FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsEasy
A technician is explaining the internal operation of a vertical speed indicator (VSI) to an apprentice. Inside the instrument case, a diaphragm is connected directly to the static line, while the case itself is connected to the static line through a calibrated restricted orifice (calibrated leak). What does this arrangement allow the VSI to measure?
- AThe absolute value of static pressure, indicating pressure altitude
- BThe time lag of pitot pressure changes, indicating acceleration
- CThe difference between pitot and static pressure, indicating airspeed
- DThe rate of change of static pressure, indicating rate of climb or descent
Show answer & explanationAnswer & explanation
Correct answer: D. The rate of change of static pressure, indicating rate of climb or descent
The diaphragm responds instantly to static pressure changes while the case pressure changes slowly through the calibrated leak, creating a pressure differential proportional to the RATE of pressure change, which the instrument displays as rate of climb or descent.
Why the other options are wrong
- A. That describes an altimeter, which measures absolute static pressure, not a rate.
- B. The VSI measures pressure rate of change, not a time-lag-based acceleration value.
- C. That describes an airspeed indicator, which uses pitot and static pressure, not two static sources.
Vertical Speed Indicator (VSI)
An instrument that displays rate of climb or descent by measuring the rate of change of static pressure using a diaphragm and a calibrated restricted leak.
- Diaphragm connects directly to static pressure and responds instantly
- Case pressure changes slowly through a calibrated leak
- Pressure differential between diaphragm and case indicates rate, displayed in feet per minute
Memory trick: Diaphragm reacts fast, case leaks slow — that lag shows how quick you go.