US vs Europe
 

Hi

A few questions for those in the know.

Millibars for Atmospheric Pressure
Does the US use a different atmpspheric reporting scale to Europe. I though millibars was used everywhere.

Transition Altitude
Switching to "flight-level" at 18,000 feet is an FAA convention; does this procedure vary in European countries or is there a standard in Europe?

Thanks

yes, inches of mercury 1013mb=29.92 inches of mercury

yes, typically 10000 feet for transition

All of Europe uses Hecto Pascal, Britain excluded they use millibars.

Most altimeters in jets include both scales, so it's not really a problem.

Transition Levels (Descent) and Transition Altitudes (Climb) vary by country and region; it is usually affected by terrain considerations.

For example, in Spain TA is 7.000' except in LEGR where it's 8.000'.

ferrydude - to be more accurate, 1013mb=29.91 inches.

The ICAO standard setting is 1013.2mb=29.92

Not that 0.2mb is THAT crucial........................:D

TA, I think, is not normally (never?) below the highest safety height at the airfield.

Thanks BOAC, I stand corrected. And as you say, .2mb won't make a pimple on a fly's arse.

The problem with the altimeter setting values are that they are "Manometric" units - used for manometric calibration - and because they are "derived units" there is no proper international standard for them.

The sea level conditions for the International Standard Atmosphere are 101.3 kPa at a temperature of 388.2 degrees Kelvin (15degrees Celsius) and a density of 1.225 Kg/m3. The sea level conditions for the American Standard Atmosphere 1976 are 2116.7 lbs/ft2 at a temperature of 59 degrees fahrenheit and a density of 0.002378 slug/ft3 which amounts to the same thing but in US units.

That 29.92 inches comes from the meterologists standard atmosphere however, by which the sea level conditions are taken as 29.92 inches of mercury and a temperature of zero degrees celsius.

Being derived units, derived from different standards, there is thus no direct equivalence between the 29.92 inches and the 1013.25 millibars of the two scales. It remains correct that for practical purposes the difference is too small to be of any concern to airmen.

It does matter to us instrument calibrator types though. Small differences at lower altitudes are magnified logarithmically as one ascends the scale, leading to much frustration for the poor calibrator who is tweaking them out by trial and error. A further complication comes from older altimeters being calibrated to older national standards that differ in their base values. For example most altimeters of British construction dating back prior to 1976 were calibrated to 'London Laboratory Conditions' - 30 inches of mercury and 60 degrees fahrenheit if my memory serves me right. It is too many years since I applied Fiducial Correction to my Casella Barometer readings using the trusty old "Z" chart...

Europe uses hectopascal and the UK uses millibars ..... which are identical, so why doesn't the UK just change the name to hectopascals?

I'm all for it. We'll make a fortune out of the airlines, stripping down their altimeters to paint out the 'mb' letters and paint in the 'hp' ;)

Will that infringe Hewlett Packard's trademark i wonder? :confused:

Topbunk
The reason is that the UK hasn't yet ratified the international agreement on units,I think.
The international system has gone completely metric and the sub units like the centimeter based ones or the "atmosphere",Bar and the likes have disappeared.
For us the pressure unit should be the "Pascal" equivalent to a
Newton per square meter,which is a ridiculously low pressure,hence the 100xPascal aka "hecto pascal" hpa.

errr - hPa actually! ;)

http://encyclopedia.thefreedictionary.com/Hectopascal