Altimeters for polar bears

Flying Safety, Nov, 2004 by Monique Yates

If polar bears could fly, they would know about cold weather altimeter settings. That's because the correction factors are required at most of the locations where polar bears frequent. Before you warm weather pilots tune out, you too have probably flown to at least one location requiring a cold weather altimeter correction. In fact, any time the ground temperature beneath an aircraft drops below 0 degrees Celsius, a correction should be made to altitudes in the departure/arrival spectrum.

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To understand the error, it helps to compare the atmosphere to the ocean surface. Many different things cause "waves" in the atmosphere, such as high/low pressure systems and temperature changes. Our altimeter surfs along a line of equal pressure. For example, over the continental U.S. at FL 190 with 29.92 set, the altimeter will follow a line of equal pressure, which will not be a constant true altitude. The true altitude (actual altitude above sea level) will vary as the aircraft moves along this line of equal pressure. We can lose up to 40 feet of our true altitude for each 10 degrees C below standard as measured at ground level at the altimeter source.

It might be easier to understand this by studying a real-life example. An air carrier was flying into Kelowna, British Columbia on the Localizer 2 DME to runway 15. The weather on the airfield was -27 degrees C. The aircraft commander opted to execute a missed approach when the ground proximity warning system (GPWS) went off for terrain closure. The instruments, however, indicated he was right on altitude. His altimeter was working as designed and set correctly, but his true altitude was dangerously lower than that indicated.

That crew could have benefited from a resource we, as military aviators, don't often think twice about--the Flight Information Handbook (FIH). (As a side note, although civilian aviators don't have the FIH, the AIM covers this same subject.) Look at page D-15 of the FIH. It contains a chart (see Chart 1) to determine a correction to be added to your altimeter for lower-than-standard temperatures. The aviator starts with the HAT/HAA (Height Above Touchdown/Height Above Aerodome) on the bottom scale and comes up the side of the chart with the temperature at the aerodrome. The correction factor provides an approximate value of how many feet to add onto the indicated altitude so the aircraft is at a true altitude closer to what the TERPster intended when the approach was designed.

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If you have never used the chart, try this example. You have a HAT of 3000 feet AGL and an indicated altitude of 5000 feet. The temperature at the airfield is -20 degrees C. Using the chart below, you will see that you need to add a correction of 420 feet onto your indicated altitude. So, you need to fly at 5420 feet indicated to really be at true altitude on this cold day.

Let's consider the aircraft that was going into Kelowna again. Remember, the airport temperature was -27 degrees C. In the intermediate segment of the approach, the TERPster only planned for 500 feet of required obstacle clearance. At the TERPS-designated altitude on the approach plate, the aircraft should have had 500 feet above any terrain or protruding obstacle. The altimeter was indicating 4400 feet. The height above the aerodrome at that point on the approach was 3000 feet. So, while the crew was confidently flying the appropriate altitudes listed on the IAP, they didn't realize they were actually only 50 feet above the terrain because they didn't apply the correction to their altimeter. It doesn't take a rocket scientist to figure out why the GPWS went off in the terrain mode.

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Sometimes it's easier to see this in an illustration.

On a colder-than-standard day, the column of air up to 4400 feet MSL is compressed and thus several hundreds of feet smaller. The compression of that column of air, and a resulting lower true altitude, is what made the GPWS go off in Kelowna.

On a warmer-than-standard day, the column of air is expanded, resulting in a higher than normal true altitude, corrections are not required because you are well above the obstacles.

AFMAN 11-217 actually addresses this problem. It states:

If the temperature is 0 degrees C or less, add corrections to ....

--The DH/MDA (Decision Height/Minimum Descent Altitude) and step down fixes inside the FAF (Final Approach Fix)

--All altitudes in designated mountainous terrain

If the temperature is -30 degrees C or less and/or the procedure turn, intermediate segment, or HAT/HAA is 3000 feet or more above the altimeter source, add corrections to ....

--All altitudes in the procedures

AFMAN 11-217, Vol 1, para 8.1.4.1

At the Advanced Instrument School (AIS), we teach, as a technique, to apply the corrections all the way through the approach. You could find yourself in a situation where the temperature is just above -30 degrees C and the HAT is just below 3000 feet. In this case, AFM 11-217 would have you wait to make the correction until inside the FAF. You could still fly significantly close to the ground even though you're outside the FAF. That is something you will have to take into consideration as the situation presents itself. Study the terrain on the approach to decide if you will adopt this technique of correcting throughout the approach. That being said, always apply the corrections from the IAF (Initial Approach Fix) inbound if the temp is -30 degrees C or less, the HAT/HAA is 3000 feet or more above the altimeter source, or you are operating in mountainous terrain.