AIM ¶ 7-3-1 — Cold Temperature Altimeter Error
AIM 7-3-1 explains how cold temperatures cause barometric altimeters to read higher than true altitude. Study the ICAO error table for checkride prep.
Your barometric altimeter is calibrated to a standard atmosphere: 15°C at sea level, decreasing 2°C per 1,000 feet. When real-world temperatures don't match this standard, your indicated altitude and true altitude drift apart.
The key rule to remember: "From hot to cold, look out below."
- When the ambient air is colder than standard, your true altitude is LOWER than what the altimeter shows. You're closer to the ground than indicated — a serious terrain and obstacle clearance hazard.
- When the ambient air is warmer than standard, your true altitude is HIGHER than indicated.
For example, at 5,000 feet on a standard day, you'd expect about 5°C outside. If it's significantly colder, your altimeter is lying to you in the dangerous direction.
The ICAO Cold Temperature Error Table (TBL 7-3-1) lets you quantify the error. To use it:
- Find the reported temperature in the left column.
- Find the height above the airport (flight altitude minus airport elevation) across the top.
- The intersection shows how many feet you may be below your indicated altitude.
This matters most on instrument approaches in cold climates, where minimum altitudes provide tight obstacle clearance.
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