Cold WX Altimetry Procedures
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Cold WX Altimetry Procedures
I would like feedback from airlines that regularly operate in very cold weather conditions and the procedures they apply with respect to pressure altimeter corrections. SOP's for instrument approaches with Airbus aircraft in very cold wx. Corrections to FAF alt, crossing alts and minima's on non precision approaches.
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As when landing at Anchorage in winter...? I can tell you that in our non-airbus jet, when passing Transition Level we set the local QNH, Millibars or Inches of Mercury, and that's it.
Why do it if it's not fun?
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Just a thought... but aren't instrument procedures designed to take into account the likely ambient temperatures in the local area, and to allow for being lower than indicated if temperatures well below ISA are common? I don't know that for a fact, but I'd kind of assume that it was the case???
FFF
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FFF
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still learning....
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SH-
This subject seems to be debated/discussed here at least once each winter. Try a thread search for "cold and altimeter" and you'll find lots of good information.
Pay special attention to what our Canadian brothers have to say. They're the experts in the field.
This subject seems to be debated/discussed here at least once each winter. Try a thread search for "cold and altimeter" and you'll find lots of good information.
Pay special attention to what our Canadian brothers have to say. They're the experts in the field.
GlueBall
No consideration given to the effects of temperature on your altimeter?
You might want to read this FAA advisory circular.
http://www2.faa.gov/language/accold.pdf
And you might want to read this article, then have a stiff drink...
http://www.bluecoat.org/reports/Long_98_Cold.pdf
You might want to read this FAA advisory circular.
http://www2.faa.gov/language/accold.pdf
And you might want to read this article, then have a stiff drink...
http://www.bluecoat.org/reports/Long_98_Cold.pdf
Last edited by compressor stall; 13th Feb 2003 at 08:21.
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cold wx altimetry
hello
long time back i got this thumb rule for altimeter correction in cold wx ops.it is applied for enroute altitudes,wonder if it si so at lower altitudes at FAF ,for eg.
TEMP CORRECTION FOR MEA ON CLOD DAYS
10% (-15 TO –30 DEG C) 20% ( BELOW –30 DEG C)
kunal
long time back i got this thumb rule for altimeter correction in cold wx ops.it is applied for enroute altitudes,wonder if it si so at lower altitudes at FAF ,for eg.
TEMP CORRECTION FOR MEA ON CLOD DAYS
10% (-15 TO –30 DEG C) 20% ( BELOW –30 DEG C)
kunal
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Add 4 feet per thousand feet to the DH/MDH (whichever you are using) for each degree below ISA. If you are using DA/MDA, only apply the correction to the height above the airfield datum.
So, if your MDH is 1000 and the MSL temp is 0, your revised MDH is 1060.
Usually it's not worth applying until you get about 30 degrees below ISA.
Don't forget that it also affect MSAs.
So, if your MDH is 1000 and the MSL temp is 0, your revised MDH is 1060.
Usually it's not worth applying until you get about 30 degrees below ISA.
Don't forget that it also affect MSAs.
Last edited by oxford blue; 13th Feb 2003 at 13:18.
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Having worked in cold weather for a long time, including the Arctic, this seemingly 'unimportant' little correction has become the demise of a few aviatiors and their passengers over the years. Yes it is important and Yes, there is a procdure and Yes there are charts for it.
Take for instance, KMSP (Minneapolis-St. Paul). They get their fair share of cool temperatures as well. (I don't have the ANC Chart at the moment).
ILS 12 L
The MDA is 1039 (200ft)
ATIS giving temperature of -30C
So, what to do? Well, for starters it's important to understand why this altimeter error occurs in the first place. No matter what type of aircraft we fly, be it a small single engine Cessna or a modern state of the art transport category airliner, the pressure altimeters are all sensitive to non-standard temperatures. With temperatures less than standard (15C) the colder it gets, the larger the errors become. This is all because the air molecules become more dense at colder temperatures and by virtue of the density it takes a smaller climb or descent in colder temperatures to affect a large reading on the altimeter in the airplane.
First off, where do we get the correct information? Well, there are several places. Jeppesen provides a nice chart on Page 244, Air Traffic Control. Transport Canada's AIP also has a chart and there are others as well.
There is a rule of thumb that can be applied to surface temperatures of above -15 C, and that is a correction of 4% height increase for every 10C below standard. The charts are more accurate.
Now back to our problem...KMSP....
The 'additives' are always made on the height above the ground, which means to say, the supposed actual true height above the ground, not the altitude with respect to MSL. This can be a source of confusion at times.
The MDA for this ILS is 1039 MSL or 200ft AGL and it is the 200 ft AGL that we want to correct.....so, into the Jepp Chart we find that at a temperature of -30C and at a height of 200 ft, the correction is 40 feet.
We now complete the calculation by adding the 40 feet to the MDA, which is:
1039
plus 40
---------
1079 MSL
Therefore, the corrected MDA for this approach becomes: 1079 ft. If we had NOT made the correction for this approach and we were flying down to limits because of poor weather, we would actually have descended BELOW the legal height limit for this approach by 40 feet and with respect to regulations, broken limits.
In a radar envioronment, corrections for maneuvering is made by ATC, although their numbers need to be checked every now and then. Take for instance, if we were maneuvering in the KMSP area, with no radar and there happened to be a procedure turn of 5000 feet MSL which is close to 4200 AGL....using the same temperature of -30C, the correction becomes important...using the chart again...the correction becomes significant. A correction of 798 feet is actually required at this height.
So.... 5000
plus 798
--------
5798 MSL
Some airline operators provide their crews with 'tailor made' charts that have corrections for all heights found on approach charts, a few being DH/MDA, Fix Crossings, MAP altitudes, transitions to approaches, Circling minima and MSAs.
This procedure is often over looked and those who have operated in cold weather operations and have never made the correction, have technically been descending below legal minima. Unless the rules have been changed recently, failure to apply the corrections, either on a simulator ride or a line check during cold weather operations in Canada, results in a FAILURE of the ride.
Take for instance, KMSP (Minneapolis-St. Paul). They get their fair share of cool temperatures as well. (I don't have the ANC Chart at the moment).
ILS 12 L
The MDA is 1039 (200ft)
ATIS giving temperature of -30C
So, what to do? Well, for starters it's important to understand why this altimeter error occurs in the first place. No matter what type of aircraft we fly, be it a small single engine Cessna or a modern state of the art transport category airliner, the pressure altimeters are all sensitive to non-standard temperatures. With temperatures less than standard (15C) the colder it gets, the larger the errors become. This is all because the air molecules become more dense at colder temperatures and by virtue of the density it takes a smaller climb or descent in colder temperatures to affect a large reading on the altimeter in the airplane.
First off, where do we get the correct information? Well, there are several places. Jeppesen provides a nice chart on Page 244, Air Traffic Control. Transport Canada's AIP also has a chart and there are others as well.
There is a rule of thumb that can be applied to surface temperatures of above -15 C, and that is a correction of 4% height increase for every 10C below standard. The charts are more accurate.
Now back to our problem...KMSP....
The 'additives' are always made on the height above the ground, which means to say, the supposed actual true height above the ground, not the altitude with respect to MSL. This can be a source of confusion at times.
The MDA for this ILS is 1039 MSL or 200ft AGL and it is the 200 ft AGL that we want to correct.....so, into the Jepp Chart we find that at a temperature of -30C and at a height of 200 ft, the correction is 40 feet.
We now complete the calculation by adding the 40 feet to the MDA, which is:
1039
plus 40
---------
1079 MSL
Therefore, the corrected MDA for this approach becomes: 1079 ft. If we had NOT made the correction for this approach and we were flying down to limits because of poor weather, we would actually have descended BELOW the legal height limit for this approach by 40 feet and with respect to regulations, broken limits.
In a radar envioronment, corrections for maneuvering is made by ATC, although their numbers need to be checked every now and then. Take for instance, if we were maneuvering in the KMSP area, with no radar and there happened to be a procedure turn of 5000 feet MSL which is close to 4200 AGL....using the same temperature of -30C, the correction becomes important...using the chart again...the correction becomes significant. A correction of 798 feet is actually required at this height.
So.... 5000
plus 798
--------
5798 MSL
Some airline operators provide their crews with 'tailor made' charts that have corrections for all heights found on approach charts, a few being DH/MDA, Fix Crossings, MAP altitudes, transitions to approaches, Circling minima and MSAs.
This procedure is often over looked and those who have operated in cold weather operations and have never made the correction, have technically been descending below legal minima. Unless the rules have been changed recently, failure to apply the corrections, either on a simulator ride or a line check during cold weather operations in Canada, results in a FAILURE of the ride.
All instrument minima are based on the ICAO standard atmosphere, which of course all altimeters are calibrated. At Cat 1 DA/MDA is going to be based on these assumptions, and as everone will be working on these assumptions - they should apply to everyone regardless of aircraft type. I know for a fact that glass cockpit Boeings do not have an automatic correction and I can safely assume the same for Airbus - as one operator I worked for applied the same rules and corrections for both types from the same entry in Ops Man A.
Another operator stipulated a correction should be made at IAS-15, ie 0 degrees C at sea level. easilt attainable in a European winter.
The 4' per thousand per degree is a good rule of thumb, but more accurate charts are available. If your operater operates into cold airfields and doesn't apply corrections, perhaps it's time to ask questions.
Another operator stipulated a correction should be made at IAS-15, ie 0 degrees C at sea level. easilt attainable in a European winter.
The 4' per thousand per degree is a good rule of thumb, but more accurate charts are available. If your operater operates into cold airfields and doesn't apply corrections, perhaps it's time to ask questions.
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This is rough but works for me: Add 10% to the height above the altimeter source, this works for - 20 celcius for DH, DA, MDA. Pay close attention to additive values in mountainous or obstacle rich terrain, for these, use the 10% rule and round up to the next 100' increment on procedure turns, fix crossings, and arcs etc. The greater the height above aerodrome and the cooler the temperature at the altimeter reporting station, the greater the error.
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There is a chart published in the UK AIP, Aerodrome Operating Minima - section (AD1.1.2). There you have Temperature against 'Altitude against source elevation' (since Altimeters are calibrated for ISA conditions), which gives you an altimeter correction. At -10 degrees C and 1000' you'd have to add 80', at -20 deg.C 120'.
Per Ardua ad Astraeus
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Just to avoid confusion, 'des', can you confirm those figures relate to aerodrome surface temperature at an aerodrome elevation of 200ft amsl (ie LGW) and flying at 1000' QNH? Otherwise they are a bit strange!
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As far as I understand it, the values in the chart should be added to the published altitudes. So, if your DA is 400' AGL @-10 deg C (don't know if Gatwick is, but with the threshold elevation of 200' your CAT1 might be), the correction to be added would be 40' = 440' DA.
Does not apply to operators with approved CAT2 or better, as they will be using their Radio Altimeter.
Does not apply to operators with approved CAT2 or better, as they will be using their Radio Altimeter.
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The way we do it is correct the minima after taking into consideration the MOC( minimum obstacle clearance) .Basically if the change is greater than 20% then you have to apply it. For example on departure the MOC is 35ft. So if the temperature correction table shows a correction greater than 7ft you have to apply it. For an ILS approach the MOC is 150 ft so if the table shows a correction greater than 30 ft again you have to apply it. The table is temperature against height above a/f elevation.
I hope that helps
I hope that helps
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The key temp is ISA deviation, assumed in the Rule of Thumb to be the same at all heights. You can have it quoted directly, or work it out from OAT/ height or airfield sfc temp/airfield elevation. 4% for every 10deg of ISA dev.
Dick W
Dick W