Gimli Glider: Metric–Imperial Fuel Miscalculation Leaves Boeing 767 Without Fuel at 41,000 Feet
What happened
On 23 July 1983, Air Canada Flight 143 — a Boeing 767 with 69 people aboard — ran completely out of fuel at 41,000 feet over Manitoba and had to glide dead-stick to an emergency landing at a decommissioned military airfield in Gimli. All 69 survived. The cause was a unit conversion error: ground crew calculated the required fuel quantity in pounds, but the Boeing 767's new fuel management system reported mass in kilograms. Canada had recently adopted metric units; the wrong conversion factor was applied and the aircraft departed with less than half the fuel it needed.[1]
What went wrong
Canada's transition to the metric system left Air Canada in an ambiguous state between unit systems. The 767 was the airline's first jet with a digital fuel quantity system displaying kilograms. Ground crew used the old factor of 1.77 lbs/litre instead of the correct 0.80 kg/litre to calculate the required volume. Three separate checks — the captain, the dispatcher, and the fuelling crew — all applied the wrong factor, because all three shared the same mistaken assumption about which units the system used. The aircraft departed with 22,300 lbs of fuel instead of 22,300 kg. The same root cause — metric/imperial unit confusion — would destroy the $327M Mars Climate Orbiter 16 years later.[1]
Lesson learned
Unit systems are a systemic failure mode, not individual error. When multiple independent checkers share the same wrong assumption, independent verification provides no protection. Safety-critical fuel calculations must display units explicitly at every step and enforce dimensional consistency in software. The Gimli Glider and Mars Climate Orbiter failures, 16 years apart, reflect the same lesson unlearned.
Sources
- [1]
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