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A vertical speed indicator connected to a too big equalizing tank results in...

  • A

    No indication.

  • B

    Mechanical overload.

  • C

    Indication too low.

  • D

    Indication too high.

Vertical Speed Indicator (VSI) shows a RATE of change in pressure. In order to achieve this the capsule (equalizing tank) is not a sealed one, like altimeters, but inside the capsule we receive the value of the atmospheric pressure from the Static port.

At the same line, there is a restrictor that feeds the case of the instrument with the value of atmospheric pressure but with a delay due to the restrictor. That leads the capsule to expand or retract not immediately with the change of the value, but with a small delay due to the delay of the restrictor.

This difference between the present Static pressure and the Static pressure of the previous moment, is translated as a Rate of climb or descent. When the pressure of the capsule is the same as the case, it means that the aeroplane is not climbing or descending, showing a value of zero differential pressure so 0 feet per minute.

If the case (equalizing tank) is too big, the atmospheric pressure in the case will delay more to be the same as the pressure at the capsule. This will lead to a faulty indication of the speed that needs the pressure to be the same in and out of the capsule.

A Vertical Speed Indicator connected to a too big equalizing tank results in indication too high.

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