In aprevious article on altimetry we discussed various matters that affect the accuracy of the barometric altimeter. The major factor being atmospheric pressure, and to compensate we dial the “altimeter setting” or QNH into the Kollsman window.
However, another factor typically ignored is the effect of temperature—perhaps because it’s less crucial compared to atmospheric pressure. Most of the time we can ignore temperature effects with two major exceptions: when calculating density altitude (discussed previously) and when conducting approaches to airports with very low temperatures. The “High to Low” limerick applies not only to pressure but to temperature. The concern is our AGL height during instrument approaches. How cold are we talking? Typically, from minus 10 degrees C (14 degrees F) to about minus 35 degrees C (minus 31 degrees F) or even less.
First let’s review the International Standard Atmosphere (ISA) in the accompanying table. This is a theoretical vertical profile of the atmosphere used as a reference for aircraft performance. A temperature is associated with different pressure altitudes (the higher the altitude, the lower the temperature) reflecting a lapse rate of approximately two degrees C per thousand feet.
Barometric altimeters are calibrated to ISA conditions. With sensitive altimeters, we can correct for non-standard atmospheric pressures through the altimeter setting. But we don’t correct for temperatures. As a result, the adjustment is to fly at a higher indicated altitude.
The diagram below shows that when the temperature is LESS than ISA, an aircraft will be LOWER than the indicated altitude. When colder, the airplane might be dangerously close to obstructions or terrain during approaches.
Airports requiring cold temperature corrections are identified on Aeronav approach charts with a snowflake icon and the critical temperature in the briefing section.
Jeppesen approach charts identify such airports with a text note. A temperature as cold or colder than the critical temperature requires corrections. About 230 airports are affected in the US, needless to say, most of these are found in Alaska (70)—and at higher elevations or in colder parts of the contiguous US (160). No one would be surprised that there are none in Florida.
The list of airports is found in Part 4, Section 1 General Section of the NTAP (Notices to Airman Publication)—issued every 28 days. It includes the ICAO airport identifier, the critical temperature and the affected segments: Intermediate, Final and Missed Approach. Most airports have only one affected segment, some two and a few have all three. There are frequent changes to this list. Also notice that the need for altitude corrections apply to the entire airport; therefore, all approaches would need altitude corrections if the temperature is below the published critical value.
Let’s review the segments of an instrument approach:
The intermediate segment of an approach begins at the initial fix (IF) and ends at the final approach fix (FAF). If there is no IF, then the segment starts when aligned and inbound to the FAF.
The final approach segment starts at final approach fix or point (“Maltese cross” for non-precision approaches or glide slope intercept on an ILS or RNAV (GPS) approach to an LPV minimum) to the missed approach point (MAP).
The missed approach segment begins at the MAP or DA/DH to the missed approach fix—usually a fix defining a holding pattern and at a charted altitude.
Identification Of Airports
As an aircraft could be lower than indicated altitude (corrected for atmospheric pressure due to very cold temperatures) the airports were selected based on loss of Required Obstacle Clearance (ROC) in two approach segments. For the intermediate segment, the ROC altitude must be at least 500 feet above the highest elevation or man-made obstruction. The ROC altitude on the final approach segment varies from 250 to 350 feet depending on the type of approach. The analysis was only done for civilian airports with a minimum runway length of 2500 feet.
ATC Reporting Requirements
When flying an approach that requires cold temperature corrections, ATC needs to know the altitude that a pilot plans to fly on the intermediate and missed approach segments. This is necessary for proper separation if another aircraft is in the vicinity that is not applying the correction. There is no need to report the altitude during the final approach segment.
Let’s review how adjustments to altitudes are made when the temperatures are below the critical temperature by looking at two airports.
My first choice is Northampton, Massachusetts (7B2), RNAV (GPS) RWY 14 because it has a low elevation (121 feet) and, the surrounding terrain is relatively flat even though there is a “small” mountain (Mount Tom 1034 feet MSL) only a few miles southeast of the field.
Only the intermediate segment is affected and, according to the “snow flake,” the critical temperature is minus 24 degrees C (minus 11 degrees F); on today’s hypothetical flight we face a minus 30 degrees C (minus 22 degrees F) temperature. The segment starts at SMUTZ (IF) and ends at EXBEB (FAF). There are no stepdown fixes between the IF and the FAF, otherwise the temperature corrections would also apply to these.
- Determine the airport elevation: 121 feet MSL.
- Locate the intermediate segment lowest altitude after SMUTZ: 2700 feet MSL at EXBEB (FAF).
- Subtract the airport elevation from the segment altitude: 2700 minus 121 = 2579 feet (round up to 2600 feet) this, in effect, would be the AGL height.
- Use the ICAO Cold Temperature Error Table. At minus 30 degrees C and 2600 feet (and interpolating the values between 2000 and 3000 feet) = 494 feet. Use 500 feet and add it to the FAF resulting in 3200 feet MSL at EXBEB. This also means staying at or above 3200 feet MSL after SMUTZ. After EXBEB, use the relevant published MDA altitudes.
One additional item is to advise ATC: “Require 3200 feet from SMUTZ to EXBEB for cold weather operations.” My second choice is Salmon, Idaho (KSMN), RNAV (GPS) RWY 17. This interesting airport has a high elevation and is located in a valley surrounded by tall mountains. Additionally, it’s one of the few airports that has all three segments requiring cold weather altitude corrections. Critical temperature is a balmy minus 11 degrees C (12 degrees F), and today we are shivering at minus 20 degrees C (minus four degrees F).
- Determine airport elevation: 4044 feet MSL.
- Locate the intermediate segment lowest altitude after CASBA: 7100 feet MSL at BUTEO (FAF).
- Subtract the airport elevation from the segment altitude: 7100 minus 4044 = 3056 feet (round up to 3100 feet) this would be the AGL height.
- Use the ICAO Cold Temperature Error Table. At minus 20 degrees C and 3100 feet (and interpolating the values between 3000 and 4000 feet) = 435 feet. Add it to the FAF resulting in 7535 (or 7540) feet MSL at BUTEO and 8035 (or 8040) feet MSL at the step down fix DIVDE. This also implies maintaining at least 8040 feet MSL until DIVDE and 7540 feet MSL to BUTEO.
Final Approach Segment
- Determine airport elevation: 4044 feet MSL.
- Locate the final approach segment altitude (MDA): 5740 feet MSL.
- Subtract the airport elevation from the segment altitude: 5740 minus 4044 = 1696 feet (round up to 1700 feet).
- Use the ICAO Cold Temperature Error Table. At minus 20 degrees C and 1700 feet (and interpolating the values between 1500 and 2000 feet) = 238 feet. Add it to the MDA resulting in a new MDA of 5978 (or 5980) feet MSL.
Missed Approach Segment
- Determine the airport elevation: 4044 feet MSL.
- Locate missed approach segment altitude = 13,600 feet MSL (at ORELE missed approach fix).
- Subtract airport elevation from segment: 13,600 minus 4044 = 9556 feet (round up to 9600 feet.
- Use the ICAO Cold Temperature Error Table at 20 degrees C and use the value at 5000 feet = 710 feet. Add it to the original altitude at ORELE resulting in a new holding altitude of 14310 feet MSL.
Note, we can ignore intermediate fixes on the missed approach segment when making corrections as only the final approach fix is considered in the ROC analysis.
Finally, we need to inform ATC: “Require 7540 feet to BUTEO and final holding altitude 14310 feet for cold temperature operations.” There is no requirement to make a report to ATC of altitude requirements on the final approach segment.
ICAO has had the procedures for CTRA for years; but it’s only recently that the FAA adopted these procedures. So they are relatively new.
Since electronic glide-slopes are not affected by pressure nor temperature, if flying an approach with a glide-slope (ILS or an RNAV (GPS) approach flown to LPV minimum) just follow the glide-slope since it will be providing reliable guidance. However, the corrected DA must be used for the MAP.
The calculations are not that difficult to do but you might not want to do them in flight right before the approach. During the flight planning phase, pre-calculate the values assuming a worse case temperature (lower) because the resulting values will be more conservative. Remember TAFs don’t forecast temperatures.
If it’s that cold, do you really need to fly? Keep in mind that very cold temperatures have other consequences such as engine starting and off-airport emergency landings. Maybe canceling the flight is a better choice.
I’d like to hear from pilots who have flown to CTRA when the temperature was cold enough to require altitude adjustments.
Luca Bencini-Tibo ATP/CFII, is a FAASTeam Lead Rep, aircraft owner and is a graduate of MIT.
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