Close on the heels of LEAPFROG came a third airlift. Operation CIRCUS, conducted on 23 April, provided for the air deployment of the 7th Marines reserve regiment, minus two battalions, across the Imjin to landing sites just to the rear of the secondary defensive line, WYOMING FORWARD. Ten helicopters carried 1,185 Marines over the river barrier to blocking positions in 90 minutes. The CIRCUS exercise illustrated that a minimum distance should be maintained between loading and unloading sites for a safe and efficient transport operation. It also pointed up that “consideration must be given to the number of aircraft assigned to each traffic pattern during short hops over a river.”[54] This successful maneuver came three days before all HRS-1 aircraft were grounded due to a defect in the tail rotors. By mid-May the problem had been corrected and the aircraft returned to flying status. [54] _PacFlt EvalRpt_ No. 4, p. 9-50. During April, Lieutenant Colonel Herring’s VMO-6 employed its 11 single-engine OE-1 observation planes for a total of 508 fixed-wing combat flights. More than half of these, 275, were for artillery spotting; of the remainder, 166 were flown for reconnaissance and 67 represented photo, weather, liaison, and area check-out maneuvers. Combat flights by the squadron helicopters[55] during the month were 110 liaison, 45 reconnaissance, and 93 evacuations. Of the total 756 combat flights performed by both fixed-wing and rotary craft, 511 were over enemy territory. [55] Rotary wing aircraft assigned were two types, HTL-4 and H03S-1. The former is a two-place, plastic-dome Bell product; the latter, the first helicopter operated by the Marine Corps, is an observation-utility, three-passenger Sikorsky-made craft. HistBr, G-3 Div, HQMC, _Marine Corps Aircraft, 1913–1965_, Marine Corps Historical Reference Pamphlet (Washington: 1967 ed.) pp. 34, 38. During that same month, Marine squadrons operating under the Fifth Air Force put a total of 2,708 planes into the air despite restrictive or prohibitive weather on 20 days. Continuing its emphasis on attacking the North Korean transportation system, the Air Force command dispatched 1,397 Marine planes on interdiction missions. Marine-piloted close air support sorties flown to assist the 1st Marine Division numbered only 56 throughout April; those piloted by Marines for 16 other UN divisions totaled 547. Not all the air sortie records were made by land-based Marine squadrons. On 18 April, VMA-312, the CTE 95.11 squadron provided by the 1st Marine Aircraft Wing, flew 80 sorties, a Korean record for a carrier-based squadron to that date and twice the daily average for the initial six months of 1952.