Flight levels vs altitude — the pressure switch at 18,000 ft

An altimeter is really just a barometer with a fancy dial. It measures ambient air pressure and converts it to a height, using a reference pressure the pilot dials in. That reference is called the altimeter setting. When you set your altimeter to the local sea-level pressure (say, 30.12 inches of mercury on a clear day), the altimeter reads true altitude above sea level. Everyone in the local area doing the same thing agrees on altitude within a few feet.

The problem is that local pressure varies from place to place — a low-pressure system over Chicago might read 29.60 inHg while a high over Denver reads 30.30. A pilot flying from one to the other would have to keep updating the altimeter setting every 100 nautical miles. That's fine for low-level flying, but at cruise altitude with a hundred jets in the same block of sky, it's a recipe for one aircraft thinking it's at 35,000 while another thinks it's at 34,800 when they're actually at the same height.

The 18,000 ft switch

So above a certain altitude — 18,000 ft in the US, 3,000 to 6,000 ft in most of Europe (varies by country) — every aircraft sets their altimeter to a standard reference: 29.92 inHg, or 1013.25 hPa. This is called QNE. Everyone is now flying at an altitude referenced to the same imaginary sea-level pressure, so everyone agrees on height relative to each other, even if that height doesn't match true altitude above the actual sea. The reading is called pressure altitude, and to distinguish it from real altitude, controllers call it a Flight Level.

FL350 means 35,000 feet of pressure altitude referenced to 29.92 inHg. On a cold day when actual atmospheric pressure is lower than standard, an aircraft at FL350 might really only be at 34,200 ft above sea level. On a hot day, it might be at 35,700. Doesn't matter — every other jet in the airspace is using the same reference, so vertical separation between aircraft is preserved. Terrain clearance is handled separately because pilots know the offset.

Why feet at all, in a metric world

The world switched to metric for most things but international aviation stayed with feet for altitude, nautical miles for distance, and knots for speed. Russia and China used to use metres for flight levels — 10,600 m instead of FL350 — but since the 2000s most Russian and Chinese controllers issue clearances in feet on international routes for compatibility. There are still some domestic metric-altitude flights in China, and they involve careful conversion at handoff to international airspace.

The reason the metric switch never happened is boring: every altimeter, every autopilot, every chart, every controller radio call, and every pilot's training is in feet. Replacing all of it globally would cost billions and gain nothing in accuracy. So feet it is, probably forever.

The one exception

Vertical separation. Below FL290 (29,000 ft), aircraft must be separated by 1000 ft vertically. Between FL290 and FL410, most of the world now uses Reduced Vertical Separation Minima (RVSM) which allows 1000 ft separation with certified altimeters — before RVSM was introduced in the 2000s, separation up there was 2000 ft. RVSM effectively doubled the useful capacity of the high-altitude cruise band, which is why airlines lobbied so hard for it.