Lithium batteries can catch fire. with disastrous consequences
United Airlines has become the second major US airline to announce it will no longer carry bulk shipments of lithium-ion batteries.
Delta Airlines stopped bulk shipments of the batteries in February.
Aviation officials believe lithium-ion batteries contributed to fires that destroyed two Boeing 747 cargo planes, killing all four crew members.
Federal Aviation Administration tests found overheating batteries could cause major fires.
In its tests, the FAA filled a cargo container with 5,000 lithium-ion batteries and a cartridge heater, which was added to simulate a single battery overheating.
The heat from the cartridge triggered a chain reaction in other batteries, with temperatures reaching about 600C.
This was followed by an explosion, which blew open the container door and set the cargo box on fire.
A second test, some months later, produced similar results, despite the addition of a fire-suppression agent.
"Our primary concerns when transporting dangerous goods are the safety of our customers, our customers' shipments and the environment," United Airlines said in a statement.
Plane crashes Experts think that batteries have contributed to several cargo plane fires in recent years.
In 2010, a Boeing 747 cargo plane operated by UPS Airlines developed an in-flight fire and crashed in an unpopulated area in Dubai. Both crew members were killed.
In the subsequent investigation, the FAA highlighted the fact that a large quantity of lithium-ion batteries had been on board.
In 2011, an Asiana Airlines cargo plane carrying 880lb (400kg) of lithium batteries crashed into the Korea Strait, killing both crew members.
The cause of the fire was never determined, but the International Civil Aviation Organisation did recommend new safety standards for the carriage of such batteries.
And back in 2006, a UPS cargo plane made an emergency landing at Philadelphia International Airport, following a fire. In that case, all crew members escaped unharmed.
The cause of the fire was never determined, but the recommendations from the National Transportation Safety Board included advice about the transport of lithium-ion batteries.
Malaysia Airlines flight 370 was also reported to have been carrying 440lb of lithium-ion batteries in its cargo, adding yet another theory to the mystery surrounding its disappearance last year.
New rules The increasing focus on battery safety will put pressure on other airlines to follow suit, as well as on the technology industry to come up with safer ways of transporting them.
Lithium-ion batteries power mobile phones, laptops and other digital devices. An estimated 4.8 billion lithium-ion cells were manufactured in 2013 and production is forecast to reach eight billion by 2025.
Shipments of rechargeable batteries on passenger planes are supposed to be limited to no more than a handful in a single box, under safety standards set by the UN's International Civil Aviation Organisation.
But a loophole permits many small boxes to be packed into one shipment, meaning that thousands of the batteries may be packed into pallets and loaded into the cargo holds of passenger planes.
No cargo fires aboard passenger airlines have been attributed to batteries.
American Airlines stopped accepting some types of lithium-ion battery shipments in February. It continues to accept small packages of batteries grouped together or packed into a single cargo container. But this has raised safety concerns because of the large number of batteries in one container.
FAA tests also revealed that lithium-metal batteries, which are not rechargeable and power devices such as cameras and calculators, could catch fire much faster than other versions.
The UN banned shipments of these batteries on passenger planes last year, and the ban came into effect in January.
About 10% of the 2.5 billion lithium-metal batteries manufactured annually are shipped by air.
Lithium-ion batteries are far more frequently shipped by air.
All three US airlines will continue to accept shipments when the batteries are packed inside or with equipment such as laptops or power tools.