Editing Gimli Glider (section)

=== Miscalculation during fuelling ===
In older aircraft with a three-person crew, the [[flight engineer]] kept a fuel log and supervised the fuelling. The Boeing 767 belonged to a new generation of aircraft that flew with only a pilot and co-pilot, but Air Canada had not clearly assigned responsibility for supervising the fuelling.<ref name="final_report" />{{Reference page|page=|pages=64–65}} On the day of the accident, two technicians and two pilots worked on the calculation in Montreal. One technician stopped after he found that he was not making any progress. Another technician was using a piece of paper in his pocket, and he stopped when he ran out of space. First Officer Quintal did the calculation by hand, and Captain Pearson checked the arithmetic with his [[Jeppesen]] [[slide rule]].<ref name="final_report" />{{Reference page|page=|pages=40–41}}

Since the FQIS was not working, Captain Pearson decided to take on enough fuel to reach Edmonton without refuelling at Ottawa.<ref name="final_report" />{{Reference page|page=26}} The flight plan showed that {{convert|22300|kg}} of fuel were required for the flight from Montreal to Ottawa to Edmonton. A dripstick check found that {{convert|7682|L}} of fuel was already in the tanks. To calculate how much fuel the airplane had to take on, he needed to convert the 7,682{{nbs}}litres of fuel already in the tanks to their equivalent mass in kilograms, subtract that figure from the 22,300&nbsp;kg total fuel that would be needed, and convert that result back into its equivalent volume.<ref name="final_report" />{{Reference page|page=41}} The density in metric units was 0.803&nbsp;kg/L, so the correct calculation would have been:

:7,682 L × 0.803 kg/L = 6,169 kg = mass of fuel already on board
:22,300 kg − 6,169 kg = 16,131 kg = mass of additional fuel required, or
:16,131 kg ÷ (0.803 kg/L) = 20,088 L = volume of additional fuel required

At the time of the accident, Canada's aviation sector was converting from imperial to metric units. As part of this process, the new 767s acquired by Air Canada were the first to be calibrated for metric units.<ref name="final_report" />{{Reference page|page=|pages=63–64}} The fueler reported that the density of jet fuel at the time was 1.77, which was in lb/L, since other Air Canada aircraft used lb. Pearson and Quintal both used the density of jet fuel in lb/L without converting to kg/L:<ref name="final_report" />{{Reference page|page=|pages=40–41}}

:7,682 L × 1.77 lb/L = 13,597 lb = misinterpreted as kilograms of fuel already on board
:22,300 kg − 13,597 kg = 8,703 kg = incorrect mass of additional fuel required
:8,703 kg ÷ (1.77 lb/L) = 4,917 L·kg/lb = misinterpreted as litres of additional fuel required

Instead of taking on the {{convert|20,088|L}} of additional fuel that they required, they took on only {{convert|4,917|L}}. The use of the incorrect conversion factor led to a total fuel load of only {{convert|22300|lb|kg|abbr=on}} rather than the {{convert|49170|lb|kg|abbr=on}} that were needed. This was less than half of the amount required to reach their destination.<ref>{{cite web |url=http://code7700.com/mishap_air_canada_143.html |title=Air Canada 143 – Accident Case Study |website=www.code7700.com |date=June 7, 2014 |access-date=September 15, 2016 |archive-date=October 8, 2016 |archive-url=https://web.archive.org/web/20161008031256/http://code7700.com/mishap_air_canada_143.html |url-status=dead}} (the source contains two typos regarding the weight of fuel already on board: ''6669'' and ''66169'', instead of the correct value of ''6169'')</ref>

The flight management computer (FMC) measures fuel consumption, allowing the crew to keep track of fuel burned as the flight progresses. It is normally updated automatically by the FQIS, but the fuel quantity can also be entered manually. Because the FMC would reset during the stopover in Ottawa, the captain had the fuel tanks measured again with the [[dripstick]]. With {{convert|11,430|litre}} of fuel in the tanks, the fueler gave a density of 1.78. Repeating the same error, Captain Pearson determined that he had {{convert|20,400|kg|abbr=on}} of fuel and entered this number into the FMC. However, he actually had just {{convert|20400|lb|kg|-1|order=flip|abbr=on}} of fuel.<ref name="final_report" />{{Reference page|page=|pages=42–43}}

The previous flight from Edmonton to Montreal had avoided the error. The fueler at Edmonton knew the density of jet fuel in kg/L, and he calculated the correct number of litres to pump into the tanks. He testified that it was a "regular practice of his" to do such calculations. When fuelling was complete, Captains Weir and Johnson checked the figures. The captain knew "from previous experience" the density of jet fuel in kg/L. He also had a working FQIS, which agreed with his calculations.<ref name="final_report" />{{Reference page|page=|pages=43–44}}