Aircraft Instrument Systems
What the instruments tell you
Candidates must understand pitot-static and gyroscopic instruments, their power sources, and their failure modes. Know how the altimeter, airspeed indicator, and vertical speed indicator use static pressure, and how a blocked pitot tube or static port affects each. Understand the vacuum or pressure-driven gyros and the electrically driven attitude and heading systems, plus the differences between traditional and glass cockpits.
Traps
A classic trap is the blocked static port: the airspeed indicator behaves like an altimeter, the altimeter freezes, and the VSI reads zero. A blocked pitot tube with the drain open causes the airspeed indicator to read zero; with the drain blocked, it acts like an altimeter. Questions also test which instruments remain reliable after a vacuum failure (the turn coordinator is often electrically driven).
Worked example
If the static port is blocked during a climb, the altimeter remains at the altitude where the blockage occurred, the VSI reads zero, and the airspeed indicator under-reads as altitude increases. The pitot heat and alternate static source are the corrective tools.
Rule of thumb
Pitot blockage affects airspeed; static blockage affects all three pressure instruments. Know which gyros are vacuum-driven and which are electric.