Learn & Understand

The Altimeter Settings Behind "From High to Low, Look Out Below"

The companion calculator converts a field elevation and altimeter setting into pressure altitude - the altitude on standard pressure. That conversion is one corner of a larger system of pressure settings that keeps every aircraft in the sky reading altitudes consistently. When that system is mis-set, or the atmosphere itself misbehaves, an altimeter can confidently display an altitude the aircraft does not actually have - and that error has driven aircraft into terrain.

An Altimeter Is a Barometer With an Altitude Scale

An altimeter does not measure height. It measures air pressure and converts it to an altitude using an assumption about the pressure at the reference level. Change that reference assumption - the setting in the little window - and every altitude the instrument shows changes with it. That is the whole reason pilots constantly reset their altimeters, and the reason there are several standard things to set it to.

The Three Settings

Above a transition altitude, everyone switches to the standard QNE setting. This is the key insight the companion calculator embodies: at high level, nobody's altimeter shows true height above the sea - but because every aircraft is using the identical standard reference, they are all wrong by the same amount, and their vertical separation from each other stays exact. Consistency beats accuracy when the goal is keeping aircraft apart.

Two Ways the Reading Goes Dangerously High

The classic mnemonic is "from high to low, look out below," and it covers two separate hazards that point the same way:

• Flying into lower pressure. If you fly from a high-pressure area toward a low-pressure area without resetting, the altimeter keeps using the old, higher reference and over-reads - it shows you higher than you truly are, so your real altitude is below what the instrument claims.
• Flying into colder air. Altimeters assume standard temperature. In air colder than standard, the atmosphere is physically compressed, so true altitude is lower than indicated. On a very cold day an aircraft on a published approach altitude can be meaningfully lower than the number in the window - which is why cold-temperature altitude corrections exist for approaches in extreme cold.

Both errors share the deadly property that the instrument reassures you while the ground rises toward you.

When the Window Was Wrong: Terrain Accidents

Controlled flight into terrain - an airworthy aircraft flown into the ground under control - has repeatedly involved altimeter reference errors: a setting left on standard during a descent, a mis-set subscale, or an uncorrected cold-temperature error on a mountainous approach. The aircraft was flying a perfectly good altitude according to its instruments; the instruments were referenced to the wrong thing. It is precisely because pressure altitude and the local setting can diverge so far that the discipline of setting and cross-checking the subscale is treated as sacred.

Reading Pressure Altitude for What It Is

Use the calculator's pressure altitude as the clean, standard-reference number it is - the right input for density altitude and for flight-level flying. Just keep the mental asterisk that pressure altitude is deliberately not your true height above the sea, and that the gap between them widens exactly when pressure is low or the air is cold, which is exactly when terrain is least forgiving.