The concept of the operation called for the ARVN units to be lifted from two hazardous landing zones over a three-day period. According to the plan 500 ARVN soldiers were to be removed from Landing Zone HOTEL on Thursday, 15 August. Landing Zone HOTEL, a small clearing which could accommodate only three UH-34Ds, was situated along a river and was crowded between two 1,000-foot-high ridgelines only five miles from the Laotian border. The steep, jungle-covered ridges generally paralleled each other less than 400 meters apart on either side of the landing zone. Slightly west of the small clearing the ridges joined to form a box canyon. The physical structure of the location dictated that the transport helicopters use the same approach and retirement routes. Due largely to the proximity of the high terrain which surrounded Landing Zone HOTEL, the ARVN adopted a Marine proposal to leave a 125-man security force on the two ridges. This force would provide cover for the helicopters conducting the final troop lift during this first phase of the retrograde movement. The 125 South Vietnamese soldiers would move cross-country to another landing zone to be picked up by helicopters following the completion of the helilift from Landing Zone HOTEL. The second landing zone, codename ZULU, was nearly as treacherous as the first. ZULU was completely encircled by a rim of hills some 500 feet higher than the floor of the landing site. In addition to the 125-man security force from HOTEL, the Marine, Army, and VNAF helicopters were scheduled to lift 200 ARVN troops and two 105mm howitzers from this landing zone on 16 and 17 August (the second and third days of the operation). An unexpected complication developed the morning the operation began when the Air Force grounded its B-26s after one of the attack bombers crashed elsewhere in the northern portion of Vietnam as a result of undetermined causes. Shortly after this crash, HMM-261 was called upon to divert a flight of helicopters to assist in search and rescue operations for the downed B-26, thus reducing even further the assets available to support the heliborne retrograde. Despite the loss of some of the air power assigned to the operation, I Corps authorities elected to proceed with the helilift from Landing Zone HOTEL as planned. After the crew of a Marine O-1B confirmed that good weather prevailed over the operations area, the first helicopters departed Da Nang on schedule. Less than half an hour after take off the Marine and Vietnamese pilots began maneuvering their aircraft between the two ridges which dominated Landing Zone HOTEL. Twice during the pickup the armed UH-1B escorts drew fire from the thick jungle on one side of the approach lanes being used by the transports. Both times they returned fire in the direction of the unseen enemy and forced him to silence his weapons. The first phase of the operation was completed without serious incident four hours after it had begun.