Your altimeter is a marvelous piece of machinery that is highly accurate throughout almost the entire flight envelope of your aircraft. Almost, but not all. When it gets very cold, the error can be enough to lower your actual altitude well below any minimums. You, ATC, or your aircraft will have to make adjustments.
I've included the math here just to show there is science behind the method. But, as a pilot, what you need to know is that when it gets cold, your altimeter puts the airplane lower than it should. In most cases the error is insignificant. The colder than 0°C it is, and the higher you are than the airport's elevation, the more significant the error. You can use a set of tables, included below and in Jepps, or if your aircraft is allowed to automatically compensate, you can do that. But if you make the corrections, you need to let ATC know. They might be giving you corrected altitudes or your correction could put you in the way of aircraft that are not correcting.
The procedures vary by country and you will have to study individual state procedures to be sure. Until recently, the requirement was less than explicit in the United States but all that has changed, see the section U.S. Procedures.
Everything here is from the references shown below, with a few comments in an alternate color.
These are the same tables reproduced by Jeppesen and in AIM. No matter where you get them, if you do a lot of cold weather flying you should either have the tables handy or have an aircraft that can do the math for you. (More about that below, using the Gulfstream G450 as an example.) Some aircraft can do the math because that's all it really is, math:
[ICAO Doc 8168 - Aircraft Operations - Vol I, Part III, §1, Ch. 4] Note.— This chapter deals with altimeter corrections for pressure, temperature and, where appropriate, wind and terrain effects. The pilot is responsible for these corrections, except when under radar vectoring. In that case, the radar controller issues clearances such that the prescribed obstacle clearance will exist at all times, taking the cold temperature correction into account.
4.1.1 Pilot’s responsibility. The pilot-in-command is responsible for the safety of the operation and the safety of the aeroplane and of all persons on board during flight time (Annex 6, 4.5.1). This includes responsibility for obstacle clearance, except when an IFR flight is being vectored by radar.
4.1.3 State’s responsibility. Annex 15, Appendix 1 (Contents of Aeronautical Information Publication), indicates that States should publish in Section GEN 3.3.5, “The criteria used to determine minimum flight altitudes”. If nothing is published, it should be assumed that no corrections have been applied by the State. Note.— The determination of lowest usable flight levels by air traffic control units within controlled airspace does not relieve the pilot-in-command of the responsibility for ensuring that adequate terrain clearance exists, except when an IFR flight is being vectored by radar.
4.1.4 Air traffic control (ATC). If an aircraft is cleared by ATC to an altitude which the pilot-in-command finds unacceptable due to low temperature, then the pilot-in-command should request a higher altitude. If such a request is not received, ATC will consider that the clearance has been accepted and will be complied with. See Annex 2 and the PANS-ATM (Doc 4444), Chapter 6.
4.3.1 Requirement for temperature correction. The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. In such conditions, an approximate correction is 4 per cent height increase for every 10°C below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above –15°C.
4.3.2 Tabulated corrections. For colder temperatures, a more accurate correction should be obtained from Tables III-1-4-1 a) and III-1-4-1 b). These tables are calculated for a sea level aerodrome. They are therefore conservative when applied at higher aerodromes. To calculate the corrections for specific aerodromes or altimeter setting sources above sea level, or for values not tabulated, see 4.3.3, “Corrections for specific conditions”.
Note 1.— The corrections have been rounded up to the next 5 m or 10 ft increment.
Note 2.— Temperature values from the reporting station (normally the aerodrome) nearest to the position of the aircraft should be used.
4.3.3 Corrections for specific conditions. Tables III-1-4-1 a) and III-1-4-1 b) were calculated assuming a linear variation of temperature with height. They were based on the following equation, which may be used with the appropriate value of t0, H, L0 and Hss to calculate temperature corrections for specific conditions. This equation produces results that are within 5 per cent of the accurate correction for altimeter setting sources up to 3 000 m (10 000 ft) and with minimum heights up to 1 500 m (5 000 ft) above that source.
H = minimum height above the altimeter setting source (setting source is normally the aerodrome unless otherwise specified)
t0 = taerodrome + L0 × haerodrome . . . aerodrome (or specified temperature reporting point) temperature adjusted to sea level
L0 = 0.0065°C per m or 0.00198°C per ft
H0 = altimeter setting source elevation
taerodrome = aerodrome (or specified temperature reporting point) temperature
haerodrome = aerodrome (or specified temperature reporting point) elevation
Note the numbers you get from this formula will not agree with the tables shown above or those in the Airman's Information Manual, which simply repeats the ICAO table. The formula is more accurate, the tables are based on a sea level airport. Using the table gives you a conservative result.
U.S. Procedures change effective January 8, 2015, as discussed in Notices to Airmen (NTAP), U.S. Cold Temperature Restricted Airports, Part 4, Section 1 and soon to be added to the Aeronautical Information Manual. This publication includes a list of U.S. airports where altimeter corrections for cold temperatures are mandatory. See also InFO 15002, Implementation of cold temperature altitude corrections at "Cold Temperature Restricted Airports" found in Notice to Airmen Publication (NTAP).
[Notices to Airmen, U.S. Cold Temperature Restricted Airports, Part 4, Section 1]
Figure: KLEB ILS 18 Profile view, from JeppView, pg. KLEB 11-1.
Flying into Lebanon, New Hampshire on the ILS Rwy 18 the ATIS reports the surface temperature is -10°C and you have been vectored to intercept the ILS at 3500 feet. What should you do?
This altitude is 3,000 feet above the altimeter setting source so, using the correction table, you need to add 290 feet to your indicated altitude. You should request to fly the intercept at 3,800 feet indicated altitude, then check your glide slope crossing at 2,930 feet and the minimum stepdown at 1,700'.
Notice that this correction, in accordance with the U.S. Cold Temperature Restricted Airports list is not mandatory at Lebanon, New Hampshire until -20°C and even then only on the intermediate and final approach segments. You can apply it, but regardless of the temperature you need to inform ATC that you are doing so. (See U.S. Procedures for more about this.)
In 2015 there was a crash of an Air Canada Airbus where the crew employed a company sanctioned procedure to increase their Flight Path Angle (FPA) to compensate for their increased Final Approach Fix altitude, which was temperature compensated. Flying the charted FPA from the higher indicated altitude at the FAF would result in arriving over the runway at too high an altitude. In theory, computing a higher FPA based on the expected indicated altitudes would get them over the threshold at the correct height. The problem with this theory is that the corrected FPA needs to be constantly updated. Airbus has since abandoned this procedure. If you've picked it up somewhere, you also need to abandon the procedure. Fly the charted FPA, even when using temperature compensated altitudes.
More about this — Case Study: Air Canada 624.
The problem with adjusting your FPA to make up for a higher temperature compensated altitude at the FAF is that the temperature compensation decreases as you descend. If you increase your angle based on the 2,000 foot correction, you will have too much angle as you descend.
In the case of Air Canada 624, company procedures had the pilots correct the altitude at the FAF, rounding up to the next even 100 feet. So they began the approach at 2,200 feet. Company rules also had them fly an increased FPA, using 3.5 degrees instead of the published 3.08 degrees. For the sake of the math, we will use the computed value of 3.45, making this a conservative examination.
Doing the math, without temperature compensation the required FPA to cross the threshold at 50 feet would have been 3.05 degrees. Flying this angle from the FAF's temperature compensated altitude (the green dashed line) would have left them too high. In theory, the temperature compensated 3.5 degreee FPA would get them to the threshold at 50 feet (the red dotted line).
But that increased FPA is only valid at the higher altitude. You can visualize the problem by recomputing everything 1 nm after the FAF. The aicraft descended from 2,000 feet at 3.45 degrees and ended up 107 feet lower than predicted. Had the crew recomputed a new corrected FPA, they would have shallowed their descent rate to 3.29 degrees. So they were now 0.16 degrees too steep.
Repeating this process at 2 nm shows them to be 134 feet lower than predicted and the error in their FPA is a little higher.
At 3 nm the errors increase.
At 4 nm the airplane is 194 feet too low.
The temperature corrected FPA did not cause the crash, the crew should never have continued the approach below the MDA without the correct visual cues in sight. But the steeper approach set them up for the visual illusions that got them in the end. They should have tracked their distance to the runway as provided in their SOPs. For more about how to avoid this FPA error and the visual illusion, see: Sitting Duck.
G450 aircraft with ASC 059 can automatically modify waypoint constraint altitudes to compensate for the effects of hot or cold weather.
[G450 Aircraft Operating Manual §2B-26-80 ¶3.A.] Temperature compensation is calculated when a landing temperature is entered by the pilot (on the LAND/GA INIT page) and a valid runway elevation is present.
Photo: Flight Config 1/2, from Eddie's cockpit.
[G450 Aircraft Operating Manual §2B-26-80 ¶3.A.] The first step in the temperature compensation activation procedure is to configure the desired temperature compensation. Temperature compensation configuration is performed on the TEMP COMP CONFIG page. The TEMP COMP CONFIG page is accessed from the FLIGHT CONFIG page 2.
Photo: Flight Config 1/2, from Eddie's cockpit.
[G450 Aircraft Operating Manual §2B-26-80 ¶3.A.] Pushing LSK 3R on the FLIGHT CONFIG page 2 displays the TEMP COMP CONFIG page.
[G450 Aircraft Operating Manual §2B-26-80 §3.A.] Temperature compensation is configured by selecting the desired temperature compensation mode. Temperature compensation is calculated when the entered landing temperature is valid for the selected temperature compensation mode (COLD or HOT & COLD) and an approach procedure is selected and activated. The compensation modes displayed on the TEMP COMP CONFIG page are as follows
Set this to COLD and keep it there. Nobody uses HOT and hardly anyone uses COLD. But with this set to COLD you will have the option of using it whenever your temperature is below 0°C.
[G450 Aircraft Operating Manual §2B-26-80 §3.B.] After the desired temperature compensation configuration is selected, an approach must be selected and activated in the active flight plan.
Photo: LAND/GA INIT page, from Eddie's aircraft.
[G450 Aircraft Operating Manual §2B-26-80 §3.C.] The landing surface temperature is entered on the LAND/GA INIT page 1. The LAND/GA INIT page is accessed by selecting the LANDING prompt (LSK 6R) on the ARRIVAL page or the LANDING prompt (LSK 4R) on the PERF INDEX page. Landing surface temperature is entered by entering the temperature value into the scratchpad and pushing LSK 4L on the LAND/GA INIT page 1.
The TEMP COMP prompt is displayed adjacent to LSK 6L when the landing surface temperature is entered. Selecting the TEMP COMP prompt (LSK 6L) on the LAND/GA INIT page displays the TEMP COMP page 1.
Photo: TEMP COMP page, from Eddie's aircraft.
[G450 Aircraft Operating Manual §2B-26-80 §3.D.] Verification and activation of temperature compensation for the approach/missed approach waypoint crossing altitudes is performed on the TEMP COMP pages. The TEMP COMP pages are accessed by pushing LSK 6L on the LAND/GA INIT page 1. The TEMP COMP pages are also accessed by selecting TEMP COMP prompt (when it is displayed) on the ACTIVE FLT PLAN page.
The left column lists the names of the waypoints in the flight plan. The middle column lists the database altitude constraints corresponding to the waypoints. The right column lists the corresponding temperature compensated altitude values. If a waypoint does not have a navigation database altitude constraint, the database altitude and baro-corrected altitude fields are blank. Pushing LSK 6R activates temperature compensation. Once temperature compensation has been activated, the prompt at LSK 6R changes to CANCEL. Selecting CANCEL turns off temperature compensation. The final entry on the last TEMP COMP page permits the flight crew to manually enter the appropriate MDA/DA. When the appropriate MDA/DA is entered by the flight crew, the temperature-corrected altitude is displayed. The flight crew uses this temperature-corrected altitude for input on the Display Controller and Flight Guidance Panel.
Photo: TEMP COMP page, from Eddie's aircraft.
[G450 Aircraft Operating Manual §2B-26-80 §3.E.] Once temperature compensation is activated, the temperature compensated altitude constraints are displayed on the ACTIVE FLT PLAN pages.
Temperature compensated altitudes from the NDB are displayed in reverse video on the ACTIVE FLT PLAN pages for pilot awareness. Angle constraints, such as the runway waypoint, are displayed as uncompensated values on the ACTIVE FLT PLAN pages for consistency with the NDB and approach charts. Pushing LSK 6R on an ACTIVE FLT PLAN page when temperature compensation is activated, displays the TEMP COMP page. If the landing surface temperature is deleted after the landing surface temperature is entered and temperature compensation is activated, the temperature compensation is canceled and the message TEMP COMP CANCELED is displayed in the scratchpad. The message is not displayed if the landing temperature is deleted prior to activating temperature compensation. If temperature compensation has been activated and then another landing temperature is entered which is outside the valid temperature range, the entry is not accepted and the temperature compensation is not changed.
Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 5.
Cold Temperature Restricted Airports, Notices to Airmen, Domestic/International, December 11, 2014.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Aircraft Service Change Number 059B, Enhanced Navigation, December 22, 2011
Honeywell Direct-To Temperature Compensation, Nov 2013, Feb 2014, and Mar 2014
ICAO Doc 8168 - Aircraft Operations - Vol I - Flight Procedures, Procedures for Air Navigation Services, International Civil Aviation Organization, 2006
InFO 15002, Information for Operators, Implementation of cold temperature altitude corrections at "Cold Temperature Restricted Airports" found in Notice to Airmen Publication (NTAP), 2/10/15
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