Well , as a so called ' redcap ' one of my responsabilities is to try to get the airplane leaving on time , and if not we always have to explain the caused delay by telex and by his specific delay code . And I can tell that there are hundreds of resons of why a flight can be late . Lets give a few examples . The flight can receive a so called ' slot ', so that way it is retricted to take-off before a certain time . This can be caused by too much traffic , bad weather , radar problems , ... on the local airport , on route or at the destination airport . And because an air traffic jam is impossible , the flights are kept on the ground till teher is spac available .On a day a few years ago i remember slots being created to flights who had to pass trough British airspace because air traffic controllers were blocked in a traffic jam on their way to their work at London , so the controllers at work couldn't be replaced in time and were getting tired , and thats why they limited the amount of traffic per hour in the London airspace ... try to explain that to your passangers
. Flights of course can be delayed by a lot of other reasons like technical problems , minimum crew rest time , late incomming flights , late crew boardings , technical breakdowns of airport or loading and servicing equipments , actions by ground staff , accidents while servicing the plane , ... What a lot of travellers don't realise is that arriving late at the boarding gate can create delays too . Cause a missing passenger who is having his luggage on board of the flight can create some serious delay , cause his bag can not travel alone on the flight for safety reasons and so has to be offloaded from the flight . I once had a missing passenger on my flight with 5 bags checked in on a flight operated by an A-330 to somewhere in africa , so we had to search for his bags . All the bags were loaded in containers ( each containing about 40 pieces ) and over 300 bags were checked-in . By our computer system we could lokalise the 5 bags loaded in 5 different containers , so we had to unload about half of the aircraft to find his bags . The flight had an on-time slot on his scheduled departure time , which it rated of course . After almost 40 mins all bags were offloaded , the plan was reloaded again and the cargo doors were closed . But at that time we received a slot with take-off only about 1 hour later . So the flight had about almost 2 hours of delay , and hundreds of passengers got angry because only 1 man did not show up at his gate . The missing passenger was found a few hours later sleeping on a seat in the airport , not knowing he caused so much troubles to others . And i can go on like this ...
So aviation is quite complicated , and delays can be caused by hundreds and hundreds of reasons .