Learn why an airspeed indicator fails, recognise pitot-static clues, respond safely in flight, and troubleshoot aircraft or simulator faults.
An airspeed indicator usually stops working because the pitot tube or static source is blocked, a pressure line is leaking or disconnected, the instrument or air-data system has failed, or an electronic display has lost power. Treat any in-flight discrepancy as unreliable airspeed first; troubleshoot only after controlling the aircraft.
In our Aviation & Real-World Flying guidance, we make one distinction first: zero airspeed while parked, and often during a slow taxi, is normal. The indicator measures the difference between pitot and static pressure, so it needs airflow to register. Our explanation of how Cessna 172 pitot-static instruments work together covers that relationship in more detail.
What does the failure pattern tell you?
The way the indication fails can identify the affected pressure source, but these clues are not proof and should not replace an approved inspection.
| Observed symptom | Likely causes | Useful cross-check |
|---|---|---|
| Zero or near-zero airspeed once the aircraft is moving | Pitot cover left fitted, blocked pitot inlet with an open drain, disconnected pitot line or failed air-data input | Check another independent airspeed source and verify that pitot covers were removed |
| Reading becomes fixed or changes strangely with altitude | Pitot inlet and drain both blocked, trapping pressure in the line | The indication may increase in a climb and decrease in a descent, behaving partly like an altimeter |
| Airspeed underreads in a climb and overreads in a descent | Blocked static source | The altimeter may freeze and the vertical speed indicator normally settles towards zero |
| Only one of several airspeed displays disagrees | Local instrument, probe, line or air-data computer fault | Compare instruments fed by genuinely independent sources |
| Electronic speed tape is blank, flagged or crossed out | Display power, data-bus, air-data computer or sensor failure | Check other data from the same computer and any standby instrument |
| Indication is slow, erratic or intermittently wrong | Water, a partial blockage, a pressure leak or an intermittent electrical/data fault | Note whether altitude, weather, heat selection or vibration changes the symptom |
What should you do if the airspeed fails in flight?
Fly the aircraft using the type-specific unreliable-airspeed procedure rather than trying to diagnose plumbing or electronics in the air. The POH, AFM, QRH or operator checklist takes precedence over generic advice.
- Maintain control. Hold a known attitude and power setting appropriate to the aircraft and phase of flight. Use published pitch-and-power values where provided.
- Cross-check independent information. Compare the standby indicator, separate pilot displays, attitude, altitude trend and engine power. GPS groundspeed is only supporting information because wind can make it differ greatly from airspeed.
- Run the approved checklist. It may call for pitot heat, an alternate static source or selection of a different air-data source. Do not improvise switch or circuit-breaker resets.
- Monitor the automation. Autopilot, flight director and autothrottle systems can react badly to false airspeed. Disconnect them if the checklist directs it or their behaviour becomes unsafe.
- Reduce the workload. Tell air traffic control, avoid icing and difficult weather where possible, and arrange a landing at a suitable aerodrome. Declare urgency or an emergency when the circumstances require it.
During the take-off roll, an indicator that does not come alive or disagrees substantially is normally a reason to reject while stopping safely remains possible. Apply the aircraft or operator’s established reject criteria rather than continuing to investigate after lift-off.
How do you troubleshoot an airspeed indicator on the ground?
Ground troubleshooting should separate an external blockage, a pressure-system leak, an instrument fault and an electrical problem without applying uncontrolled pressure to the system.
- Record the exact symptom. Note whether the indication was zero, frozen, intermittent or merely different from another source, together with altitude, weather, icing and heat selection.
- Check covers and plugs. Confirm that every pitot cover and static-port cover has been removed. Detailed pre-flight inspections should catch the most common cause of a zero indication.
- Inspect the openings visually. Look for ice, insects, tape, paint, dirt, water or physical damage. Do not insert wire, blow into a pitot line or apply compressed air; that can damage instruments and air-data sensors.
- Compare connected instruments. A frozen altimeter and dead VSI alongside bad airspeed strongly suggest a shared static problem. One bad instrument among several independent displays points towards a local unit or data source.
- Check electrical indications. For glass cockpits, verify the required buses, air-data equipment and displays are powered. Do not repeatedly reset a tripped circuit breaker unless the approved procedure permits it.
- Have the system tested properly. Appropriately authorised maintenance personnel can perform calibrated pitot-static leak and instrument tests, inspect drains and lines, and confirm the repair without overstressing the system.
An intermittent fault should not be treated as fixed simply because the indication returns. Dispatch must be determined from the aircraft’s equipment requirements, approved maintenance data and any applicable minimum equipment list.
Can an electrical failure stop the airspeed indicator?
A conventional diaphragm-type airspeed indicator does not need aircraft electrical power, but a glass-cockpit display, air-data computer or electronic standby instrument does. An electrical failure can also disable pitot heat, allowing ice to block the probe even though the mechanical indicator itself remains powered pneumatically.
This distinction is easy to miss in training aircraft. Our guide to Cessna electrical and instrument power explains why a traditional round-dial airspeed indicator may keep working after the radios and powered displays fail.
Why can icing or rain make the airspeed stop working?
Ice can obstruct the pitot opening, drain hole or static source, while water can cause lagging or erratic pressure if drainage is compromised. Pitot heat protects only the equipment designed to be heated; it does not automatically clear every static blockage.
Rain alone should not disable a serviceable system with effective drainage, so a persistent weather-related fault still needs inspection. We cover the characteristic failure patterns in our detailed explanation of icing and weather effects on airspeed indications.
What if the airspeed indicator fails only in a flight simulator?
A simulator-only failure usually comes from modelled pitot icing, a deliberate failure setting, an incorrect aircraft state, missing instrument power or an add-on-specific maintenance fault.
- Confirm that airflow should register. A stationary aircraft and a slow taxi can legitimately show zero or very little indicated airspeed.
- Check the aircraft state. Remove modelled pitot covers and other ground equipment. Complex add-ons may preserve these items between sessions.
- Check weather and heat. If icing is present, operate probe or pitot heat as the simulated aircraft requires. Correcting the switch may not restore the indication instantly.
- Review failures. Clear intentional failures in both the simulator and any add-on maintenance or tablet system. Some aircraft store wear and failure states separately from the base simulator.
- Verify electrical power. Glass-cockpit speed tapes need the relevant display, sensor and air-data buses. Compare the main display with any independently powered standby instrument.
- Isolate the aircraft or add-on. Test a default aircraft in a clean session without third-party instrument modifications. If that works, the problem is probably confined to the original aircraft, saved state or add-on configuration.
For Microsoft Flight Simulator, our practical checks for blank or frozen MSFS instruments also cover pitot heat, failures, electrical power and standby-instrument cross-checks.