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.