QNH correction for instrument approach
You'd have a devil of a job proving it though.
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PANS OPS Volume 1 ICAO DOC 8168 is the reference.
Here is a quote I posted earlier;
My highlight.
The UK AIP is still awaiting the appropriate text to be inserted.
However, if you get a cold snap and the temperature at LL is -10, then the minimum safe altitudes have to be increased by the cold weather corrections.
I as a pilot will do it for own navigation flying, but if being vectored, then as the person responsible for terrain separation, the controller must make the corrections. I will be checking of course!
Regards,
DFC
Here is a quote I posted earlier;
This chapter deals with altimeter corrections for pressure, temperature and, where appropriate, wind and
terrain effects. The pilot is responsible for these corrections, except when under radar vectoring. In that case, the radar
controller issues clearances such that the prescribed obstacle clearance will exist at all times, taking the cold
temperature correction into account.
terrain effects. The pilot is responsible for these corrections, except when under radar vectoring. In that case, the radar
controller issues clearances such that the prescribed obstacle clearance will exist at all times, taking the cold
temperature correction into account.
The UK AIP is still awaiting the appropriate text to be inserted.
However, if you get a cold snap and the temperature at LL is -10, then the minimum safe altitudes have to be increased by the cold weather corrections.
I as a pilot will do it for own navigation flying, but if being vectored, then as the person responsible for terrain separation, the controller must make the corrections. I will be checking of course!
Regards,
DFC
Unless the controller is also the met observer, he/she should not have to make ANY corrections; the met observer should do this prior to displaying the QNH on the controller's display's.
Where there is no qualified met observer, any QNH passed by the ATSU (whether ATC, FISO or A/G) will not have any correction added, but in this case the airfield will not have a published instrument approach procedure; if a pilot chooses to design his own private procedure, then it's up to him/her to work out any correction necessary.
Where there is no qualified met observer, any QNH passed by the ATSU (whether ATC, FISO or A/G) will not have any correction added, but in this case the airfield will not have a published instrument approach procedure; if a pilot chooses to design his own private procedure, then it's up to him/her to work out any correction necessary.
However, if you get a cold snap and the temperature at LL is -10, then the minimum safe altitudes have to be increased by the cold weather corrections.
aceatco, retired
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This seems a bit of commonsense (from Doc 8168 Vol 1)
Presumably we've never bothered in the UK.
4.3.6 Small corrections
For practical operational use, it is appropriate to apply a temperature correction when the value of the correction
exceeds 20 per cent of the associated minimum obstacle clearance (MOC).
For practical operational use, it is appropriate to apply a temperature correction when the value of the correction
exceeds 20 per cent of the associated minimum obstacle clearance (MOC).
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Unless the controller is also the met observer, he/she should not have to make ANY corrections; the met observer should do this prior to displaying the QNH on the controller's display's.
Once again, controler should never correct the QNH, nor the pilot.
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Yes Vintage ATCO, that means when the correction to the Sector Safe Altitude or Radar Vectoring Altitude (providing 1000ft obstacle clearance) reaches 200ft it needs to be applied.
Don't have the chart to hand but it does have to be icy cold. However, there have been times in the UK where the temperature has been below -10 over the last 30 years. Especially in Scotland where they do vector traffic from time to time.
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Chevvron,
Please have a read of the Flight Information Handbook next time you are in work. Altimeter corrections for the UK military are in there.
Regards,
DFC
Don't have the chart to hand but it does have to be icy cold. However, there have been times in the UK where the temperature has been below -10 over the last 30 years. Especially in Scotland where they do vector traffic from time to time.
-------------
Chevvron,
Please have a read of the Flight Information Handbook next time you are in work. Altimeter corrections for the UK military are in there.
Regards,
DFC
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QNH - Temperature Correction?
It has always been my understanding, that the QNH is the barometric pressure, when set on a subscale of an aircraft altimeter, will give the aircraft's height above mean sea level.
There are a large number of Met Reporting Stations, all feeding their data into the Met Bureau, who from the readings given, determine the "Area QNH". Aerodrome QNH is either given by ATC, or the Terminal Area Forecast.
The only temperature correction that I am aware of, is to apply the temperature when determining "Density Altitude", for take off and landing distance calculations.
On first contact with Tower or Approach, the QNH and Temperature are given, either by ATC, or on an ATIS broadcast.
Years ago, I saw a fully laden B707 waiting for one degree drop in Temperature, in order to achieve take off on the runway distance available.
"QNH is QNH, and Temperature is Temperature". Combined, calculation of Density Altitude is done.
What do you think.
There are a large number of Met Reporting Stations, all feeding their data into the Met Bureau, who from the readings given, determine the "Area QNH". Aerodrome QNH is either given by ATC, or the Terminal Area Forecast.
The only temperature correction that I am aware of, is to apply the temperature when determining "Density Altitude", for take off and landing distance calculations.
On first contact with Tower or Approach, the QNH and Temperature are given, either by ATC, or on an ATIS broadcast.
Years ago, I saw a fully laden B707 waiting for one degree drop in Temperature, in order to achieve take off on the runway distance available.
"QNH is QNH, and Temperature is Temperature". Combined, calculation of Density Altitude is done.
What do you think.
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It has always been my understanding, that the QNH is the barometric pressure, when set on a subscale of an aircraft altimeter, will give the aircraft's height above mean sea level.
Treating altimeter values as real altitudes is good for navigational purposes, but not for measuring vertical distances accurately.
QNH will give you the real altitude above MSL only and always when you are on the ground. The higher you are and the more air differs from ISA, the more QNH-set altimeter will lie you.
BUT:
All procedures, separations, MSA, AMA, MRVA and so on - are already corrected for this issue. The only expception are condtions with ground temperature below -15C (in my country, maybe other limiting values are used elsewhere), when obstacle/terrain clearance altitudes should be increased. There's official table for it at DOC-8168 and a lot of unofficial tables in pilots' suitcases.
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Outside the UK you do not know if a descent altitude on the QNH is temperature corrected or not until you check the approach plates/information or by asking the ATC if you are not sure.
One controlled civil airport on our route network does and we know it.
The other is a military airfield which dosn't. When we get a descent clearance they state that the altitude is not temperature corrected. We make a temperature correction and read back "descending to ****ft, temperature corrected".
At uncontrolled airports we make our own corrections.
The ICAO table can be used, or for ease of calculation our SPM states,
"Add 10% to altitude when AD temperature is between -5c to -25c
Add 20% to altitude when AD temperature is between -25c to -40c
Add 30% to altitude when AD temperature is below -40c"
All DA´s and MDA's are corrected as above. As most of our AD's are located at, or close to MSL the correction will only be about 60-80ft for the NDB/DME approaches down to -25c.
One controlled civil airport on our route network does and we know it.
The other is a military airfield which dosn't. When we get a descent clearance they state that the altitude is not temperature corrected. We make a temperature correction and read back "descending to ****ft, temperature corrected".
At uncontrolled airports we make our own corrections.
The ICAO table can be used, or for ease of calculation our SPM states,
"Add 10% to altitude when AD temperature is between -5c to -25c
Add 20% to altitude when AD temperature is between -25c to -40c
Add 30% to altitude when AD temperature is below -40c"
All DA´s and MDA's are corrected as above. As most of our AD's are located at, or close to MSL the correction will only be about 60-80ft for the NDB/DME approaches down to -25c.
Last edited by whitehorse; 14th Nov 2007 at 16:04. Reason: spelling
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I wouldn't have a clue if it was corrected where I work or not. Nor would I have a clue where to look.
If someone wants to fly higher than I have cleared them just ask, although, the way I vector you are always high on the approach anyway!!
If someone wants to fly higher than I have cleared them just ask, although, the way I vector you are always high on the approach anyway!!
