Aviation Topic of the Week
By Michael Oxner, January 4, 2004


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This week's topic:
Cold Weather Altimeter Errors

In keeping with the final topics of 2003, and with the brumal weather anticipated for January, we move on to another issue experienced in Canada's winter, Cold Weather Altimeter Errors.

Altimetry Basics
    Measuring Altitude with Barometric Pressure
    Standard Atmospheric Conditions
    Correcting For Pressure Changes
Cold Weather Effects
    Temperature vs. Height Correction Chart
    Example
    Calculations in AIP
Using Corrections in Cold Weather
    Procedures

Altimetry Basics

Our old friend the altimeter is one of the essential instruments for IFR flight. I think we can all understand the value of it. Certainly ATC can. In Canada, the transition level from standard pressure to altimeter setting use comes at 18,000 feet ASL. Almost any time an aircraft in flight will talk with ATS personnel in Canada below this altitude, the pilot will be issued an altimeter setting. The information is so critical that it's often issued several times in a short time period, such as when given descent en route by ACC staff, then re-issued by terminal staff on initial contact, and once more by tower staff. The pilot should check the instruments each time a setting is issued to verify they are set correctly. I haven't yet seen it written anywhere that these settings should be read back, though it is a very common practice, and in my opinion, a very valuable one. I have caught several readbacks that were incorrect which could have contributed to a problem while on approach. From a control standpoint, when a pilot reads back an altimeter to me, I read the display as he does to verify what he says matches that, rather than listening for differences between what he says and what I remember saying to him.

The problem with the standard aircraft altimeter is that it measures altitude indirectly by measuring atmospheric pressure. It is known that as altitude increases, barometric pressure decreases. Accordingly, altimeters sensing a decrease in barometric pressure will indicate a rise in altitude. This holds true even if the aircraft is parked on the tarmac at an airport. If a storm system moves through with a low barometric pressure, the altimeter will show a rise. Conversely, when the system passes and fair weather follows, the barometric pressure will often rise and the altimeter will indicate a drop in altitude. As such, the altimeter must be corrected for atmospheric pressure in order to read a useable altitude. A standard reference point must be used to compare to when correcting altimeters.

ICAO adopted a model for what would be termed the ICAO Standard Atmosphere. Conditions were accepted for use and comparison when creating this model. The standard atmosphere consists of the following conditions, all measured at Sea Level, according to the AIP, AIR 1.5.2:
An altimeter will only read correct altitude when standard atmospheric conditions exist. Which is to say, almost never. The good news is that the errors presented are often small, and they are generally consistent. So if an aircraft is indicating 2,000 feet and another aircraft right above him is showing 3,000 feet, they should be 1,000 feet apart, even if they aren't actually at 2,000 and 3,000 feet exactly.

Of the conditions above, the most prominent effect on an altimeter's ability to accurately reflect height is the barometric pressure. For that reason, there is a little window on the standard altimeter which contains a subscale. This is graduated in increments corresponding to barometric pressure, whether measured in inHg or mb (millibars). Pilots can adjust this subscale to correct for changes in barometric pressure from region to region as he flies based on weather reports from stations along his route of flight. For better or worse, this is the only direct method of correcting indicated altitude directly on the gauge.

Also, temperature can have a profound effect on altimeter readings. If the temperature is warmer than standard, the altimeter will read an altitude that is lower than the true altitude of the aircraft, which means the aircraft will be higher than shown. Cold weather, on the other hand, leads to an opposite error, meaning the aircraft is lower than shown. This error is more critical.

Cold Weather Effects

In areas like Canada, conditions of extreme cold enter the climate regularly in the winter. The problem becomes one of error induced by the extreme difference between outside air temperature and the ICAO standard atmosphere. As such, the pilot could believe he is at a safe altitude, but is actually much lower than what is showing on the gauge.

Just how much of an error? The error is proportional to the height of the aircraft above the station reporting the barometric pressure (altimeter setting) and the difference between actual and standard temperature. I've reproduced the following chart from the Canada Air Pilot, the IFR pilot's bible of instrument approach and IFR procedures at aerodromes in Canada. It is also available in the AIP, RAC Figure 9.1.

Aerodrome
Temperature (°C)
 Altitude Correction Factor
0
 20
 20
 30
 30
 40
 40
 50
 50
 60
 90
 120
 170
 230
 290
-10
 20
 30
 40
 50
 60
 70
 80
 90
 100
 150
 200
 290
 390
 490
-20
 30
 50
 60
 70
 100
 100
 120
 130
 140
 210
 280
 430
 570
 710
-30
 40
 60
 80
 100
 130
 130
 150
 170
 190
 280
 380
 570
 760
 950
-40
 50
 80
 100
 120
 170
 170
 190
 220
 240
 360
 480
 720
 970
 1210
-50
 60
 90
 120
 150
 180
 210
 240
 270
 300
 450
 600
 890
 1190
 1500
Height above Aerodrome->
 200
 300
 400
 500
 600
 700
 800
 900
 1000
 1500
 2000
 3000
 4000
 5000

Once again, please don't use this table for real world flight. I believe I reproduced it accurately, however errors may exist. For real world flight, use current, real world publications, ALWAYS. Thanks.

The concept of the above chart is that the values above are to be added to the altitudes as required to provide an appropriate indicated altitude to ensure obstacle clearance. For example, the minimum safe altitude for a given area near an aerodrome with a weather reporting station is 2,000 feet, and the field elevation is 500 feet, with a current temperature reported as -20°C. Looking at the chart above, cross reference the column representing 1,500 feet above the aerodrome with the row corresponding to -20 for temperature, and the value you have to add to your altitude will be 210 feet. This means that in -20 temperatures, for this area, the altimeter would have to indicate 2,210 feet to ensure the aircraft is actually flying at 2,000 feet, which provides obstacle clearance. While I didn't find it directly stated in the AIP, the example they gave indicated that the altitude correction is to be rounded up to the next nearest 100 foot increment. So, in the above example, 2,210 should be rounded up to an altitude of 2,300 feet. If anyone did find this, please point it out to me and I'll add a line to next week's topic about it. Readers, please check back next week in case someone finds the reference for me.

Also, for those who demand exacting science, I'll refer you to the AIP, RAC 9.17.1. There are formulae there for much more accurate data than is provided in the chart above. They are kind of complex, but certainly not unworkable. I will not reproduce them here, since the above chart will suffice for online simming. One more note, I'm not even sure if Flight Simulator provides for such an altimeter error in the model. Does anyone know the answer to this?

Using Corrections in Cold Weather

The most critical phase of flight when it comes to obstacle clearance is the approach phase. On departure, you're leaving the earth from a known position. On approach, you're often flying in bad weather and can't tell precisely where you are other than looking at your instruments. In such a case, you don't know for sure how high above obstacles you are, only that you're above them according to charts. You know you're above them because of your altimeter. As such, it has to be accurate.

The AIP, RAC Figure 9.1, gives direction for what to add the above corrections to and when. When it's extremely cold, correction factors have to be applied to all published altitudes to ensure obstacle clearance. This applies to quadrantal altitudes, DME arcs, etc. Unless otherwise published, the destination aerodrome elevation is to be used as the elevation of the altimeter source.

With respect to altitude corrections, use the following procedures:
  1. As always, a pilot may accept or refuse an IFR altitude assignment. Also, the reason for refusal shall be stated. In the case of refusal being for cold weather considerations, the pilot shall indicate this to ATC and request another assignment. This is for en route as well as for descent for approach.
  2. IFR altitude assignments shall not be corrected for temperature. That is to say that if ATC says, "Maintain 3,000", the pilot, upon acceptance of such a clearance, shall fly at an indicated altitude of 3,000 feet.
  3. Radar vectoring altitudes are corrected for cold weather temperature errors and no correction factors are to be applied to altitude assignments while on radar vectors.
  4. If applying an altitude correction to procedure turn, fix crossing, or missed approach altitudes, ATC is to be advised of the magnitude of the correction factor being applied.




I'm getting cold as I write this just thinking about the subject. I'm not a winter fan, can you tell? Thanks for taking the time to read. Any feedback on this or any other week's topic can be addressed to me at my e-mail address, moxner@nbnet.nb.ca.