On a July evening in 1983, Air Canada Flight 143, a nearly new Boeing 767, was cruising high over Canada when both engines flamed out. The aircraft had simply run out of fuel, roughly halfway to its destination. What followed became one of aviation’s most famous saves: the captain, an experienced glider pilot, dead-sticked the airliner onto a disused runway at Gimli, Manitoba, and everyone aboard survived. The aircraft was thereafter nicknamed the Gimli Glider.
The reason a modern jet departed with about half the fuel it needed was not a leak or a technical exotic. It was a unit conversion error, made at the worst possible moment in Canadian history to make one.
Caught between pounds and kilograms
Canada was in the middle of converting to the metric system, and the 767 was among the first aircraft in the fleet whose fuel was measured in kilograms rather than pounds. On the day of the flight, the aircraft’s fuel-quantity computer was unserviceable, so the fuel load had to be worked out by hand from a dripstick measurement of volume in liters.
Converting liters to mass requires multiplying by fuel density, and here the error slipped in: the calculation used a density factor appropriate to pounds per liter, about 1.77, instead of kilograms per liter, about 0.8. Since a kilogram is about 2.2 pounds, the arithmetic made the fuel appear more than twice as heavy as it was. The crew believed they carried over 20,000 kilograms; the tanks actually held roughly half of what the flight required.
Gliding a jet with no engines
When the engines quit at altitude, the 767 lost its main electrical generation and most hydraulics, leaving limited instruments and a small air-driven turbine for essential systems. The captain’s gliding experience and the first officer’s knowledge of the abandoned Gimli airfield, a former air-force base he had once served at, combined into an improvised approach, including a dramatic forward slip to lose altitude. The runway was in use for motor racing that day, yet the aircraft landed with a collapsed nose gear and no loss of life, and it was repaired and returned to service for decades afterward.
The quiet lessons
The subsequent investigation spread responsibility across the system: an ambiguous handling of the faulty fuel gauge, procedures that left a safety-critical conversion to manual arithmetic, and a metrication transition that had pounds and kilograms coexisting in the same airline. The fixes were systemic, clearer procedures, better training, and eventually cockpit and refueling practices that leave far less room for a density factor to be plucked from the wrong system.
The Gimli Glider endures as the friendliest possible version of a hard lesson: unit conversions are safety-critical, so when a calculation involves two measurement systems, slow down and check which one every number belongs to.
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