No other weather phenomenon causes as much chaos for airports as a snowstorm. Not only does it make things more dangerous for aircraft moving around on the ground, it also poses a huge risk to aircraft in the air and attempting to land as well.
There are three main reasons why snowstorms are so dangerous: reduced visibility, slippery surfaces, and icing.
Flying through clouds has rarely been an issue for pilots and their aircraft. Ever since the early 1930’s pilots have been trained to be able to safely pilot their aircraft from one place to another without needing to ever look out the window. This is extremely useful in situations where the pilot can’t see outside their cockpit, such as when they are flying through clouds or snow. In these situations pilots need to rely on their aircraft’s instruments to get them safely to their destination, hence why it is called “instrument time” or “flying on instruments.”
When pilots can’t see out the window they need to follow a very specific set of rules, called the “Instrument Flight Rules” or “IFR.” These regulations were designed to keep everyone in the air safe by keeping airplanes further apart from each other than normal (to ensure they don’t accidentally run into each other in a cloud) and requiring pilots to use very specific instructions called an “instrument approach” to land at their destination airport. These approaches are specifically designed for each airport and the surrounding city or landscape so that airplanes trying to land won’t accidentally run into any tall buildings or mountains in the area.
Even with the best navigational instruments and the best trained pilots the Instrument Flight Rules still require the pilots to be able to see the runway for takeoff and landing. As aircraft instruments have improved over the years to be more accurate airplanes have been able to get closer and closer to the runway without needing to see the ground, but only a human pilot is reliable enough to safely land an aircraft on a runway. At the moment a human pilot typically needs to be able to see the runway from 200 feet above the ground and 1/2 mile from the end of the runway in order to legally continue the approach and landing, which is why when the cloud cover at an airport drops below 200 feet flights will typically need to cancel or divert to other airports.
Since 1968, some aircraft with very special equipment, specially trained pilots, and designated airports, have been able to use an “autoland” system which allows the flight computer to actually land the aircraft without human intervention. During these approaches and landings the systems are so finely tuned that flight attendants will require passengers to power off all of their electronic devices so that no stray signals will interfere with the instruments. While the computer might be able to get the airplane on the ground, no system has yet been designed to get the aircraft from the runway to its parking spot at the terminal without a human pilot being able to see outside. That’s why even with these systems aircraft still need the clouds to be at least 50 feet off the ground and at least 600 feet of horizontal visibility to land. Otherwise airplanes would land and get stuck on the runway.
Snow storms are notorious for creating situations with poor visibility. Heavy snow clouds can often be very close to the ground, sometimes below the 200 feet required for normal instrument approaches. The large snowflakes can obstruct the pilot’s vision keeping them from seeing the runway until it is too late, meaning the aircraft would be unable to land. And even when the snowstorm has stopped snow blowing across the runway can reduce the visibility to dangerous levels.
Slowing Down and Stopping
Once aircraft are able to see the runway and land, the next question is: should they?
On a normal sunny day with a dry runway, a normal Boeing 737 needs about 4,000 feet of runway to safely come to a stop. That usually isn’t an issue as even at DC’s Reagan National Airport (consistently ranked as one of the most difficult airports to land an airplane) the shortest runway offers a cool 5,204 feet.
If your runway isn’t perfectly dry then the airplane can’t stop as quickly, and as a result it needs more room to stop. On a compacted snow runway the same 737 needs an additional 800 feet to come to a stop, putting it dangerously close to the end of that 5,200 foot strip. If there’s a little bit of ice, or water, or slush, then the pilot needs another 2,400 feet to bring the plane to a stop — well past the end of that runway.
As you’d expect, larger aircraft need longer runways to stop. A Boeing 747 would need 7,400 feet of runway to come to a stop on a good day, significantly more when it has been snowing and the runway is wet.
Ice and snow don’t just increase the distance it takes for an airplane to slow down — it also makes the airplane much more difficult to control. Patches of ice on the runway can lead to uneven braking during touchdown, sometimes causing the airplane to spin. Even when the plane is safely on the ground, ice patches on the taxiway can still cause the aircraft to slip off the pavement.
Ice: An Airplane’s Worst Enemy
When there’s a snow storm blowing through getting rid of the snow and ice on the runway is the easy part. Getting it off the airplanes is the harder and much more important consideration.
Airplanes can only take off and fly when the lift generated by their wings is greater than the weight of the aircraft, and when the thrust of their engines is greater than the drag produced by the aircraft body. Which is why it is so critical in aircraft design and maintenance to keep the weight as low as possible, keep the body aerodynamic to reduce drag, and maintain the shape of the wings to create as much lift as possible. Ice and snow ruin all three of these important characteristics.
First and foremost, snow and ice is heavy. The Boeing 737 has about 1,340 square feet of surface area. If the entire wing were covered in just a half inch of ice — barely enough to be visible — that would add an additional 418 pounds (55.875 cubic feet of water) to the weight of the aircraft. That’s not even counting the ice accumulating on the rest of the body of the aircraft.
In a world where airlines are using lighter paper in their in-flight magazines to save gas, that extra weight could mean the airplane is now too heavy to make it to its destination.
That is, if the aircraft can even get off the ground. Snow and ice not only add weight but they also add drag by changing the shape of the aircraft. Once covered in ice and snow that sleek and shiny jet liner now looks more like a Chia Pet, covered in a fluffy white shag carpeting. All those extra nooks and crannies slow the aircraft down because they increase drag. If there isn’t enough thrust in the aircraft’s engines to overcome that extra drag then the aircraft might not be airborne for very long, if ever.
Ice isn’t just a problem for aircraft taking off and landing, though. Ice can accumulate on an aircraft any time they are in a cloud. Usually aircraft have systems designed to remove the ice and keep the airplanes flying, but if those systems aren’t working properly or if the pilots aren’t paying attention the ice can build up and cause issues with the third force we discussed: lift.
Wings create lift because of their unique shape, which uses the air flowing over the wing to causes pressure under the wing than over the wing and suck the airplane upwards (apologies to my flight instructor for the way over simplified explanation). If the shape of the wing is altered, such as when a good amount of ice builds up on its surface, the wing can’t produce the same level of lift. If the ice builds up past a critical point the wing cannot hold the airplane in the air any longer.
Like I Said, The Worst
I think you get the picture. Snow storms and ice storms are pretty much the mortal enemy of the airplane. They make everything more difficult, and as a result airlines just don’t want to operate anywhere near them. And as a passenger, I can’t blame them. I’ll happily drive or take a later flight if I know things will improve.
It can take ages to recover from a snow storm. Once the snow on the ground is removed it can still be days or weeks of below freezing weather, which means ice can still be an issue on the ground and in the air. And the clouds can linger causing visibility issues for pilots trying to land.
My best advice: be patient. And keep an eye on the forecast.