Airspeed Indicators - Knots Vs MPH ????
 

I was just reading the thread regarding the "PA28R Glide Speeds", and without hijacking that thread I was wondering about Airspeed Indicators...

In Australia we use Knots displayed on the AI. But I see some refer to Miles Per Hour (MPH) on the their aircraft AI's. Is this just an American thing, or all around the world??

I've been involved in aviation for 22 years now, but have always dealt in Knots, tho some Flight Manuals have had some readings in MPH. Without being too nieve, what is the world standard?? Who uses what?? I wish that would just use the one speed to simplify things when reading international posts :ooh:

Cheers, KP

In general all US built aircraft before around 1974 had airspeed indicators in MPH. In the mid 1970's GAMA (General Aviation Manufacturing Association) recommended all manufacturers standardize on knots and so most but not all switched (Maule for example still to this day uses MPH calibrated airspeeed indicators). Unless the Australia CAA mandated all Aus registered aircraft have airspeed indicators in knots, there should be plenty of older aircraft still flying with their original MPH airspeed indicators, there certainly are in Canada, including my 1974 Grumman AA1B. I have no idea what the situation is in Europe.

I fly three different types. One has MPH, one has knots and the third has both. All three built in Europe.
Some manufacturers like to use KPH in advertising blurb. Makes it sound faster.

Fortunately, though our ASI main scale figures are in MPH,, there is also a scale in Kts with slightly smaller numbers just inside the MPH ring of numbers...........(1970 Arrow)

And in the U.K. at least, microlight a/c are commonly mph.

[That's flex and 'proper' fixed wing, nowadays with quite useable cruise speeds & h.p.]

Robins (French) have the outer scale, with all the bright colored markings on it, in km/h. There's a dull inner scale in knots.

Gliders do everything in metric. Speeds, but altitudes as well.

Speed is actually not the worst issue. You only have three ways of measuring them, with a fixed ratio between them (disregarding Mach numbers for now). But you can measure fuel in:

- liters
- imp. gallons
- US gallons
- pounds
- kilos
- tonnes

And of course the relation between the first three and the last three is not fixed, but is dependent on the specific gravity of the fuel. Which, with Jet-A, is not a strict given but can vary a little.

In the UK both the ASI and the vario are in knots, and the altimeter is in feet. However we record cross-country distances in kilometers.

I know of a K21 that was imported from Spain and believe the ASI and altimeter were changed for instruments in knots and feet - I think they left the mechanical vario alone.

Generally older GA aircraft (as mentioned before) have ASIs in MPH. I teach on Cessna's and of the 12 aircraft in the fleet, three have MPH on their ASIs. Although not entirely correct we use the KT numbers and add 10 to get a rough speed in MPH. For a Cessna 150 this works out well. We also have a number of Arrows and of those two are the Arrow II variant with ASIs in MPH again. The others are newer and are calibrated in KT. For those Arrows the POH has everything in MPH so we use the proper figures from the appropriate POH.

To answer the original question, I believe that knots, mile per hour or km per hour are all acceptable units of speed. Feet seem almost universal for height except in eastern europe.

Why actually do we use knots? The definition of nautical mile (one minute of arc on a great circle) is fairly irrelevant when flying from Norwich to Perth (for example). We could as easily use statute miles and mph (or km if we want to be good Europeans). From the point of view of the POH the unit is fairly irrelevant - you just fly the numbers, which I believe in Yaks is in km/hour.

For navigation purposes. You're right, it is not very relevant when flying short distances but that is the reason. Back in the pre-WWII days a DC-3 might have an ASI in MPH on the left (captain's) side of the cockpit as he did the flying, and an ASI in KT on the right (co-pilot's) side of the cockpit as he did the navigating. This made for confusing situations when changing seats.

I would make a wild guess and suggest that our maps were produced by people who had produced nautical charts, hence distances in nautical miles and the graduation of some ASIs in knots.

Well, given that we still use aeronautical charts marked in lat and long, and 1 nautical mile is a minute of latitude, that's a fair guess! It's just that we never renamed it the air-mile, and that name is now taken!

Up until the 1950s, most British and American aircraft used mph, but there's been a steady move to knots - a degree of standardisation, but also possibly the removal of navigators from cockpits. In Europe, knots is normal in bigger aeroplanes, but kph is common in gliders, microlights and some light aeroplanes.

Except in sales demonstrator aeroplanes, which are almost always in mph, because the prospective purchaser thinks they fly faster.

Personally I have a slight preference for knots, but have flown knots, m/s, mph and kph and so long as you know the limitations in display units, and your cruise speed, it really doesn't matter.

Altimeters on the other hand, in anything but feet, I hate with a vengeance, not least because of the confusing when in mixed airspace with aeroplanes whose altimeters are more conventionally in feet.

G


N.B. Note to original poster. AI = Attitude indicator, ASI = Airspeed indicator. Standard industry abbreviations.

I believe that is the "geographic" nautical mile which will vary as the earth is an oblate spheroid and not a perfect sphere.

The International Nautical Mile is defined as 1852 metres.

Much more logical to have the ASI calibrated in Kts (nautical miles per hour) methinks.

A kilometre is one ten thousandth of the distance from the Pole to the Equator, hence 10,000 km = 5,400 nautical miles.