Instrument Systems: Pitot-Static, Gyroscopic, Magnetic
Flashcards on how flight instruments work, their power sources, and how failures present in IMC.
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Questions Covered in This Set
11 cards to master
Which instruments are driven by the pitot-static system?
Airspeed indicator, altimeter, and vertical speed indicator — the ASI is the only one using both ram air and static pressure.
What does the ASI actually display?
The difference between ram (pitot) pressure inside the diaphragm and static pressure in the case — dynamic pressure, calibrated in knots.
Why does the VSI lag 6–9 seconds?
Its capsule gets static pressure directly while the case receives static through a calibrated leak; the needle shows the pressure lag between the two.
Blocked pitot tube with drain hole open — what happens?
Airspeed drops toward zero: dramatic and unambiguous.
Blocked pitot tube AND drain (icing) — how does the ASI behave?
Trapped pressure makes it act like an altimeter: IAS increases in a climb, decreases in a descent regardless of actual speed (Birgenair 301, Aeroperú 603).
What is the signature of a blocked static port with a clear pitot?
Altimeter frozen at blockage altitude, VSI frozen at zero, ASI reads low in a climb and high in a descent — two dead instruments plus a lively ASI.
Effects of selecting the alternate static source in an unpressurized aircraft?
Cabin static is slightly lower than ambient, so the altimeter reads slightly high and the ASI slightly fast.
No alternate static source — what is the emergency fix?
Break the VSI glass: it vents cabin static, restoring altimeter and ASI, but the VSI then reads backwards.
Which gyro principle serves the AI and HI, and which serves the turn coordinator?
AI and HI use rigidity in space; the turn coordinator uses precession (force produces a reaction 90° later in the direction of rotation).
Typical steam-gauge power split, and why it matters?
Vacuum pump drives AI and HI; the turn coordinator is electric — so a single failure can't blank all attitude information, enabling partial-panel flight.
What instruments remain after a vacuum failure?
Electric turn coordinator, the pitot-static trio (ASI, altimeter, VSI), and the magnetic compass.
