The first F-111As, hot off the assembly lines, were sent to Vietnam in March 1968 for the Combat Lancer operation. Six tactical bombers of the 428th TFS had the chance to prove their worth in the battle zone, but the experiment was disastrous: three of the six were destroyed during unescorted missions at various times. Initially it was assumed they had been shot down by the enemy, but it later became clear that they had crashed because of structural faults. The detachment, having carried out 55 missions, was recalled to the US, where criticism of this already controversial aircraft reached a new peak. Yet the F-111A did eventually prove itself, again in Vietnam. On September 27, 1972, 48 F-111As of the 429th and 430th TFS of the 474th TFW arrived in Southeast Asia; they immediately went into action to help check the accelerating advance of the North Vietnamese. In five months, that is up to the end of the hostilities, they carried out over 4,000 sorties dropping about 74,000 tons of bombs with high results, and the loss of only six machines. It is worth noting that 3,980 of these 4,000 missions were effected by means of TFR (Terrain Following Radar), a radar system capable of guiding the plane at a height of only a few meters, encompassing the slightest unevenness of terrain, without intervention by the pilot, and flying beneath the net of the SAM missile radar systems. Such feats in Southeast Asia were crucial in establishing and later restoring the reputation for quality which this variable-sweep wing fighter-bomber from General Dynamics was intended to possess from the start. In action once more in April 1986, attacking targets in Tripoli and Benghazi, Libya, the F-111s confirmed their right to be considered the spearhead of the USAF tactical bombing operations.