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Cold Wx Altimeter Correction???

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FoxHunter

Well-known member
Joined
Jan 4, 2002
Posts
679
This subject has been discussed at length at www.pprune.org. It seems to be a subject never mentioned here in the USA. My understanding is that there is a 4% error in all barometric altimeters for each 10 degrees away from ISA at the reporting datum. (Temperature at the station giving you the altimeter setting) If the temperature at the reporting station is -25C the correction is 16%. If your altimeter reads 1000' above reporting datum elevation your true altitude is 160' lower than indicated. If your altimeter reads 5000' above the reporting datum your true altitude is 800' lower. If ATC assigns you an altitude they should have accounted for the low temperature. If you are reading the altitude off the approach plate you have to make the correction. If you fly the published min alt,or fly to the published mins you are actually going too low.

The folowing was was one of the sources referenced over on PPRUNE.

The Chilling Result of Cold Temperatures on Barometric Altimeters
Flight Safety Foundation ALAR Steering Committee Meeting
19 March 1998
Captain Howard A. Long
Air Line Pilots Association
Project Leader, CFIT & ALAR Working Group
Barometric altimeters on modern aircraft with digital Air Data Computers are very accurate most of
the time and we rely on these altimeters in every aspect of today’s air travel. For those of us in the
Unites States, we diligently set the local altimeter settings prior to each takeoff and each approach
but we do not routinely make any corrections for non-standard temperatures. Somewhere in my
engineering background, I certainly knew that temperature affects air density but my aviation
training has not included a procedure for making temperature corrections.
In ICAO PANS-OPS, Volume 1, in Section 3.5.4 titled “Final approach segment – precision
approach – ILS” there is a table that shows altitude corrections to be made by the pilot when the
airport temperatures are 0° C or below. Because this section applies only to a precision approach,
my first impression was that it was only necessary to make adjustments to DH and the FAF for an
ILS. The discussion below clearly shows that corrections needed are much more extensive.
Only recently have I started to understand the significant deficiencies associated with barometric
altimeters in extremely cold conditions. As you know, barometric altimeters are calibrated to
indicate true altitude under international standard atmosphere (ISA) conditions. Any deviation
from ISA will result in an erroneous reading on the altimeter. We routinely set the altimeter for the
local pressure when we are below the transition level but most of us in the United States do not
consider the effect of cold temperatures. It never occurred to me that altimeter errors in cold
conditions would be so large to exceed terrain clearance margins on departures or arrivals.
I finally understood cold temperature errors concerning barometric altimeters when I reviewed a
recent incident by a southern operator who was making a non-precision approach to an airport in
the interior of British Columbia. This approach was conducted in an MD-80 aircraft when the
temperature at the airport was –27° C. This aircraft was cleared for an approach by Vancouver
Center and told to contact the tower for landing clearance. A short time later the aircraft abandoned
the approach and came back to the center frequency telling of a GPWS warning. The crew then
successfully completed a different non-precision approach and landed safely.
Later, when that crew was questioned concerning the altitudes flown during the first approach, it
became obvious that the crew had not applied a temperature correction to the procedure turn
altitude. The published procedure turn altitude is 4900 feet above field elevation and with the field
temperature of –27° C, the crew should have added 800 foot as the temperature correction to that
published procedure turn altitude. A quick scan of the approach plate revealed that the mountain
just east of the localizer was where the GPWS terrain warning occurred. It was estimated that the
aircraft missed the top of the mountain by approximately 150 feet, which was confirmed by the
crew’s reported radio altimeter readings during the GPWS warning.
While Canada has been applying cold weather altimeter corrections for many years, the FAA is just
now doing research on this problem but presently there is no official procedure for implementation
of temperature corrections in the US. As each air carrier expands its sphere of operations, the risk
increases that their pilots may encounter a situation similar to the incident above, where cold
weather altitude corrections are critical. A few degrees cooler that day in British Columbia and that
aircraft would likely have hit the top of the mountain.
Page 2
The Chilling Effect of Cold Temperatures on Barometric Altimeters
Let’s ask the question again, “Why didn’t the local altimeter setting give that crew adequate
protection for any approach to that airport?” As stated above, barometric altimeters are accurate
only under ISA conditions. Any deviation in either pressure or temperature from ISA conditions
will result in an erroneous reading on the altimeter. The local altimeter setting provides the
necessary correction for the non-standard pressure in the local area but it does not correct for the
effect of non-standard temperature for altitudes above field elevations. The pilot must make the
altitude corrections if the temperature at the source of the altimeter setting is 0° C or below.
When the temperature is higher than ISA, the true altitude will be higher than the indicated altitude
and that will cause little or no problems on any approach. It is only when the temperature is lower
than ISA, that the true altitude will be lower than the indicated altitude. The altimeter error is
relatively minor down to 0° C but it can be significant with extremely cold temperatures especially
when you are dealing with the minimum altitudes required on some approaches to airports in
mountainous areas. In the incident in British Columbia with the procedure turn altitude
approximately 5000 feet above the field elevation, if the temperature had been –50° C, it would
have required a correction of 1200 feet rather than 800 feet.
Flying into cold air has much the same effect as flying into a low-pressure area; that is, the aircraft
is lower than the altimeter indicates. It is the temperature at the source of the local altimeter setting
and the height above the elevation of the altimeter source that determines the correction to be
applied. Remember, “Cold & low, look out below!” I had heard that before but without
procedures to follow for cold temperatures, it fell on deaf ears.
In cold temperatures, altitude corrections must be applied by the pilot to all MEA’s, sector
altitudes, procedure turn altitudes, FAF crossing altitudes, DH, MDA and missed approach
altitudes. These corrections need to be made anywhere cold temperatures are encountered, not just
in mountainous terrain. We need to look at the data again on past accidents that may have been
affected by cold temperature altimeter errors.
Radar vectoring altitudes are corrected for temperature in Canada but there is no guarantee in the
rest of the world. I wasn’t able to confirm whether there is any cold temperature correction of
radar vector altitudes in the United States but my guess is that they do not correct for temperature.
They may accommodate for it in other ways. The main issue here is that pilots need to be aware of
the local practice on radar vector altitudes and operate accordingly. The bottom line is that the pilot
has the responsibility to refuse a radar vector altitude if he does not think it provides adequate
terrain separation.
Cold temperature errors affect Enhanced GPWS and most likely to GCAS in the same manner as
they affect barometric altimeters. Unless temperature corrections for the altitudes used by EGPWS
and GCAS are automated or greatly simplified, these new systems will always have a flaw that
may someday come back to bite someone.
ALPA is fully supporting the FAA’s efforts toward making cold temperature corrections a standard
part of our operations in the US. We would endorse the effort that Canada has done and would
hope that the FAA will soon provide similar procedures for US carriers to follow. We need to go
beyond that and see that changes are made to ICAO so that it is clear that cold temperature
corrections are applicable to all approaches, not just precision approaches. In addition, it must be
clear to all pilots which altitudes must be corrected.
 
Look in the canada correction charts.
 
This subject has been discussed at length at www.pprune.org. It seems to be a subject never mentioned here in the USA.

I'm glad you brought this up as it is something of which all professional aviators should be aware even though their present flying environment may not require it. It can become critical in some places at certain times of the year.

I believe it's most important in the terminal area in mountainous terrain in the winter. Terrain clearance can be compromised if altimeters are not corrected in very low temperatures.

I think one reason altimeter corrections for temperature don't get much attention here in CONUS is the relatively high Transition Altitude/Level standardized at 18,000'/FL180, well above the highest terrain. In Europe, I understand TA's/TL's are frequently much lower and vary from airspace to airspace. I'm guessing if one pilot flying level at 5000' in the dead of a Scandinavian winter who has corrected his altimeter for cold temperature encounters another pilot flying level at FL060 who didn't correct his altimeter for the cold, they could end up trading paint.

This scenario also illustrates why everybody should be on the same page. Either everyone corrects for temperature or no one corrects for temperature. If no one corrects for temperature then TERP/PANS-OPS instrument procedure design criteria should provide adequate terrain clearance to allow for uncorrected altimeters and Transition Altitudes should not be the same as Transition Levels(e.g. - TA of 18,000' and a TL of FL200.)
 
Technique for Cold Weather Correction

Multiply the altitude (1000's of feet) you are above the altimeter setting station by the number of degrees below standard temp (deg C.), then multiply the result by a constant of 4. This will give you the difference between indicated altitude and true altitude.

Here's the formula:

Alt. x temp dev. x 4 = Delta

Example:

You are flying into an airport that's 1000 ft. MSL. Your MDA is 1,500 ft. MSL.
This means you'll be 500 ft., or .5K, above the altimeter setting station at MDA. The reported temp at the station is -10 deg. C; standard temp of course would be 13 deg. C, therefore temp dev. is -23 deg. C.

Now perform the calculation:

.5K x (-23) x 4 = Delta = -46.

This means that at MDA, indicating 1,500 MSL, you will actually be at 1,454 MSL, or 454 AGL instead of 500 AGL (assuming a perfect altimeter installation).

This method was taken from the DoD Flight Information Publications several years back, I don't know if it's still contained there.
 
When we fly survey lines, it's very important to compute temp into the actual flight altitude. Just last week it was -15C at 8000, so we had to fly at about 8,600 feet to have the same terrain foot print. In the summer, it's reversed.
 
FL420 said:
This subject has been discussed at length at www.pprune.org. It seems to be a subject never mentioned here in the USA.

I'm glad you brought this up as it is something of which all professional aviators should be aware even though their present flying environment may not require it. It can become critical in some places at certain times of the year.

I believe it's most important in the terminal area in mountainous terrain in the winter. Terrain clearance can be compromised if altimeters are not corrected in very low temperatures.

I think one reason altimeter corrections for temperature don't get much attention here in CONUS is the relatively high Transition Altitude/Level standardized at 18,000'/FL180, well above the highest terrain. In Europe, I understand TA's/TL's are frequently much lower and vary from airspace to airspace. I'm guessing if one pilot flying level at 5000' in the dead of a Scandinavian winter who has corrected his altimeter for cold temperature encounters another pilot flying level at FL060 who didn't correct his altimeter for the cold, they could end up trading paint.

This scenario also illustrates why everybody should be on the same page. Either everyone corrects for temperature or no one corrects for temperature. If no one corrects for temperature then TERP/PANS-OPS instrument procedure design criteria should provide adequate terrain clearance to allow for uncorrected altimeters and Transition Altitudes should not be the same as Transition Levels(e.g. - TA of 18,000' and a TL of FL200.)

My understanding is that you would never correct an altimeter for the cold if the Level/Altitude was issued by ATC. ATC should have adjusted the lowest usable Altitude/Level to take in account the low temperature.The problem with very low temperatures is the lack of terrain clearance. You do have to correct any published min alt in order to be sure of expected terrain clearance. If the procedure calls for PT at min 3000' AAL with the temp at ISA-40 it should be flown at a min of 3480', shows crossing the FAF at 2000' should be adjusted to 2320' to a MDA of 500', MDA should be adjusted to 580'. If you fly a Cat 1 ILS approach to the normal Baro mins of 200' at ISA -40 you are really going to 168'. The way I see it is that to be legal the mins have to be adjusted to 232' for the known altimeter error. Now I realize that most do not make the correction, and it will never be an issue, unless something bad happens and the light bulb goes on in the NTSB or some Law Firm that wants to attibute blame.
 
FoxHunter said:
My understanding is that you would never correct an altimeter for the cold if the Level/Altitude was issued by ATC.

I think you are probably correct, at least in the USA.

ATC should have adjusted the lowest usable Altitude/Level to take in account the low temperature.

However; my life and career would have ended long ago had I lost situational awareness and blindly followed ATC instructions. At least five times over the years.
 
FL420 said:
This subject has been discussed at length at www.pprune.org. It seems to be a subject never mentioned here in the USA.

I'm glad you brought this up as it is something of which all professional aviators should be aware even though their present flying environment may not require it. It can become critical in some places at certain times of the year.

I believe it's most important in the terminal area in mountainous terrain in the winter. Terrain clearance can be compromised if altimeters are not corrected in very low temperatures.

I think one reason altimeter corrections for temperature don't get much attention here in CONUS is the relatively high Transition Altitude/Level standardized at 18,000'/FL180, well above the highest terrain. In Europe, I understand TA's/TL's are frequently much lower and vary from airspace to airspace. I'm guessing if one pilot flying level at 5000' in the dead of a Scandinavian winter who has corrected his altimeter for cold temperature encounters another pilot flying level at FL060 who didn't correct his altimeter for the cold, they could end up trading paint.
It's not a QNE/QNH transition level issue. It has to do with the possiblility that an airplane with the correct (for the pressure) QNH might hit trees on approach because the altimiter was not adjusted for extremely low temps.
 
I truely believe this is why my boss died years ago. When I was waiting for him at the airport the temperature was -35F. When coming in at night with the airport in sight we had our own visual stepdowns. It's a big mountain area. He hit the top of the mountain by 50 feet. 400 feet lower than he should have been.
 

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