I 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?!
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). The boat enjoys a nice increase in speed because its speed through the water is added to the speed of the river current:
Boat speed + river current speed = speed relative to the ground. 15kts + 10kts = 25kts.
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 the boat, it takes it 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
The boat heading downstream cruises along at 25 knots while the upstream boat crawls at 5 knots. Quite a difference!
Airplanes fly 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?
Conventional airplanes can’t actually fly backwards, but they can move across the ground backwards. I did 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, it only cares about the speed of the air flowing over the wings), 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.
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 windy.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.
Breaking the Sound Barrier?
On February 9, 2020, a British Airways 747 broke the record for fastest sub-sonic flight from New York to London. The flight was a remarkable 4 hours 56 minutes, breaking the previous record by 17 minutes.
Check out the aircraft ground track. The wind barbs in the center of the Atlantic indicate winds around 200 knots (230 mph). The aircraft had quite a push. [Learn more about wind barbs and winds aloft]
The flight reached a top speed of 825 mph across the ground. The speed of sound at cruise altitude was 674 mph (the speed of sound changes with temperature). So, did the aircraft break the sound barrier? It was certainly moving across the ground faster than the speed of sound…
BA flight 112 was flying at its normal cruise speed of around Mach .88 (88% of the speed of sound). The airflow over the wings was sub-sonic, so no shock wave, and no sonic boom. The aircraft did not break the sound barrier. The airplane flew happily within its normal operating envelope.
As demonstrated earlier, airplanes fly within a body of fluid. If it had been a boat, the 747 was cruising at the normal speed on a river that was flowing at over 230 mph toward London.
More About Winds
Fascinated by winds and air mass movement? Learn more about Winds and Temperature Aloft Forecasts!