Hectopascals
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.
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.
Just need to finally abolish inches of mercury for pressure and temperatures in Fahrenheit.
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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!!
Several chaps/esses lost out and others gained when this happened.
Just realised that is probably explained by HH's post - sorreee!!
Avoid imitations
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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.
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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.
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.
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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.
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.
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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.
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.
Just need to finally abolish inches of mercury for pressure and temperatures in Fahrenheit.
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.
Only half a speed-brake
Originally Posted by UK CAA, CAP 413 2.69
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
-Altimeter Settings, including units when value is below 1000 hectopascals
Originally Posted by ICAO Doc 4444 PANS-ATM 4.5.7.5 READBACK OF CLEARANCES
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.
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.
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)
Originally Posted by CAP 413 Chapter 3 General Phraseology
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.
Last edited by FlightDetent; 30th Dec 2017 at 13:03.
Only half a speed-brake
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.
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)
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)
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.