The smallest missing item on a plane can cause huge delays for travellers.
Missing crew, misbehaving passengers and striking air controllers can all cause delays or diversions to a flight.
But you may be surprised to learn that something as small as a packet of paracetamol or a missing screw on a coffee machine could also halt a journey.
MailOnline Travel spoke to British pilot Neel Patel about the document that decides whether a plane is ready to go or if gets delayed.
According to Patel, every aircraft has a Minimum Equipment List (MEL), which is a series of documents that "describe certain elements that you're allowed to dispatch the aircraft with".
Patel told MailOnline Travel: "When there's something wrong with the aircraft the mechanics will check the MEL to see whether they can send the aircraft away."
If everything on the checklist is present and correct, a plane can be dispatched, or allowed to travel.
The contents of this check list will vary according to the type of aircraft, the airline and whether the plane is carrying passengers or not.
Manufacturers of the aircraft will release the original document, the Manufacturer's Minimum Equipment List (MMEL).
Each airline will then produce its own MEL based on what the manufacturers have advised.
In general, airlines will be "more restrictive", according to Patel, meaning that more equipment is needed on a plane before it's sky-worthy.
However, the airline's MEL is considered a sensitive document and is generally not available to the public.
But as Patel explains, if an item on the airline's MEL is not up to the standard dictated by the document, the plane could end up being delayed.
Perhaps one of the most frequently stolen items from a plane are the life jackets and there are generally spares on board.
If there aren't enough however, the plane would be forced to wait until more can be brought on board.
In some cases, missing spare parts or extended repair time could mean that an aircraft is classified as "inoperative", even if it's a small issue.
For example, a missing screw holding a coffee machine into place could mean the whole machine is classed as not working when in fact, there's nothing wrong with the equipment itself.
But if the airlines MEL specifies that the coffee machine must be working, and a replacement screw cannot be found immediately, then the issue could end up delaying the plane.
Patel said that food and drink on board can also delay flights and this will again depend on the airline.
He explained: "From an operational point of view, an airline might not dispatch an aircraft without certain catering. They might have to wait three hours for that catering to be ordered."
This could happen because some food items had expired or some drinks have unexpectedly ran out.
It's likely to be particularly important for airlines where premium service is a priority.
Items for the crew is also a consideration.
If the airline is supposed to provide crew with food and drink, and most airlines do, then these need to be present and correct.
Patel said: "Even if there's water on board, but there's no water for the crew, the aircraft could be held up."
Another important item on the MEL is the first aid kit.
The MEL will generally define the acceptable standards of the first aid kits on board.
However, if an aircraft is missing, for example, a packet of paracetamol from one of the first aid kits, and that item is required to satisfy the MEL, the could be held up until a replacement one can be found.
Patel says that everything on board has to be counted in and out and there are maintenance tasks going on all the time.
While some of the items on the MEL might seem improbable, the document is, designed to ensure the safety of passengers on board.