In the 1990s, the FAA began researching the impact that cold temperatures have on obstacle clearance for pilots on instrument approaches. Several pilots had reported that when flying approaches in very low temperatures that they found themselves dangerously close to terrain. This dangerous situation was caused by the aircraft altimeter being affected by extremely cold temperatures and causing the aircraft’s true altitude to be lower than the indicated altitude.
The errors caused by high and low temperatures in barometric altimeters was well known at this time, but the dangerous implications of cold temperatures and maintaining obstacle clearance, especially when in instrument meteorological conditions, was not fully understood. There was consensus among aircraft operators that the safety implications of pilots on instrument approaches unknowingly flying lower than their altimeter indicated required action.
After the aviation industry raised this concern, the FAA and MITRE developed a systematic process to identify segments of instrument procedures that could be at risk of losing obstacle clearance in unusually low temperatures. The study investigated all the instrument approach procedures at civil airports with runways of at least 2,500 feet in the continental United States and Alaska.
The FAA’s risk-based process identified the coldest temperatures at these airports over the last five years and calculated the probability of a pilot exceeding the required obstacle clearance on a nonstandard day. If the probability surpassed 1 percent on a segment of the approach, that segment received the temperature restriction. The procedures most impacted were those found at high true altitudes, such as in mountainous areas, and those located in areas routinely seeing very cold temperatures, such as northern Alaska.
The list of impacted airports became known as the Cold Temperature Restricted Airport list. Eventually, several hundred procedures were identified as requiring at least one segment correction for cold temperatures. The temperature correction procedure became mandatory for all pilots in September 2015.
AOPA has advocated for a procedure to be in place that will allow pilots to correct for unusually cold temperatures when conducting an instrument approach. It can be unsafe to fail to make the necessary correction because the pilot may unknowingly be dangerously close to terrain or obstacles. AOPA is significantly involved in improving the existing procedure and is committed to working with the FAA to ensure the correction procedure is user friendly.
Below is a presentation that will explain more about the safety implications of cold temperatures on altimeters, how the Cold Temperature Restricted Airport list changes, and when and how to make the required calculation.
What is the NTAP and where can I find it?
The NTAP is the Notices to Airmen Publication. It is published on a 28-day cycle and includes permanent notams, graphical notams, and procedures like that published for the Cold Temperature Restricted Airports (CTRA). It is the default location for the CTRA procedure as it allows changes to be made on the 28-day cycle versus other publications, like the Aeronautical Information Manual, which update infrequently. Pilots are encouraged to check the NTAP regularly for new procedures and changes to existing ones. The NTAP can be found on the FAA’s website.
How does the procedure account for temperature inversions, such as when the surface temperature is much colder than at altitude?
The procedure does not account for temperature inversions, which would only affect the intermediate segment. It is important for pilots to be aware of and make the altimeter corrections for the cold temperatures anytime it is required. Pilots could still lose their required obstacle clearance (ROC) near the surface as the cold temperatures will impact their altimeter as they descend. There is no accurate way at this time to provide at-altitude temperatures for each airport, so pilots must rely on the temperature as reported on the surface.
Are there any plans to note together with the snowflake symbol the segment(s) that require a correction on the approach plate?
Due to the instrument approach procedure review cycle, the cost of making minor changes, and the large number of procedures required to be updated to show the snowflake symbol, there was hesitation to identify the segment of the approach on the plates as well. The segments were first thought to change more often than any other aspect of the CTRA procedure and thus providing the most accurate information could only be done using the NTAP. Receiving user feedback is important so please let AOPA know your thoughts using the contact information below.
Should I round up or down when making my correction?
Rounding is up to the individual pilot. Best practice would be to round up or down to the nearest hundred for intermediate and missed approach segments. Final approach segments would be best to be rounded up if the exact number is not to be used.
It has always been safe flying approaches at this airport, why is this change necessary now?
Based on the risk identified through modeling and pilot reports, very cold temperatures can result in a pilot’s altimeter erroneously indicating a higher altitude than the pilot is at. This issue is much more dangerous when in instrument meteorological conditions and close to the ground. The threat of the loss of obstacle clearance has always been present, but now a procedure is available to mitigate that threat. Many organization and countries have had cold temperature correction procedures for years, including the U.S. Air Force and Canada.
My airport does not have weather reporting so I use Real Time Mesocscale Analysis (RTMA). Why does the RTMA temperature for my airport not match the actual temperature when I land?
As some airports may not have weather observations (AWOS, ASOS, etc.) and those with reporting may have the temperature measuring sensor fail, the need arose for a product to be available to report temperature. The RTMA product provided by the National Weather Service was designed to fulfill this requirement for aviation. The RTMA surface temperature report provides a simple hourly report of surface temperature at select airports and regions, every hour, 24 hours a day.
The temperature RTMA provides is based on a complex model and on gridded analyses at 2.5 kilometer resolution. The large resolution, difficulty to account for inversions, issue of vast terrain differences (such as an airport in a valley), and other local considerations, such as being on a coast, can lead to the RTMA temperature varying from the actual temperature at the airport. The National Weather Service is continuing to refine and improve this product’s modeling.
There are several step-downs on the intermediate segment, is there an approved way to apply a quick correction to all of them?
Yes, the correction to the lowest published altitude on the segment of the approach being flown may be used on all other altitudes of that segment.
In order to fly the corrected altitude on the intermediate segment I would have to climb, why is this the case?
There are cases where a step-down fix on the intermediate segment may have a corrected altitude that is higher than the previous segment’s (i.e., initial segment) altitude. This occurs because of the calculation used to determine which segment may result in the loss of required obstacle clearance (ROC) due to temperature and the amount of ROC the initial segment has versus another segment, like the intermediate segment. It is important that the corrected altitude is flown and air traffic control is notified of your need to fly this altitude for cold temperature operations.
The approach I am flying has a procedure turn, where does the intermediate segment start?
The intermediate segment begins following the procedure turn inbound (i.e., proceeding to the final approach fix) and the aircraft properly aligned with the final approach course. If a pilot is required to make altitude corrections for the intermediate segment, they would apply the correction to the procedure turn completion altitude and any step-downs after this point to include the final approach fix or final approach point. Exception: Procedures that do not have an IF or FAF (normally due to the navaid being on the field), there is no need to apply a correction to the intermediate segment (i.e., procedure turn completion altitude).
To offer feedback on the information provided or on the FAA’s procedure, please send an email to AOPA.