Editing Gimli Glider (section)

===Fuel-quantity indication system===
{{More sources|section|date=October 2024}}
The amount of fuel in the tanks of a Boeing 767 is computed by the FQIS and displayed in the cockpit. The FQIS on the aircraft was a dual-processor channel, each independently calculating the fuel load and cross-checking with the other. In the event of one failing, the other could still operate alone, but in that case, the indicated quantity was required to be cross-checked against a [[floatstick]] measurement before departure. If both channels fail, no fuel display was seen in the cockpit, and the aircraft would be considered unserviceable and not authorized to fly.

Because inconsistencies had been found with the FQIS in other 767s, Boeing issued a service bulletin for the routine checking of this system. An engineer in Edmonton duly did so when the aircraft arrived from Toronto following a trouble-free flight the day before the accident. While conducting this check, the FQIS failed, and the cockpit fuel gauges went blank. The engineer had encountered the same problem earlier in the month when this same aircraft had arrived from Toronto with an FQIS fault. He found then that disabling the second channel by pulling the [[circuit breaker]] in the cockpit restored the fuel gauges to working order, albeit with only the single FQIS channel operative. Without any spares, he repeated this temporary fix by pulling and tagging the circuit breaker.

A record of all actions and findings was made in the maintenance log, including the entry: "SERVICE CHK – FOUND FUEL QTY IND BLANK – FUEL QTY #2 C/B PULLED & TAGGED...".<ref>{{cite book |last=Stewart |first=Stanley |title=Emergency, Crisis on the Flightdeck |year=1992 |publisher=Airlife Publishing Ltd |isbn=978-1-85310-348-3 |page=123}}</ref> This reports that the fuel gauges were blank and that the second FQIS channel was disabled, but it does not make clear that the latter fixed the former.

The aircraft flew from Edmonton to Montreal on the day of the accident. Before departure, the engineer informed the pilot of the problem and confirmed that the tanks must be verified with a floatstick. In a misunderstanding, the pilot believed the aircraft had been flown with the fault from Toronto the previous afternoon. The flight to Montreal proceeded uneventfully, with fuel gauges operating correctly on the single channel.

On arrival in Montreal, a new crew boarded for the return flight to Edmonton. The outgoing pilot informed Captain Pearson and First Officer Quintal of the problem with the FQIS and passed along his mistaken belief that the aircraft had flown the previous day with this problem. In a further misunderstanding, Captain Pearson believed he was also being told that the FQIS had been completely unserviceable since then.

While the aircraft was being prepared to return to Edmonton, a maintenance worker decided to investigate the problem with the faulty FQIS. To test the system, he re-enabled the second channel, at which point the fuel gauges in the cockpit went blank. However, before he could disable the second channel again, he was called away to perform a floatstick measurement of fuel remaining in the tanks, leaving the circuit breaker tagged (which masked the fact that it was no longer pulled). The FQIS was now completely unserviceable, and the fuel gauges were blank.

On entering the cockpit, Captain Pearson saw what he was expecting to see—blank fuel gauges and a tagged circuit breaker. Pearson consulted the [[master minimum equipment list]] (MMEL), which indicated that the aircraft was not legal to fly with blank fuel gauges. Still, due to a misunderstanding, Pearson believed it was safe to fly if the amount of fuel was confirmed with measuring sticks.<ref name="ASN">{{cite web |url=http://aviation-safety.net/database/record.php?id=19830723-0 |title=Accident Boeing 767-233 C-GAUN, Saturday 23 July 1983 |author=<!--Staff writer(s); no by-line.--> |date=2016 |website=[[Aviation Safety Network]] |access-date=November 3, 2024}}</ref>

The 767 was still a very new aircraft, having flown its [[maiden flight]] in September 1981. C-GAUN was the 47th Boeing 767 off the production line and had been delivered to Air Canada fewer than four months prior to this flight. In that time, 55 changes had been made to the MMEL, and some pages were blank pending development of procedures.

Because of this unreliability, maintenance personnel authorizing flights had become standard practice. To add to his misconceptions about the aircraft's flying condition since the previous day, reinforced by what he saw in the cockpit, Pearson now had a signed-off maintenance log, which had become customarily preferred over the MMEL.