Tragic Anniversary, TWA Flight 800

July 17 -- Five years have passed since a fuel tank explosion brought down TWA Flight 800, sending 230 passengers to their deaths.

Since the tragedy, the Federal Aviation Administration has received recommendations on how to prevent this kind of accident from happening again. There have been some safety improvements, but critics say more needs to be done.

The disaster occurred just 11 minutes into the flight, when a spark ignited the center fuel tank, causing an explosion.

Flight 800 blew up in the air while pilots nearby watched two fireballs go down to the water. At first, it seemed certain to have been a bomb. After all, planes don't just blow up in flight.

Cause: Hot Fuel Vapors

But this 747 did blow up in mid-air and the National Transportation Safety Board concluded the cause was hot fuel vapors, which exploded in the jet's center fuel tank. Investigators have never found the source of the spark that caused the blast.

Today the reconstructed plane sits in a hangar on Long Island.

Since the accident, the FAA has issued 40 directives to help eliminate any sources of ignition.

"Aircraft flying today are safer because of the aggressive program we've done than they were before the TWA accident and they will be safer in the future," Tom McSweeney, of the FAA said.

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Some industry experts, such as Bernard Loeb, the former safety chief for the NTSB, said more needs to be done in order to protect passengers.

"The concept of engineering out the ignition sources is the basis of the certification of the airplane now and has been for years," Loeb said. "Despite that we've had explosion after explosion … obviously it doesn't work."

There have been three fuel tank explosions since 1990. The first one was in the Philippines and it was followed by the 1996 TWA Flight 800 crash. The latest such explosion occurred just four months ago in Thailand.

Solution: Inert Nitrogen Gas

Loeb insists the real solution is to pump inert nitrogen gas into the fuel tank. That way the vapors can't catch fire.