A short-circuit in a lithium-metal battery likely caused the fire that damaged an Ethiopian Airlines Boeing 787-8 last year and certification requirements must be improved, the UK Air Accident Investigation Branch (AAIB) says in a new special bulletin.
The fire at London Heathrow airport on 12 July 2013 was probably the result of a botched electrical installation of the aircraft’s emergency locator transmitter (ELT) - the Honeywell Rescu 406AFN . This led to short-circuiting, causing its battery to overheat and start a process known as thermal runaway, which generates intense heat.
The fire spread in the aircraft’s rear upper fuselage while the aircraft was empty and unattended until the fire crews arrived, causing extensive damage to the 787’s composite materials hull.
The AAIB explained this ELT is powered by a five-cell lithium-metal (not lithium-ion) battery, and describes what it found: “The internal battery pack had experienced severe disruption, exhibiting evidence of a very high-energy thermal event, consistent with having a thermal runaway. All five cell cases had been breached and burnt material had been ejected into the battery compartment and outside of the ELT case.”
Evidence of the improper installation of the ELT is described in the report: “Significantly, the ELT battery wires were found…crossed and pinched together between the battery cover-plate and the ELT case…the cover plate was noticeably bulged in this location as the wires prevented it from sitting flush against the ELT case.”
To address the possibility of the ELT battery failure being the secondary effect of a fire caused by something else, the AAIB quotes from its initial report: “The absence of any other aircraft systems in this area containing stored energy capable of initiating a fire, together with evidence from forensic examination of the ELT, led the investigation to conclude that the fire originated within the ELT battery.”
The AAIB says that initial actions, inspections and modifications designed to ensure another such incident is unlikely to occur in the 787 fleet have been carried out satisfactorily.
“It is recommended that the [US Federal Aviation Administration] develop enhanced certification requirements for the use of lithium-metal batteries in aviation equipment, to take account of current industry knowledge on the design, operational characteristics and failure modes of lithium-metal batteries," the AAIB says.
Honeywell has since changed its process to ensure that the wiring problem cannot be repeated, the AAIB says. Inspections across the 787 fleet last year found tripped wires on 28 other ELTs, including one that had short-circuited in a way that did cause a thermal runaway event.
The AAIB bulletin compares the circumstances surrounding the incident on the Ethiopian 787 to certification issues raised last year about lithium-ion batteries on commercial aircraft. The 787 fleet was grounded for more than four months after two lithium-ion battery fires erupted in January 2013. The investigation uncovered deficiencies in the testing the FAA uses to determine if lithium-ion batteries are safe, prompting the US National Transportation Safety Board to recommend that the FAA impose more rigorous testing on such power systems.
The lithium-metal batteries involved in the Ethiopian 787 battery fire last year are chemically different than lithium-ion batteries, but the certification standards show similar deficiencies, the AAIB says. The current certificated standards for such lithium-metal batteries are government by a technical standard order (TSO), and therefore do not require manufacturers to test the batteries as integrated with the ELT or as installed on the aircraft, the AAIB says.
As a result, the AAIB recommends that the FAA and industry meet to reconsider whether the TSO process is the best way to prove that lithium-metal batteries are safe.
In March, the AAIB performed a more robust test than the TSO process requires on a lithium-metal battery. That showed the Honeywell ELT can overheat and cause a thermal runaway event, the AAIB says.
Boeing and Honeywell were not immediately available to comment on the AAIB report.