Section 7. Altimeter Settings
2-7-1. CURRENT SETTINGS
a. Current altimeter settings must be obtained from direct-reading instruments or directly from weather reporting stations.
b. If a pilot requests the altimeter setting in millibars, ask the nearest weather reporting station for the equivalent millibar setting.
c. USAF/USA. Use the term “Estimated Altimeter” for altimeter settings reported or received as estimated.
2-7-2. ALTIMETER SETTING ISSUANCE BELOW LOWEST USABLE FL
a. TERMINAL. Identify the source of an altimeter setting when issued for a location other than the aircraft's departure or destination airport.
b. EN ROUTE. Identify the source of all altimeter settings when issued.
c. Issue the altimeter setting:
1. 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:
2. 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.
3. 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.
4. 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.
5. 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.”
d. 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.
2. Approach charts for many locations specify the source of altimeter settings as non-FAA facilities, such as UNICOMs.
e. 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.
f. Department of Defense (DOD) aircraft which operate 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 their restricted areas, MOAs, and ATC assigned airspace areas.
g. When the barometric pressure is greater than 31.00 inches Hg., issue the altimeter setting and:
1. En Route/Arrivals. Advise pilots to remain set on altimeter 31.00 until reaching final approach segment.
2. Departures. Advise pilots to set altimeter 31.00 prior to reaching any mandatory/crossing altitude or 1,500 feet AGL, whichever is lower.
2. Flight Standards will implement high barometric pressure procedures by NOTAM defining the geographic area affected.
3. 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.