The vertical speed indicator
Navigation · Chapter 23 · DGCA CPL and ATPL
How the VSI turns a pressure difference a few seconds old into feet per minute, why it lags, and how the IVSI cures the lag.
Remember
- Static only: capsule gets it now, the case through a metering unit (capillary + orifice).
- Climb: capsule lower than case → UP. Descent → DOWN. Level → 0.
- Lag: a few seconds after starting or stopping a climb.
- Static blocked → zero. Pitot blocked → no effect. Ground check: within 200 ft/min.
- IVSI: dashpot (accelerometer) cures lag; over-reacts in turbulence; false climb in a steep turn.
How can a pressure gauge show a rate of climb?
Two pressures, a few seconds apart
- Static, two ways • The VSI (vertical speed indicator) takes static two ways. Straight into the capsule: the pressure now • And through a metering unit, a tiny leak called the choke, into the case: the pressure a few seconds ago
- Level: equal • Same pressure both sides: the needle sits at 0
- Climb • Static falls; the capsule feels it first • The case is still higher → capsule squeezed → needle UP
- Descend • The reverse: capsule pressure higher → needle DOWN
- Fly it • Move the slider; watch the case air lag behind
Why is the VSI a few seconds late?
The lag
- Start a climb • The aircraft is already climbing; the needle creeps up • The case air needs time to leak through the coil
- Level off • The aircraft is level, but the needle still shows a climb for a few seconds: this delay is lag
Draw it as a box: box, oval, choke
The box, VSI style
This lesson is part of the Navigation plan. See plans
What can make a VSI lie?
Errors and blockages
This lesson is part of the Navigation plan. See plans
How does the IVSI get rid of the lag?
The IVSI
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 23
Test yourself
This lesson is part of the Navigation plan. See plans