Why Low Clouds and Poor Visibility Cause Delays

Airplanes fly through clouds all the time. Heck, thanks to the super intelligent autopilot systems installed on most airplanes they can even land themselves. But despite all of that technology one of the things that regularly causes the most delays is low clouds and poor visibility. But why?

Spacing, The FAA, and the Big Sky Theory of Aviation

Once upon a time there was no Air Traffic Control System within the United States. When commercial aviation was still in its infancy there just weren’t enough flights in the skies to make anyone concerned. Airlines would coordinate their schedules to make sure that generally no two airliners would be in the same place at the same time, local tower controllers would regulate airplanes taking off and landing to make sure that no one ran into each other in that very small area, and for the rest of the flight pilots relied on the “big sky” theory to keep them safe.

The idea here was that the sky was so very vast and there were so very few airplanes that running into each other was statistically improbable. And if they ever did meet, the pilots could “see and avoid” the other aircraft to keep everyone safe.

For decades the system worked, until in 1956 two fully loaded airliners — United flight 718 and TWA flight 2 — collided over the Grand Canyon.

Two years later in 1958 the U.S. Government established the Federal Aviation Administration (FAA) with the objective to design a system to make sure that such an accident never happens again. They worked together with other international organizations to design the rules and regulations that govern commercial aviation to this day.

One of the fundamental rules of this air traffic control system is the idea of keeping “spacing” or separation between aircraft. Accidents and mistakes can happen, but this spacing provides a buffer zone that allows for the mistake to be identified and fixed before another catastrophe happens.

The amount of “spacing” required between aircraft depends on the weather conditions. When visibility is good and there’s not a cloud in the sky pilots don’t need as much space between aircraft, often they are even allowed to maintain “visual separation” between themselves, meaning they can fly in fairly close formation as long as everyone can see each other. When there are clouds and poor visibility that visual separation isn’t possible and the required separation is increased.

How does this impact you, dear passenger? Well, bad weather means increased spacing requirements. Increased spacing requirements means fewer airplanes can fly through the same space in a given period of time. Which, in turn, means that fewer airplanes can take off and land at a given airport.

Especially at busy airports, the schedules are set up with the assumption of good weather. So if there’s poor weather and fewer airplanes can land then there are nearly guaranteed to be delays.

Spacing can slow things down, but it can also bring things to a screeching halt.

Approach Minimums and Why They Matter

Clouds happen, and airplanes still need to land even when the skies aren’t clear. To bring airplanes safely in to land even when they can’t see where they are going airlines and airports developed “approach procedures” that detailed a process for safely landing an aircraft at a specific runway using only the instruments within the aircraft, also called “instrument approach procedures.” The first instrument approach used by a commercial flight was in Pittsburgh, PA in 1938.

The concept behind an instrument approach procedure is to safely guide the aircraft down low enough that the pilots are able to see underneath the clouds, identify the runway, and land visually.

Some approach procedures are more accurate and reliable than others. As a result, in order to maintain the highest level of safety for everyone involved, less accurate approach methods aren’t allowed to get as close to the ground as more accurate methods. And depending on the training of the aircraft crew they might have additional requirements that try to reduce the risk of these instrument approach procedures by not going as low.

But in order to even attempt these approach procedures, the weather needs to be good enough that a pilot might be able to successfully see the runway and land from the lowest altitude they are allowed to descend. These weather requirements are called “approach minimums” and involve visibility, cloud cover, and other factors. If the weather at an airport is below these approach minimums then pilots will wait, either in a “holding pattern” in the air or decide to delay their departure from their original airport, until the weather improves.

Instrument approaches have gotten better and better over the years, to the point where an aircraft can now land itself automatically. So why can weather still slow down and stop aircraft?

Getting from the Runway to the Gate

Getting down is half the battle. Once you’re on the ground you still need to navigate from the runway to the gate. And all of the support vehicles need to be able to safely move around and pick up baggage, refuel the aircraft, and more.

Navigation systems are great in the air, but there hasn’t been a system designed yet that can safely guide an aircraft from the runway to the gate without seeing outside the cockpit. One day this technology will exist, but we just aren’t there yet.

In the meantime visibility will still have a major impact on the ability for aircraft to land at airports, even in an age where autoland systems are common.

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.