A maximum authorized altitude (MAA) is a published altitude representing the maximum usable altitude or flight level for an airspace structure or route segment. It is the highest altitude on a Federal airway, jet route, RNAV low or high route, or other direct route for which an MEA is designated at which adequate reception of navigation signals is assured. MAAs represent procedural limits determined by technical limitations or other factors such as limited airspace or frequency interference of ground based facilities.


In controlled airspace, pilots must maintain the altitude or flight level assigned by ATC, although if the ATC clearance assigns “VFR conditions on-top,” an altitude or flight level as prescribed by Part 91.159 must be maintained. In uncontrolled airspace (except while in a holding pattern of 2 minutes or less or while turning) if operating an aircraft under IFR in level cruising flight, an appropriate altitude as depicted in the legend of NACO IFR en route high and low altitude charts must be maintained. [Figure 3-21]

When operating on an IFR flight plan below 18,000 feet MSL in accordance with a VFR-on-top clearance, any VFR cruising altitude appropriate to the direction of flight between the MEA and 18,000 feet MSL may be selected that allows the flight to remain in VFR conditions. Any change in altitude must be reported to ATC and pilots must comply with all other IFR reporting procedures. VFR-on-top is not authorized in Class A airspace. When cruising below 18,000 feet MSL, the altimeter must be adjusted to the current setting, as reported by a station within 100 NM of your position. In areas where weather-reporting stations are more than 100 NM from the route, the altimeter setting of a station that is closest may be used. During IFR flight, ATC advises flights periodically of the current altimeter setting, but it remains the responsibility of the pilot or flight crew to update altimeter settings in a timely manner. Altimeter settings and weather information are available from weather reporting facilities operated or approved by the U.S. National Weather Service, or a source approved by the FAA. Some commercial operators have the authority to act as a government-approved source of weather information, including altimeter settings, through certification under the FAA’s Enhanced Weather Information System.

Flight level operations at or above 18,000 feet MSL require the altimeter to be set to 29.92. A flight level (FL) is defined as a level of constant atmospheric pressure related to a reference datum of 29.92 in. Hg. Each flight level is stated in three digits that represent hundreds of feet. For example, FL 250 represents an altimeter indication of 25,000 feet. Conflicts with traffic operating below 18,000 feet MSL may arise when actual altimeter settings along the route of flight are lower than 29.92. Therefore, Part 91.121 specifies the lowest usable flight levels for a given altimeter setting range.


When the barometric pressure is 31.00 inches of mercury or less and pilots are flying below 18,000 feet MSL, use the current reported altimeter setting. This is important because the true altitude of an aircraft is lower than indicated when sea level pressure is lower than standard. When an aircraft is en route on an instrument flight plan, air traffic controllers furnish this information at least once while the aircraft is in the controller’s area of jurisdiction. According to Part 91.144, when the barometric pressure exceeds 31.00 inches Hg., the following procedures are placed in effect by NOTAM defining the geographic area affected: Set 31.00 inches for en route operations below 18,000 feet MSL and maintain this setting until beyond the affected area. Air traffic control issues actual altimeter settings and advises pilots to set 31.00 inches in their altimeter, for en route operations below 18,000 feet MSL in affected areas. If an aircraft has the capability of setting the current altimeter setting and operating into airports with the capability of measuring the current altimeter setting, no additional restrictions apply. At or above 18,000 feet MSL, altimeters should be set to 29.92 inches of mercury (standard setting). Additional procedures exist beyond the en route phase of flight.

The lowest usable flight level is determined by the atmospheric pressure in the area of operation. As local altimeter settings fall below 29.92, pilots operating in Class A airspace must cruise at progressively higher indicated altitudes to ensure separation from aircraft operating in the low altitude structure as follows:
Current Altimeter Setting      Lowest Usable Flight Level
  • 29.92 or higher                  180
  • 29.91 to 29.42                  185
  • 29.41 to 28.92                  190
  • 28.91 to 28.42                  195
  • 28.41 to 27.92                  200
When the minimum altitude, as prescribed in Parts 91.159 and 91.177, is above 18,000 feet MSL, the lowest usable flight level is the flight level equivalent of the minimum altitude plus the number of feet specified according to the lowest flight level correction factor as follows:

Altimeter Setting            Correction Factor
  • 29.92 or higher             none
  • 29.91 to 29.42            500 Feet
  • 29.41 to 28.92            1000 Feet
  • 28.91 to 28.42            1500 Feet
  • 28.41 to 27.92            2000 Feet
  • 27.91 to 27.42            2500 Feet