Cold temperature corrections

[supsup]

Does ATC make corrections for cold weather? ie. MVA. I believe they do in Canada.

 

[Singlecoil]

It is my understanding that they do not. I wrote a NASA report on this several years ago after getting a ground prox warning while being vectored on a cold day over the mountains near Yakima, WA. I asked that same question to the approach controller and he had no idea what I was talking about. I tried to explain the cold temperature altimeter error and it was clear to me that he was not aware that the altimeter and hence the mode C would be reading incorrectly on a cold (or any non-standard) day.

 

[Jar Jar]

wow i had a ground prox go off last week over mountainous terrain, very cold weather and never had that problem there before. Thought it was odd, thankfully it was day vmc at the time so we thought just a glitchy gpws. How much of a difference is caused by these temperature errors, is it hundreds of feet at times?

 

[Singlecoil]

wow i had a ground prox go off last week over mountainous terrain, very cold weather and never had that problem there before. Thought it was odd, thankfully it was day vmc at the time so we thought just a glitchy gpws. How much of a difference is caused by these temperature errors, is it hundreds of feet at times?

Yes. Definitely hundreds of feet, hence the ground prox. It all depends on the height of the mountains above the station that is reporting the altimeter setting. Like many things in aviation, this concept was discovered because people died. The fact that ATC isn't aware of it and MVA's aren't corrected for it is a shame but I'm sure it will get fixed when there is an accident.

There is a chart for this in the front of your Jepps or NOS charts. Here is one as well: http://bathursted.ccnb.nb.ca/vatcan/fir/moncton/WeeklyTopics/Archives/20040104/CurrentTopic.html

For an airport in mountainous terrain, when the airport temperature is -30 the error at 4000 AGL is 760 feet. Therefore, if there is a mountain near the airport at 4000 feet AGL, you will drive right into it when your altimeter reads 4759 AGL.

 

[Flyin2low]

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.

 

[unwashed]

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:

• The air is a perfectly dry gas
• The barometric pressure is 1013.2 hPa (29.92 inches of mercury (inHg))
• The temperature is 15°C
• The rate of temperature fall with height is 1.98°C per 1000 feet up to the height at which the temperature becomes -56.5°C and then remains constant..

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.

​

The concept of the above chart (chart has been removed by me) 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, [email protected].

 

[flyburg]

Hi,

ATC in most countries in Europe make the altitude corrections.

In our manuals we also have charts to determine the altitude to ad to MSA's and DH/MDA in case of cold weather.

A quick rule of thumb: add 4 feet per degree below standard per 1000 feet. In your example of -20 and altitude of 1500 AGL, 35 times 4 times 1.5 would be 210 (what do you know, the chart works).

Greetings