Boeing spent more than 60,000 hours testing changes to the lithium-ion battery system for the 787 Dreamliner, which has been grounded in the wake of incidents on two planes, the chief project engineer for the project said Friday, according to CNN.

At a news conference in Tokyo, Mike Sinnett said the company “may never get to the single root cause” of the battery issues, which included a small fire on the front of the battery box of a plane that was on the tarmac at Boston’s Logan International Airport on January 7. He said he believes improvements to the system will prevent problems.

One of the changes is a new fireproof, stainless steel enclosure around the batteries, which Sinnett said will prevent fires. He said engineers also minimized potential combustion sources in the lithium-ion batteries, and if there were a release of gas from the batteries, the enclosure would keep it from the rest of the airplane and vent vapors outside the airplane.

Boeing has also proposed several solutions to prevent the battery malfunction, including limiting the voltage range to reduce the energy in the batteries, adding four additional tests for the batteries, and changing the charging unit.

“I get asked if the airplane is safe,” Sinnett said. “My answer is absolutely. This airplane is among the safest airplane our company has ever produced.”

There are 50 grounded Dreamliners worldwide, and Boeing has orders for several hundred more.

Boeing President and CEO Ray Conner apologized to passengers, the airlines and the Japanese people for the problems plaguing the company’s newest and most advanced commercial jetliner.

Conner also repeated assurances that the plane is safe: “When the plane returns to the skies, I plan to fly on the very first flight.”

Boeing executives could not give a definitive time frame for Dreamliner’s return to service, as it is contingent on safety approvals.

The fleet has been grounded since January following the two battery-related incidents in Boston and in Japan. No passengers or crew were hurt in either incident.

“In neither event was the airplane structure damaged,” Sinnett said.

The Federal Aviation Administration must approve the redesign before the Dreamliner can carry passengers again.

Earlier this week, the agency approved a proposal for the battery system. The company will have to successfully complete all required tests and analysis.

A group of engineers and technical experts examined potential causes for battery failure and identified about 80 possible factors such as overcharging and overheating, Sinnett said.

When the lithium-ion battery overheats, the cell’s contents vaporise. The vaporisation looks like smoke, he said.

Boeing has suggested an electrical insulator that wraps around each battery cell to isolate each cell from each other and the battery case.

“In the design changes we made to the battery, we’ve minimized potential ignition sources, we have eliminated the ability for oxygen to begin or sustain combustion. If a battery fails, I’m very, very confident we will not have a fire as a result,” Sinnett said.

The batteries are not essential during flight as they are mainly used on the ground during maintenance activity, Sinnett said, such as providing navigation lights and braking while the airplane is towed.