Altitude and vertical speed from pressure

Does changing QNH move the aircraft?

Distinguish instrument reference from actual height above terrain.

Pressure translated through a reference atmosphere

An aneroid altimeter senses static pressure and maps it to a height scale using a standard-atmosphere relationship. The selected subscale shifts the reference. Standard setting gives pressure altitude; QNH normally makes the indication approximate aerodrome elevation on the ground; QFE makes it approximate zero at its stated datum. None of these settings directly measures geometric clearance above terrain along the route.

Increasing the selected pressure raises the indicated altitude without moving the aircraft. Near sea level a supplied examination approximation may use 27 or 30 ft per hPa, but the pressure-height relationship is nonlinear and the approximation must be stated. Use the exact correction or approved table where required. The laboratory uses a standard atmosphere for pressure generation and labels its local setting approximation separately.

Temperature and source errors remain

When the air column is colder than standard, pressure surfaces are closer together. At an indicated height above the setting datum, true height is lower than the standard calibration suggests. A correct local pressure setting does not remove this temperature effect. Aircraft position error, static-system blockage or leakage, mechanical error and response lag can independently corrupt the indication.

Use a provided correction table by identifying its temperature, height reference and units, then applying the correction in the stated direction. A correction above an aerodrome datum is not automatically a correction to total altitude above sea level. GNSS altitude uses a different measurement and vertical reference; datum and integrity limitations prevent treating every satellite altitude as interchangeable with a barometric clearance. Compare sources only after understanding those references.

Why can the VSI briefly disagree with the manoeuvre?

A rate instrument must distinguish a trend from a transient.

A pressure difference created by a restriction

A conventional VSI connects a diaphragm directly to static pressure and allows the case pressure to equalise through a calibrated leak. During a climb static pressure decreases; the two sides respond at different rates, producing a difference proportional to the pressure-change rate within the design range. When altitude is steady, pressures equalise and the indication returns to zero.

This lag is part of the measuring principle rather than evidence that the aircraft continues climbing after level-off. An instantaneous VSI uses additional acceleration-sensitive input to improve initial response, while an inertial vertical-speed source derives information from inertial measurements. Their behaviour during turbulence and acceleration differs. Know which source the display actually uses before interpreting a brief change as a sustained flight-path trend.

Convert and cross-check

Vertical speed may be displayed in ft/min or m/s. One metre per second is about 196.85 ft/min. A momentary inertial indication, pressure lag or turbulence-induced fluctuation should be compared with attitude, altitude trend and performance rather than chased with repeated large pitch corrections. The correct control technique follows the aircraft’s instrument flying guidance and the conditions.

A blocked static line removes a conventional VSI’s ongoing pressure input; after equalisation it generally reads zero despite a real climb or descent. A leak can make the indicated trend respond to the wrong pressure environment. In the model, apply a step in actual climb rate and watch the indicated rate approach it over time. Then level off and observe decay to zero. The displayed time constant is synthetic and explains filtering rather than specifying an installed instrument.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.