Sec. 91.121 - Altimeter settings.

(a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating --

(1) Below 18,000 feet MSL, to --

(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;

(ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or

(iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure; or

(2) At or above 18,000 feet MSL, to 29.92&sec; Hg.

(b) The lowest usable flight level is determined by the atmospheric pressure in the area of operation as shown in the following table: 

------------------------------------------------------------------------
                                                                 Lowest
                                                                 usable
                   Current altimeter setting                     flight
                                                                  level
------------------------------------------------------------------------
29.92 (or higher).............................................       180
29.91 through 29.42...........................................       185
29.41 through 28.92...........................................       190
28.91 through 28.42...........................................       195
28.41 through 27.92...........................................       200
27.91 through 27.42...........................................       205
27.41 through 26.92...........................................       210
------------------------------------------------------------------------

(c) To convert minimum altitude prescribed under §§91.119 and 91.177 to the minimum flight level, the pilot shall take the flight level equivalent of the minimum altitude in feet and add the appropriate number of feet specified below, according to the current reported altimeter setting: 

------------------------------------------------------------------------
                                                              Adjustment
                  Current altimeter setting                     factor
------------------------------------------------------------------------
29.92 (or higher)...........................................       None
29.91 through 29.42.........................................        500
29.41 through 28.92.........................................      1,000
28.91 through 28.42.........................................      1,500
28.41 through 27.92.........................................      2,000
27.91 through 27.42.........................................      2,500
27.41 through 26.92.........................................      3,000
------------------------------------------------------------------------