At an airport in Rome recently, 336 Air Transat passengers sat in a plane on the tarmac for six hours. Why can't passengers just leave an aircraft that is delayed? CBC News explains.

What causes these delays?

There are several common causes.

"Ninety-nine per cent of these very long [delays] that we're talking about are caused by nature, we have no control over," said Ross Aimer, CEO of Aero Consulting Experts and a former airline captain.

In particular, snow and ice can be a big factor, and the time it takes to de-ice the plane. And that causes backlogs for other flights.

Mechanical issues can take a long time to diagnose, as well as the time needed to retrieve the parts for a repair and to fix the problem.

Customs services may be full or customs officials unavailable.

Or there may be no gates available where airplanes can park.

"If the airport is very busy at that time, and maybe a multitude of delays have caused a domino effect, then we have to wait it out until the gate comes available," said Christina Ling, lead flight attendant instructor at the Canadian Tourism College.

Do passengers have rights to services?

Yes. Canada's new passenger bill of rights came into force July 15. The section on tarmac delays says airlines must guarantee that passengers have:

Access to working washrooms;

Proper ventilation and heating or cooling;

Food and drink in reasonable quantities; and

Ways to communicate with people outside the plane, where feasible.

The U.S. Department of Transportation sets out similar rules, which include:

Providing passengers with a snack and drinking water no later than two hours after the aircraft leaves the gate'

Providing working toilets and comfortable cabin temperatures;

Obtaining enough food and drinking water to provide a serving to all passengers; and

Providing passengers with notifications regarding the status of the delay every 30 minutes, including the reasons for the delay.

Why not just let the passengers leave?

One of the reasons for delays is the same reason that an airline can't unload passengers: there may not be a gate available. As well, there may be no teams available to handle the passengers.

It may also be difficult to predict the length of the delay. If it's short, it will take time to round up the passengers.

And if a plane leaves its position, it may lose its place in line to take off, which would cause further delays. "I just lost that slot time.. And the next one could be eight hours from now," Aimer said.

Security is an issue, too, if passengers have been let off the plane and allowed to go back inside the airport.

"What if a passenger goes past security zone or something, and then we have to find that passenger again?" Ling asked.

And an airline may lose passengers who decide they don't want to wait any longer.

"Somebody decides, 'You know the hell with it. I'm not gonna go now.' By law you can't take off. You have to go on and take that passenger's bags off the airplane," Aimer said.

"Imagine how long that takes on a 747 or big airplane. You have to take all the bags off, identify that person's bag because now it's a security issue."

And in the U.S. at least, if airlines offer passengers the opportunity to get off the airplane during a tarmac delay, the airline is not required to let them back on.

Are there rules about waiting times?

Yes. After a three-hour tarmac delay at a Canadian airport, the plane must return to the gate to allow passengers to disembark, unless it's not possible "for safety, security, air traffic control or customs reasons."

However, a plane can stay on the tarmac for up to 45 extra minutes if it is likely that it will take off within that period and the airline is able to continue providing comfort services to passengers.

For domestic flights in the U.S, if a delay is approaching the three-hour mark, airlines are required to begin to move the airplane to a location where passengers can get off safely. (It's four hours for international flights). The exceptions are similar to those in Canada.