9.instrument-failures. Instrument Failures
Instrument failure during IFR flight is one of the most demanding emergencies a pilot can face. The ability to recognize a failed instrument quickly, partially panel fly the aircraft, and continue safely to a suitable landing is a core instrument flying skill. Failures generally fall into three categories: pitot-static system failures, vacuum (or pressure) system failures affecting gyroscopic instruments, and electrical failures affecting electric gyros, avionics, and (in glass cockpits) primary flight displays.
Pitot-Static System Failures
The pitot-static system feeds the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI). Common failures include:
- Blocked pitot tube, drain hole open: ASI drops to zero because ram air can no longer reach the diaphragm.
- Blocked pitot tube and drain hole (typical icing case): ASI behaves like an altimeter — it reads higher as the aircraft climbs and lower as it descends, because trapped pressure expands or compresses with ambient static pressure changes.
- Blocked static port: ASI, altimeter, and VSI all become unreliable. Altimeter freezes at the altitude where the blockage occurred, VSI reads zero, and ASI reads inaccurately (low in a climb, high in a descent).
The primary remedy for a static blockage is the alternate static source. Selecting it usually draws cabin pressure, which in unpressurized aircraft is slightly lower than outside static pressure, so the altimeter will read slightly higher, the VSI will momentarily show a climb, and the ASI will read slightly faster than actual. The POH provides correction tables. If no alternate static source is installed, the pilot may break the glass on the VSI as a last resort; the VSI then becomes unusable but the altimeter and ASI work off cabin static.
Use pitot heat as a preventative measure any time visible moisture or icing conditions are anticipated.
Vacuum/Gyro Failures
Most light aircraft drive the attitude indicator (AI) and heading indicator (HI) with an engine-driven vacuum pump. The turn coordinator (TC) is normally electric and provides redundancy. Vacuum failure indications include:
- Suction gauge reading low or zero.
- Slow, subtle precession of the AI and HI — often the most dangerous failure because it can lead the pilot into a graveyard spiral before being recognized.
- Disagreement between the AI/HI and the TC, magnetic compass, ASI, altimeter, and VSI.
When a vacuum failure is suspected, cover or disregard the AI and HI and fly partial panel using:
- Pitch: altimeter, VSI, and airspeed indicator.
- Bank: turn coordinator and magnetic compass.
- Power: tachometer/manifold pressure and airspeed.
Declare an emergency, request vectors, and request a non-precision approach (or PAR/ASR if available) suitable for partial-panel flight. Make small, controlled control inputs; chasing the needles is the most common partial-panel error.
Electrical Failures
A complete or partial electrical failure can disable the TC, navigation radios, transponder, pitot heat, and in glass cockpits, the PFD/MFD. Indications include a discharging ammeter, low-voltage warning light, or loss of avionics. Initial actions:
- Reduce electrical load (turn off non-essential equipment).
- Reset the alternator field/master per POH.
- If unrecoverable, plan for battery-only endurance (typically 30 minutes or less in light aircraft).
- Squawk 7600 if radios fail; squawk 7700 for a full emergency.
Glass Cockpit Failures
Electronic flight instrument systems (EFIS) introduce new failure modes. A complete PFD failure typically defaults information to the MFD via reversionary mode. Loss of the AHRS removes attitude and heading; loss of the ADC removes altitude, airspeed, and vertical speed. The aircraft must have a functioning standby attitude indicator, altimeter, and airspeed indicator for IFR flight under 14 CFR 91.205(d). Treat standby instruments as the primary reference once the EFIS is unreliable.
Recognition and Cross-Check
The key to handling any instrument failure is a disciplined scan and cross-check. No single instrument tells the whole story; corroborate every indication with at least one other source:
- Attitude indicator vs. TC + altimeter + VSI.
- Heading indicator vs. magnetic compass.
- Airspeed vs. expected pitch and power.
When instruments disagree, identify which are unreliable, cover them if possible, and continue flying the aircraft on the remaining valid instruments — aviate, navigate, communicate.
Example: In cruise at 6,000 ft, the ASI begins climbing as you climb and falling as you descend, while altimeter and VSI behave normally. This is a classic blocked pitot tube with open drain hole — wait, both blocked scenario (ASI acting as an altimeter). Apply pitot heat, fly known pitch and power settings for cruise airspeed, and advise ATC.
Proficiency in partial-panel work is required during the instrument practical test, and it should be practiced regularly under the hood with a safety pilot or in a simulator.