Most of the world uses the metric system — meters, liters, and grams — while a few countries (the U.S., Liberia, and Myanmar) primarily employ the “customary” or “imperial” system — feet, gallons, and pounds. Canada was one of the latter countries until 1970 when the nation began to change over. Metrication (as it was called) took some time, though — fifteen years, maybe more. One of the industries to change over late in the process was the airline industry, which given the expense and longevity of the equipment makes sense. On July 23, 1983, Air Canada Flight 143 was one of the earlier flights using the new metric units.

Unfortunately, Air Canada was undergoing a second change at the same time — smaller flight crews. Typically, there were three core members of the crew — the pilot, the co-pilot, and a flight engineer. Flight 143, through, didn’t have a flight engineer; the pre-flight fueling protocol therefore fell to the pilots. That protocol required them to convert volume (liters) into mass (kilograms or pounds, depending on the system you use) in order to figure out how much fuel you need to add to your plane. And they did exactly what they should have done — except they got the labels wrong. Instead of figuring out how many liters the plane needed to hit the required payload of 22,300 kg, the crew calculated how many liters were needed to hit 22,300 pounds. Wikipedia explained the math:

The volume of jet fuel varies with temperature. In this case, the mass of a litre of fuel was 0.803 kg, so the correct calculation was:

• 7,682 L × 0.803  kg/L = 6,169 kg  : fuel already onboard
• 20,088 L × 0.803  kg/L = 16,131 kg : fuel to be transferred to plane
• 27,770 L × 0.803  kg/L = 22,300 kg : fuel for flight

Between the ground crew and pilots, they arrived at an incorrect conversion factor of 1.77, which was the weight of a litre of fuel in pounds. This was the conversion factor provided on the refueller’s paperwork and which had always been used for the airline’s imperial-calibrated fleet. Their incorrect calculation was:

• 7,682 L × 1.77 lb/L = 13,597 lb : fuel already onboard
• 4,916 L × 1.77 lb/L = 8,703 lb : fuel to be transferred to plane
• 12,598 L × 1.77 lb/L = 22,300 lb : fuel for flight

Instead of 22,300 kg (27,770 L) of fuel, they had 22,300 pounds (12,598 L) on board — 10,100 kg, about half the amount required to reach their destination.

Half the fuel needed, of course, means you’re only getting halfway to your destination. Which, when the vehicle in question is an airplane cruising 12,500 meters (41,000 feet) above the ground… well, that’s a problem.

Luckily, the pilot and co-pilot were better at the flying part than the math part of the job. As the New York Times reported a week after the incident, the pilot had ten years of glider training under his belt, and his co-pilot had trained at Gimli during his days with the Canadian Air Force — and therefore knew the surroundings quite well. Together, the pair were able to land the 767 — gliding the last 60 miles (100 km) — touching down just an hour or so before nightfall. The plane — the Gimli Glider, as it’d henceforth be known — suffered some damage to its nose and blew out some tires, as seen above, but the passengers were pretty much okay. Ten people suffered minor injuries but, miraculously, there were no fatalities.