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As happens every year at this time, everyone should be doing a review of their winter operational procedures, and dusting off the cobwebs from a summer of flying in a temperate climate.

Having discussed the coming winter with many fellow pilots and controllers over the past few weeks, I’ve found a recurring general theme. Nearly everyone can list hazards of icing, winter weather, slippery runways, and additional human factors, but whenever the topic of cold weather altimeter error comes up, I see more long faces than I should. Discussions in online forums show that most individuals have an idea of the implications that cold weather has on altimeter readings, but most can’t get all the details correct, so here is our brush-up situation.

Cold weather altimeter error is operationally similar to flying from an area of high pressure to low pressure; the altimeter reads higher than it really is. The degree to which the altimeter misreads must be corrected by the use of charts available in the Transport Canada Aeronautical Information Manual (TC AIM) RAC Figure 9.1. Simple really, but there are common misconceptions about this procedure.

Firstly, this and other altimeter corrections are not done by ATC, but are the pilot’s responsibility. Radar vectoring altitudes assigned by ATC are, however, already corrected for cold temperatures. This correction is done by airspace planners while establishing all minimum safe altitudes for use by ATC.

Secondly, any correction applied to a published altitude should be relayed to ATC. There is no minimum altitude correction that can be brushed under the carpet. Even the smallest corrections can make a big difference.

Corrections calculated by pilots are to be used to ensure obstacle clearance during final approach fix crossings, procedure turns, or missed approaches.

For those who have never used an altitude correction chart, here is an example of how the Canadian chart works. The minimum safe altitude for our example aerodrome with weather reporting is 3 000 ft, and the field elevation is 1 000 ft; therefore, the height above elevation of altimeter setting is 2 000 ft. The current aerodrome temperature is -30°C. Looking at the Altitude Correction Chart below, find the column representing 2 000 ft above the aerodrome with the row corresponding to -30°C for temperature, and the value required to be added to your altitude is 380 ft. To ensure that a published altitude of 3 000 ft will truly provide obstacle clearance, the altimeter must then be reading 3 380 ft. Additionally, in examples shown in the current TC AIM, the corrected indicated altitude is rounded to the next higher 100-ft increment, so our example would become 3 400 ft.

Sound like a small correction? Is it worth pulling out charts to cross reference while briefing the approach? In an accident report published by the Canadian Aviation Safety Board [now the Transportation Safety Board of Canada (TSB)], the hazards of failing to correct for even the smallest temperature error are clear. Fortunately, there were no fatalities in this incident:

“The helicopter was dispatched at night, in IFR conditions...The crew descended on the inbound leg to 150 ft, with reference to the pilot’s altimeter. The helicopter struck the sea ice and was destroyed by post-impact fire. The crew had not applied a temperature correction to the minimum descent altitude [approximately 40 ft to as much as 100 ft. —Ed.], and this omission—combined with the known 50-ft error in the pilot’s altimeter—accounted for the mistaken belief the helicopter was higher.” (A81W0134)

A combination of high terrain or obstacles and low aerodrome temperature can easily wear down safety margins on your approach. Our above example has an error of 400 ft, meaning we would have no terrain clearance if we flew the published altitudes uncorrected.

So how can you know if your feet are cold? The following are the guidelines in the TC AIM.

According to TC AIM RAC Figure 9.1—Altitude Correction Chart:

With respect to altitude corrections, the following procedures apply:

IFR assigned altitudes may be either accepted or refused. Refusal in this case is based upon the pilot’s assessment of temperature effect on obstruction clearance.

IFR assigned altitudes accepted by a pilot shall not be adjusted to compensate for cold temperatures, i.e. if a pilot accepts “maintain 3 000,” an altitude correction shall not be applied to 3 000 ft. Radar vectoring altitudes assigned by ATC are temperature compensated and require no corrective action by pilots. When altitude corrections are applied to a published final approach fix crossing altitude, procedure turn or missed approach altitude, pilots should advise ATC how much of a correction is to be applied.

NOTES: The corrections have been rounded up to the next 10-ft increment.
Values should be added to published minimum IFR altitudes.
Temperature values from the reporting station nearest to the position of the aircraft should be used. This is normally the aerodrome.

Everyone knows the old saying “high to low, look out below.” As we enter another winter flying season, let’s add another reminder phrase to our repertoire, “hot to cold, don’t be so bold.” Don’t get cold feet in your altimeter this year!

John Tomkinson is an active air traffic controller in Toronto Center and a private pilot. He is also an aviation staff writer for www.aviation.ca.