Instrument panel
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Airspeed indicator: measures pitot pressure minus static pressure (dynamic pressure). Needs both.
Altimeter: a sealed aneroid capsule inside a case fed with static pressure. Static only.
VSI: compares static pressure now with static pressure a few seconds ago (via a calibrated leak). Static only, and it lags a few seconds.
The small numbers under each instrument show the real value and the error. They are hidden during the Mystery Failure quiz.
Pitot-static system
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Click the pitot inlet, the drain hole or the static port to block or clear it. The ice symbol marks a blockage.
Pitot line (total pressure) goes to the airspeed indicator only. Static line goes to all three instruments.
Moving dashes mean air is flowing because the pressure is changing. TRAPPED means that line is sealed off and holds its old pressure.
Simplified schematic. Pressures in hPa, ambient pressure from the ISA standard atmosphere.
Fly the aircraft
Blockages and switches
Training
Load a scenario or set your own blockages, then fly. The analysis below updates live and explains every instrument.
A hidden failure has been set and the schematic is hidden. Climb, descend and change speed, watch the instruments, then choose your diagnosis.
Symptom matrix
| Blockage | Airspeed | Altimeter | VSI |
|---|---|---|---|
| Pitot inlet, drain open | Drops to zero | Normal | Normal |
| Pitot inlet and drain | Acts like an altimeter: climb reads high, descent reads low, speed changes show nothing | Normal | Normal |
| Static port | Climb reads low, descent reads high (overspeed risk) | Frozen | Zero |
| Pitot, drain and static | Frozen | Frozen | Zero |
| Alternate static (cabin air) | Reads slightly high | Reads slightly high | Brief climb indication, then normal |
With pitot inlet and drain blocked the airspeed rises while you climb. A pilot who trusts it pulls the nose up to "slow down", while the real speed decays toward a stall. Cross check pitch attitude and power: pitch + power = performance.
How the three instruments work
The pitot tube faces the airflow and picks up total pressure (static + dynamic). The static port sits flush on the fuselage side and picks up only the ambient static pressure.
Airspeed indicator: total minus static. Altimeter: static only, converted to altitude with the standard atmosphere. VSI: rate of change of static pressure.
What the drain hole does
A small hole in the pitot tube lets water run out. If only the inlet ices over, the trapped air escapes through the drain and the pitot pressure drops to static: the airspeed goes to zero.
If the drain is blocked as well, the pitot pressure stays trapped. That is what turns the airspeed indicator into an altimeter.
Static port blocked: why the airspeed errs
The static pressure is frozen at the altitude where the port closed. Climb: the real pressure outside drops, the pitot pressure drops with it, but the static reference stays high, so the airspeed under-reads.
Descend: the real pressure rises and the airspeed over-reads. The indicator might still look fine while you already exceed Vne.
Alternate static source
A valve in the cockpit connects the static line to cabin air. Because air flowing around the fuselage lowers the cabin pressure a little, the altimeter and airspeed read slightly high, and the VSI shows a short climb when you switch.
Opening vents or heater changes the cabin pressure. Check the POH for the correction table of your aircraft.
Preflight and in flight
Preflight: remove the pitot cover, check the pitot inlet, the drain hole and the static ports for insects, dirt, tape or wax. Check pitot heat (briefly, it gets very hot).
In flight: pitot heat ON before entering visible moisture near or below freezing. With unreliable instruments: fly pitch and power, use alternate static, cross check GPS (groundspeed is not airspeed) and tell ATC.
Why it matters: real accidents
Northwest 6231 (1974): pitot heads iced with pitot heat off, the airspeed rose in the climb, the crew pulled up and stalled. Birgenair 301 (1996): a blocked pitot tube, probably an insect nest, gave false airspeed on one side.
Aeroperú 603 (1996): static ports still covered with tape after cleaning, altimeters and airspeed unreliable at night over water. Air France 447 (2009): pitot probes iced at cruise altitude and airspeed indications became inconsistent.
Pitot only affects the airspeed. Static affects all three. Frozen altimeter plus VSI on zero means: think static first.