How did the first RH-53D minesweeper helicopters destroy mines in Haiphong?

The mining of Haiphong harbor began on May 9, 1972. The order established that all entrances to North Vietnamese ports would be mined to prevent access to them and North Vietnamese naval operations from them. Naval mining had been studied many times, but had always been rejected because of the risk of provoking intervention by the Soviet Union or the People's Republic of China. It was adopted when Quảng Trị province was starting to collapse under the North Vietnamese Easter Offensive. A protocol of the agreement later obliged the United States to neutralize the American mines in North Vietnam's coastal and inland waters.

The helicopters trained for the operation off Charleston, South Carolina, where it was found that the Marine Corps pilots' inexperience in towing the heavy Mark 105 hydrofoil minesweeping sleds posed a risk. Operations ran from February 6 to July 27, 1973, and Task Force 78 helicopters swept Haiphong's main navigation channel on February 27, 1973. On March 9 a mine exploded —the only one to go off during End Sweep— as a minesweeping helicopter passed near it.