Pressure altimeters are subject to mechanical, elastic, temperature, and installation errors. Thru manufacturing and installation specifications, periodic test and inspections required by 14CFR Part 43, these errors are accounted for and mitigated. Generally speaking ,any error greater than +/- 75’ is considered grounding for IFR flight. Your Airframe/Operations manual will contain specifics. Pressure altimeters are calibrated to indicate true altitude under ISA conditions, and any deviation from ISA will result in an erroneous altimeter reading.
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.
A FAA Information For Operators (#15002 Dated 2/10/15) gives some background information, a detailed timeline for implementation and expected operator actions. Here is a link:
Updated Aeronautical Information Manual (Paragraph 7-2-3) explains the principals in expanded detail and directs the reader to “Table 7-2-3”. This is a reprint of the correction table published in PANS-Ops, Volume 1. A draft of the new 2015 Instrument Procedures Handbook has similar updated/expanded cold weather altimeter correction information. Here is a link
A New PIC Responsibility, “Cold Temperature Restricted Airport”
This study resulted in a NOTAM (Domestic and International NOTAMS, Part 3, Section 2, Page 4GEN-14) directing pilot determined correction be applied to specific segments of approaches at airports in Alaska, California, Colorado, Idaho, Illinois, Indiana, Iowa, Maine, Massachusetts, Michigan, Minnesota, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oregon, Pennsylvania, Utah, Vermont, Washington State, West Virginia, Wisconsin, and Wyoming. Here is a link:
This list may be found at the bottom of the Terminal Procedures Basic Search page. Here is a link:
Operationally here’s how to comply with this new FAA requirement:
1. Cold Temperature Restricted Airports goes into effect 17SEP15
- 2. Use the current local altimeter setting. Do not make an altimeter change to accomplish a cold temperature correction.
- 3. The affected airports list should be reviewed to determine relevance to each operator’s operations, Pilots should calculate a cold temperature altitude correction at any airport with a runway length of 2,500 feet or greater that is included in the Cold Temperature Restricted airport list. Pilots operating into an airport with a runway length less than 2,500 feet, may make a cold temperature altitude correction in cold temperature conditions.
- 4. Pilots will need to identify which segments of the associated approaches will require an altitude correction and at what temp to begin correcting.
- 5. To help with airport identification. A “Snowflake icon” will be incrementally added to airport approach plates, beginning Mar 5, 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.
- 6. Temperatures for Cold Temperature Restricted Airports are completely separate from the temperatures published on area navigation (RNAV) approaches. Temperature restrictions on RNAV approaches for LNAV/VNAV minima must be followed, even if it is warmer than the temperature associated with the “snowflake” icon.
- 7. Pilots without temperature compensating aircraft should calculate and make a manual cold temperature altitude correction to the designated segment(s) of the approach using the AIM 7-2-3, ICAO Cold Temperature Error Table. There is one exception: KMDW, Chicago Midway Intl, Runways 22L and 22 R. Cold temperature altitude corrections will ONLY be applied to approaches for runways 22L and 22R.
- 8. 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.
- 9. 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”
- 10. 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.