Vertical Speed Indicator (VSI)

Learning objective: You decide whether a climb/descent reading is real — the classic vertical speed indicator (VSI / Variometer) compares now-static with delayed static, and with blocked static it reads 0.

60 seconds: You are flying level and you pitch up. The attitude indicator nods at once; the VSI lags, then shows climb in ft/min. Remember: the VSI is not a gyro and not pitot — only static plus a calibrated leak. Nose-up can still be a descent; rate lives here, not on the horizon.

The capsule sees current static. Behind a tight leak sits a reservoir of a moment ago. Climb: outside pressure falls faster than the reservoir → climb. Descent: the reverse. Level, pressures equalise → 0.

The classic needle lags a few seconds behind attitude (teaching order of magnitude, not AFM seconds). An IVSI adds an accelerometer so the first kick is quicker — if fitted, type.

QNH/QNE are set on the altimeter, not the VSI. The VSI has no subscale.

Fully blocked static: no pressure change reaches the capsule → VSI ≈ 0. Alternate static per AFM. Pitot blockage alone does not move the VSI.

“The VSI shows pitch.” That is wrong. People see nose-up and expect climb. Pitch is attitude (AI); whether you actually climb is a pressure change.

Exam knowledge

• VSI = static versus delayed static (calibrated leak) → ft/min.
• No pitot, no gyro, no QNH subscale.
• Static blocked → VSI ≈ 0.
• Trend can lag pitch (teaching).
• Nose-up ≠ climb. Confirm on VSI / altimeter.
• IVSI = extra accelerometer, if fitted.

Source: typical EASA PPL 020 pitot-static teaching. Lag times and IVSI design are school teaching / type, not a universal AFM.

Check yourself

1. How does a classic VSI produce climb/descent?

• A) Pitot pressure scaled to ft/min
• B) A QNH subscale coupled to the altimeter
• C) Static in the capsule versus delayed pressure behind a calibrated leak
• D) A vertical-axis gyro measuring pitch rate

2. Fully blocked static — VSI reads?

• A) Zero
• B) Full climb
• C) Full descent
• D) Still correct, because the leak equalises internally

1. C — static only, calibrated leak.
2. A — no pressure change, no difference. ASI blockage errors are another lesson.