US flight-safety regulators have issued an advisory bulletin warning cargo carriers of the importance of correctly restraining heavy vehicle loads.
It recommends that operators review their approved weight and balance control programmes, with particular emphasis on vehicle carriage.
The US FAA has not given a specific reason for the formal safety alert, dated 17 May, but it comes less than three weeks after the unexplained National Airlines Boeing 747-400F crash on departure from Bagram in Afghanistan.
National Airlines had previously confirmed that the aircraft's cargo had included military vehicles being transported out of the country.
Investigators have not disclosed any findings from the inquiry into the 29 April accident, which killed all seven crew members, and the FAA has not indicated whether its bulletin is related.
But it refers to the "potential safety impact" of carrying and restraining heavy vehicle special cargo loads.
The bulletin aims to "re-emphasise current policy and guidance" concerning weight and balance control procedures, as well as loading instructions and processes.
Heavy vehicle loads are non-standard and irregular and require careful consideration of the limitations of the aircraft, it states, as well as the characteristics of the cargo.
Vehicles might need special handling or restraint devices other than a regular pallet or container, it adds, and there are often "overlapping limitations" to be considered when determining a safe restraint configuration for such cargo.
"It is the air carrier's responsibility to maintain the highest level of safety," says the bulletin. "Proper cargo loading is essential for safe flight operations."
The safety alert advises the importance of using multiple restraint points but cautions that asymmetrical tie-downs - including mixing different tie-down materials or devices - can alter load distribution to the point of tie-down failure.
Carriers should also check they have accurate weight data for the vehicle, use qualified loadmasters to ensure correct procedures are followed, and not exceed the capacity of any tie-down point.