why do the airplanes sometimes go the other direction?

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I live near Harry Reid Airport (FKA McCarren airport) in Las Vegas, directly west of the airport. My apartment faces west. The vast majority of the time, I can look out my window or walk my dog and see planes lined up to come in for a landing (sometimes 7 or more at once!) and watch them fly eastward overhead on their descent. But sometimes, every now and then, the planes go the opposite direction and I see them going WEST and gaining altitude (so clearly they have taken off). WHY do they seemingly randomly switch the runway direction? Weather and wind does not seem to play any factor as I’ve tried to pay attention to if I only see them going the other way when it’s cloudy or windy and no… It truly seems random. Why do they do this? It baffles me.

In: 49

Typically airports will try to have aircraft take off into the wind, so if the direction of wind shifts the airport can switch up the direction aircraft take off and land.

Aircraft need to move a certain speed through the air in order to take off so if they move into the wind they are getting a boost, while if they take off with the wind at their back they need to accelerate even more. Similarly if aircraft are landing with the wind at their back they will need to contact the ground moving faster than if they were flying into the wind.

When possible, planes try to take off and land into the wind. This is because it allows the plane to maintain more lift at a slower speed, and the headwind helps the plane slow down once it’s on the runway. So when the winds shift directions, deepening on the layout of the runways, airports will often change which runway direction planes use for landing and takeoff so they can still land and takeoff with a headwind. The wind might not be apparent to you but it could still be enough to benefit the planes.

It has to do with the direction of the wind.

As a pilot, you want to want to take off and land with a “headwind”, that is, the wind in your face rather than at your back. This forces more air across the wings and control surfaces of the plane, giving it more lift, which means they don’t need as much actual speed to take off and land.

For this reason, most airports will have at least one runway that runs roughly north-south, and at least one that runs roughly east-west. That way, no matter what direction the wind is blowing, there’s a runway that runs in roughly the right direction to give pilots a headwind.

I’m a Vegas plane watcher too and sometimes when the wind is light, traffic will take off to the east. Since this cuts down on taxi time from Terminal 1 to the end of the runway (it can be almost 2 mile long taxi going to the “usual” end.)

Also, flights going East save fuel and time because they don’t have to turn 180° to head East.

This happens famously at LAX Airport in Los Angeles. 95% of the time the wind is from the west or southwest off the ocean (in fact the airport was put there on the coast because of the reliable west wind), and the planes both take off and land pointing west. I live 3 miles south of it, with a north facing window, so I can clearly see the planes.

On days when it’s raining, storming, or there’s an offshore wind (common in the fall and winter), they reverse the airport, and planes land from the ocean and take off toward the city. They also do a half reverse each night to reduce noise pollution, both landing and taking off over the ocean.