A student pilot notices that the vertical speed indicator (VSI) shows a brief reversed indication for a second or two immediately after abruptly leveling off from a steep climb. What is the best explanation for this phenomenon?
- AInstrument lag inherent to the diaphragm and calibrated leak design of the VSI
- BA blocked pitot tube causing erroneous static pressure readings in the VSI
- CExcess moisture in the static line freezing momentarily during the maneuver
- DA malfunctioning gyro precessing due to the rapid pitch change
Show answer & explanationAnswer & explanation
Correct answer: A. Instrument lag inherent to the diaphragm and calibrated leak design of the VSI
The VSI operates using a diaphragm that compares current static pressure to static pressure delayed by a calibrated leak (capillary/restricted orifice) inside a case, producing an inherent lag of six to nine seconds behind actual vertical speed changes. During abrupt maneuvers, this can cause a brief, momentarily reversed or lagging indication, sometimes called 'reversal error' or lag error, until the instrument catches up.
Why the other options are wrong
- B. A blocked pitot tube affects the airspeed indicator, not the VSI, which uses only static pressure.
- C. While moisture/icing can affect static systems, the described brief reversal during normal maneuvering is a known inherent lag characteristic, not necessarily an icing malfunction.
- D. The VSI is not a gyroscopic instrument; it operates purely on differential static pressure through a diaphragm.
Vertical Speed Indicator (VSI) Lag
The VSI compares current static pressure to a delayed pressure from a calibrated leak inside its case, causing inherent lag of several seconds and a brief reversal indication during abrupt pitch changes.
- Uses a diaphragm and calibrated leak (capillary restriction)
- Typical lag is 6-9 seconds behind actual vertical speed
- Abrupt maneuvers can cause brief reversed indications ('reversal error')
Memory trick: VSI's calibrated leak makes it a few seconds behind the truth