On Mon, 03 Apr 2017 19:47:12 +0300, Tauno Voipio wrote:

> On 3.4.17 17:29, Martin Gregorie wrote:
>
>> Nope - I wouldn't have a clue about my weight and height in Imperial
>> units, but then NZ went metric before the UK did - and did it faster,
>> cheaper and with a lot less hassle than happened here. It was quite a
>> shock to arrive here and have go back to using outdated Imperial units.
>>
>> Small side note: actually, I do use some of the old units due to the
>> pervasive ICAO influence on aviation. As a glider pilot I measure speed
>> and rate of climb in knots, altitude in both feet and meters and
>> distance in km (because the Silver, Gold and Diamond badges measure
>> distance in km and height in meters).
>
> In aviation, there is some sense in distances in nautical miles (1 arc
> minute on great circle) and speeds in knots.
>
> Altitudes are measured in feet. The explanation was that on the
> altimeter 1000 m / revolution was too coarse and 100 m / revolution too
> fast.
>
> Vertical speeds are measured in feet/minute.
>
I know that are - in powered aircraft, but the variometers in UK gliders
are always calibrated in knots because it simplifies calculation:
dividing sinking speed by airspeed (ASIs are also calibrated in knots)
gives glide ratio without needing conversion factors - something you need
to know when calculating a final glide home.

By contrast, all European gliders are metric - rates of climb and height
in metres, speed in km/h, distances in km, which makes the glide ratio
calculation a bit harder.

OTOH all European powered civil aircraft use nautical miles, feet and
knots in conformity with ICAO conventions.

> As a born metric system user, I see aviation as a huge mess of all kinds
> of units, especially as American made aircraft have the manuals in
> inches and pounds.
>
Yes, you're right, but it can get even worse, with older American
aircraft often have airspeeds measured in miles per hour rather than
knots.

I think the ICAO approved units is a left-over from the aftermath of WW2.
The ICAO was set up in 1944 with 52 countries signing the convention, so
my guess is that the use of nautical miles, knots, feet and pressures
stated as mm of mercury rather than bars dates from about then and which
countries had operating civil airlines at the time, and so, with air
traffic control needing the use a single system worldwide and the
immediate post war dominance of US and British civil aircraft makers that
seemed like the most sensible approach. And, once set, why change? Look
up the "Gimli Glider" incident to see the problems you can get from
changing measurement units!


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martin@   | Martin Gregorie
gregorie. | Essex, UK
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