Hectopascals
 

Air pressure seems to be measured now in what sounds like Hectopascals. Is that different from the old measurements of QFE or QNH?

The previous unit was millibar.
Old QNH 1013 millibar
New QNH 1013 hectopascal
So it is only different name.

The CAA, against much opposition, decided a couple of years ago to implement this as it is the ICAO 'standard', otherwise it's exactly the same unit.
Pilots don't like it, controllers and FISOs don't like it, but the CAA insist on using it even though it makes RTF more complex as it doesn't 'roll off the tongue' like 'millibars'.
There is a possibility it may officially be abbreviated sometime in the future but the decision has yet to be made on this.

Yes, the Department for Pointless Change has struck again !

Isn't it only quoted over the radio for values < 1000?

Anyway bar is no longer directly related to atmospheric pressure, it was redefined equivalent to 100,000 pascals so really has no relevance anymore.

In so doing its value was reduced by 1.3%, which is why a 'standard atmopshere' is 1013 hPa and not 1000.

AFAIK, 1 bar is equal to 1000mb, which is equal to 1000hPa.
1 hPa is equal to 100 Pascals.
So 1 bar is equal to 100,000 Pascals, as you say.
But 100,000 Pascals is not equal to one hectoPascal!
100,000 Pascals is equal to 1000 hectoPascals.
Possible slip of a decimal point?

There was some french meeting redefining SI units and asking that they be called by the name of their inventor some time ago

But I thought his christian name was Blaise!!

Well if we're being picky, 1 bar is equal to 1000 mbar. The mb is a completely different unit (of area, in fact) with little or no relevance to aviation.

I was listening to ATC today when the pilot repeated back the air pressure figure without saying the word hestopastcals. ATC made the guy say the word.

So the controller was doing his job. And the word is "hectopascals". not your version!

Believe El Bunto has the right info. I believe it’s stop confusion between say 997 hectopascals and 29.97 inches. At least that’s what an ATC chap told me. It’s probably in CAP413?

Ah yes; the good old millibarn!

Thanks for the correction and Merry Christmas!

Actually, hectoPascals with a capital “P”.

I heard that a pilot used to altimeter settings in inches stupidly set digits passed as millibars on the same scale. So we've all had to change, rather than re-educate the few.

We have to say the word when the pressure is below 1000 as saying 999 can be mistaken by a US aircraft as 29x99 ! Yes I know !!!

Blame the French! Is there any metric unit commemorating someone else? Oops, I think Newtons, but who uses them?

GF

It was a very good idea to standardise and adopt ‘hectopascals’ as the unit of pressure, which most of the rest of the world already used.

The more UK-only idiosyncrasies we can eliminate the better.

While there was some resistance at the time of change, that soon disappeared. It’s just what we use now, and has had no adverse impact.

But don't hold your breath awaiting a post like that from the USA. :O

I’ll start with a :confused: , just in case you are posting in jest....then, (as someone who still uses newtons at times) off the top of my head I’d point out that the names of the basic SI units were actually deliberately chosen to commemorate the leading early researchers in their field.

From memory here is a very much none exhaustive list of metric units - yes, you can blame the French for some ....but not all of them:

Tesla, Watt, Joule, Ampere, Ohm, Volts, Curie, Hertz, Rontegen..........for the benefit if our American friends there was a fermi for a while but I think that’s no longer an official SI unit, but that reminds me of the Angstrom.......

I still haven't recovered from GMT becoming UCT (I would have felt much better about it if I could have got on the global wandering committee discussing it though)

That wouldn't have been too bad, had the French not stuck their oar in and insisted that we all call it their version, UTC.

There seems to be a general CAA campaign to reduce the number of 'differences' the UK files with ICAO procedures.

Yes, and it’s a damn good one.

c.f. ‘Behind’ vs ‘After’ for conditional line-ups.

Just need to finally abolish inches of mercury for pressure and temperatures in Fahrenheit.

Some years ago we used the cgs (centimetre, gramme, second) system of units but the basic unit was too small and we went to the mks (metre, kilogram, second) system to address this.

Several chaps/esses lost out and others gained when this happened.

Just realised that is probably explained by HH's post - sorreee!!

off topic .....
 

A millihelen is a unit of beauty required to launch exactly one ship.

Precisely. That's why ATC had to get anally retentive about everyone repeating the word Hectopascals.

Absolutely. We could get rid of split infinitives too, while we're at it.

They don't do it in the ROI or any other country from what I'm told. It's only because of 'pressure' from a certain source we still do it here.

It becomes pretty useful when the QNH is 992 hPa.. the American crew read back "nine ninety-two" and you get them confirm it's 992 hPa and potentially spotted a 600 feet level bust!!
It happens.. not regularly, but it does. Especially when they come over the Atlantic half asleep at 5am!! :)

Agreed, the word millibars would have achieved a similar result, but I think hectopascals, being a mouthful, stands out more and it makes it even more obvious that is't not inches of Hg.

So why do we measure:

speed = knots,
route distance = nautical miles.
altitude = feet
visibility = metres
wind = knots.

I noticed in Flight International, many years ago, aircraft range was published in Km's??? and speed in kph, but the aircrafts instruments and FMC's still used Kts & nm's. Duh!
And there is huge resistance in EU to use Meters for altitude instead of feet; wind in m/s; like eastern europe; and visibility in feet like USA.

Thus I think there is more to sort out in our 'supposed metrication' standardisation than Hpa v Mbar. It's only numbers, anyway. We managed to change, and get used to - slowly - runway lengths changing from feet to metres, but I found visibility in feet difficult to assess. All you had to do was compare the numbers to the minima, same as metres.

I think a lot of people need to get out more.

Yes, it’s more clunky than it used to be but it’s hardly some seismic shift in ATC.

Bigger fish.

With regard to Rat 5 post.

Certainly when navigation was done, by using charts, nautical miles made some sense.

I recall the first a/c I flew (Forney ercoupe?) had an asi in mph.

Perhaps knots were used as they are quite a large unit of measurement rather than metres per second (if we are being strictly SI).

Maybe feet for altitude as, the alternative, metres would lead to multiples of 3 if one wanted a similar vertical separation.

I suspect though it was all to do with what had traditionally been used when long distances were covered by ships.

It is interesting that temperatures in TAFs and METARs in the USA are in Celsius. I'm not sure when that was introduced.

It's unlikely that hectopascal would ever be introduced in the US though - that would require replacing every GA altimeter - roughly 150,000 of them!

The same logic applies in Canada, where we have been officially metric since the 70s.

dated MAY 2016, extracted from the website.

Fifteenth Edition — 2007 AMDT 5

The mandatory read-back of units below 1000 mb seems to be a UK thing - I did rather thorough search of the ICAO paperwork.

I would consider it a constitucionalized best-practice against confusion with american operators used to inches, for reasons discussed and explained above. Anyone suffering from this UK requirement is actively participating on a safety net preventing trans-atlantic aircraft crashing into other traffic. Let's now make an informed decision how much to get aggravated :)

The rule was first implemented in CAP 413 edition 15 amendment 01 in December 2004, together with the "degrees" added after heading values ending in zero. Similar requirement exists for ATC personnel communicating over telephone, intercom of even relying company messages - readback and its verification included.

Today's wording (since JUL 2013 or older)

You are joking, I assume.

First part is supposed to be ironic, the second rather serious. Should have said "getting much closer than permissible to other ...". Hope this clarifies and sorry for the confusion.

Like I said at #31, we're the only country that does it.
As for the 'degrees' thing, I adopted the TC 'best practice' of giving heading instructions as +5s ie 235, 185 etc during my last couple of years as an ATCO 'cos I was too lazy to keep saying degrees.