As COVID restrictions relax across the country, I bet most of us are thinking of travelling. For those who may have recently travelled by air, you might recall thinking to yourself: "Why is my delayed, AGAIN?".
Having studied and now beginning my career in the airline industry, I have finally figured out the answer to this annoying question myself!
Obviously, there is not one situation that casuses a flight to be delayed every time, however, some reasons for delay are more common than others. I will highlight and briefly explain the most common scenarios below:
1. Your flight is not actually delayed, but right on time.
What do I mean by "not actually delayed"? Well, the published departure time for all airlines is actually the time that the aircraft brakes are released. For most jet aircraft, this means that the departure time is when the aircraft is being pushed back from your gate, and not the time you lift into the sky.
Airlines us this "breaks release time" as the departure time because this is the value used to calculate many figures across the industry; from the flight crew's pay, legal time limitations for the flight, to the amount paid to the airport for parking fees! For convience, the "breaks release time" or "OUT" time of an aircraft became the departure time posted by airlines and seen on websites and those big schedule boards in the terminals.
However, this doesn't mean that you will be late for arrival for every flight. The time it takes for a plane to taxi (drive) to the runway and take-off, and the time to land and taxi (drive) to the gate is all accounted for in the estimated time enroute. Therefore, that "arrival" time you see on websites and airport schedules denotes the "breaks set" or "IN" time for your flight, and not the time that the aircraft wheels touch the ground.
2. Weather
Sometimes, mother nature has different plans for everyone.
Air travel is probably the most weather-dependent method of travel. Different countries, airlines, and even airports each have its own limitations and regulations regarding safe weather conditions that must be met for a flight to take place. On days which may be foggy, windy, stormy, or all of the above, you can often find the entire "departures" board in the terminal red with delays and cancellations.
On those bad weather days, you might see another aircraft depart next to your plane, but your captain just informed you that the weather does not meet the weather requirements for takeoff. What is the flight crew talking about? Well, each aircraft may have its own limitations too! Some aircraft (those large, intercontinental ones carrying hundres of people) have equipment in their avionics suite that cost millions to purchase and tens of thousands to maintain each year. This equipment is what allows certain flights to depart when the weather is poor.
Active weather which you can't see at your departure airport might also limit your plans. Afterall, the plane needs an acertained landing point; so if your destination or any point enroute is covered by bad weather such as thunderstorms, airlines will halt departures until landing becomes more possible.
3. Ground handling delays (baggage, food, etc.)
The time between an aircraft's parking and pushback from the gate is known as the turn-time. During this time, there are many pieces around the plane that must move in unison. If any of them suffer a delay, then the flight can be late for departure.
There are too many to list each in detail, so here are a few of the most common aspects in with a brief summary:
Inbound passengers: a flight with many passengers requiring a wheelchair or a lot of carry-on bags may take longer to deplane.
Grooming: A specialized team will clean the aircraft. This does not happen on all flights, but almost all international flights will be sanitized and vacuumed.
Catering: Food containers from the inbound flight and garbage will be taken off the plane which fresh food and drinks will be loaded into the gallies.
Bags: Ground handlers need to remove all inbound bags and cargo, and then load all outbound bags and cargo if passengers check-in late, this may cause a delay in the bags getting to the aircraft and subsequently, late in loading the bags onto the plane.
Fuellers: Fuelling takes time for jumbo-jets and larger propellor aircraft. Many airlines prohibit boarding while fuelling is taking place for passenger safety.
Boarding: Missing passengers or any mix-up with the tickets can lead to further delays. Flights may also be delayed because of connecting passengers who are marking their way through the airport.
Paperwork: This includes the flight plan, passenger list, weight and balance calculations, and many other details and safety requirements for the flight.
An article written by Kelly Jamieson perfectly summarizes and outlines all the hidden aspects of aviation that must each perform flawlessly for every aircraft to depart on time. If you have the time and are interested, I would highly recommend taking a look. Here is the master flow chart by her:
4. Missing passenger
Why doesn't the flight just depart without the passenger?
The primary reason that a missing passenger may cause longer delays is because if the passenger has checked-in a piece of luggage, that luggage cannot depart on the flight without the passenger on board. This rule was implemented as a direct response to a tragic bombing incident on a trans-oceanic flight in 1985, where an individual placed a bomb within a suitcase, checked the bag into the aircraft, but did not board the plane himself. Therefore, if a passenger checks in a bag but does not show up for boarding, the respective bag must be offloaded. In large jets, this could mean sorting through over 300 bags to find the correct tag, which may be buried 30m into the belly of the aircraft. In most situations, the passenger has gotten lost in the terminal, is casually shopping at a duty-free, or relaxing at the lounge. For airlines, it is simply easier to find the passenger than go through the hassel of offloading a bag.
4. The plane was late getting to you.
Airlines generally plan their schedules quite tightly since a plane does not generate any revenue while sitting on the ground. Therefore, any delay that the aircraft or flight crew had to deal with earlier in the day would likely affect a subsequent flight, or subsequent chain of flights. These delays can happen because of any aforementioned reasons. It can also be due to in-flight situations on the inbound flight, such as a reduced tailwind which pushes the plane faster, or an increased headwind which slows the plane down.
Why wouldn't the airline just use a spare plane or crew?
While it is true that airlines have reserve crew and reserve aircraft on standby, these pieces of the puzzle are located at the airline's main hub or bases. This means unless you are departing from your airline's major base of operations, the aircraft and crew will still need to travel to you in order to make the return trip. If the inbound flight is the root cause of the delay, then in most situations activating a reserve crew and aircraft will take longer than going with the originally scheduled personnel.
"Many systems must align for ONE flight to depart... Next time your flight leaves on time, consider it a complete miracle!" - Kelly Jamieson, MA and airline pilot on the B787.
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