Flight Instruments: Pitot-Static, Gyros and the Compass

What you'll learn: how each instrument works and what failures look like. Source: PHAK Ch. 8.

Pitot-static instruments

Instrument	Uses	If blocked
Airspeed indicator	Pitot (ram) and static pressure	Pitot blocked (with drain blocked): acts like an altimeter, reading higher as you climb
Altimeter	Static pressure	Static blocked: freezes at the altitude where it blocked
Vertical speed indicator	Static pressure (with a lag)	Static blocked: reads zero

Many airplanes have an alternate static source. Using it inside the cabin usually makes the altimeter and airspeed read slightly high.

Altimeter tips

"High to low, look out below": flying from high to low pressure (or warm to cold air) without resetting makes the altimeter read higher than your true altitude.

Airspeed markings

White arc: flap operating range
Green arc: normal operating range
Yellow arc: caution, smooth air only
Red line: V_NE

Gyroscopic instruments

Attitude indicator: rigidity in space (often vacuum-driven)
Heading indicator: rigidity in space; precesses, so align with the compass about every 15 minutes
Turn coordinator: precession; shows rate of turn and roll (often electric)

Magnetic compass errors

Variation: true vs magnetic north (on the sectional).
Deviation: the airplane's own magnetic fields (compass card).
Acceleration error (ANDS): Accelerate North, Decelerate South, on east/west headings.
Turning error (UNOS): Undershoot North, Overshoot South in the northern hemisphere.

Check yourself

2 questions. Pick an answer to see why it's right or wrong.