The next phase of the helilift from ZULU on 17 August was characterized by increasing concern for security around the landing site. The general scheme for protecting the helicopters during this critical stage of the exercise was to establish two perimeters, one around the rim of high ground which surrounded the zone and another around the immediate landing site. The outer perimeter would be withdrawn first, leaving the inside ring of troops to deny the enemy direct access to the landing zone while the force from the outer perimeter boarded the helicopters. Once the Vietnamese soldiers were withdrawn from the rim of hills, the area within 300 meters of the close-in defenses would be automatically cleared for air strikes. Even with these precautions the helicopters would be extremely vulnerable to any enemy force that might rapidly occupy the high ground above Landing Zone ZULU following the withdrawal of the outer perimeter. Accordingly, once the troops from the outer defenses were staged for the helilift, the transport helicopters would be directed by the airborne ASOC to tighten the landing interval between aircraft from the usual five minutes to as short a time span as possible. By landing in such rapid succession, the dangerous final stage of the operation could be accomplished more quickly.
Two hours after the helilift began on Saturday morning, the air liaison officer at ZULU reported that the outer perimeter had been withdrawn and that all remaining Vietnamese troops were in positions around the landing zone. At this point the operation, how in its most critical phase, began to experience agonizing delays. First, a loaded helicopter arrived at the assembly area with a rough running engine. Fearing that the fuel in the TAFDS had somehow become contaminated, Lieutenant Colonel Shook instructed all HMM-261 pilots to check their aircraft's fuel strainers while their passengers disembarked at the assembly point. No evidence was found to indicate that the fuel contained contaminants, but the operation was slowed at the exact point where the intensified helilift was to have begun. Another minor delay occurred after a helicopter flying near the landing zone reported having drawn enemy ground fire. The approach and departure routes were adjusted slightly so that the transport helicopters would not fly over the area and VNAF T-28s were directed to attack the suspected enemy position. Shortly after the air strike ended the air liaison officer at the landing zone reported more enemy activity only 500 meters from his position. This momentary crisis was resolved when the American air liaison officer personally directed armed UH-lBs to neutralize the target area. Finally, the airborne ASOC passed instructions to proceed with the operation, whereupon HMM-261 and VNAF helicopters began spiraling down into the landing zone. The escorting UH-1B gunships provided continuous protection for the transport helicopters by flying concentric but opposite patterns around them. One after another the transports landed, took on troops, climbed out of the landing zone, and turned toward Thuong Due. Less than five minutes after the stepped-up helilift began, the last troops were airborne. The crew chief of the helicopter which embarked the final ARVN heliteam then dropped a purple smoke grenade into the empty landing zone to signal all other aircraft that the lift was complete.
The three-day heliborne retrograde from the Laotian border proved to be one of the most efficient helicopter operations conducted by the Marines in the Republic of Vietnam during the early 1960s. Its success was due largely'to detailed planning, particularly the South Vietnamese plans for the ground defense of both landing zones. These plans and their subsequent execution led a grateful Colonel Gomez, the task element commander, to declare: "This was the first time in our experience that a helicopter-borne withdrawal had been treated as a retrograde operation rather than an administrative lift. Without a sound retrograde plan the operation might well have failed."7
Although this observation was correct, it should be added that the close coordination between the airborne ASOC, the operational aircraft, and the air liaison officer on the ground had contributed to the successful execution of the plans. These agencies were instrumental in coordinating the bilingual, multiservice effort, particularly when it was beset with difficulties in its critical final stage.
HMM-261's combat support missions continued at a normal rate following the completion of the mid-August retrograde helilift. A month later, on 16 September, Lieutenant Colonel Shock's squadron lost its second UH.-34D in a crash 25 miles west-southwest of Hue. The helicopter, which had