A pilot abruptly pulls back on the yoke to begin a climb. Immediately afterward, the vertical speed indicator (VSI) briefly shows a slight descent before settling into an accurate climb indication a few seconds later. What causes this brief erroneous indication?
- AA leak in the static line causing altimeter and VSI disagreement
- BThe inherent lag of the VSI's calibrated leak mechanism responding to the pressure change
- CPrecession error identical to that found in the attitude indicator
- DPitot tube icing affecting airspeed and vertical speed together
Show answer & explanationAnswer & explanation
Correct answer: B. The inherent lag of the VSI's calibrated leak mechanism responding to the pressure change
The VSI operates using a diaphragm connected to a calibrated leak that measures the rate of change of static pressure; this mechanism has an inherent few-second lag, which can cause a brief, momentarily reversed or inaccurate indication (a 'zero delay' effect) immediately after abrupt pitch changes before stabilizing. This is a normal characteristic, most pronounced in standard (non-instantaneous) VSIs.
Why the other options are wrong
- A. A static leak would cause persistent errors, not a brief momentary reversal.
- C. Precession error is specific to gyroscopic instruments like the attitude indicator, not the VSI.
- D. Pitot icing affects airspeed, not the VSI, which uses only the static system.
VSI Lag
A brief delay in vertical speed indicator response caused by the time needed for pressure to equalize through the instrument's calibrated leak, sometimes producing a momentary reversed indication after abrupt pitch changes.
- Standard VSI has several seconds of lag
- Instantaneous VSI (IVSI) uses accelerometers to reduce lag
- Momentary reversed indication is normal, not a malfunction
Memory trick: The leak takes a beat to catch up—like a slow leak taking time to equalize