True altitude in the standard pressure region
Finding true altitude in the SPR (TC AIM AIR 1.5.5):
- 29.92 set gives pressure altitude, not indicated altitude.
- Terrain clearance needs true altitude: get indicated altitude another way.
- Altimeter at 29.92 reads true altitude only if (TC AIM AIR 1.5.4):
- ICAO Standard Atmosphere conditions exist, or
- aircraft is at the one level where 29.92 is the correct setting.
Otherwise:
| Route | Method |
|---|---|
| Airport along route | Momentarily reset altimeter to nearest airport's setting |
| Water or barren land, no airports along route | Use forecast mean sea level pressure for the time and place |
- Sea level pressure unknown, pressure altitude used as indicated: "true" altitude is only true above the 29.92 level, wherever that sits relative to mean sea level.
Trap: 29.92 is true altitude only in ISA conditions or at the one level where 29.92 is correct.
References
- TC AIM AIR 1.5.4
- TC AIM AIR 1.5.5