§91.121

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§ 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" Hg.

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

Current altimeter setting Lowest usable 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:

Current altimeter setting Adjustment 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
Part §91 Subpart B — Flight Rules
§91.101 Applicability • §91.103 Preflight • §91.105 Crewmembers at stations • §91.107 Seatbelts • §91.109 Flight instruction • §91.111 Near other aircraft
§91.113 Right-of-way • §91.115 Right-of-way on water • §91.117 Speed Limits • §91.119 Minimum altitudes • §91.121 Altimeter • §91.123 ATC instructions • §91.125 Light signals
§91.126 Class G • §91.127 Class E • §91.129 Class D • §91.130 Class C • §91.131 Class B • §91.133 Restricted areas • §91.135 Class A
§91.137 TFRs • §91.138 Hawaiian TFRs • §91.139 Emergency traffic • §91.141 Presidential TFRs • §91.143 Space ops • §91.144 High barometric pressure
§91.145 Air shows & and sporting events • §91.146 Charity flights • §91.147 Paying-passenger flights
§91.151 VFR Fuel reserves • §91.153 VFR flight plans • §91.155 VFR weather minimums • §91.157 Special VFR • §91.159 VFR cruising altitude • §91.161 DC ADIZ
§91.167 IFR Fuel reserves • §91.169 IFR flight plans • §91.171 VOR test • §91.173 ATC clearances • §91.175 IFR Takeoffs and landing
§91.177 IFR Minimum altitudes • §91.179 IFR cruising altitudes • §91.180 RVSM • §91.181 Course flown • §91.183 IFR comm. • §91.185 IFR radio failure
§91.187 Malfunctions in IFR • §91.189 Cat II & III ops. • §91.191 Cat II & III manual • §91.193 Certain Cat II ops.