Why did Air Canada passengers end up 400 miles further away from London?

Airline passengers should always make two assumptions when boarding a flight. The first concerns baggage: if you check anything in, wave the case goodbye for the last time as it disappears along the conveyor belt. Assuming you will never see your luggage again sounds excessively pessimistic. Almost always your possessions will be returned to you – usually at the destination airport, sometimes days later when they turn up in a van. But the possibility that your precious stuff may be on a one-way journey to the Unclaimed Baggage Center in Scottsboro, Alabama, should inform your packing. Anything you cannot do without – for sentimental, medical or financial reasons – should stay with you in your cabin luggage.

The second assumption of misfortune is that your flight will be diverted. Touching down at the wrong airport can happen for all manner of reasons: running short of fuel, technical issues, offloading disruptive passengers…

I have no data to confirm it, but I sense that diversions are increasing. The story of the summer comprises storms across Europe and air traffic control restrictions.

August began with this combination causing widespread diversions of Gatwick-bound flights to a range of airports. In the early hours of Friday morning, a Tui flight from Paphos in Cyprus to London Gatwick landed surprisingly at Stansted. As always with a diversion, passengers who did not have cars parked at Gatwick were at an advantage – worth bearing in mind when you are considering how to reach your departure airport.

At around the same time, easyJet passengers from Hurghada to Gatwick were waiting around in a near-deserted terminal at Milan Malpensa – or Silvio Berlusconi International Airport, as we must now call it.

Italy had not featured in the passengers’ plans. But Gatwick-Hurghada is a round-trip of around 5,000 miles. The pilots and cabin crew are able legally to operate the return journey with a planned time of around 12 hours between leaving Gatwick and returning to the Sussex airport. But even mild disruption – in this case air traffic control slot delays – means the journey cannot be completed within the crew’s maximum permitted hours. The long-established workaround is for a replacement crew to be positioned to pick up the flight at Milan. The same applies for links such as Sharm el Sheikh to Manchester. Such long sectors are particularly prone to diversion on the homeward leg, so build in that possibility when making plans for your return.

Madeira’s spectacular airport is in a class of its own in terms of construction – partly on stilts on the raw southern edge of the Atlantic island – and its vulnerability to adverse weather. Always best to be prepared for a diversion to mainland Portugal or even back to your starting point.

Returning to the departure airport is a relatively common decision, even if the plane is well into its journey. If a plane needs attention, the airline would much rather have the aircraft at its base – where all the engineers and spares are based. If a non-critical fault develops a few hours into a journey, the captain may be ordered to turn around rather than continue to their destination. For passengers on the affected plane, as well as those waiting at the far end for the return leg, this is extremely frustrating. But the airline will often judge it as the least bad option.

What, though, was Air Canada doing with its flight AC868 on 1 August? This is the daytime link from Halifax in Nova Scotia to London Heathrow. The Boeing 737 Max had “gone oceanic” beyond the eastern end of Newfoundland and had about four hours flying time remaining when it turned back. But the crew did not divert to the nearest airport at Gander or return to Halifax. Instead they flew an extra hour beyond Halifax to Montreal. The passenger ended the flight 400 miles further away from London than when they began.

What was the reason? Montreal is a much bigger Air Canada base, and therefore has more scope for fixing problems. Montreal airport has many more nearby hotel rooms than does Halifax, which could be a factor if a night stop was necessary. It turns out, though, that the passengers’ daytime flight was about to become an overnight trip. “Air Canada’s flight on 1 August from Halifax to London diverted to Montreal due to a mechanical issue with the aircraft,” the airline told me.

“The decision to divert to Montreal was made so that our customers could be accommodated on flights from Montreal to London also departing on 1 August.”

I have some skin in the game here: on Sunday morning I am booked on that very flight, and could really do without going west. But I am now assuming I shall do just that.

Simon Calder, also known as The Man Who Pays His Way, has been writing about travel for The Independent since 1994. In his weekly opinion column, he explores a key travel issue – and what it means for you.