Headwinds and Tailwinds

Headwinds-tailwinds-splashdentist3I was sitting in The Chair as the dentist poked and scraped my freshly cleaned teeth. He was commenting about his upcoming Southwest Airlines flight to Phoenix…

Dr. S: Ken, my flight from Louisville to Phoenix is scheduled for 4 hours and 10 minutes…
Ken: uhh huh.
Dr. S: …and the return flight is 3 hours, 20 minutes.
Ken: uhh huh.
Dr. S: Is there really that much difference between flying east and west?!?
Ken: uhh huh. Wahhn you hlying eassh you hahh a hailwinn.
Dr. S: Well, that’s what I thought but I didn’t realize it was that much!
Ken: Uhh huh.

Clean teeth and free from the Cavity Creeps. But what about my dentist’s question? His westbound flight takes nearly an hour longer than his flight back home. Why is there so much difference in the flight time? He knew that winds had something to do with it. But an hour difference for a relatively short flight?!

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To understand the effects of wind on an airplane, it helps to think about a motor boat on a river. In the animations below, the river is flowing left to right at 10 knots (about 12 mph). Both boats are traveling through the water at 15 knots. Both motors have the exact same throttle setting.

The first boat is traveling the same direction as the water (downstream). He enjoys a nice increase in speed because he combines his speed through the water with the speed of the river current:

Boat speed + river current speed = speed relative to the ground.   15kts + 10kts = 25kts.

currentflow

Downstream1

In the next animation, the boat is traveling upstream and again moving through the water at 15 knots. Since the water current is flowing towards him, it takes him longer to travel the same distance along the river bank. We’ll have to change our formula and subtract the water speed:

Boat speed – river current speed = speed relative to the ground.   15kts – 10kts = 5kts

currentflow

RiverBed-upstream

The boat heading downstream cruises along at 25 knots while the upstream boat crawls at 5 knots. Quite a difference!

Airplanes travel in moving bodies of fluid, too!

Like boats, airplanes also move through a large body of fluid…  air! If our aircraft’s speed through the air is 450 knots and the air mass is moving at 160 knots in the same direction (a tailwind), our speed across the ground will be a smoking fast 610 knots (700 mph).

The downside is when we fly the opposite direction; the wind works against us. Now we subtract the headwind from our airplane’s speed: 450 knots – 160 knots = 290 knots across the ground. 610 knots vs. 290 knots – yuck!. Depending on the distance of travel, the time difference between a westbound and eastbound flight can be anywhere from a several minutes to a few hours!

Airplanes flying backwards?

slowflight

Going nowhere fast!

Conventional airplanes can’t actually fly backwards, but they can move across the ground backwards. I used to do this demonstration with my students when I was a flight instructor in the dark ages. It’s really cool.

If you fly a small airplane into a 50 knot headwind then slow the plane down until its airspeed is about 45 knots, something interesting happens.

Aircraft speed – wind speed = ground speed     45kts – 50kts = -5kts  

That’s a negative 5 knots of ground speed!  The headwind is faster than the airplane’s speed through the air. The airplane still flies just fine (it doesn’t care that there’s a strong headwind), but when you look down at the ground, you will be moving backwards. EEEK!

Here’s a video demonstrating this. These guys achieve several seconds of zero ground speed and a little bit of negative ground speed. Backwards Flying!

Flight planning and wind

When pilots and dispatchers plan flights, we take a very close look at the high altitude winds. The weather chart below shows the location of “jet streams” (the green lines) in North America and the Atlantic. Jet streams are narrow bands of strong winds. During winter in the northern hemisphere, they can reach as high as 200 knots.  If I’m crossing the Atlantic ocean eastbound to Europe, I want to fly close to the jet stream so I get a nice push to save time and fuel. Flying the other direction, I want to stay away from these strong winds.

High Level Prog

High Level Significant Weather forecast for 7 Feb, 2014

Wind speed vs. altitude

Wind speeds and direction can change quite a bit as we change altitude.  Airline dispatchers look at the winds aloft forecasts for several different altitudes to find the best combination of routing and altitude for each flight.

The following images are taken from windyty.com (a great website for exploring winds at different altitudes). It’s easy to see the difference between the wind speeds and patterns at 16,000 feet and 34,000 feet.

FL160-winds1

Winds aloft forecast for 16,000 feet. Screen shot from: www.windyty.com

FL340-winds1

Winds aloft forecast for 34,000 feet. Screen shot from: www.windyty.com

As I told my dentist: Have fun on your westbound flight. More time to relax and enjoy the scenery and two servings of peanuts!

AeroSavvy is written by Ken Hoke. Since 1984, Ken has loitered the skies in many vehicles, most notably the classic Douglas DC-8. He currently frustrates air traffic controllers in the US, Asia, and Europe as a Boeing 767 captain for a package express airline.
Ken can be reached here or any of these fine social media outlets:   

8 thoughts on “Headwinds and Tailwinds

  1. Alan

    Really well explained. When I don’t understand the complicated technical jargon on Wikipedia I look for sites like yours. I’m currently reading about the recent Dubai Emirates accident and this helped me understand head & tailwinds in the accident report. Could you write about indicated airspeed and true airspeed? Thank again for the wonderful easy to understand information.

    Reply
  2. Julia

    Thanks a lot for explaining all this. This is like ground school for dummies and I truly appreciate it. If you’re not already, you should really think about becoming an instructor 👍

    Reply
    1. Ken H. Post author

      Hi Julia,

      Thank you for the kind words. I appreciate the feedback. I was an instructor for several years and really enjoyed it!

      Thanks for reading!
      Ken

      Reply
  3. Hamza S.

    For a math paper, I was actually comparing flights from New York to Los Angeles to flights from Los Angeles to New York, and there was a 5 hour difference between the two! (8 hours vs 3 hours) I was so surprised I jumped out of my chair! Until I realized that there exist time zones.

    Reply

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