During a rapid transition from level flight into a descent, a pilot notices the vertical speed indicator (VSI) briefly shows a slight climb indication before settling into the correct rate of descent. What causes this momentary lag error?
- APitot tube blockage cross-coupling airspeed and vertical speed indications
- BStatic port icing that reverses the pressure sense temporarily
- CThe VSI's diaphragm and calibrated leak require time for internal pressure to equalize with the changing static pressure, producing a brief lag during rapid pitch changes
- DMagnetic dip acting on the VSI's internal compensating weight
Show answer & explanationAnswer & explanation
Correct answer: C. The VSI's diaphragm and calibrated leak require time for internal pressure to equalize with the changing static pressure, producing a brief lag during rapid pitch changes
The VSI measures the rate of static pressure change by comparing pressure inside a diaphragm to pressure in the instrument case, which is released through a calibrated leak. Because this leak restricts airflow, there is a brief lag during rapid pitch changes, sometimes producing a momentarily reversed or delayed indication before settling on the correct value.
Why the other options are wrong
- A. The VSI uses static pressure only; pitot blockage does not directly affect it.
- B. Icing would cause sustained erroneous readings, not a brief transient lag in a functioning system.
- D. Magnetic dip has no effect on a pressure-based VSI mechanism.
VSI Lag Error
The VSI's calibrated leak and diaphragm design inherently cause a brief lag or momentary reversed indication during rapid changes in pitch or vertical speed, before settling to the correct rate.
- Caused by the calibrated leak restricting airflow
- More pronounced during abrupt maneuvers
- Instantaneous VSI (IVSI) reduces this lag using accelerometers
Memory trick: The leak takes a breath before it tells the truth.