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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.
LOWEST USABLE FLIGHT LEVEL
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
Figure 3-21. IFR Cruising Altitude or Flight Level.
3-17
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
OPERATIONS IN OTHER COUNTRIES
When flight crews transition from the U.S. NAS to
another country’s airspace, they should be aware of differences not only in procedures but also airspace. For
example, when flying into Canada regarding altimeter
setting changes, as depicted in Figure 3-22 on page 3-18,
notice the change from QNE to QNH when flying northbound into the Moncton flight information region
(FIR), an airspace of defined dimensions where flight
information service and alerting service are provided.
Transition altitude (QNH) is the altitude in the vicinity
of an airport at or below which the vertical position of
the aircraft is controlled by reference to altitudes (MSL).
The transition level (QNE) is the lowest flight level
available for use above the transition altitude. Transition
height (QFE) is the height in the vicinity of an airport at
or below which the vertical position of the aircraft is
expressed in height above the airport reference datum.
The transition layer is the airspace between the transition altitude and the transition level. If descending
　
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