Flight Delayed by “Flow Control” – What Is It?

Things were going so well.

You made it through airport security without getting groped. You arrived at your gateĀ  with enough time to get one last bathroom break before boarding. You weren’t crushed to death in the mad dash to board when your group was called. And, most importantly, your airplane pushed back from the gate exactly on time with you safely tucked away in your seat.

Then things went horribly wrong.

You probably didn’t notice that your airplane took a left turn on that taxiway instead of a right, bringing it to a holding pad instead of the runway. But when the pilot came on the speaker and told you that “due to flow control” into your destination airport the aircraft can’t take off for a while you became keenly aware that you would now be cooling your heels for a while on the ground.

That’s the worst case scenario, stuck on an airplane for an unknown period of time because air traffic control won’t let you leave. But “flow control” can also strike when you are at the gate before you board.

But, why? Why did air traffic control pull this terrible trick on you?

Let me explain.


The All-Knowing ATC System

Every regularly scheduled commercial flight in the United States is required to file a flight plan for the flight they intend to operate. Wikipedia has some great information on what a flight plan is and the various parts (and no, it’s not the terrible 2005 movie starring Jodie Foster), but the reason why it is important for this topic is the timing. Every flight plan includes not only the expected departure time for the flight, but also how long the flight is expected to take and when they will actually arrive at the destination airport.

Flight plans give the FAA a very accurate picture of how many aircraft intend to reach a specific airport in the future.

Airports can only handle so many airplanes in a given time frame. Not only is there a limit to the number of aircraft that the controllers themselves can safely manage, there are legal requirements as well for how much space needs to be maintained between aircraft. This is called “spacing” and is designed to provide enough time for pilots and controllers to identify potentially hazardous situations and fix them before an accident can occur.

The spacing required between aircraft isn’t a fixed number, either. Spacing requirements are increased when the weather gets bad, which gives pilots and controllers a wider margin of error. Spacing can also vary depending on the kind of aircraft — large and fast aircraft require more spacing than small and slow aircraft to maintain the same margin of safety.

Or, alternatively, things could just be so miserable that the airport is completely closed. For example, if there’s a huge snowstorm or thunderstorm over the airport they may decide to stop releasing flights in that direction.

Based on the weather conditions at the airport and the number and type of aircraft arriving, the air traffic controllers can determine how many aircraft they can safely handle in a given period of time.

Armed with that information, as soon as your flight files their flight pan with the local air traffic control office the ATC system can determine if there’s enough space for your airplane to land when it arrives. If there isn’t enough space (due to poor weather or heavy traffic or any other of a variety of reasons) they can determine roughly how long you would need to wait before it was your turn to arrive.

Can’t You Do That Before We Leave?

Flight plans are typically filed around two hours prior to departure, but pilots are often in the dark about ATC’s plans until after they have already pushed back from the gate.

Just because you have filed a flight plan doesn’t mean that’s what ATC wants the flight to do. Depending on a whole number of circumstances, from weather to other air traffic in the area, ATC will often adjust the flight plan and re-route planes to make them easier to handle.

Usually a pilot only gets this information moments before they push back from the gate and start the engines. As part of the workflow pilots will make sure they have the latest weather report for the airport and any notices that have come out (called a Notice to Airmen or NOTAM) then call the tower to ask for their clearance.

There’s a difference here to note: a “flight plan” is what the pilot intends to fly, a “clearance” is what ATC will allow them to fly.

This is the point at which the pilot will get their first idea of what ATC has in mind for their flight. Sometimes the pilot gets their clearance while still at the gate, other times the pilot is told that their clearance is “on request” meaning that ATC isn’t done analyzing their flight plan and they will get it in a few minutes.

In either case the pilot typically has no idea before all the passengers are on board the plane and they are ready to go. There’s an effort called NextGen where ATC will be able to send flight plans digitally to each aircraft well in advance, but not every airplane is equipped with that system yet.

But Why Am I Delayed On The Ground?

So your flight is being asked to patiently wait its turn. Why not fly slower? Or just get there and then wait your turn? There’s two very good reasons.

First: things may get worse.

Delays are estimates. Sometimes the weather can deteriorate even further and the delays will need to be much longer to accommodate everyone. The airline might even realize that the delay is so long that it makes more sense to cancel the flight. Or maybe they want to bring everyone back to the gate so they can wait comfortably in the lounge instead of in their cramped seat.

In these cases it makes sense to be on the ground. Airplanes can return to the gate if needed, take on more gas to account for future delays, and have a lot more options than if they were in the air.

Second reason: fuel.

Airplanes only carry as much fuel as they need to complete a flight, plus a little more than 45 minutes of reserve fuel. Carrying any more would be expensive and wasteful. So as soon as that airplane pushes back from the terminal and the engines start turning that airplane only has a certain amount of time it can be airborne before it needs to refuel.

Flying slower would eat up more fuel. Airplanes fly as efficiently as possible to burn as little fuel as they possibly can. Fuel costs money and airlines, as I’m sure you can tell, don’t like spending money they don’t have to. So any change from the most efficient route, whether changing speed or anything else, will increase the fuel use for that flight. Increased fuel use means more money spent and a profitable flight can quickly turn into a money pit.

Not to mention the whole running out of gas issue. There aren’t any gas stations in the clouds. At least, not for civilians.

This Sucks

I know. Been there, both as a passenger and a pilot. Trust me, the pilots don’t like it either. They only get paid for the time they are flying, so sitting on the ground twiddling their thumbs doesn’t help anyone in this situation. Just know that there’s a very good reason why you are sitting on the ground instead of in the air, and eventually things will get better.