Captain Peter Hamilton and First Officer Donald Rowland had flown on various flights together before, and had an ongoing discussion on when to arm the ground spoilers. They both agreed they did not like arming them at the beginning of the final approach, fearing it could lead to an inadvertent spoiler deployment. The captain preferred arming them on the ground, while the copilot preferred arming them during the landing flare.

The flare is executed just above the runway, causing the aircraft's nose to rotate up. That ensures the nose wheel does not contact the runway first, and it also reduces the rate of descent so that the main wheels will not impact the runway too hard. The thrust of the engines is reduced at the same time, causing the speed of the aircraft to slow significantly.

The pilots made an agreement that, when the captain was piloting the aircraft the first officer would deploy the spoilers on the ground, as the captain preferred, and when the first officer was piloting the aircraft the captain would arm them on the flare as copilot preferred.

On this particular instance however, the captain was piloting the landing and said, "All right. Give them to me on the flare. I have given up." This was not their usual routine. Sixty feet from the runway, the captain began to reduce power in preparation for the flare and said, "Okay" to the first officer. The first officer immediately deployed the spoilers on the flare, instead of just arming them. The aircraft began to sink heavily and the captain, realizing what had happened, pulled back on the control column and applied full thrust to all four engines. The nose lifted, but the aircraft still continued to sink, hitting the runway with enough force that the number four engine and pylon broke off the wing. There was also a tail strike at this time. Realizing what he had done, the first officer began apologizing to the captain. Apparently unaware of the severity of the damage inflicted on the aircraft, the crew managed to lift off for a go-around, but the lost fourth engine had torn off a piece of the lower wing plating and the aircraft was now trailing fuel, which ignited. The first officer requested a second landing attempt on the same runway but was told it was closed due to debris and was directed to another runway.

Two and a half minutes after the initial collision, the outboard section of the right wing above engine number four exploded, causing parts of the wing to break off. Six seconds after this explosion, another explosion occurred in the area of the number three engine, causing the pylon and engine to both break off and fall to the ground in flames. Six and a half seconds after the second explosion, a third explosion occurred, destroying most of the right wing, including the wing tip. The aircraft then went into a violent nose dive, striking the ground at a high velocity of about 220 knots (407 km/h) and killing all 100 passengers and the nine crew members on board.

The mishap was the first Air Canada accident involving fatalities since November 1963, when another DC-8, Flight 831, also bound from Montreal to Toronto, crashed with a loss of 118 lives. Wreckage, body parts, bits of clothing and women's pocketbooks were strewn for more than 90 metres (100 yards) beyond the impact spot. The plane dug a furrow eight or ten feet deep, less than 60 metres (200 feet) from the home of the Burgsma family, in which 10 persons lived, with the crash explosion blowing in their windows.

The crash occurred in a farm field located near Castlemore Road and McVean Drive in Brampton, Ontario. The memorial and witness accounts at the time report the crash site was at Woodbridge. This was because in 1970, prior to urban sprawl and changes in municipal boundaries, the site was closer to Woodbridge than Brampton.

Investigation

A Board of Inquiry published their official report on January 29, 1971. The accident was attributed to pilot error. In the official report, eight recommendations were provided, including that the activating lever for the spoilers should be designed in such a way that it could not be activated while the DC-8 is in flight, that the manufacturer should reinforce the structural integrity of the DC-8's wings and fuel tank, and that Air Canada training and operating manuals should clarify the operating procedures around spoiler arming and deployment.

Aftermath

Though it is customary for airlines to retire a flight number after a major incident, Air Canada continues to use Flight 621 for a flight from Halifax to Toronto. Air Canada no longer operates a flight from Montreal to Los Angeles with a stopover in Toronto.

Recovery and identification of bodies proceeded slowly after the crash due to the need to excavate the crash crater to a significant depth. More than 20 of the passengers were United States citizens, all of them listed as being from Southern California.

On 30 July 1970, 49 identified and 3 non-identified victims were buried at Mount Pleasant Cemetery and in May 1971 a stone monument was erected at the grave site with all 109 names inscribed on it. In 1979 Air Canada also added an additional memorial at Mount Pleasant Cemetery.

In June 2002, Paul Cardin, a resident of Castlemore, entered the farm fields comprising the former crash site of Flight 621. He had been inspired by a Toronto SUN article written by feature columnist Mike Strobel in November 2001 which had revisited the 1970 Air Canada crash. Mr. Cardin made some disturbing discoveries at the time which included aircraft wreckage and possible human bone shards. Peel Regional Police Detective-Sgt Frank Roselli and other officers of the Homicide Division were dispatched to investigate Mr Cardin's findings when he reported them on June 27. It was later determined that the bones were not of recent origin, and had indeed come from the crash site. Continuing research of the crash site by archeologist Dana Poulton (D.R. Poulton & Associates Inc), and Friends of Flight 621 (a Brampton-based advocacy group founded by Paul Cardin), produced hundreds of additional human bone fragments.

Memorial Garden dedication in 2013

Since the crash the surrounding area of the crash site itself has experienced significant residential urbanization. In January 2007, the landowners in conjunction with the property developers filed an application to designate a section of the crash site as a cemetery and memorial garden. On July 7, 2013, the memorial was officially opened at the site  near Degrey Drive in present-day Brampton.

The small memorial park, approximately a third of a hectare in size (~3,000 M2) contains lilacs and 109 markers of polished white granite arranged in a random configuration within a bed of black granite paving stones. A polished black granite plaque noting all the victims' names is mounted on a large pink granite boulder. A Candevcon Corporation spokesman, Diarmuid Horgan, said at its official opening that it was hoped the park would help victims' families find peace.