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I'm sure this was asked somewhere, I just can't find it. So if it's a dupe, let me know.

Anyway, I'm one of those guys that actually looks at the flight information when I'm flying. I fly transatlantic 3-4 times a year and they show it on the personal screens.

They display the groundspeed, headwind/tailwind, temperature etc. I'm wondering how to calculate the actual speed based on that. Is it the groundspeed plus/minus the headwind/tailwind? Meaning, if groundspeed is 500mph, and the headwind is 200mph, does that mean we're only traveling 300mph? Meaning it will take us 10 hours to travel 3000 miles? Or are there more factors involved and more math calculations?

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  • $\begingroup$ Would this question over at Physics SE help to answer your question? $\endgroup$ – Peter Kämpf Jan 2 '17 at 15:11
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    $\begingroup$ This question covers the difference between "GPS speed" (groundspeed) and airspeed. Does it answer your question? $\endgroup$ – Pondlife Jan 2 '17 at 15:18
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I'm wondering how to calculate the actual speed based on that.

Which actual speed? All of the speeds you mentioned are actual speeds. None of them is in any way artificial, virtual, or otherwise invented.

Is it the groundspeed plus/minus the headwind/tailwind?

That would be airspeed, i.e. the speed at which the plane moves relative to the air around it.

Meaning, if groundspeed is 500mph, and the headwind is 200mph, does that mean we're only traveling 300mph?

No. If groundspeed is 500mph into a 200mph headwind, this means that the plane has to fight against the headwind, i.e. its airspeed is 700mph.

Have you ever gone swimming where there is current? Can you imagine riding your bike on a treadmill?

Meaning it will take us 10 hours to travel 3000 miles?

No. If the groundspeed is 500mph, it will take you 6 hours to travel 3000 miles over ground.

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  • $\begingroup$ But you deduct a headwind and add a tailwind, this aircraft would be moving 300mph over ground? $\endgroup$ – user13555 Jan 2 '17 at 16:17
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    $\begingroup$ I don't follow. The OP said the groundspeed is 500mph. We don't need to compute it. We already know it. $\endgroup$ – Jörg W Mittag Jan 2 '17 at 16:28
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    $\begingroup$ @user13555 You deduct a headwind from airspeed to get groundspeed. If you're starting with groundspeed, you add a headwind to get airspeed. But you wouldn't do that unless you were using GPS to determine your speed and had no airspeed indicator on the aircraft, in which case you should be asking yourself if your plane is even airworthy. $\endgroup$ – Throw Away Account May 16 '17 at 8:19
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How do head- and tailwinds affect airspeed?

They don't. Airspeed is the aircraft's speed relative to the air it's travelling through. The aircraft doesn't care whether that air is moving relative to the some other object such as the ground, because the aircraft is only interacting with the air. (Similarly, when you walk to the back of the plane to go to the bathroom, you don't care how fast the plane is moving through the air or across the ground, since you're interacting only with the plane and not with the air.)

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When most non-pilots ask for the aircraft speed, they are usually wanting to know the groundspeed. This is the actual speed the aircraft is taking over the ground, hence it has already taken the wind into account.

If you remove the wind component from the groundspeed, you are left with the "true airspeed", which is the speed of the aircraft relative to the air around it.

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The Answer:

It's quite a common confusion when explaining speed to passengers - when someone asks "how fast are we going" they usually mean "how fast are we going over the ground" in which case you want Knots Groundspeed. This makes sense as both where you took off from, and where you land are (hopefully!) on the ground, and so it's your speed over the ground that determines travel time.

The Extra Bit:

Remember that Knots are Nautical Miles per Hour - so they don't correspond directly to Miles per Hour as you will know them. A knot is approximately 1.15 Mph, so if you were travelling at 500Kts, that would be 575Mph.

The Caveat:

Of course, this means that head/tailwinds can have a huge effect on your actual speed. If I'm flying at 500kts (airspeed) into a 100kt headwind, my groundspeed will only be 400kts.

I hope this helps!

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  • $\begingroup$ "you want Knots Groundspeed" Honestly, you probably want to give the groundspeed in a unit the passengers are familiar with: (statute) miles or kilometers per hour. $\endgroup$ – David Richerby Jan 3 '17 at 16:16
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If we take your sample problem of the aircraft flying at an airspeed of 500 mph and a 200 mph headwind (VERY strong headwind even for the jetstream but occasionally does happen) dead on the nose and steady, then, yes, your ground speed will be 300mph. If wind is sustained for entire duration, then it will take 10 hours to fly 3000 statute miles.

Generally headwinds or tailwinds are not straight on the nose or tail, so a little trigonometry will be required to determine the direct headwind or tailwind component of the wind. Let's say in the example above that you are flying at 500 MTAS on a ground track of 065 magnetic. The upper level winds at cruising altitude are 200 MTAS coming from 081 magnetic. Since the difference between your ground track and the wind is 16 deg, your true headwind component is 200 * cos(16 deg) or 192.25 MTAS, which will yield a groundspeed of 307.75 mph.

Modern flight and navigation software can compute this automatically for the pilot based upon a gps ground track, indicated airspeed, OAT and altitude. Previously, graphical methods using an E6B flight computer were used to calculate this as well as headings to hold a course on.

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  • $\begingroup$ My memory is failing me if they display the airspeed or groundspeed. Based on the other answers, it seems groundspeed already takes headwinds into account, so then it'll be easier to figure out length of flight etc. $\endgroup$ – Mennyg Jan 3 '17 at 13:59
  • $\begingroup$ Modern PFDs show groundspeed, true airspeed, and indicated airspeed, in different places. But the speed that appears next to the artificial horizon is IAS. $\endgroup$ – Throw Away Account May 16 '17 at 8:24
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To answer this question you need some aviation basics in speed measuring: In aviation we use several different speeds whith different names.

  1. KIAS (knots indicated airspeed) is the speed which is indicated in the cockpit. It's the aircrafts relative speed through the surrounding air. This speed is crucial for the flying performance of an aircraft 'cause it's the speed which is used to describe the minimum speed and similiar important airspeeds.
  2. KTAS (knots true airspeed) is the corrected IAS. This speed is not important for your question so I won't go any further on that.
  3. GS (groundspeed). This describes the aircrafts speed relative to the ground. So if an airplane has a groundspeed of 100kts it will fly 100nm per hour relative to the ground. This speed is influenced by the head- and tailwind, the GS will be higher than the IAS/TAS if the aircraft experienced tailwind and vice versa.

So the answer is: the speed you, as a passenger, see at the IFE System is the groundspeed. Therefore it is already influenced by head- and tailwind and it shows the movement relative to the ground in nautical miles per hour (knots).

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I know nothing about flying an aircraft, but on a recent trip on a Dreamliner to the Carribean I found myself regarding the very helpful maps and flight information accessible through the in flight entertainment system and puzzling over the expressions used such as ground speed, tail wind, head wind and wind speed. Cross winds weren't mentioned but I'm sure they could well have been.

Here's my layman's theory which made sense to me during the flight:

For passengers, the aeroplane's speed relative to the ground is the most important. It is ground cover we want after all, isn't it, to get us from home to our holiday destination and back in the most direct and efficient way possible.

However it seems to me that a moving aeroplane has two states:

  1. a terrestrial state: when it is still in contact with the ground
  2. airborne state: when it has left the ground

When moving forward for take off with its wheels still in contact with the runway a head or tail wind or indeed a side wind or an oblique wind will affect its motion as it would a car or any other terestrial vehicle designed to cover ground while remaining in contact with it.

However, once an aircraft has left the ground i.e. no longer has its undercarriage in contact with the runway, it is entirely at the mercy of the body of air in which it is suspended as it travels though it at several hundred mph. Here an aircraft experiences no more pressure on its nose cone at a constant speed through the air whatever the speed at which and direction in which the air itself may be travelling; head, tail or any other 'winds' surely become irrelevant; although the speed at which the body of air may be travelling is extremely relevant.

For example, if the body of air supporting the aeroplane is travelling at 50 mph in the same direction as the aircraft which is doing 400 mph the ground speed will be cumulative - namely 450 mph but the plane will still only be doing 400 mph relative to the body of air in which it is traveling. If the body of air has a terestrial speed of say 100 mph in the opposite direction to an aircraft travelling at 400 mph, the aircrafts speed relative to the ground - its ground speed - will be subtractive - only 300 mph.

If the body of air in which the aircraft is suspended is moving in a sideways direction' either directly or obliquely, relative to the terestrial course the plane needs to follow in order to describe a relatively direct route from take off to landing the plane won't be 'blown' off course as such but if no corrections are made and if all speeds are steady then the aircraft will arrive at a different destination from the one intended.

For example if an aircraft maintains a steady speed of 500 mph with respect to the ground i.e. has no help or hinderance from the body of air which is supporting it moving in a favourable or unfavourable direction but has a direct sideways motion of say a steady 20 mph left to right and the distance of terestrial travel is 2000 miles, after the plane has travelled for the 4 hours necessary to cover the 2000 miles it will arrive 80 miles to the right of its desired destination.

In these same steady conditions the plane would need to aim at a point 80 miles to the left of its desired destination and then it would arrive at its desired destination. The tail, head and cross winds are a different story though once the (for all intents and purposes) static ground is about to come up and meet the aircraft at considerable speed and as it prepares to leave it's airbourne state and assume its terrestrial state.

It's then, I guess, that the boffins invoke their equations for approach speed relative to the ground, air movement speed - be it head, tail or side wind and runway length in order to make the transition to a smooth landing.

I know this forum is to answer a specific question; please accept this as the offering of a complete layman but it does seem to me to be scientifically sound - indeed is my own answer to a question I have asked myself but I think this would appear to cast some light on the role head or tail winds play in aviation scenarios.

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    $\begingroup$ I would highly recommend editing your post to improve the formatting, a big mass of text is hard to read. It has been flagged as low quality and it may get removed if not improved. $\endgroup$ – Notts90 Dec 12 '18 at 21:23
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    $\begingroup$ Hi chris dolman, welcome to aviation.stackexchange. Your text is very hard to read because of the lack of formatting. If you allow me, here is some advice for writing better answers: 1) be to the point; side stories about going to the Caribbean, past work experience, are not relevant to the answer. 2) Cut your answer up into a number of paragraphs of at most 4-5 sentences. Leave blank line spaces between the paragraphs. 3) If you are not really sure about the topic, it's better to restrict your answer to what you know. If you are unsure about some parts of the answer, just state that. $\endgroup$ – DeltaLima Dec 14 '18 at 7:37
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    $\begingroup$ Hi chris dolman: I've improved the formatting and changed a couple of sentences to save your answer from being deleted by the community. It could do with further improvement, but I leave that up to you. Again, welcome at aviation.stackexchange. $\endgroup$ – DeltaLima Dec 14 '18 at 7:49

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