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Project (LOREP) Annual Report ofField Activities 1992-1993 (Reynolds, 1993). 3.2 Flight Plans Most of the flights were planned to take place in supercooled clouds under instrument flight rules (IFR) conditions. Federal Aviation Regulation 91.177 (a)(2)(i) specifies a minimum of 600 m (2000 ft) above the highest surface features within 2.5 km (5 statute miles) for IFR flight over mountainous terrain. This would preclude sampling at the low elevations where seeded plumes were expected to pass over the target ridge. A waiver was obtained from the regional office of the Federal Aviation Administration (FAA) to permit flights to within 300 m (1000 ft.) of the highest terrain within 2.5 km of the flight path during IFR flights. Additionally, clearances were obtained for a block of airspace, increasing safety and sampling flexibility. The flight plan used during research missions is shown in Figure 1. The flight leg flown between point PI and point P2, upwind of the target area, was a valley track flown at 2500 m (8300 ft. MSL) on a northwest heading. The flight leg between point P3 and point P4 was a ridge track flown at 2680 m (8800 ft. MSL) on a southeast heading. During VFR flights, these same tracks were flown as low as 2100 m (7000 ft. MSL). At times the aircraft would be directed by the Project Operations Center to fly short legs further downwind between points P5 and P6. During IFR conditions this flight leg WOll;ld be flown at 2600 m (8500 ft. MSL). The SF 6 release locations are designated in Figure 1 as SF 6 1 and SF 6 2. Typically, a flight would takeoff from the Reno airport and climb to the west over the Sierra Nevada mountain range. The aircraft would then descend (from 3960 m to 2500 m) into the area of operations entering at the point labelled PI in Figure 1. Each flight lasted approximately 3 hours and completed 8 valley and ridge passes. The aircraft logged 35 research flight hours during the project, completing 10 flights on 10 separate days. 4