Actual speed of aircraft

I’ve got this question that’s been nagging me for years. How do they calculate the speed of an aircraft or a spacecraft? I’ve given this a lot of thought and here’s what I see as the problem(s).

Let’s assume that a car starting at point A on the surface of the Earth (a sphere) travels 70 miles to point B in a straight line for one hour. Thus, 70 miles per hour. Now if an airplane is above the same spot at one mile overhead, then the circumference of the circle has been increased and it takes longer for the airplane to travel the same distance. So if the airplane covers the same distance in let’s say 1/2 hour, you might think that the plane traveled 140 mph, but it would have had to travel faster because it was one mile high over the Earth.

So when they announce that a plane is travelling 500 mph how is that being calculated?

I believe I’ve heard reference to “ground speed” (or similar) and “air speed.” E.g., an airplane heading into a strong wind is going 300 mph (effective speed, as if it were on the ground), but its instruments read 500 mph (its air speed).

I have never heard of it being computed as anything other than the speed as if the plane were at ground level for ordinary purposes of navigation. The difference between that speed and one calculated taking into account the difference in altitude for a plane at 40000 ft would be about 19 parts in 10000 or 0.19%.

In the example you gave the difference would be about 25 parts in 100000 or about 0.025%. If that turns out to make a difference it isn’t difficult to compute. I would think the difficulty would be in knowing the circumferance of the earth along the path in question to the precision needed to make the computation.

OK - but that’s for airplanes. Now let’s look at spacecraft. I’ve been told that some of these beast travel at 18,000mph. Sound’s a bit high, but who am I to argue. Is this speed calculated the same way, for if it is, then the “actual” speed of the spacecraft would be much higher.

If the earth’s circumference is about 25,000 miles then a spacecraft traveling at 18,000 mph would circle the earth in about 90 minutes. And that’s just how fast a spacecraft in low-level orbit takes.

In the case of spacecraft what is meant is that if you drop a marker at time zero and measure the distance from the marker along its path, one hour later that distance will be 18000 miles.

You see, in one orbit a spacecraft can only travel the circumferance of the earth, about 24000 miles, when measured as ground covered per one orbit no matter how fast its velocity along its path.

For example, a geosynchronous satellite covers no ground in any 24 hour period.

As David Simmons has pointed out, the difference in speed based solely on altitude is insignificant.

But “airspeed” vs “groundspeed” is a completely different animal, as iwakura43 mentioned.

I think that the OP’s question has been answered by the altitude mathematics, but allow me to clarify this part:

Note: Simplified discussion ahead!
This is most likely groundspeed. It is the airspeed of the aircraft corrected for many factors, including altitude and wind. Without getting too technical, air density decreases as you climb in altitude. Airplanes measure airspeed with instruments that measure the flow of air into them (called pitot tubes). As air density decreases, so does the Indicated Airspeed. If an airplane climbs using a constant speed climb (meaning constant Indicated Airspeed) it must actually fly faster in order to maintain the same Indicated Airspeed for increasing altitude. The airplane’s actual airspeed corrected for altitude is called “True Airspeed”.

There are many ways to calculate True Airpseed (TAS), including straight math, “whiz wheels”, and aircraft components. Most modern jets have TAS displays. As an example a jet cruising at 250 knots Indicated Airspeed at 35,000 feet might have a True Airspeed of 410 knots. This means that in a no-wind situation the aircraft would be moving at 410 knots across the ground.

One knot = one nautical mile per hour. Aviation distances are in nautical miles (except for visibility, for all of you Doper pilots checking my facts!). One nautical mile = 6000 feet. One statute mile (what everyone else is used to) is 5280 feet. This is where the difference between knots and MPH comes into play.

500 Knots = 500 NM/Hour= 500*6000ft/hr = 3 million ft/hr

3 million/5280 = 568.18 Miles Per Hour.

So 500 knots = 568 MPH in a no-wind situation. Add in a headwind or tailwind and the speed will increase or decrease, and will become your groundspeed.

Since wind is in knots, the correction is made before the conversion to MPH. For example, 500 KTAS (Knots True Air Speed) and a 50 knot tailwind equals 550 Knots Groundspeed. This equates to 625 MPH Groundspeed.

And for the record, MPH is used only when relating to non-flying folks like passengers or news audiences. Knots are used exclusively between pilots and controllers.

The Cessna 150 airspeeds are in MPH on the gauge, as well as on some Piper Warrior I’s (those whose airspeed indicators haven’t been upgraded). Probably a few other old fogey planes as well. Not that I expect a Big Iron Pilot to know that off-hand. :slight_smile: But it was rather annoying when I was filing flight plans and dealing with ATC with a C150 and had to always be converting the MPH on the gauge to knots. Not too annoying - at one point I did have the equivalents memorized for the C150 flight envelope.

And then there are the ultralights… well, they could be registering airspeed in furlongs or cubits for all I know.

But Pilot141’s explanation (which did have a warning label about simplification, after all) is a good one. As usual.

But here’s my question:

Here in the States we talk about speed in knots (as Pilot141 mentioned, but in other countries (Europe, for example) do they ever have reason to convert to KPH - or just “when relating to non-flying folks like passengers or news audiences”?

Also - we talk about elevations (of terrain, clouds, aircraft, whatever) in feet - is the convention outside the US to use meters? I think this may be the case, but I’m not sure.

(Obviously, I have not flown outside the US, yet)

That’s a geostationary satellite. All of those are posted at the equator, and don’t move relative to the ground.

A geosynchronous satellite is placed in an elliptical orbit at the same altitude as the geostationary ones. The upshot is that a geosynchronous satellite appears to cross the same point on the ground constantly, moving north/south each day.


One nautical mile equals 1.15 statute miles; so 500 knots is 575 smph.


I learned to fly in dad’s 1970 Cessna 172. Its airspeed indicator was calibrated in miles per hour.

Slight hijack: I’ve always pronounced “pitot” as “PEE-Toe”. I think I’ve heard the correct pronunciation is “pee-TOE”. No one I know uses the ostensible correct pronunciation, and no doubt the pitot tube will continue to be called by the common pronunciation; but can anyone give me the Straight Dope on this? (Regardless, I’ll still say “PEE-toe”.)

My dictionary says 'pi:təə (if those last two characters don’t come out, they’re schwa and upsilon), so that’s PEE-toe, as you say. The dictionary capitalises the P, as Pitot tube, as it is named after the French physicist Henri Pitot. In French, the pronunciation would, I imagine, be more like “pih-TOH”, with a short unstressed “i” sound as in “pit”.

I’m in the avionics biz, and every engineer I know who knew what one was calls it PEE-toe.

Well, most parts of the world use feet, knots and flight levels. I’ve flown all around Europe, Africa and the Pacific and the only place I’ve had to use meters was in the former Soviet Union. I’m sure there are other places that still use meters, but I’ve never been there.

One difference in Europe is that they use hectopascals for alitimeter settings instead of inches of mercury. Instead of 29.92 it’s 1012 (or whatever) hPa. On older US-made altimeters you have to do the conversion yourself and set in/Hg. Newer “steam” altimeters have dual windows and you just dial in whichever scale you are using. Glass displays usually have a button that switches the altimeter between in/Hg and hPa.

Any orbit around the Earth with a period of 1 day is geosynchronous. Geostationary is a special case of geosynchronous, with the additional requirements that the orbit be equatorial and circular. A geosynchronous satellite with an eccentric but not inclined orbit would, in fact, wander in the sky over a point, but only east-west, not north-south. A satellite with an inclined but circular orbit would move in a figure 8 pattern relative to the Earth. And for a satellite with nonzero inclination and nonzero eccentricity, the figure 8 would be asymmetric.

No, in French, as a French name, the “i” would be prounouced as an English “ee” sound - French does not have a “short i”. But the emphasis would most likely be on the second syllable for “pee-TOH”

On the other hand, no matter that the word came from French, at least in North America it has been (for lack of a better term) anglicized into “PEE-toh”. There are several other (at least) aviation terms derieved from French but given an anglicized pronounciation in the US (and I think Canada, too).

Another example (in my crude phoenetics, with “JZ” being that French “j” or “g” sound that isn’t quite the English sound for the same letters):

US - PYE-leh-tage
France - pee-loh-TA(JZ)E

US - EM-pen-age
French - em-pen-A(JZ)E

US - FYOO-suh-lage - or- FOOS-lage
French - fyoo-seh-LA(JZ)E

You can see here that, again, the emphasis shifted to the first syllable and the vowels anglicized. It’s not an unusual pattern for French words immigrating into English. So, really, while you can argue that there’s a more correct French pronounciation, the truth is, in American English the American pronounciation is more correct (whatever that really means) when speaking English in the United States. They’re still loan words, even if they haven’t changed that much.

Or, as t-shirt of one of my friends says: “English doesn’t borrow words from other languages. English follows other languages into dark alleys, knocks them down, and goes through their pockets for loose grammar”