Another tactical improvement about this same time dealt with artillery flak suppression in support of close support aircraft. Two refinements made in the procedure in the late spring of 1953 involved firing of HE rounds during the actual run of planes over the target. Basically, the plan consisted of releasing a TOT or VT concentration on the most lucrative enemy antiaircraft positions within a 2,500-yard circle around the strike area. A continuous rain of HE-fuzed projectiles was placed on these targets for a three-minute period, during which Marine planes made their runs. Favorable results were achieved in that new system tended to keep enemy antiaircraft gunners off-balance for a longer period of time and thus decreased the danger to friendly attacking aircraft. On the other hand, pilots quickly noted that this became an “unimaginative employment of an unvarying flak suppression schedule which Communist AA gunners soon caught onto and turned to their own advantage.”[443] [443] _PacFlt EvalRpt_ No. 6, Chap. 10, p. 10-70. With respect to squadron hardware, Marine combat potential increased substantially during the spring months with the phasing out of F7Fs in Night Fighter Squadron 513 and introduction of the new F3D-2 twin-jet Skyknight intruder. By late May the Allocation of 24 of these jet night fighters had been augmented by 4 more jets from the carrier USS _Lake Champlain_ and the squadron “assumed its primary night-fighter mission for the first time in the Korean War.”[444] While the sturdy, dependable Tigercats[445] made their final contribution to the United Nations air effort early in May with the experimental NCAS program, the new Skyknights continued the squadron’s unique assignment inaugurated in late 1952 as night escort to Air Force B-29 bombers on their strike missions. Not a single B-29 was lost to enemy interceptors after 29 January 1953. The capabilities of the skilled Marine night-fighters were noted in a “well done” message received by the CO, VMF(N)-513 in April from the Air Force.[446] [444] _Ibid._, p. 10-99. [445] Also characterized by squadron members as the “tired old Tigercats” in reference to the war-weary, 1945-vintage aircraft. VMF(N)-513 ComdD, May 53, p. 6. [446] CO, 19th Bomber Group (Col Harvey C. Dorney, USAF) msg to CO, VMF(N)-513 (LtCol Robert F. Conley), n.d., reading: “19th Bomber Group Airborne Commander and crews participating in attack on Sinanju Bridge Complex, 11 April, have high praise for night fighter protection. All feel that without their protection severe damage or loss of B-29’s would have resulted.” VMF(N)-513 ComdD, Apr 53, p. 6.