On November 24, 2003, about 1900 mountain standard time, a Eurocopter AS 350 B3, N846BP, made a hard forced landing at the Yuma MCAS International Airport, Yuma, Arizona. The commercial pilot and a crewmember (observer) sustained no injuries. The helicopter was owned and operated by the United States Border Patrol (USPB), Air Operations, El Paso, Texas, and it was substantially damaged during the public-use flight. Visual meteorological conditions prevailed during the nighttime patrol flight that was performed under the provisions of 14 CFR Part 91. A company flight plan was filed. The flight was originating at the time of the accident. The operator indicated that a few seconds after taking off, a mechanical malfunction occurred about 20 feet above ground level. The pilot made a forced landing during which the helicopter touched down hard. During the subsequent examination of the helicopter, the flex coupling between the engine drive shaft and the main gear box drive was found separated. The three bolts in the flex coupling assembly were found sheared. A further examination of the bolts revealed that they were devoid of three required cotter pins. The American Eurocopter Corporation's U.S. manager of accident investigation reported to the National Transportation Safety Board investigator that company records indicated that the accident helicopter was delivered to the USPB with the cotter pins installed. A subsequent examination of the broken components in the wreckage revealed evidence that at some point in time cotter pins had been installed in the bolts. However, no cotter pins were found in the wreckage. American Eurocopter’s accident investigation manager opined that the component examination performed by Eurocopter France's materials laboratory indicated that both sides of the flats on the recovered nuts revealed evidence of marks consistent with mechanical damage (see attached report). Maintenance records indicated that, following the helicopter's sale to the USBP, USBP mechanics performed a module 1 change. In addition, other maintenance was performed such as installation of an air conditioner. This maintenance may have involved manipulation of the drive. Maintenance or handling of the drive, if performed in accordance with Eurocopter's maintenance manual, would have required removing the three cotter pins and unscrewing (untorquing) the self locking flex coupling bolts. Regarding the effect of having the flex coupling disconnect, according to American Eurocopter's manager of accident investigation, when the bolts sheared, their disconnection from the drive shaft to the coupling flange link resulted in a total loss of drive to the rotors. The manager further reported that when properly installed, the subject self locking nuts are torqued, and then they are cotter pinned. Even if the cotter pins are not installed, when properly torqued the nuts should not unscrew.