Helicopter Crash Fuel Tanks Wreckage: the Bulli helicopter crash. Photo: Australian Transport Safety Bureau All helicopters similar to those involved in a fiery crash that killed four retired scientists will be grounded at the end of the month unless vital changes are made to their fuel tanks. The Civil Aviation Safety Authority has issued the directive after a preliminary report into the March 21 crash at Bulli Tops found that the R44 helicopter had not had its all-aluminium fuel tanks replaced despite repeated warnings from its manufacturer. The tanks have been found to rupture and caused fuel-led fires in two other recent fatal crashes. Owners have been given an April 30 deadline to replace them with flexible fuel tanks. Victim: Dr Tony Farmer Photo: Supplied Following the release of the Australian Transport Safety Bureau report into the Bulli crash, CASA said it will ground any of the helicopters that do not comply. The ATSB report estimates that about 100 helicopters will not have the new tanks fitted by then. "All R44 helicopter operators have been given ample notice of this requirement, with the first fuel tank service bulletin issued by Robinson in December 2010," a statement from CASA said. The ATSB report found that shortly after landing the helicopter was seen lifting off and turning to the right when the main rotor struck branches of a tree, causing the helicopter to roll on its side. A fire started on the grass underneath and, although people nearby attempted to rescue the four on board, the sudden intensity of the blaze made it impossible for them to get close. The bureau preliminary report, released on Friday, found that the circumstances of the accident were consistent with two other recent R44 crashes in Australia that involved "low-energy impacts" that resulted in the fuel tanks "being breached and a fuel-led fire." "R44 accidents result in a significantly higher proportion of post-impact fire than other similar helicopter types," the bureau's report says. All owners of R44 helicopters must replace their all-aluminium tanks by April 30. The bureau estimates the owners of about 100 Australian R44 helicopters will not have done so by then. "The ATSB remains concerned at the significant risk that many R44 helicopters will not comply with the service bulletin and has recommended that the [Civil Aviation Safety Authority] take further action to ensure compliance," it says. Former CSIRO deputy chief of operations Tony Farmer, retired head industrial physicist ? Gerry Haddad, CSIRO fellow Don Price and former senior research scientist John Dunlop died in the crash at Bulli Tops at midday on March 21. The helicopter went down in a grassed clearing outside the Panorama House restaurant and function centre. The bureau's report said that on that day the weather was fine and visibility was good. The pilot held the appropriate licence and had spent almost 22 hours flying that model of helicopter, including to Bulli Tops. The other two recent fatal crashes involving fuel tank ruptures on R44 helicopters have also occurred in NSW. On February 4, 2011, a flight instructor and a passenger were killed when the helicopter they were in collided with the runway at Cessnock Aerodrome and caught fire. Exactly a year later Australian film writer and producer Andrew Wight and fellow filmmaker Mike deGruy were killed when an R44 they were in crashed shortly after lift-off from Jaspers Brush, near Nowra.