Flight delays are an unavoidable frustration of air travel. Last year around Europe, each plane was late by an average of nearly two minutes, adding up to more than 19 million minutes of delay in total - that's 316,666 hours and more than double the figure from 2017.
While it might seem sensible to assume that each hold up is a result of a major aviation issue, Telegraph Travel has been told that sometimes delays are actually due to the most minor of complications.
AJW Group tests, refurbishes and repairs aircraft components for more than 1,000 airlines, from Air Astana to United Airlines, and including British Airways, Easyjet and Lufthansa, across 117 countries, so knows a thing or two about preparing a plane to fly.
CEO Christopher Whiteside says that though there are many obvious reasons planes are kept on the tarmac, others are much more subtle, and borderline ridiculous. "Many are safety related, but some are simply crucial to delivering the passenger experience," he said.
We asked him to select five of more unusual faults that might have led to thousands of hours of delays around the world.
The coffee machine
"A lack of hot coffee does not break an aircraft but it does lead to seriously grumpy passengers (and crew)," he said. "In the long run, this can do serious damage to an airline's level of inflight sales, customer satisfaction, and ultimately, its reputation.
"For this reason, take-off is frequently delayed so that faulty coffee machines can be fixed or replaced. Although safety is always an airline's top priority, they are paying increasing attention to the passenger experience."
The pilot's seat
"It may sound obvious but daily checks of the pilot's seat take place to ensure that it is fully functional," said Christopher. "Any issues that arise with either the manual or electronic operation of the seat must be fixed before the aircraft can take off.
"Firstly, the pilot's seat must be firmly located in the seat tracks. Any sign of looseness means that the aircraft cannot take off. It takes time to get the pilot's seat just right and if any single part malfunctions, all areas have to be reset and thoroughly checked."
The emergency slides
A plane's emergency slide. Photo: Alamy
"All aircraft slide rafts have a set shelf life that must be strictly adhered to," he said. "Maintenance programmes ensure that all slide rafts are fully functional before they are placed on board, and that they are removed from an aircraft well in advance of their expiry dates.
When an emergency slide is activated by accident
"If a slide raft is accidentally deployed during the tight turnaround between take-offs, it must be removed, and a new unit fitted before the next scheduled flight. Reassuringly, an aircraft is not considered airworthy without in-date slide rafts on board."
A cabin crew seat
"It might be something that you have never thought about, but if a seat designated for cabin crew is unable to be used, a member of the cabin crew will not be able to fly.
"Airlines have strict passenger to crew ratios they are required to stick to for the safety of all onboard. If the airline is a member of crew short due to a broken seat, these ratios are unlikely to be met and the aircraft will not take off. This could mean a lengthy delay for passengers."
A fuel leak
Christopher said: "A fuel leak should never be ignored no matter where it occurs. On an aircraft, the consequences of a fuel leak in the air could be even greater than they would be on the ground.
"The tiniest of leaks can cause serious issues and threaten the safety of everyone on board. It goes without saying that if there is a leak the aircraft will not be flying.
"On an aircraft, fuel tanks are located in the lower wing. These areas are checked daily, along with the engine nacelles. This ensures that any issues can be identified and swiftly fixed before they get to the stage of causing a leak."
The Telegraph, London