Learning objective: You decide in seconds which instrument still counts — a red OFF flag kills that gyro; a pitot-static lie arrives with no flag and hits ASI, altimeter, VSI.
60 seconds: You are flying IMC. OFF on the attitude indicator. The turn coordinator is alive; ASI and altimeter still move. You do not keep flying that horizon “a bit inaccurately”. Remember: flag = gyro power dead. No flag, but needles frozen or acting like an altimeter = pitot-static. Typical school: AI/DG vacuum, TC electric — two separate deaths.
Red in teaching = warning, do not use this system. Yellow = caution, procedures, often still time. Glass CAS captions only from the type AFM, no G1000 fiction.
Two failure families later items will cite:
| Family | What you see | What you do |
|---|---|---|
| Gyros (AI, DG, TC) | often an OFF flag, suction or DC gone | that gyro invalid; the other power source may still live |
| Pitot-static (ASI, ALT, VSI) | no gyro flag; needles lie or freeze | think drain/ram/static; alternate static per AFM |
The magnetic compass needs neither suction nor bus. Magnetos neither: Master accidentally OFF → electrics dead, engine keeps running. Starter silent: check Master ON first. Alternator offline: battery only, remaining time AFM/load, no universal minutes. Ammeter ≈ 0 on a full battery can be normal — confirm with the voltmeter.
A DC mark on the instrument means bus power, not vacuum.
“Red flag on the horizon — the TC confirms attitude, I’ll keep that picture.” That is wrong. People want one attitude picture. The TC shows rate, not pitch. And a pretty PFD with no power does not paint attitude — attitude comes from the gyro/AHRS.
Pitot-static fails you catch by contradiction without a flag: ASI climbs like an altimeter, ALT/VSI ok → ram trapped. ALT and VSI dead, ASI twisted → static. Detail lives in the ASI lessons; here only the split.
Exam knowledge
- OFF flag = this gyro invalid, not “roughly OK”.
- Typical: AI/DG vacuum, TC electric — separate failures.
- Pitot-static: no gyro flag; ASI/ALT/VSI lie.
- Magnetos and compass independent of the bus.
- Red warning / yellow caution = teaching; glass captions = type AFM.
- Master OFF: engine may still run. No invented amps/minutes.
Source: typical EASA PPL 020 gyro-power and bus teaching; FAA AFH fail flags. Voltage examples are school only; procedures AFM.
Check yourself
1. How do you spot an electrically powered flight instrument, and what does failure mean?
- A) Vacuum attitude and heading gyros always carry the marking “DC”
- B) A failed ASI shows the ram/drain/static table as a fail flag in the glass
- C) Electric instruments (electric AI, electric turn coordinator, GPS) are often marked “DC” and die with Master off or a flat battery; vacuum AI/DG are not “DC” and independent of the bus
- D) On any generator failure the vacuum attitude indicator dies first
2. Master accidentally OFF / electrical failure in flight — what lives, what dies?
- A) Magnetos fail at once and the engine stops, because ignition is on the bus
- B) Only the PFD remains; altimeter, ASI and vacuum gyros go to zero without the bus
- C) The failed gyro shows an OFF flag, pitot failure a red “PITOT” — that is the only picture this lesson teaches
- D) Magnetos keep running (engine lives). Mechanical instruments (altimeter, ASI, vacuum AI/DG, magnetic compass) keep running; radios/nav and electric gyros are affected
- C — “DC” means bus power. Vacuum instruments are not DC-marked.
- D — magnetos independent. Classic mechanical/vacuum instruments remain. OFF flags are gyro teaching, not the pitot-static table.