Problems with the 787’s lithium-ion battery have sparked questions about why the FAA in 2007 granted Boeing a “special condition” to allow use of the batteries on the plane, despite the fact that they are highly flammable and hard to extinguish if they catch fire.

Boeing designed a special system that was supposed to contain any such fire and vent toxic gasses outside the plane, but the two recent incidents have raised questions about whether that was a good decision.

It remains unclear what caused the batteries to fail, but when it announced plans to ground U.S.-based 787s, the FAA said both battery failures released flammable chemicals, heat damage and smoke - all of which could affect critical systems on the plane and spark a fire in the electrical compartment.

The FAA has said it will keep the 787s grounded until airlines demonstrate that the battery system is safe and complies with safety regulations.