Washington: The US Federal Aviation Authority (FAA) has allowed Boeing to carry out flight tests of its 787 Dreamliner in order to help it check its troubled electrical system and batteries which is suspected to be the root cause of battery fire in one of its planes last month.
    
The entire Dreamliner fleet had been grounded ever since the January 7 incident. In a statement late on Thursday, the FAA said these flight tests will allow Boeing to gather additional data.     

"These test flights will be an important part of our efforts to ensure the safety of passengers and return these aircraft to service," Transportation Secretary Ray Lahood and FAA Administrator Michael Huerta said in a joint statement.
    
They said the primary purpose of the test flights will be to collect data about the battery and electrical system performance while the plane is air-borne.
    
These test flights, however, will be subject to a number of restrictions including extensive pre-flight testing and inspections and in-flight monitoring in order to ensure the highest levels of safety.
    
The flights will be conducted in defined airspace over unpopulated areas, they said.
    
Earlier in the day the National Transportation Safety Board (NTSB), which is carrying out probe into incident, said it has zeroed down on the root cause.
    
NTSB Chairman Deborah Hersman said the flight data recorder data showed the battery voltage had unexpectedly dropped from a full charge of approximately 32 volts to approximately 28 volts.
    
This drop is consistent with the charge voltage of a single cell, she said.
    
"Based upon findings from the examinations and identifying thermal and mechanical damage, we believe that the evidence points to a single cell as the initiating event," she said.
    
Hersman said the short-circuit was caused in cell number six of the battery which was the root cause of the fire.
    
"At this time, we've been able to rule out two possible causes. We have ruled out the mechanical impact damage to the battery. All mechanical damage to the cells and the battery case occurred after the initiating the short-circuiting in cell number six," she said.
    
The board, in its probe, has also ruled out external short-circuiting of the cells or battery, she said, adding several potential causes for the short-circuiting in cell number six are being examined.
    
"We are looking for evidence of contamination, electro folds, wrinkles and pinches in the assembly of cells and the battery. And because the 787 battery is really a collection of eight individual cells packaged together in a box, we're looking at the total design of the battery", she said.
    
The NTSB also blamed the FAA certification process for having given approval for the battery.

(Agencies)

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