Vertical Speed Indicators

A Vertical Speed Indicator (VSI) is a sensitive differential pressure gauge, which records the rate of change of atmospheric pressure in terms of rate of climb or descent, when an aeroplane departs from level flight.

Principle of Operation

The principle employed is that of measuring the difference in pressure between two chambers, one of which is enclosed within the other, as shown below.

The pressure of the atmosphere is communicated directly to the inner chamber (capsule) and through a calibrated choke or capsule case to the outer chamber. If the pressure changes, as in a climb or descent, the lag between the capsule and outer chamber will result in a pressure differential across the metering unit, which is a direct measure of the aeroplane's rate of climb or descent.

Movement of the capsule is transmitted via a mechanical linkage to a pointer, which moves against a calibrated dial on the face of the instrument, where the indications are arranged in a logarithmic scale. This allows the scale in the range 0 to 1000 feet/minute to be more easily interpreted while, at the same time, allowing smaller changes in the vertical speed to be registered in that range. This allows the flight crew to achieve a given flight profile more easily.

The metering unit of the VSI provides a pressure differential across the capsule case for any given rate of climb or descent, whilst compensating for variations in temperature and pressure of the atmosphere with changes in altitude. The compensation is achieved by incorporating in the metering unit both an orifice and a capillary, whose sizes are chosen so that indicator readings remain correct over a wide range of temperature and altitude conditions.

Operation of the VSI

In level flight the pressure inside the capsule and the case will be the same, so the pointer will remain in its horizontal position, indicating a zero rate of climb. If the aeroplane climbs, the static pressure in the capsule will decrease at a quicker rate than that in the casing and the capsule will collapse slightly, and will cause the pointer to indicate a rate of climb.

Conversely if the aeroplane descends the static pressure in the capsule will increase at a higher rate than that in the casing and the capsule will expand slightly, causing the pointer to indicate a rate of descent.

Errors of the VSI

The VSI can suffer from the following errors:

Lag

When an aeroplane is suddenly made to climb or descend, a delay of a few seconds occurs before the pointer settles at the appropriate rate of climb or descent, which is due to the time required for the pressure differential to develop. A similar delay will occur in the pointer showing a zero rate of climb or descent when the aeroplane resumes level flight.

Instrument Error

This error is due to the manufacturers' tolerances. However, in service the instrument pointer can be re-adjusted to the zero position using a screw adjustment.

Manoeuvre Induced Error

Errors induced by manoeuvres or flight in turbulence can cause any pressure instrument to misread for up to 3 seconds at low altitudes and up to 10 seconds at high altitudes. The times for the VSI may be even longer. Thus, during any manoeuvre involving a change of attitude, absolute reliance must not be placed on the VSI, with pitching resulting in the greatest error.

Faults of the VSI

The following faults will have an adverse effect on the VSI reading:

Blockages

A blockage in the static line will render the instrument completely unserviceable, and the pointer will register zero regardless of the aeroplane's vertical speed.

Breakage or Leakage in the Static line

A breakage or leakage in the static pressure supply line will cause the static value to change as the breakage occurs, e.g. if the breakage occurs in a pressurised section of the aeroplane the VSI will initially show a high rate of descent and will then stabilise to give a zero indication. This reading will be maintained until the aeroplane descends below the cabin altitude pressure.