Mode-C Off (Stop Squawk Altitude)
Mode-C off is an ATC instruction to disable altitude reporting of the transponder, while the squawk code is still transmitted.
Source: ICAO Doc 4444 PANS-ATM, Chapter 8; ICAO Doc 9432.
When required?
1. Altitude encoder defect
- Transponder sends wrong altitude (e.g. instead of 5000 ft transmits 50,000 ft).
- ATC detects defect: substantial deviation from pilot-reported altitude.
- ATC instructs: "Stop squawk altitude" — pilot switches Mode C off.
2. Interference with other traffic
- Altitude reporting error causes radar conflicts.
- ATC can reduce one aircraft to Mode A to separate.
3. Test / maintenance
- During maintenance work on the altitude encoder.
Phraseology
ATC instruction
"DEMRA, stop squawk altitude" or "DEMRA, stop Mode Charlie".
Pilot read-back
"Stop squawk altitude, DEMRA."
Resumption
ATC when all OK: "DEMRA, resume squawk altitude".
Pilot: "Resume squawk altitude, DEMRA."
Cockpit action
Mode switch
Switch to Mode A (ON) instead of Mode C (ALT):
- Code is still transmitted.
- Altitude is not transmitted.
What happens?
- ATC still sees the code (squawk).
- But no 3D information anymore → ATC loses vertical separation.
- → VFR flights may continue with Mode A; IFR is handled with extra caution.
What the pilot must do additionally
Immediate altitude reporting
Pilot must now actively report altitude:
- At each position report.
- On altitude change (climb / descent).
- On ATC query.
Example
Pilot leaving an altitude (instead of Mode C automatic):
- "DEMRA, leaving flight level 100, descending to flight level 80."
Subsequent defect correction
- After flight: have altitude encoder serviced.
- Maintenance unit: encoder calibration and test.
- Only after repair: use Mode C again.
Mode-switch comparison
| Switch position | Function |
|---|---|
| STBY (stand-by) | Transponder on, not transmitting |
| ON / A (Mode A) | Transmits code, NO altitude |
| ALT / C (Mode C) | Transmits code + altitude |
| S (Mode S) | Transmits code + altitude + extended data |
Mode-C off = switch from ALT to ON (or STBY per POH).
Special cases
Mode-S test
With Mode-S transponders (modern):
- Altitude info is contained in Mode S data.
- "Stop squawk altitude" can be non-trivial.
- ATC may request Mode A + IDENT routine instead.
Pilot can no longer Mode A
If transponder defective to Mode A:
- Pilot reports ATC: "Negative Mode C, transponder defective."
- ATC decides procedure.
Practical significance
Mode-C off is rare in modern airspace:
- Defective altitude encoders are rare in certified aircraft.
- Follow POH procedure on defect.
Cross-reference
- Subject 090 Lesson "Modes": Mode A/C/S theory.
- Subject 020: pitot-static and altitude encoder.