why it sits at the center of the T. Depending on the aircraft, the gyro is spun by engine-driven vacuum suction or by an electrical source.
The altimeter is, at its core, a barometer with a dial calibrated in feet. Because atmospheric pressure at a given altitude changes with weather, it has an adjustable Kollsman window that lets a pilot dial in the current local altimeter setting. "Altitude" actually refers to several distinct values, and the altimeter only ever shows one directly.
| Type | What it is |
|---|---|
| Indicated altitude | The number read directly off the altimeter with the current local setting dialed in |
| True altitude | Actual height above mean sea level |
| Absolute altitude | Height above the ground directly below the aircraft, in feet AGL |
| Pressure altitude | Height above the standard datum plane, read with 29.92 dialed in |
| Density altitude | Pressure altitude corrected for non-standard temperature |
The altimeter shares its static pressure source with the airspeed indicator and VSI, which is why a static system problem affects all three at once.
The turn coordinator does two jobs: a miniature airplane symbol shows rate of roll and rate of turn together, and an inclinometer ball underneath shows whether that turn is coordinated. Unlike the attitude indicator, its gyro is mounted at roughly a 30-degree angle to the aircraft's longitudinal axis, making it sensitive to rate of roll as well as rate of turn. A standard rate turn, 3 degrees per second, a full 360 in exactly two minutes, is marked directly on the instrument. A centered ball means the turn is coordinated; displaced to the inside means slipping, to the outside means skidding. "Step on the ball" is the standard memory aid.
The heading indicator uses a gyroscope's rigidity in space, oriented to display compass heading instead of pitch and bank. Internal friction and the earth's own rotation cause it to drift at a rate of roughly 15 degrees per hour, which isn't a malfunction, it's inherent to how the instrument works, and needs periodic realignment against the magnetic compass, roughly every 15 minutes. Because the compass itself has its own errors in turns and during acceleration (remembered by the mnemonics UNOS and ANDS), realignment is best done in stable, straight-and-level, unaccelerated flight.
The VSI measures the rate at which static pressure is changing, displayed as feet per minute of climb or descent. Because it depends on a rate of change rather than an instantaneous value, it typically lags a few seconds behind an actual change, which makes it most useful as a trend instrument confirming a climb or descent is stabilizing, rather than the first instrument that shows a change has started.
Three of the six instruments, airspeed, altitude, and vertical speed, all draw from the same physical plumbing: a pitot tube for ram air pressure and one or more static ports for ambient air pressure. The pitot line runs only to the airspeed indicator; the static line runs to all three. Because the static line feeds three instruments at once, a static system problem rarely affects just one gauge.
The attitude indicator and heading indicator both rely on a spinning gyroscope, and in most piston singles, that spin comes from suction, the vacuum system. An engine-driven pump lowers pressure inside each gyro instrument's case, and filtered air drawn in through a nozzle accelerates the rotor. The turn coordinator's gyro, by contrast, is almost always electrically driven, a deliberate redundancy: a vacuum pump failure takes out the attitude and heading indicators but leaves the turn coordinator working.
| Failure | Effect |
|---|---|
| Pitot tube blocked, drain hole open | Airspeed indicator drops to and stays at zero; altimeter and VSI unaffected |
| Pitot tube and drain hole both blocked | Airspeed indicator behaves like a second altimeter, rising in a climb and falling in a descent |
| Static port blocked | Altimeter and VSI both freeze; airspeed reads low above that altitude and high below it |
| Alternate static source selected | Altimeter and airspeed both read slightly higher than actual, VSI shows a momentary climb |
A vacuum pump failure is dangerous specifically because it's gradual. The gyro doesn't stop instantly, it spins down slowly, and the attitude indicator can continue giving a plausible, subtly wrong picture for several minutes. Most aircraft with a vacuum system include a suction gauge or warning flag; checking it periodically is the most direct way to catch a failing pump before the gyro instruments start visibly drifting.
One useful way to organize a scan: control instruments, primarily the attitude indicator, show current pitch and bank and are used to set and hold a desired attitude; performance instruments (airspeed, altimeter, heading indicator, turn coordinator, VSI) show the result of that attitude. An effective scan keeps returning to the attitude indicator between glances at each performance instrument, rather than moving eyes around the panel in a fixed loop.
The full PDF includes all diagrams (a real six-pack panel photo, labeled instrument faces) and 16 practice questions with a worked answer key.
This guide is intended for educational and training purposes only. It is not a substitute for official FAA publications, an authorized ground school, or qualified flight instruction, and should not be used as a sole source for real-world flight planning, dispatch, or operational decisions.