- Current altimeter settings must be obtained from direct‐reading instruments or directly from weather reporting stations.
FAA Order JO 7210.3, Chapter 2, Section 10, Wind/Altimeter Information.
- If a pilot requests the altimeter setting in millibars, ask the nearest weather reporting station for the equivalent millibar setting.
- USAF/USA. Use the term “Estimated Altimeter” for altimeter settings reported or received as estimated.
- TERMINAL. Identify the source of an altimeter setting when issued for a location other than the aircraft's departure or destination airport.
- EN ROUTE. Identify the source of all altimeter settings when issued.
(If the altimeter is one hour old or less),
THE (facility name) ALTIMETER (setting).
(If the altimeter is more than one hour old),
THE (facility name) ALTIMETER (setting) MORE THAN ONE HOUR OLD.
- Issue the altimeter setting:
- To en route aircraft at least one time while operating in your area of jurisdiction. Issue the setting for the nearest reporting station along the aircraft's route of flight:
14 CFR Section 91.121(1) requires that the pilot set his/her altimeter to the setting of a station along his/her route of flight within 100 miles of the aircraft if one is available. However, issuance of the setting of an adjacent station during periods that a steep gradient exists will serve to inform the pilot of the difference between the setting he/she is using and the pressure in the local area and better enable him/her to choose a more advantageous setting within the limitations of 14 CFR Section 91.121.
- TERMINAL. To all departures. Unless specifically requested by the pilot, the altimeter setting need not be issued to local aircraft operators who have requested this omission in writing or to scheduled air carriers.
- TERMINAL. To arriving aircraft on initial contact or as soon as possible thereafter. The tower may omit the altimeter if the aircraft is sequenced or vectored to the airport by the approach control having jurisdiction at that facility.
- EN ROUTE. For the destination airport to arriving aircraft, approximately 50 miles from the destination, if an approach control facility does not serve the airport.
- In addition to the altimeter setting provided on initial contact, issue changes in altimeter setting to aircraft executing a nonprecision instrument approach as frequently as practical when the official weather report includes the remarks “pressure falling rapidly.”
- If the altimeter setting must be obtained by the pilot of an arriving aircraft from another source, instruct the pilot to obtain the altimeter setting from that source.
- The destination altimeter setting, whether from a local or remote source, is the setting upon which the instrument approach is predicated.
- Approach charts for many locations specify the source of altimeter settings as non-FAA facilities, such as UNICOMs.
- When issuing clearance to descend below the lowest usable flight level, advise the pilot of the altimeter setting of the weather reporting station nearest the point the aircraft will descend below that flight level. Local directives may delegate this responsibility to an alternate sector when Optimized Profile Descents (OPD) commence in sectors consisting entirely of Class A airspace.
- Department of Defense (DoD) aircraft that are authorized to operate in restricted areas, s, and ATC assigned airspace areas on “single altimeter settings” (CFR Exemption 2861A), must be issued altimeter settings in accordance with standard procedures while the aircraft are en route to and from the restricted areas, s, and ATC assigned airspace areas.
The DoD is responsible for conducting all “single altimeter setting” operations within the boundaries of s, restricted areas, and s. Under an LOA, the DoD provides safe altitude clearance between DoD aircraft and other aircraft operating within, above, and below the s, restricted areas, and s with appropriate clearance of terrain.
FAA Order JO 7610.14, Appendix 4, Document 4, Grant of Exemption No. 2861A - Single Altimeter Setting For Frequent Transit of FL180.
- When the barometric pressure is greater than 31.00 inches Hg., issue the altimeter setting and:
- En Route/Arrivals. Advise pilots to remain set on altimeter 31.00 until reaching final approach segment.
- Departures. Advise pilots to set altimeter 31.00 prior to reaching any mandatory/crossing altitude or 1,500 feet AGL, whichever is lower.
ALTIMETER (setting), SET THREE ONE ZERO ZERO UNTIL REACHING THE FINAL APPROACH FIX.
ALTIMETER (setting), SET THREE ONE ZERO ZERO PRIOR TO REACHING (mandatory/crossing altitude or 1,500 feet AGL, whichever is lower).
- Aircraft with Mode C altitude reporting will be displayed on the controller's radar scope with a uniform altitude offset above the assigned altitude. With an actual altimeter of 31.28 inches Hg, the Mode C equipped aircraft will show 3,300 feet when assigned 3,000 feet. This will occur unless local directives authorize entering the altimeter setting 31.00 into the computer system regardless of the actual barometric pressure.
- Flight Standards will implement high barometric pressure procedures by defining the geographic area affected.
- Airports unable to accurately measure barometric pressures above 31.00 inches Hg. will report the barometric pressure as “missing” or “in excess of 31.00 inches of Hg.” Flight operations to or from those airports are restricted to VFR weather conditions.