Instantaneous Vertical Speed Indicator (IVSI)

The Instantaneous Vertical Speed Indicator (IVSI) is also sometimes referred to as the Inertial Lead Vertical Speed Indicator (ILVSI). The basic construction of this instrument is shown on the next page. It consists of the same basic elements as the conventional VSI, but it is additionally fitted with an accelerometer unit that is designed to create a more rapid differential pressure effect, specifically during the initiation of climb or descent.

The accelerometer comprises of two small cylinders or dashpots, which contain inertial masses in the form of pistons that are held in balance by springs and their own mass. The cylinders are connected in the capillary tube system leading to the capsule and are thus open to the static pressure source.

Operation of the IVSI / ILVSI

If a change in vertical motion is initiated, the resultant vertical acceleration, i.e. due to the change in vertical velocity along the aeroplanes vertical axis, will produce a force and the pistons will be displaced from their neutral position. The pistons are arranged so that one responds to nose up pitch changes (positive G) by reducing the capsule pressure, whilst the other will respond to nose down pitch changes (negative G) by increasing the capsule pressure.

This will in turn create an immediate pressure change inside the capsule, and will produce an instantaneous movement of the indicator pointer, and in the correct sense, to the initiation of a climb or descent manoeuvre. The errors are generally the same as those affecting the conventional VSI, although the lag and induced manoeuvre errors will be virtually eliminated, with the instrument providing a rapid response to changes in the aeroplanes vertical flight path.

The accelerometer output will decay after only a few seconds, and the pistons will automatically return to their neutral position, by which time a steady pressure differential will have been established across the metering unit. The instrument will then continue to behave like a conventional VSI.

The IVSI is however affected by the acceleration forces, i.e. g-forces, which act on the pistons during steep turns when the angle of bank is in excess of 40°, and may produce a false reading, known as ‘Turning Error'.