The same principle brings the VSI's defining fault: it takes the leak a few seconds to establish the difference, so the plain instrument lags. The instantaneous VSI (IVSI) patches the lag with an accelerometer: small spring-held pistons (dashpots) feel the vertical acceleration at the very start of a climb or descent and pump an immediate indication into the capsule line, holding the needle honest while the pressure difference catches up.
Three VSI types compared
Barometric VSI — capsule, case and leak alone: simple and smooth, but lagging by several seconds.
Instantaneous VSI (IVSI) — the same instrument plus the accelerometer pump: immediate response, at the price of feeling accelerations that are not climbs, the subject of the turbulence section below.
Inertial VSI — no capsule involvement in the displayed rate: the vertical speed is computed by the inertial reference unit from its accelerometers, stabilised against the barometric signal, and drawn on the electronic display. Immediate and smooth: the modern transport standard.
The VSI's errors
The catalogue is short and familiar:
Static system errors — whatever position error corrupts the static source flows into the rate indication during speed and configuration changes.
Instrument errors — the mechanism's tolerances and friction.
Time lag — the plain VSI's built-in delay, the error the IVSI and the inertial type exist to remove.
The VSI with a blocked or leaking static line
A blocked static line defeats the VSI completely: with no new pressure arriving, capsule and case equalise through the leak and the needle settles on zero, whatever the aircraft is doing — arguably the most insidious failure indication on the panel, because zero is a perfectly plausible reading. A leak in the static line has the alternate-static flavour: the momentary pressure step shows as a transient climb or descent indication, and thereafter the VSI reads rates referenced to the leak's location, close to correct in an unpressurised hull, meaningless inside a pressurised cabin whose pressure is scheduled by the pressurisation controller rather than the climb.
Reading the VSI
The classic display is a circular dial with the needle at rest pointing left at the 9 o'clock zero: climb swings it up, descent down, with the scale compressed toward the extremes so the everyday rates get the most needle travel. Electronic displays draw the same information as a small vertical tape beside the altitude tape, often with the numeric rate beside it.
Turbulence, manoeuvring, and which VSI to trust
In turbulence the two barometric types trade places. The plain VSI's lag now works as a filter: it smooths the bumps and shows something like the mean path. The IVSI's accelerometer feels every gust, so the needle dances, and the proper technique is to read its average and fly attitude rather than chase the needle. In manoeuvring the IVSI has a subtler vice: its accelerometer cannot tell a vertical acceleration from the load factor of a turn, so rolling briskly into a level turn shows a momentary false climb (and rolling out, a false descent), because the load factor of the turn presses on the accelerometer exactly as an upward acceleration would. With either type the manoeuvring technique is the same: pitch and bank come from the attitude indicator, and the VSI is consulted once things are steady, remembering that the plain type lags (lead the level-off) and the instantaneous type overshoots transiently.
The ASI's units
Airspeed arrives in three units. Knots (kt), nautical miles per hour, the aviation standard; statute miles per hour (mph), surviving on some light aircraft; and kilometres per hour (km/h). The relationships: 1 kt = 1.852 km/h (the nautical mile in metres), 1 mph = 1.609 km/h, and 1 kt ≈ 1.15 mph.
How the ASI works
The ASI is the pitot-static lesson's subtraction built in metal: an open capsule fed total pressure (where the altimeter's capsule was sealed, this one has an inlet) sits in a case fed static pressure, so the capsule's expansion follows exactly the difference, the dynamic pressure. The linkage and dial are calibrated to convert that pressure into a speed on one fixed assumption: ISA sea-level density. That assumption is why what the instrument shows is indicated airspeed rather than the speed through the air: at altitude the same dynamic pressure means a faster true airspeed, the ladder of speeds that Principles of Flight climbed and the air-data computer climbs automatically in this module's final lesson.
The ASI's errors, and when each matters
Pitot-static system errors — faults of the pressure sources themselves (leaks, partial blockages, icing): considered whenever the sources are suspect, and fatal to the reading when full blockage strikes (below). Note the naming: the altimeter's list filed position error under this heading; the ASI's syllabus list gives position error its own entry two bullets down.
Instrument errors — mechanism tolerances; always present, calibrated small.
Position errors — the static source's location error, changing with configuration and angle of attack: considered at every speed, corrected by the flight manual's table or the air-data computer, turning indicated into calibrated airspeed.
Compressibility errors — at high speed the air piles up compressed in the pitot mouth and the instrument over-reads the dynamic pressure: negligible below about 200 knots, growing through the middle speeds, material above roughly 300 knots TAS or in high-altitude cruise, corrected toward equivalent airspeed.
Density errors — the ISA sea-level calibration meets non-ISA air: present whenever the density is not the ISA sea-level value, growing with altitude, the correction that turns equivalent into true airspeed.
Displays, and the correction table demonstrated
Three presentations:
Pointer — the classic round dial.
Vertical straight scale (speed tape) — the presentation of electronic flight displays, with bugs (movable reference markers) and a trend arrow showing where the speed is heading.
Digital values on the head-up display — projected on the glass in the pilot's forward view.
The position-error table works exactly as the altimeter's: enter with configuration and speed, apply the correction. Worked once: flaps down, indicated 120 kt, the table row reads +4 kt, so the calibrated airspeed is 124 kt.
The colour codes
White arc — the flap operating range, from the stall speed in landing configuration (VS0) up to the maximum flap extended speed (VFE).
Green arc — the normal operating range, from the clean stall speed (VS1) to the maximum structural cruising speed (VNO).
Yellow arc — the caution range above VNO, up to VNE, to be entered only in smooth air.
Red line — the never-exceed speed VNE; on transport aircraft its moving equivalent is the barber's pole, a striped pointer riding at VMO/MMO as it changes with altitude.
Blue line — on multi-engine piston light aeroplanes, the best rate-of-climb speed with one engine out (VYSE): the speed the surviving engine's climb is flown at.