To put it politely, we were recently taken to task for using mph instead of knots when reporting on an aviation speed record because, among other things, “mph is for cars not aeroplanes.”
If there ever was grist for the mill, the prop wash surrounding the units of measure regarding aircraft speed is it. That’s because, like all bottomless debates, there is no authority supreme enough to decree we’re all going to use one measurement over another. Oh, the ICAO can recommend all they want, but to the horror of concrete-specific personalities worldwide, the convention is we all have to get along as each of us uses the measurements most comfortable to us. In other words, both knots and miles are acceptable in aviation, or when in Rome…(use kilometers).
Still, a brief examination of the situation shows just how confusedly human we remain. We can perhaps start by observing that from the beginning in English speaking countries, namely England and the United States, aviation speeds were universally given in miles per hour—that is, statute miles per hour—up to the jet age. At that point Mach numbers became more useful to the fast movers, so that part of aviation added “Mach 1,” “.82 Mach” and so on to the lexicon. In the later 1970s knots entered the general aviation world in a meaningful way, this after taking over commercial and military aviation in the decades prior along with ultimately stillborn attempts to align the United States with international standards. The result is we sport aviators converse to this day in a mixture of knots and miles.
Why Knots?
Good question. Why did knots ever come to aviation anyway? It is, after all, a measure of nautical miles traveled, which by the sound of it shouldn’t have much to do with piloting aircraft. Well, the military uses both ships and airplanes in common operations, so a universal measurement would seem expedient in their line of work, which might explain why they started using knots.
Plus, you’ve got to give it to the globe-trotting sailors as they’ve worked out a good system using nautical miles. Turns out each nautical mile equals one second of latitude, which therefore means each minute (60 seconds, you’ll recall) of latitude equals 60 nautical miles. As marine—and aviation—charts are marked with lines of latitude and longitude, this makes estimating distances and speeds rather easy. A quick eyeballing of degrees of latitude multiplied by 60 gives the nautical miles involved, and estimating the time involved to travel that distance is easily done if you know how fast you’re covering ground in knots. Advantage knots.
Where figuring distances and speeds from a lat and long perspective falls apart is when transferring nautical distances and speeds to any other system of measurement, including the also logical—and by the way, most popular—scientific and universally adopted system extant, the metric system. Or SI, for Le Système international d’unités, to be pedantic. And guess what? The continental Europeans and much of Asia have been using kilometers per hour to keep track of their airplanes for over 100 years.
Let’s also note that speeds are just the beginning of aviation’s absolute train wreck of measurements. Wind speeds come in both mph and knots or, if you’re overseas, they might be presented in meters per second. Visibility is given in statute miles, altitude in feet (or meters) and barometric pressure is expressed in everything from inches of mercury to hectopascals. Magnetic and true compass headings intermingle promiscuously and I could go on, but for once won’t.
So, here we are talking about little airplanes we build and fly ourselves in the greater aviation system. A quick poll around the KITPLANES® office resulted in everything from “Knots!” to “Whatever.” As the magazine is mainly freelance written, we get a pretty broad spectrum of opinions from my fellow contributors. Some use knots, most use mph and a few use both in the same story, meaning the poor managing editor has to pick one or the other. Of course, it’s also possible to give both knots and (mph) or mph (knots) but that’s clumsy reading, and we’re supposed to be enjoying our hobby, not slogging through an operations manual.
Personally, I started flying when knots were something sailors and maybe airline captains related to. Every little airplane on the field had airspeed indicators calibrated in mph, the instructor talked about mph, we navigated in statute miles and flew indicated mph in the pattern and all that. Furthermore, I drove a car to the airport that had a speedometer marked only in mph—no km/h in the background—and life was pretty simple.
My initial training being in the ’70s, just a year or so later the new Cessna trainers had airspeed indicators in mph with knots in a little triangle window under the pointer, and a bit after that the airspeed indicators were in knots with mph in the little window. We kept talking and flying in mph, though.
Today, my 71-year-old certified puddle jumper obviously has a mph airspeed indicator along with honest-to-cancer radium numerals on the tachometer—and my 40-year-old Experimental also has a mph airspeed indicator. I still drive to the airport in vehicles calibrated in mph (although km/h is often available) and to come right to the point, I live in a statute-mile world where all my charts, maps, odometers, speedometers and airspeed indicators read in the same units. I’ve also become reasonably adept at multiplying knots by 1.15 in my head when I want to convert the reported performance of an airplane or estimate how long it will take to get to a favorite destination in it. Occasionally I’ll think in knots if all the measurements are given that way, but I have to admit I’m a mph kind of guy and have the white beard to prove it.
What About the Manufacturers?
Even for their slowest models, Beechcraft, Cessna, Cirrus and Piper all talk knots. But then there’s the sport aviation industry of kit manufacturers, and yep, they almost all speak in mph. We can only speculate why, but two ideas seem plausible. The first is mph is a larger number than the equivalent knots, so a speed is more impressive in mph to the admittedly irrational human mind. After all, would you like to purchase a kit promising to yield an airplane that can fly 96 knots or one that goes 110 mph? They’re the same speed, of course, but the larger number just sounds faster.
The other reason we think aircraft kit manufacturers like to talk in mph is because if their prospective customer is a non or newly minted pilot then it’s a cinch they’ll relate to mph much better than knots. While a newbie kit buyer could be a 777 captain, he’s at least as likely to be a regular Joe who drives a car with a mph speedometer and is clearly thinking in mph and hasn’t transitioned to knots.
If nothing else, it’s a fact the kit manufacturers use mph in their brochures, advertisements and other customer outreach.
If you’re in the pleasure craft market, it’s 10 to one your customers are thinking mph. Heck, even small, trailered powerboats have speedometers calibrated in mph. And that’s a marine application, an arena with thousands of years of traditions and a highly specialized lexicon where a ceiling is the wall, a wall is a bulkhead, the floor is a deck and on and on. But the small-boat builders know their customers aren’t a bunch of mariners waterskiing on their day off but rather sportsmen reveling in their boat’s performance.
Which might give us a way to look at the knots versus mph situation: knots for pros and mph for sportsmen. Or hang it all and go metric.
Great, humorous article, with which I can identify. Thank you.
I’m a fairly new pilot and learned in C172’s calibrated in kts, except for two in the fleet that were calibrated in mph. And I’ll admit that the few times I forgot that my ASI was in mph made for some tense approaches. Then I bought a 1946 Taylorcraft with and ASI calibrated in mph. Since I grew up in the US, you’d think I felt right at home, but I still mentally translate my speed into kts. And, just to make things even more confusing, I now live in Canada so my vehicle (and all the road signs) are in kph.
Ain’t life grand?
Dateline October 1, 1999
“NASA lost its $125-million Mars Climate Orbiter because spacecraft engineers failed to convert from English to metric measurements when exchanging vital data before the craft was launched, space agency officials said Thursday”
Yep…
Turns out a nautical mile is a minute of arc (not a second), and 60 makes up a degree (not a minute)
People need to just “Lighten up, Francis.”
A lot of airplanes are still out there flying around with airspeed indicators marked ONLY in MPH, not in Knots (or both.)
The limitations for my 1963 Cessna 205 are in MPH, those for my Glasair are in MPH, and though the Cessna has a dual indicator marked in both, many planes are not like that.
People need to worry about what matters. As long as both people know which units are being used, it doesn’t matter which they use.
These arguments are almost always started by people with near-zero actual relevant flying experience.
I think I’ll mark my next plane’s airspeed indicator in angstroms per femtosecond. Just to fire up some more of the aviation Karens.
Personally, I think it would be pretty cool to measure our airspeed in km/hr. At a cruise speed of 115 mph, my airplane sounds pretty slow. But when you say 185 km/hr, its a whole different feeling…
On the other hand, it we switched to metric (sorry – “SI”) for speeds, I guess we would also have to switch to measuring fuel consumption in ” liters / 100 km”… That analysis has never made much sense to me (probably because I’ve measured miles per gallon forever). But it just seems really odd to calculate fuel consumption for a flight based on how many quarts of gasoline it takes to travel 65.6 miles… (That’s the Imperial equivalent to liters per 100 km. Remember, a liter is roughly 1.05 quarts, so you have to multiple the 100km by that…)
Finally, a man after my own heart with regard to the liters/100km thing.
Fuel measurement in aircraft is per hour, not per ‘distance’.
In Australia, we use knots, nautical miles for flight distances, litres for liquids, feet for altitude and kilometres for other distances. It is a mish mash but we all are used to it.
Good write-up.
When flying IFR, communications with ATC work best in one standard, i.e. knots.
When I perform data analysis for a flight test, I always convert to knots (and feet per second) because that is how my systems are set up. Also, that is what is expected to be used in Flight Manuals for airplanes that fly IFR a lot.
What a mess! wind in MPH vs flight in Knots 😉 Oh well.
The simple fact is that I don’t measure distance between two points in knots but in miles and if I am trying to determine the time it will take me to traverse a given distance, I will have to make multiple conversions to arrive at the answers. If everything were using a common value, such as miles per hour the calculation would be a lot simpler for all of us. Instead, we must try and convert the winds which are presented in knots to miles per hour along with anticipated airspeed (knots or mph) to determine how fast in miles we are going. Seems rather stupid to have to make all those calculations. The knots should have been left out to sea.
Several years ago I was flying a Blanik sailplane that had a variometer calibrated in m/s. I had no idea what a m/s was, but I was in a thermal climbing at 4.5 m/s, the max value on the vario was 5, and the ground was getting very small, very fast. Turns out I was climbing at about 900 ft/m. The nice thing with varios is that 1m/s is about 2 kts, which is about 200 ft/min. It doesn’t really matter what units the vario is calibrated in.
Great article. Loved the information. Just hope someday I get to a point in life where I can spend time arguing about the little things. (-: