Altimeter error (non-standard air)
Altimeter always assumes standard atmosphere: 15 °C, 29.92", cooling 1.98 °C/1000 ft. Real air differs: indicated altitude wrong even with correct setting.
Pressure (TC AIM AIR 1.5.3):
| Condition | Altimeter | True altitude |
|---|---|---|
| Fly toward lower pressure, or hold old too-high setting | Over-reads | Below indication |
| Fly toward higher pressure, or too-low setting | Under-reads | Above indication |
- Size: 1,000 ft per 1 in.Hg of setting error.
- Formula:
true = indicated - (set - local) x 1000. - Worked: set 29.80, local 29.20, indicated 6,000 ft: 6,000 - 600 = 5,400 ft.
Other errors:
| Cause | Effect on true altitude | Detail | Cite |
|---|---|---|---|
| Warmer than ISA | Above indication | Subscale corrects surface pressure only, not mean temperature of air column below | TC AIM AIR 1.5.3, 1.5.4 |
| Extreme high pressure | Higher than indicated | Subscale sets only 28.0 to 31.0" Hg; very cold, dry air can push sea-level pressure above 31.00", past most altimeters | TC AIM AIR 1.5.3, 1.5.9 |
| Near mountains | Below indication | Wind speeding around peak or through valley lowers local pressure (Bernoulli's Principle); altimeter over-reads where terrain is closest | TC AIM AIR 1.5.6 |
- Altimeter reads true altitude only on the ground at that airport, with its current setting on subscale (TC AIM AIR 1.5.4).
Example (TC AIM AIR 1.5.3):
- Set 29.80 at 500 ft field.
- Subscale not reset until 25 NM from 1,000 ft destination (29.20).
- Crossed 4,800 ft ridge at 6,000 ft indicated.
- Clearance 600 ft, not 1,200 ft expected.
Trap: "From high to low, look out below." Altimeter reads high; true altitude lower.
References
- TC AIM AIR 1.5.3
- TC AIM AIR 1.5.4
- TC AIM AIR 1.5.6
- TC AIM AIR 1.5.9