Chalk’s Ocean Airways has voluntarily grounded its remaining fleet of four Grumman Turbo Mallards after investigators found a serious fatigue crack in the wing spar of the Mallard that crashed off Miami Beach on Monday. Meanwhile, NTSB investigators spent yesterday poring over flight and repair records and scrutinizing Chalk’s maintenance program, developed for the salt-water environment and rough landings the amphibians encountered each day. Although salvage crews recovered the cockpit voice recorder yesterday along with most of the wreckage, the device malfunctioned and revealed no clues. The airplane carried no flight data recorder. The 1947 Mallard G-73T (N2969) had taken off Monday shortly before 2:40 p.m. for a flight to Bimini in the Bahamas, when only seconds later it broke apart in an inferno and fell into Government Cut channel. All 18 passengers and two crewmembers died in the crash.