Hectopascals

[Spanish eyes]

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

[pilotms]

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

[chevvron]

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.

[kcockayne]

Yes, the Department for Pointless Change has struck again !

[El Bunto]

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.

[eckhard]

it was redefined equivalent to 100,000 pascals ( one hecto pascal )

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?

[MD83FO]

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

[finncapt]

But I thought his christian name was Blaise!!

[DaveReidUK]

AFAIK, 1 bar is equal to 1000mb

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.

[Spanish eyes]

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.

[HEATHROW DIRECTOR]

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

[BBK]

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?

[eckhard]

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.

Ah yes; the good old millibarn!

Thanks for the correction and Merry Christmas!

[eckhard]

And the word is "hectopascals". not your version!

Actually, hectoPascals with a capital “P”.

[ShyTorque]

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.

[scarecrow450]

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 !!!

[galaxy flyer]

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

GF

[Gonzo]

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.

[DaveReidUK]

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

[wiggy]

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

GF

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.......

[Senior Paper Monitor]

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)

[DaveReidUK]

I still haven't recovered from GMT becoming UCT

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.

[chevvron]

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.

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

[Gonzo]

Yes, and it’s a damn good one.

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

[ATNotts]

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

[Heathrow Harry]

https://en.wikipedia.org/wiki/International_System_of_Units

[finncapt]

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!!

[Dubaian]

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

[ShyTorque]

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

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

[DaveReidUK]

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

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

[chevvron]

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?

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.

[LEGAL TENDER]

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.

[RAT 5]

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.

[good egg]

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.

[finncapt]

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.

[India Four Two]

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

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.

[FlightDetent]

ATS messages ... to be read back in full by the pilot ... If a readback is not received the pilot ... will be asked to do so.
-Altimeter Settings, including units when value is below 1000 hectopascals dated MAY 2016, extracted from the website.

4.5.7.5.1 The following items shall always be read back: ...
c) ... altimeter settings ...

4.5.7.5.2 The controller shall listen to the readback to ascertain that the clearance or instruction has been correctly acknowledged by the flight crew and shall take immediate action to correct any discrepancies revealed by the readback. 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)3.4 For all transmissions, the word ‘hectopascal’ shall be appended to figures when transmitting a pressure setting below 1000 hPa, or in cases where confusion or ambiguity may result.

[DaveReidUK]

Anyone suffering from this UK requirement is actively participating on a safety net preventing trans-atlantic aircraft crashing into other traffic.

You are joking, I assume.

[FlightDetent]

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.

[chevvron]

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)
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.

[DB6]

If you accept as a principle of R/T the desirability of getting the maximum amount of information over in the minimum amout of air time, you will see how moronic the use of 'hectopascals' is.

[mgahan]

The following (paraphrased) excerpt from an old interview with Spike Milligan - I think it was with Harry Secombe and Terry Wogan in 1987 - seems relevant to this discussion.

SM: My father had piles.
TW: You mean hemorrhoids?
SM: No he had them before they went metric.

MJG
Who was responsible for issuing the notice of change from millibars to hectopascals to the RAAF in 1987.

[dastocks]

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)
GMT was fine in an age when clocks weren't too accurate. It was handy to know how many seconds there were going to be in a given calendar year even if no one knew exactly how long each second was because this could only be determined retrospectively via astronomical observation.

Now that we live in the age of very accurate atomic clocks it is more useful to have seconds of a standard length and then add a leap second every now and then to keep UTC within a second of the 'mean time' UT1. UT1 is the modern equivalent of GMT, determined by astronomical observation.

[chevvron]

If you accept as a principle of R/T the desirability of getting the maximum amount of information over in the minimum amout of air time, you will see how moronic the use of 'hectopascals' is.

Like I said at #3, the CAA are considering using an abbreviation.

[flyingtincan]

Back at post 33 RAT 5 said
So why do we measure:

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

When I was learning the penny dropped when I realised that if I heard "feet" that was up or down and when I heard "metre" or "nautical miles" that was in front or behind. i.e to differentiate between vertical or horizontal distance.

[RAT 5]

When I was learning the penny dropped when I realised that if I heard "feet" that was up or down and when I heard "metre" or "nautical miles" that was in front or behind. i.e to differentiate between vertical or horizontal distance.

That's an interesting theory, and very personal. I'm not advocating anything as I was so used to the current units; but it no longer matters, professionally. If west EU measures visibility & runways in meters, and metrication has become the European norm outside UK veggie markets, and east EU uses meters for altitude & wind speed, and I've flown French GA a/c with kph ASI's, it would be possible to go fully metric in EU if range was also quoted as km's. However, the might of EU, west & east including Russia, would lose out to the power & influence of USA, China, Japan and southern hemisphere, and the status quo will remain. So metrication in our industry, is a part-time affair and at the end of the day it's only numbers.
Hap became possible because it was at no cost to instrument manufacturers. The same with changing Kelvin to Celsius. There is the battle between Watts & Joules, that may still be on going. Not good for anglo-french relations. Fortunately Newton holds sway in one field.
And who gives a peck or a bushel about it all anyway. 'Chain' them up in the 'yard' next to the horse with 14 'hands' and a 'cubit' of mane, drinking a pint and telling the rest of the world to go forth & multiply.
Happy New year, EU version, but at differing times.

[DaveReidUK]

There is the battle between Watts & Joules, that may still be on going.

What "battle" is that ?

The two have different dimensions, so you can't substitute one for the other.

[jensdad]

Is there any metric unit commemorating someone else?

GF


Good old Mr Celsius had a unit named after him :)


edited to add: Sorry, I just read the first page of the thread, not realising there were another two pages !

[RAT 5]

Watts to joules calculation
The energy E in joules (J) is equal to the power P in watts (W), times the time period t in seconds (s):
E(J) = P(W) × t(s)

Sorry Dave; I was forgetting about the 'time' parameter. I'd been confused when they'd tried to metricate bhp. I should have stayed awake and the back of class.

[PDR1]

Yes, because 550 foot-pounds per second is such an intuitive unit, isn't it...

:ugh::ugh:

The point is that the Watt is a unit of Power while the Joule is a unit of Energy. Power and energy are completely different things - power is the rate of delivery of energy. You would never suggest that miles were being replaced by the m/sec...

PDR

[DaveReidUK]

I'd been confused when they'd tried to metricate bhp. I should have stayed awake and the back of class. Sadly, I too can remember doing power calculations in hp.

In fact I recall having the hp/kW conversion ratio marked on my slide rule. :O

[India Four Two]

chevvron,

I received your PM but I’m unable to reply via PM or your email address (on PPRuNe).

Please email me at the address I previously sent you.

[chevvron]

chevvron,

I received your PM but I’m unable to reply via PM or your email address (on PPRuNe).

Please email me at the address I previously sent you.

I'm still unable to access PMs. Been off again for a couple of days but posted the minimum 5 postings so at least I'm partially back on line.

[dixi188]

Straight out of my head.

746 Watts = 1 Horse Power.

[ZOOKER]

It could have been much worse. Imagine if the early research on pressure had been carried out not by Pascal, but by by Mendeleev, Gay Lussac, or Avagadro? :E

'hectoBoyles' even?