We've all moaned and groaned over flights cancelled due to weather. It's maddening to miss your connection (or even your whole trip) because of bad weather, but incidents like today's crash of Delta 1086 in a snowy New York City are exactly why they make that tough call that can affect millions of travelers.

If for some reason you haven't heard yet, 24 people received minor injuries when a Delta Air Lines MD-88 from Atlanta to New York-LaGuardia skidded off of Runway 13 on landing late this morning. The plane veered left off the runway and onto the grass, crashing into an embankment on the airport's boundary with Flushing Bay. All passengers and crew managed to evacuate the damaged aircraft, which lost its left wing (causing a fuel spill) and took a nasty hit to the nose when it impacted the embankment.

Poor Weather at LaGuardia

New York is one of many cities in the eastern United States on the receiving end of a sprawling, slow-moving winter storm that dropped up to two feet of snow on parts of Kentucky last night. As such, conditions weren't all that great at LaGuardia when the plane landed a little after 11:00 AM, or 16:00 UTC.

Almost all official weather stations around the world produce weather data in a condensed format called a "METAR," short for METeorological Aviation (or "Aerodome") Report. Here's LaGuardia Airport's METAR taken shortly after Delta 1086 crashed:

KLGA 051622Z 01008KT 1/4SM R04/3000VP6000FT SN FZFG VV011 M03/M05 A3012 RMK AO2 SFC VIS 1/2 P0003

The report was taken at 11:22 AM with temperatures around 27°F and winds blowing from 10° (the north) at around 10 MPH. Conditions at the time were overcast at 1,100 feet with moderate snow falling and freezing fog present over the field. The station reported visibility of one-quarter of a mile, but an observer added a remark at the end of the report saying that the surface visibility was one-half of a mile.

Two things stand out from that report:

The aircraft had a slight tailwind on landing. Planes landing on Runway 13 are facing roughly 135°, or southeast, and winds were blowing from 10°, or north, so winds were blowing towards the back-left part of the plane as it landed. Planes almost always take off and land into the wind because tailwinds inflate their speed—if that tailwind shifts or stops blowing, you suddenly lose some of your forward speed, and during landing, a sudden loss of any forward speed can be disastrous.



(Correction: As many emails and comments have gleefully pointed out, I was wrong about tailwinds. Tailwinds aren't necessarily dangerous, they just require a longer stretch of runway to take off or land. I was thinking of the danger of tailwinds with regard to microbursts. Planes take off and land with a headwind because it increases air speed and shortens the runway length required.)

(Correction: As many emails and comments have gleefully pointed out, I was wrong about tailwinds. Tailwinds aren't necessarily dangerous, they just require a longer stretch of runway to take off or land. I was thinking of the danger of tailwinds with regard to microbursts. Planes take off and land with a headwind because it increases air speed and shortens the runway length required.) Temperatures were cold enough that the snow and freezing fog were readily sticking to exposed surfaces at the time.

Of course, we won't know for sure the exact cause of the crash until the National Transportation Safety Board (NTSB) releases their report a year from now, but planes don't just skid off of runways for no reason. Given the conditions at the time, it's safe to speculate that hazardous weather conditions played at least a contributing role in the plane's runway excursion.

It's annoying as hell to get stuck in an airport because of a cancelled flight, but incidents like the one in New York this morning are exact reason that airlines ground thousands of planes during severe winter storms.

Slick Runways

Aside from traffic management, one of the main reasons that airlines cancel flights during hazardous weather is the condition of runways at affected airports. It goes without saying that aircraft are heavy and they're going very fast when they come in for a landing. On a good day, aircraft need ample room to use their brakes, spoilers (the flaps that extend on the top of the wing), and reverse thrust to slow the aircraft to a safe speed so they can begin taxiing to the gate. When runways are slippery from rain, snow, or ice, they need a much longer stretch of pavement in order to maintain traction and safely slow down.

There are some airports in the world where space is virtually no issue. The runways at Denver International Airport are all more than two miles long. LaGuardia, stuck on the northwest corner of Long Island with water to its north and millions of people to its south, doesn't have that luxury. Each of LaGuardia's runways stretch about 7,000 feet, or a little over 1.3 miles long. When you combine the complicated approach pattern required to land at the airport along with the relatively short runway lengths, pilots aren't afforded much room for error.

Within the past two years, several commercial aircraft have slid off of runways and taxiways at airports around the United States due to icy conditions. In January 2014, a Delta Connection jet from Toronto slid into the grass at New York-JFK after encountering an icy patch on a taxiway. Another Delta jet slid off the runway during a snowstorm in Madison, Wisconsin, back in December 2013. A United flight slid into the grass after landing on an icy runway in Cleveland in February 2013. Back in 1994, a Continental Airlines jet slid off a runway and into a ditch during a snowstorm after aborting its takeoff from LaGuardia.

Aircraft Icing

Another major concern aviators run into during winter storms is icing on the aircraft itself. Even a small amount of ice on an airplane can spell disaster—some pilots will request their planes to be deiced after a light morning frost let alone a significant bout of freezing rain or snow. Air must flow smoothly over an airfoil in order for the plane to take off and maintain controlled flight. Any significant buildup of ice on any the surface of an airplane—especially the wings—can disrupt the airflow and cause the plane to lose speed, stall, and potentially crash.

CORRECTION: As several commenters have pointed out (1, 2), the cause of the Colgan Flight crash discussed in the next paragraph was not due to ice, but rather pilot fatigue. Icing was discussed during the investigation, but found not a significant factor in the accident. I apologize for my error. A better example would have been Air Florida 90, a flight that crashed in the Potomac River after taking off from National Airport in Arlington, Virginia, during a snowstorm. In that crash, the pilots failed to recognize and mitigate ice buildup on the aircraft and its engines, preventing the plane from gaining altitude before it crashed into the 14th Street Bridge. The original paragraph appears below, unedited.

No American air carriers have experienced a fatal accident since Colgan Flight 3407 crashed in a neighborhood near Buffalo, New York, on February 12, 2009. The turboprop flew through severe icing on its approach into Buffalo from Newark, allowing a crust of ice to form on the aircraft's wings. The overworked and overtired pilots didn't properly respond to the ice buildup on the aircraft, causing the plane to go into an irreversible stall.

Airlines can conduct marginal operations when pure snow is falling, but if any sleet or freezing rain starts to mix in, forget about it.

Low Visibility

Low visibility is a pretty good reason to cancel flights. It helps when the pilot can see where he or she is flying your plane at several hundred miles per hour. The inability to see more than a few hundred feet in front of you is more common during dense fog than winter storms, but visibility can dip pretty low in heavy snow.

Commercial pilots are trained to fly their planes under two different sets of conditions: VFR (Visual Flight Rules) and IFR (Instrument Flight Rules). The former allows pilots to use their skills and visual judgement during takeoff and landing, while the latter requires pilots to strictly adhere to their instruments when visibility is too low to safely maintain control of the aircraft.

On December 16, 1997, an Air Canada regional jet carrying 42 people crashed in Fredericton, New Brunswick, when pilots attempted to abort their landing due to dense fog. The pilots were unable to regain altitude, clipping the runway with one wing and crashing into the edge of a wooded area. Amazingly, nobody was killed, and zero deaths is even more incredible when you see the picture of the tree that tore through the cabin.

Don't Yell at the Airlines for Bad Weather

Winter weather isn't the only reason that flights are cancelled. Severe thunderstorms over the airfield present a whole new set of weather-related issues, and they often result in planes circling or remaining grounded until the worst of the weather has passed through.

Almost everyone who has to fly on a regular basis runs into someone who inevitably yells at a gate agent because bad weather cancelled their important flight to Yuk Yuk, Mississippi. "Aww, it's not so bad! Come on! It's just a little snow! What's your problem!?"

Airlines cancel flights during bad weather because airplanes have a nasty habit of crashing when runways are icy or the wind is blowing too strong. They lose boatloads of money when planes sit empty at the gate, but their coffers take a bigger hit when they have to pay out million-dollar settlements to the families of craters in the ground.

It's too early to know for surethat hazardous weather conditions are the reason that the front of a commercial jet is hanging over the edge of Flushing Bay this afternoon, but it's a safe bet that snow and ice on Runway 13 was at least a major contributing factor in the crash. If you ever get caught in the airport during a snowstorm and can't figure out why your flight got cancelled over a few measly inches of snow, look no further than the banged-up shell of Delta 1086 for a compelling explanation.

[Images via the AP]

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