On 23 July 1983, an Air Canada Boeing 767 ran out of fuel at 41,000 feet over Ontario. Both engines flamed out. The crew glided the powerless jet to an abandoned airfield at Gimli and landed it with no engines and no major injuries, earning the aircraft its lasting name: the Gimli Glider. The cause was not weather, not sabotage, not mechanical failure. It was arithmetic.
The fuel had been calculated in the wrong units, pounds where kilograms were needed, while Canada was midway through converting to metric. The aircraft left the gate with roughly half the fuel everyone believed was on board.
That is the entire case for Annex 5 in a single story. Aviation is a global system in which numbers cross borders constantly, and a number is meaningless, or worse, dangerous, unless everyone agrees what unit it is in. Annex 5 to the Convention on International Civil Aviation is the standard for units of measurement used in air and ground operations. It is short, plain, and one of the most quietly important rulebooks in the entire system.
One table, agreed by everyone
The core of Annex 5 is a standardized table of units to be used across aviation, so that altitude, speed, distance, weight, pressure, temperature, and time mean the same thing to a pilot in Riyadh, a dispatcher in Frankfurt, and a controller in Atlanta. The aim is not elegance for its own sake. It is the elimination of the silent assumption: the moment when one person writes a number meaning one thing and another reads it meaning something else.
Annex 5 adopts the International System of Units (SI), the modern metric system, as the basic standard for aviation. SI is the foundation: the meter, the kilogram, the second, and the units derived from them, such as the newton for force and the pascal for pressure. If the story ended there, aviation would be cleanly metric. It does not, and the reasons are a study in how a global standard makes peace with history.
The units that refused to go metric
Despite SI being the baseline, Annex 5 explicitly keeps a set of non-SI units for aviation use, because the operational world was built around them and changing them would create more risk than it removes. Three are worth knowing by heart:
- The foot for altitude, elevation, and height. The entire global system of cruising levels, terrain clearance, and vertical separation is expressed in feet, and every altimeter, chart, and clearance assumes it.
- The nautical mile for distance. Tied directly to a minute of latitude, it maps naturally onto navigation across the curved surface of the Earth.
- The knot, one nautical mile per hour, for speed: the natural partner of the nautical mile.
This is the answer to the question in the title. Altitude is in feet but a runway length is given in meters because Annex 5 is a deliberate hybrid. It keeps SI as the destination while permitting the deeply embedded aviation units that the world's cockpits, charts, and procedures already depend on. The Annex has long carried the intention to move fully to SI eventually. In practice, that transition has been slow for a good reason: a half-finished switch, the exact condition that downed the Gimli aircraft, is more dangerous than either consistent state.
Figure 1 — The deliberate hybrid
The deliberate hybrid. SI is the baseline; feet, nautical miles and knots stay because the world's cockpits were built on them.A unit is a contract. The number on one side only means something if both sides signed the same one.
The standard reference points
Beyond distance and speed, Annex 5 fixes the references that keep the rest of the operation coherent.
Time is kept in Coordinated Universal Time (UTC), so that a flight crossing many time zones, and the controllers handing it along, all work to a single clock. A flight plan filed for departure at a UTC time means the same instant everywhere, with no zone arithmetic to get wrong. Temperature is given in degrees Celsius, which feeds directly into performance calculations: hot air is thin air, and takeoff numbers depend on it. Pressure is given in hectopascals (hPa), and this one is more than a convention.
Why pressure units are a safety matter
Altimeters do not measure height directly. They measure air pressure and convert it to an indicated altitude. That makes the altimeter setting a critical exchanged value. Below a defined transition altitude, crews set the local sea-level pressure, QNH, so the altimeter reads height above sea level. Some operations use QFE, referenced to the aerodrome, so it reads height above the field. Above the transition level, everyone sets the same standard pressure, 1013.2 hPa (29.92 inches of mercury), so that all aircraft in the cruise share one reference and their flight levels are mutually consistent. The entire scheme collapses if the pressure value is passed in an unexpected unit or misheard. A setting wrong by a small amount puts the aircraft hundreds of feet from where the crew believes it is, and near terrain or other traffic that is the difference between routine and catastrophe. Consistent pressure units are not bookkeeping. They are vertical safety.
Figure 2 — Two pressure references, one handover
Two pressure references, one handover. QNH keeps you honest against the ground; the standard setting keeps everyone honest against each other.What happens when units collide
The risk Annex 5 manages is specifically the risk of mixed or mis-converted units. It shows up in a few recurring shapes: fuel figured in one mass unit and loaded as another, as at Gimli; weight and balance computed in pounds on an aircraft documented in kilograms; fuel uplifted in liters but planned in gallons; a clearance or a chart altitude read in the wrong reference. The danger is sharpest exactly where Annex 5 permits a mixed environment, where SI and non-SI units legitimately coexist, because that is where a person can plausibly grab the wrong one. The defense is partly the standard and partly discipline: label the unit, confirm the unit, and never assume it.
How states adopt it, and where exceptions persist
States implement Annex 5 through their national operating rules, and the picture is mostly harmonized at the cockpit level: feet, knots, and nautical miles are effectively universal in the air, whatever a country uses on the ground. Saudi Arabia, through the General Authority of Civil Aviation (GACA), aligns with the ICAO unit system. The United States, through the FAA, uses the same aviation units in the air while remaining a largely non-metric country on the ground, which makes aviation an island of partial metrication inside it. Europe, under the European Union Aviation Safety Agency (EASA), likewise flies in feet and knots despite being thoroughly metric on the ground.
The pragmatic exceptions are the interesting part, and they are real. A few states have used meters for altitude: China's airspace is built on metric flight levels, and several states in the Commonwealth of Independent States long did the same before moving toward feet-based levels in recent years. Where metric and feet-based airspace meet, aircraft must convert their cruising levels at the boundary. It is a managed procedure, but it is exactly the kind of seam where an error can creep in. Annex 5 accepts that a single global flip to SI is not realistic in the near term, and instead manages the coexistence as carefully as it can.
Weight, balance, and the load sheet
The Gimli accident was about fuel, but the same unit discipline runs through the whole question of mass. An aircraft has to be loaded within strict weight limits, and its center of gravity kept inside a defined envelope, or it will not perform or handle as certified. All of that is computed on a load sheet, and every figure on it, the empty weight, the fuel, the passengers, the cargo, the limits themselves, has to be in a consistent mass unit. An aircraft documented in kilograms and loaded against numbers worked in pounds is the Gimli error wearing different clothes, and it has caused tail strikes and rejected takeoffs of its own.
Automation has reduced this risk without removing it. Modern flight management systems, fuel-quantity computers, and electronic load sheets do the conversions internally and consistently, which is a real safeguard. But automation brings its own failure mode: the moment a human has to enter a number by hand, a fuel figure read off a bowser, a cargo weight from a manifest in unfamiliar units, the old danger is back, now hidden inside a system everyone trusts. Annex 5 cannot legislate attention, but by fixing the units it removes one whole category of ambiguity the human never has to resolve under pressure.
Where it still falls short
For a standard this old, the open problems are remarkably consistent with the day it was written.
- The incomplete SI transition. The long-stated intent to move fully to SI remains unfinished, and a permanently half-converted system carries a permanent low-level risk that a clean system would not.
- Mixed-unit cockpit and ground environments. Where crews, fuelers, loaders, and dispatchers work in different units, every handoff is a conversion, and every conversion is a chance to be wrong.
- Conversion errors. The Gimli lesson never fully expires. Each new aircraft type, each cross-border operation, each fuel uplift in an unfamiliar country is another opportunity for pounds to be mistaken for kilograms, or liters for gallons.
There is a temptation to see units of measurement as the most boring corner of aviation regulation, a footnote to the real rules about how to fly. The Gimli Glider is the rebuttal. A modern jet, two qualified pilots, an airworthy aircraft, and the thing that nearly killed everyone aboard was a number in the wrong unit. Annex 5 is the standard that turns numbers into shared facts. It is dull in exactly the way that load-bearing things usually are.
Glossary
Center of Gravity
The balance point of the loaded aircraft, which must stay inside a defined envelope for the aircraft to perform and handle as certified.
Flight Level (FL)
An altitude flown on the standard pressure setting of 1013.2 hPa, expressed in hundreds of feet: FL350 is 35,000 feet on the standard setting.
Foot (ft)
The retained non-SI unit for altitude, elevation and height throughout world aviation.
Hectopascal (hPa)
The SI-derived pressure unit used for altimeter settings. 1013.2 hPa equals 29.92 inches of mercury.
Knot (kt)
One nautical mile per hour: the retained unit for speed.
Load Sheet
The document computing the aircraft's weight and balance for a flight. Every figure on it must be in one consistent mass unit.
Nautical Mile (NM)
The retained unit for distance, tied to a minute of latitude on the Earth's surface.
QFE
An altimeter setting referenced to the aerodrome, so the altimeter reads height above the field.
QNH
The local sea-level pressure setting, so the altimeter reads altitude above sea level.
SI (International System of Units)
The modern metric system: the baseline standard Annex 5 adopts for aviation.
Transition Altitude / Transition Level
The boundary where crews change between the local QNH setting and the shared standard setting of 1013.2 hPa.
UTC (Coordinated Universal Time)
The single clock all aviation runs on, regardless of time zone.
Sources
- Convention on International Civil Aviation (ICAO Doc 7300), Article 37.
- ICAO, Annex 5 to the Convention on International Civil Aviation, Units of Measurement to be Used in Air and Ground Operations.
- Final Report of the Board of Inquiry into the Air Canada Boeing 767 (C-GAUN) fuel exhaustion accident, Gimli, Manitoba, 23 July 1983.