All US freight carriers should install fire suppression systems on their aircraft to contain fires in the cargo hold, says the US National Transportation Safety Board, following its investigation of a post-flight fire that destroyed a UPS DC-8 at Philadelphia International airport.

Investigators were not able to determine the cause of the fire on 7 February, 2006, but say it most likely started in one of three cargo containers. Contributing to the aircraft hull loss, said the board, were inadequate certification test requirements for smoke and fire detection systems and the lack of an on-board fire suppression system.

The board also issued the US Federal Aviation Administration with six additional recommendations, including a request for "clear guidance" to operators of airports, large passenger and cargo aircraft and fractional services on how flight crew or first responders should react to evidence of a fire when no onboard alerts are available.

According to the safety board, the UPS pilots had the choice of four checklists for troubleshooting a smell the first officer described as "like burning wood" when the aircraft was descending through 9,375m (31,000ft) on the approach to Philadelphia. "None covered what the crew was seeing," said an investigator, adding that one of the checklists they completed brought more air into the cargo hold, fuelling the fire.

The aircraft's smoke alarms ultimately sounded at 1,088m (3,600ft) and smoke entered the cockpit just as the aircraft touched down. The crew evacuated the aircraft with no injuries.

The board also issued six recommendations to the Pipeline and Hazardous Materials Safety Administration , which oversees transport of hazardous materials by air.

These included a call to analyze "the causes of all thermal failures and fires involving secondary and primary lithium batteries and, based on this analysis, take appropriate action to mitigate any risks determined to be posed by transporting lithium batteries, including those contained in or packed with equipment, on board cargo and passenger aircraft as cargo checked baggage or carry-on items".

Investigators found secondary, or rechargeable, lithium batteries in the debris, but could not connect the devices with the origin of the fire.

According to the NTSB, there have been 15 fire incidents involving primary (non rechargeable) and secondary lithium batteries since the UPS incident, but only 12 incidents in the 10-year period before the fire.

The PHMSA allows carriers to transport smaller secondary lithium batteries, the most common being laptop computer batteries, both in cargo and in the cabin.

Source: Flight International