On 2017-04-03, Tauno Voipio  wrote:

> On 3.4.17 22:32, Martin Gregorie wrote:

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

And I don't know whether an STC even exists to let you replace your
airspeed indicator with one calibrated in knots.  I started looking
into it once, but quickly retreated.  At least most such indicators
have an inner scale in knots, which comes in handy for navigation -
but all the numbers in the POH are in miles per hour, so you're stuck there.

>> 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

Inches of mercury over here.  One slight compensation is that one inch
of mercury equals about 1000 feet at lower altitudes, which is a handy
rule of thumb.

>>                                          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!

Shortly after that incident a cartoon appeared showing a refueling tech
kneeling on the wing of an airliner with a ruler and calling out to the
passengers inside: "How many feet in a liter?"

> Right. The point with the altimeter scale is moot, as current
> IFR aircraft are quickly having glass cockpits with band-style
> altimeter and airspeed indicators.

Not quite moot, unless you have an upscale aircraft or are willing to
fork out a 5-digit sum to upgrade.  There are thousands of small aircraft
with mechanical altimeters whose owners won't upgrade without a fight.
Mode C transponders digitize altitude in hundreds of feet; all hardware
and procedures in the entire North American ATC system are hard-wired
into feet.  I shudder to think of the nightmare it would be to cut over.

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