The American system of measurement and its units—feet, miles, pounds, and gallons—are quite bizarre. They are random, unintuitive and have no logical relationship to one another, which makes conversion from one type to another a nightmare. In the 1970s, the United States tried to push the country to metric and even passed a law that directed federal agencies to adopt the metric system. But the legislation was not strong enough and many industries chose not to convert. However, some industries, having realized the benefit of the metric system, decided to switch on their own volition.
Now there are two different systems at work, and anytime two different industries using two different system of measurement have to coordinate, extra care needs to be taken to ensure the units are correctly converted. Otherwise the results could be disastrous, just like Air Canada found out in 1983.
Air Canada Boeing 767 C-GAUN, “the Gimli Glider”, taxing at San Francisco International in February 1985.
On the evening of July 22, 1983, an Air Canada Boeing 767 C-GAUN arrived in Edmonton, Alberta, following a flight from Toronto and spent the night at the tarmac. During the stopover, a technician performing a routine check of the airplane and discovered that the plane’s fuel gauges were not working. The technician tracked the problem down to a defective channel in the fuel-quantity indication system (FQIS), a computer that takes reading from the fuel sensors in the tank and converts it into the various units required by other aircraft systems, including the fuel gauges. The FQIS uses two redundant channels, but a failure in only one channel causes the entire FQIS to fail. The technician managed to solve the problem by disabling the faulty channel, and the gauges blinked back to life. The technician logged the temporary fix in the aircraft’s logbook, and also left instruction that the fuel levels need to be checked manually, as the protocol required as additional safety measures since one of the processor channels was inoperative.
The next morning, Captain John Weir and co-pilot Captain Donald Johnson were told about the problem. Since the FQIS was operating on a single channel, a manual fuel drip measurement was taken to double check the amount of fuel in the tanks, and no discrepancy was found. The plane flew to Toronto and then to Montreal without incident.
At Montreal, the airplane was taken over by Captain Bob Pearson and First Officer Maurice Quintal. Before handing over the airplane, Weir described the problem to Pearson, but the latter got the impression that not only the FQIS was at fault, but the gauges themselves had been blank.
As they waited for a dripstick test, a ground engineer climbed into the cockpit to inspect the FQIS. He found the circuit breaker that the technician at Edmonton had pulled out to disable the faulty channel, and in order to check his diagnosis, pushed the breaker back into channel 2. Immediately, the fuel gauges went offline. The technician logged his observation in the logbook, but before he could pull out the circuit breaker, he was called back out of the aircraft to assist in the drip check currently in progress. The technician left the craft forgetting to return the circuit breakers to their original positions. When Captain Pearson and his First Officer Maurice Quintal arrived in the cockpit minutes later, they found the fuel gauges blank, as they had expected.
The dripstick test involves pulling out a hollow tube from the underside of the fuel tank located on the wings of an aircraft. When the top of the dripstick is withdrawn below the level of the fuel, fuel enters it and drips through a hole in the cap. Graduations on the tube indicate the depth of the fuel in the tank in centimeters. This value is converted into volume in liters and then into kilograms by multiplying this figure by the density of jet fuel. Traditionally, on all Air Canada aircrafts, the density was measured in pounds per liter, but this particular aircraft, the 767, was an entirely new model to enter service which used the metric system instead of imperial. But the technicians didn’t know that.
During calculation, the fueler consulted his documentation, which still reported all figures in the Imperial system, and found that the density of jet fuel was 1.77 pounds/liter. Using that conversion factor, the technicians calculated that the airplane needed 4,917 liters of additional fuel for the flight from Montreal to Ottawa to Edmonton. The correct conversion factor should have 0.803 kg/liter, which would have yielded a figure of 20,088 liters. The dripstick check found that 7,682 liters of fuel were already in the tanks, which meant that when the plane took off from Montreal on July 23, 1983, it had less than half the amount required to reach their destination. A working fuel gauge would have shown the discrepancy, but the fuel gauges were offline and without them there was no other way for the pilots to know the real amount of fuel the airplane was carrying.
The plane reached Ottawa without incident. Some passengers disembarked, others boarded, and again the ground technicians carried a fuel check, making the same exact error. Everything was found to be in order, and the air traffic controller gave the go ahead, and once again the plane took off destined for Edmonton.
Shortly after 8 pm, when Flight 143 was cruising over Red Lake, Ontario, at 41,000 feet the plane ran out of fuel. At first the pilots assumed an instrumentation error and turned off the alarms. But minutes later, both the engines flamed out. The 767 was one of the first jets to use an electronic instrument system powered by its engines. This meant that when the engines stopped working, all the instruments went dark. However, the jet was equipped with a ram air turbine (RAT) that dropped out of the fuselage and generated enough power to function emergency instruments and also provided some hydraulic support for the crew to be able to maneuver the plane.
Pearson and Quintal decided to divert the plane to Winnipeg, 65 miles away. But after making some calculation, Quintal announced that they would never make to Winnipeg. Instead, he suggested that they try to land at a decommissioned military airbase in the town of Gimli, some 45 miles from their position, near the shores of Lake Winnipeg.
Gimli had two 7,200-foot runways, but the pilots did not know that the runways had been converted to a race track complex, now known as Gimli Motorsports Park. It included a road race course, a go-kart track, and a dragstrip. As fate would have it, it was race day and the track was full of cars and campers.
Without main power, the pilots used a technique called ‘gravity drop’ to lower the landing gear and lock it into place. The main gear locked into position, but the nose wheel was too light to drop by gravity alone and was only partially extended.
As the plane approached the runway, the pilots realized they were coming in too high and fast. If they didn’t loose altitude fast, they would overshoot the runway entirely. The pilots briefly considered making a 360° loop to reduce speed and altitude, but they decided that they did not have enough altitude for the maneuver. Pearson, an experienced glider pilot, decided to execute a forward slip to increase drag and reduce altitude. A forward slip is a technique commonly used in gliders and light aircraft to descend more quickly without increasing forward speed. It is almost never used in large jet airliners. Admiral Cloudberg explains how a slip can be executed:
A slip can be induced on any aircraft by steering the nose in one direction with the rudder, while banking in the opposite direction with the ailerons to compensate. This allows the plane to maintain its present course while skidding or slipping with one side facing into the oncoming air and the forward wing pointed at the ground. Pointing the side of the fuselage into the airstream in this manner generates enormous drag which will cause the plane to descend while also keeping its forward airspeed in check.
With both of its engines dead, the plane made hardly any noise as it approached the drag strip. Pearson could only hope people got out of the way. As the gliding plane closed in on the decommissioned runway, the pilots noticed two boys were riding bicycles within 1,000 feet of the projected point of impact. Captain Pearson later said that the boys were so close that he could see the looks of sheer terror on their faces as they realized that a large aircraft was bearing down on them.
As the plane touched down, the nose gear collapsed back into the wheel well, causing the aircraft’s nose to slam into the tarmac and then scrap along the ground. The friction helped slow the plane down and kept it from crashing into the crowds surrounding the runway. All 61 people on board survived.
After an investigation by Air Canada, Captain Pearson was demoted for six months, and First Officer Quintal was suspended for two weeks for their role in the fuel miscalculation and making the erroneous and reckless decision to fly with blank fuel gauges. These suspensions were overturned following an appeal. Two years later, Pearson and Quintal were awarded the first ever Fédération Aéronautique Internationale Diploma for Outstanding Airmanship.
The airplane was repaired and returned to service with Air Canada, where it continued to fly for 25 years until its decommission in 2008. The Gimli Glider was scrapped in 2014.