MONTREAL -- Canada's two biggest airlines saw a far higher proportion of their flights delayed this summer than many of their American peers, according to figures from an aviation data firm.

Overall, only half of Air Canada's 31,168 flights were on time between June 19 and July 16, statistics provided by Cirium reveal.

In comparison, 64 per cent of WestJet's 14,998 flights touched down late.

Flights are considered on time if they reach the airport within 15 minutes of their scheduled arrival time.

The numbers contrast with on-time performances that range between 68 per cent and 83 per cent for the five biggest airlines in the United States. Alaska Airlines, which ranked first, operated a similar number of flights to Air Canada in the four-week period at 32,193.

The stats also point to a recent drop in punctuality as flights ramp up, with Air Canada notching a 70 per cent on-time performance in May and 53 per cent in June. WestJet scored 67 per cent last month.

Several reasons underlie the ongoing tarmac tardiness, which falls well below the general industry goal of 80 per cent on-time flights.

To meet soaring summer demand -- Air Canada now transports 140,000 passengers a day -- airlines are running their planes too frequently, said John Gradek, a lecturer at McGill University's aviation management program. The practice can result in hitches that hold up travellers across the country.

"The existing airplanes are working harder," he said. That leaves less time for maintenance between flights and overnight.

"If you don't give the mechanics enough time to fix the snags on a regular turn, those snags will catch up to you. And then you now have an unserviceable airplane that cannot be dispatched."

Half of Air Canada's 18 A330 wide-bodies and most of its 76 Airbus A319, A320 and A321 narrow-bodies -- more than 40 per cent of its fleet -- are well over two decades old.

"These airplanes are long in the tooth need a lot of maintenance, need a lot of TLC to fly reliably," Gradek said.

Crew shortages can also play a role in peak season. It's harder to find a spare pilot or flight attendant when each aircraft is flying more -- or when delays pile up, exceeding the "duty times" that employees are allowed to work within a given period.

Air Canada acknowledged the strain on its network amid the throes of peak travel season.

"As with any system, when it is operating at full capacity it may slow processes down and take longer to recover when issues arise. That said, many of the delays were relatively short," spokeswoman Angela Mah said in an email.

The figures show that about 93 per cent of Air Canada trips, and 98 per cent of WestJet trips, hit the gate less than two hours late, a far better performance than last summer.

Some weekends in June and July of 2022 saw Air Canada top the global chart for delays, repeatedly hitting two-thirds and far outpacing other carriers as complaints of hours-long hold-ups or cancellations littered social media. Toronto's Pearson airport also ranked as the delay capital of the world on multiple occasions.

This summer, thunderstorms have caused their share of problems, Air Canada noted. Of the 40 days between June 1 and July 10, 31 saw "weather-related disruption" versus 20 days in the same period in 2019, the Montreal-based company said.

"Our Montreal global hub experienced numerous thunderstorms in recent weeks, including multiple red alerts -- necessitating halts to operations due to lightning -- which have led to delays. Similar situations have occurred frequently in the eastern part of North America, where we have a significant volume of flights," Mah said.

She also pointed to the dearth of air traffic controllers -- in Canada and the U.S. -- that means planes are often left waiting to land.

That time circling above the runway can also tack on hours to flight crews' shifts each week, pushing them closer to their 28-day cap and leaving less leeway for them to plug labour gaps by month's end.

"Following these events, Air Canada's focus is to get aircraft and crew back on track and get our customers on their way as soon as possible," Mah said.

WestJet said its "summer readiness plan" has made boarding more efficient and co-ordination with ground handlers smoother. However, the Calgary-based carrier also cited thunderstorms and "third-party delays incurred from service partners across the aviation ecosystem" as reasons for delays.

WestJet is calling for "shared accountability" across the sector -- from air traffic controllers to security screeners -- rather than pinning blame for disruptions on airlines.

Nonetheless, carriers have fewer outside factors to blame than during the post-pandemic travel tumult of 2022.

While an aviation labour shortage last summer meant fewer security screeners, border officers and baggage and ground handlers, those areas are now largely back to pre-pandemic levels, according to the companies and agencies behind them.

At Toronto's Pearson airport, 91 per cent of passengers cleared security within nine minutes in the first week of July -- a far cry from the snaking lines and bulging terminals of the year before -- Canada's transport security agency says.

The average wait time for passengers at customs was seven minutes, according to Pearson. And baggage wait times are down by more than a fifth -- though luggage is typically the airline's responsibility, often contracted out to third parties.

Meanwhile, just one flight with passengers on board was held -- for nine minutes -- between July 3 and 10. "This situation represents a marked improvement over last summer when holding passengers at the gate was a regular practice due to overcrowding in the customs hall," Pearson said in a release last week.

As of May, Air Canada made up 47 per cent of the domestic air travel market and WestJet served 28 per cent, amounting to three-quarters of the total, according to Cirium.

This report by The Canadian Press was first published July 25, 2023.

Correction:

An earlier version had an incorrect figure for the proportion of WestJet's flights that were late.