SOME BASIC IDEAS
JUST EXACTLY WHAT IS “STANDARD”
The International Standard Atmosphere, “ISA” is an atmospheric model of pressure, temperature, density, and viscosity in the Earth's atmosphere. This is set by the International Organization for Standardization, ISO. The U.S. Standard Atmosphere, International Standard Atmosphere and World Meteorological Organization standard atmospheres are the same as “ISA” for altitudes up to 104,986ft. The ICAO published their "ICAO Standard Atmosphere" in 1993. It has the same model as the ISA, but extends the altitude to 262,500ft
PRESSURE DIFFERENCES CAUSE “TRUE” ALTITUDE PROBLEMS
A current altimeter setting is one provided by approved direct reading or remote equipment or by the most recent routine hourly weather report. Care should be exercised when using altimeter settings older than 60 min or when pressure has been reported as falling rapidly. From Pilot Training 101 we have heard the saying: “HIGH TO LOW LOOK OUT BELOW”. Dramatic effects most commonly occur when crossing a frontal boundary from an area of high pressure to an area of low pressure. If the altimeter is set to the local altimeter setting in the area of high pressure and the aircraft flies to an area of low pressure, without changing the altimeter setting, the altimeter reading will be too high, possibly by hundreds of feet.
WIND DIFFERENCES CAUSE “TRUE” ALTITUDE PROBLEMS
The procedure designer for routes normally increases minimum Obstacle Clearance over mountainous areas. The combination of strong winds and mountainous terrain can cause local changes in atmospheric pressure due to the Bernoulli effect. This occurs particularly when the wind direction is perpendicular/across mountain crests or ridges. It is not possible to make an exact calculation, it is up to the pilot-in- command to evaluate whether the combination of terrain, wind strength and direction are such as to make a correction for wind necessary. Corrections for wind speed should be applied in addition to the standard corrections for pressure and temperature, and ATC needs be advised of this operational need.
TEMPERATURE DIFFERENCES CAUSE “TRUE” PROBLEMS
When temperatures are extremely cold, true altitudes will be significantly lower than indicated altitudes published for IFR such as MEA, MOCA, MVA. These are minimum altitudes that meet obstacle clearance requirements when ISA conditions exist, the aircraft altimeter is set to the current altimeter setting for that aerodrome and the wind is calm. Under conditions of extreme cold, corrections to the published altitudes should be applied to ensure adequate obstacle clearance.
AUTOMATION TO THE RESCUE?
Non-standard temperatures can result in a change to effective vertical paths and actual descent rates while using aircraft Baro-VNAV equipment for vertical guidance on final approach segments. A higher than standard temperature will result in a steeper gradient and increased actual descent rate. Conversely, a lower than standard temperature will result in a shallower descent gradient and reduced actual descent rate. The final approach path vertical path angle is safeguarded against the effects of low temperature by the design of the procedure. Baro-VNAV procedures are not permitted when the aerodrome temperature is below the promulgated minimum aerodrome temperature for the instrument approach procedure. Some states allow for operational approval of flight management systems, equipped with approved cold temperature compensation for the final approach. In this case, the minimum temperature can be disregarded provided it is within the minimum certificated temperature limits for the equipment.
THE ICAO’S RESPONSE
Since 2004, ICAO Document #8168, PANS-Ops states that the Pilot-in-Command is responsible for altimeter corrections due to non-standard to pressure, temperature and, where appropriate, wind effects on altimeter indications. Here are some other noteworthy bullets
* Minimum altitudes are to be calculated by the state, Annex11
* Published by NOTAM or AIP, Annex 15, Appendix 1
* ATC Radar vectors are to be temperature compensated, Doc#4444
* The PIC shall be advised on how the minimum altitude were determined, Doc#8168
The state calculated minimum safe altitudes must be adjusted when the ambient temperature on the surface is much lower than ISA. 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. For colder temperatures, a more accurate correction should be obtained from a table found in PANS-Ops, Volume 1, Part III, Section 1, Chapter 4.
When using this table keep in mind:
* These tables are calculated for a sea level aerodrome and are conservative when applied at higher than seal level aerodromes.
* The corrections have been rounded up to the next 10ft increment.
* Temperature values from the reporting station nearest to the position of the aircraft should be used.
* There is a two-page equation for your use in specific case…if desired
THE EUROPEAN RESPONSE
Currently, there is not a European-wide common procedure to deal with adjustments to Minimum Altitudes. Some regulators do not specify adjustments to MSAs and consequently ATC providers do not apply a temperature correction to published MSAs for cold temperatures. It is the flight crew responsibility according to the provisions of ICAO PANS OPS. For example: the RAF advises flight crews to add 1000ft to the MSA when the temperature is - 30 °C or colder.
EuroControl directs ATC controllers to issue clearances such that the prescribed obstacle clearance will exist at all times until the aircraft reaches the point where the pilot will re-join the flight planned route, or a published route or instrument procedure. There is not a clear definition of how this is determined or when and where to apply this in operation. EuroControl is not a regulatory authority for how minimum altitudes are designated or cold temp compensated. By EuroControl and ICAO definitions, the objectives of the air traffic control service do not include prevention of collision with terrain. The state over which you are flying determines the need and specific application of cold temp compensation.
With respect to altitude corrections, the following procedures apply to operational pilots:
* IFR assigned altitudes may be either accepted or refused. Refusal in this case is based upon the pilot’s assessment of temperature effect on obstacle clearance.
* IFR assigned altitudes accepted by a pilot should not be adjusted to compensate for cold temperatures, i.e., if a pilot accepts “maintain 3000”, an altitude correction should not be applied to 3000ft.
* Radar vectoring altitudes assigned by ATC are temperature compensated and require no corrective action by pilots.
* When altitude corrections are applied to a published final approach fix, FAF crossing altitude, procedure turn or missed approach altitude, pilots should advise ATC
In 2014 EuroControl published a set of guidelines for cold temp corrections. In this document, EuroControl enforces the idea that the flight crew is responsible for any necessary cold temperature corrections to all published minimum altitudes/heights including the altitudes/heights for the initial and intermediate segments. ATC rules and procedures do not suggest relieving pilots of their responsibility to ensure that any clearances issued by air traffic control units are safe in respect to terrain clearance. Here is a link to the guidelines.
THE FAA’S RESPONSE
Since 1992 the FAA has been studying a method to publish when, where and how to describe and publish cold weather altimetry errors. The FAA conducted a risk analysis to determine if current 14 CFR Part 97 instrument approach procedures, in the United States National Airspace System, place aircraft at risk during cold temperature operations. The parameters for this study were:
1. Only specific airports in the USA were considered at risk and evaluated
2. A five year “look-back” period was used for evaluation
3. The coldest MEAN low temp, in the coldest month was identified
4. Anticipated barometric altimetry system errors were factored in
5. Intermediate, Final and Missed Approach segments were evaluated for each published instrument approach procedure
6. If probability was greater than 1% that the Required Obstacle Clearance, ROC used on procedure segment altitudes would be lost… a temperature restriction was applied to that segment.
FAA in Washington DC has published Information to Operators #17014. This “InFO” cancels and replaces InFO #16023, dated 12/19/16. It contains the addition and subtraction of airports to the Cold Temperature Restricted Airports list. Here is a link:
Several of the Alaska and Northeast US tech stop and divert fields commonly used are impacted by this requirement. New in for 2017 is the inclusion of several military fields as well. The entire list of 2017 updated airports requiring Cold Weather Compensation found in the Notices to Airmen Publication, International/Domestic, “NTAP”, Part 4, Graphic Notices, Section 1, published 17AUG17. Here is a Link to the “NTAP”.
Temperatures for Cold Temperature Restricted Airports are completely separate from the temperatures published on RNAV approaches. Temp restrictions on RNAV approaches for LP or LNAV/VNAV minima must be followed, even if it is warmer than the temperature associated with the “Snowflake” icon. In other words, “APPROACH NOT AUTHORIZED BELOW XX˚” is a separate limitation when using LNAV/VNAV or LP lines of minima.
Pilots must not correct altitudes published on Standard Instrument Departures, SIDs, Obstacle Departure Procedures, ODPs and Standard Terminal Arrivals, STARs. ATC will not apply a cold temperature correction to radar vectoring altitudes. Pilots must be cleared by ATC to apply a cold temperature compensation to an ATC assigned altitude or when flying on a radar vector in lieu of a published missed approach procedure.
Pilots wishing to correct only the identified segments below the critical temperature OR without an automatic “Temp Comp” feature in the FMC must use the previously published method of cold weather temperature compensation. This is called the “Segments Method”. Pilots using the “Segments Method” will need to reference the list of affected airports. Pilots may correct only the required segment(s) indicated in this NTAP’s restricted airports list. Calculations will be made based on the altitude at the Final Approach Fix (FAF)/Precision Final Approach Fix (PFAF), the Minimum Descent Altitude or Decision Altitude (DA) and the Missed Approach (MA) final holding altitude. The calculations made at these fixes will be used to make altitude corrections on the other fixes in the applicable approach segment(s). For example:
All altitudes from the FAF/PFAF up to but not including the intermediate fix (IF) altitude. Calculate correction by taking FAF/PFAF altitude and subtracting the airport elevation. This number will be used to enter the height above airport in the ICAO table until reaching the reported temperature. Round this number as applicable and then add to FAF altitude and all step−down altitudes.
Calculate correction by taking the MDA or DA for the approach being flownand subtract the airport elevation. This number will be used to enter the height above airport in the ICAO table until reaching the reported temperature. Use this number or round up. Add this number to MDA or DA/DH, as applicable, and any applicable step−down fixes in the final segment.
Missed Approach Segment:
Calculate the correction by taking the final missed approach (MA) holding altitude and subtract the airport elevation. This number will be used to enter the height above airport in the ICAO table until reaching the reported temperature. Round this number as applicable and then add to the final MA altitude only.
Here are some more key points to remember using this method:
1. Only airports with a runway length of 2,500 feet or greater were evaluated
2. A critical temperature is identified that when, “at or below” , temperature compensation MUST be used on the identified approach segments
3. A “Snowflake icon” on the approach chart indicates there has been a critical temp and segment for this airport combination.
4. A manual calculation using the AIM 7-2-3, ICAO Cold Temperature Error Table is needed to determine the required add-on.
5. No extrapolation above the 5000ft column required. Pilots should use the 5000ft “height above airport in feet” column for calculating corrections of greater than 5000ft above reporting station.
6. ATC will require the pilot to report what altitude they are flying vice the published altitude.
“ALL SEGMENTS METHOD”
FMC’s with “Temp Comp” available in your FMC the cold temp compensation procedure is somewhat more simplified. This is described as the “All Segments Method”. Like the name implies, this involves all segments form the Initial Approach Fix, IAF to the Missed Approach Holding Fix. Here are the items to remember:
1. Specifically look for a “Snowflake” icon and a cold weather compensation temperature limit on the approach chart. There is no need to reference the restricted airports list.
2. If below the Critical Temperature for Cold Weather Correction, pilots will select “Temp Comp” on the FMC and may correct all altitudes from the IAF altitude to the missed approach final holding altitude
3. ATC will require the pilot to report what altitude they are flying vice the published altitude.
When using either the “Segments” or “All Segments” methods, pilots need to calculate the correction at the published MDA or DA and then add this to published altitude inside the Final Approach Fix (The FAF is included in the “All Segments” and “Segments” calculation already).
Here’s an example.
Given: OAT =−24C, MDA = 7080ft, TDZE = 6606ft
Difference is (7080 – 6606) = 474ft
Use the table in AIM 7−2−3, find 474ft at −24C and the add-on correction is approximately 80ft. It is permissible to round up. In this case to 100 ft. Now add corrections to the MDA and any stepdown fix altitudes in final segment.
Here is the completion to the example:
Published FAF Altitude = Do not Correct
Published Stepdown fix = 8400 + 80 = 8480ft
Published MDA = 7080 + 80 = 7160ft
Pilots still must meet the requirements in 14 CFR Part 91.175 in order to operate below the corrected MDA or DA. Pilots must see and avoid obstacles when descending below the MDA. You are correct if you think this procedure may increase your “Effective” descent minimums.
1. There is still a requirement to correct for colder than standard temp
2. The list of affected US airports was modified for 2017
3. FMC "Temp Comp" is authorized as an “All Segments”
4. Published altitudes inside the FAF at affected airports are to be manually cold temperature corrected.
5. Use the current local altimeter setting. Do not make an altimeter change to accomplish a cold temperature correction.
6. To help with airport identification. A “Snowflake icon” has been incrementally added to airport approach plates since 2015. The icon indicates a cold temperature altitude correction will be required on an approach when the reported temperature is, “at or below” the temperature specified for that airport.
7. No extrapolation above the 5000ft column required. Pilots should use the 5000ft “height above airport in feet” column for calculating corrections of greater than 5000ft above reporting station.
8. Pilots will add the correction determined from the table to the identified segment altitude, fly at the new corrected altitude and advise ATC. This should be done on initial radio contact with the ATC issuing approach clearance. ATC requires this information in order to ensure appropriate vertical separation between known traffic. For example: “Require final holding altitude, 10600 ft. on missed approach for cold temperature operations”
9. Not all ATC radar vectors provide a cold temperature correction to Minimum Vectoring Altitudes. Pilots must ensure terrain separation and request higher altitude when needed. For example: the USA, UK and Norway do not compensate for cold temperature corrections while radar vectoring. Countries using cold temperature corrections when radar vectoring are Russia, France, Japan, Denmark and the Netherlands.