More on Cold Weather Altimeter Errors

The pressure altimeter used in aircraft is calibrated to read altitude based on measurements of barometric pressure. Because of the number of variables in air which can affect barometric readings, a baseline model had to be developed so we could determine the amount of deviation. The ICAO Standard Atmosphere has a number of conditions attached to it. Included are the following factors:

The temperature at sea level is 15° Celsius
Temperature drops at a rate of 1.98 C° per thousand feet until the height where it reaches -56.5°C and then remains constant
Barometric pressure at sea level is 29.92 inHg
The air is a perfectly dry gas

It is a reasonable analogy to compare the pressure levels (which would be indicated as altitudes on an altimeter) to the markings on a ruler stood on its edge. Each mark on the ruler could be said to represent 1,000 feet, for our purposes. Have a look at the diagram at right and see what I mean. The green aircraft is indicating 3,000 feet, and actually flying at 3,000 feet. Similarly, the red aircraft is also flying and indicating 6,000 on his altimeter.

In warmer air, the chart above changes a little, as the pressure levels will actually expand. This means there is more actual distance between the pressure levels than there is when it is warmer. it's as if the ruler still has the same number of marks on it, but it has stretched so its top still meets the top of the column of air, and its bottom is still at sea level. Have a look at the diagram to the right to see the effects of warmer than standard conditions.

The warmer conditions mean that an aircraft indicating 4,000 feet on his properly calibrated altimeter that is set to a proper altimeter setting may actually be flying at 4,200 feet -- higher than the pilot believes he is. This isn't too much of an issue, since the hills don't climb with temperature, and all the other pilots in the area would experience, in theory, the same amount of error. It also means that the pilot indicating 5,000 feet might actually be flying at 5,230 feet. The higher the altitude, the more the error. Thus, an aircraft may have more difficulty reaching its service ceiling on its altimeter on a hotter than standard day. It may very well be at that altitude, but its altimeter would actually read lower.

The diagram to the right shows the same two aircraft from the above diagram. The green aircraft is at the same height above sea level as he was in the previous diagram, but now he is indicating 2,500 feet instead of 3,000. The red aircraft, on the other hand, is flying an indicated altitude of 6,000 feet. He is actually higher than this, as evidenced by his increased distance above sea level. To emphasize this effect, I've placed these two diagrams, along with the third from the next section, in a table at the bottom of this page.

Now we get to the other end of the scale, where the effects are a little more dangerous. In a similar fashion to the way warmer air expands the ruler, colder air shrinks it. Again we still have the same number of marks on the ruler, but the overall length of the ruler, representing the top of the atmosphere and sea level, is actually shorter. This means that an aircraft indicating 3,000 feet may actually be flying only at 2,800 feet. Similarly, an aircraft indicating 10,000 feet would be somewhere closer to 9,200 feet. Remember, the altimeters when set to local station pressure would still indicate higher altitude. Thus, in order to ensure the plane is flown at the required altitude, like a minimum segment altitude for an approach, the pilot would have to calculate the altitude correction factor and add it. Therefore, with the situation above at 3,000 feet, the pilot would have to add the 200 feet, and make the altimeter read 3,200 to ensure he meets the obstacle clearance altitude of 3,000.

In the diagram at right, once again the green aircraft is at the same actual elevation above sea level, which is, as it has been, 3,000 feet. With a colder than standard atmosphere, he is now indicating about 3,800 on his altimeter. The more likely situation is the red aircraft, however. He is diligently flying his 6,000 feet on his altimeter, indicated once again by the pressure level lines on the diagram. If you look at the comparison below with the three diagrams side-by-side, you'll see that he's actually lower than he was in the other two diagrams, despite paying particular attention to the gauge. This is why cold weather altimeter correction factors must be applied.

As promised, here they are, side by side. Remember, the green aircraft is always at the same actual level, while the red aircraft is always at the same pressure level, which corresponds to indicated altitude. In all three situations, station pressure at sea level is the same setting, while the temperature in each diagram is vastly different form the ICAO Standard Atmospheric Model. While you're comparing diagrams, compare the vertical distances between pressure levels, and the number of pressure levels visible in each image. All three images are the same dimensions.

This is, perhaps, a simplistic overview of it, but it may be more than we really need to get into for this topic. Remember to add corrections as required (discussed in the first topic of 2004), to ensure adequate obstacle clearance, and only where required. Any thoughts? Send me an e-mail at moxner@nbnet.nb.ca. Thanks again for reading and writing!

Warmer than Standard	ICAO Standard	Colder than Standard