The drivers are under scrutiny since two recent crashes that left 8 people dead and 46 injured.

Several recent crashes have occurred while pilots were wearing them.

Two recent crashes, in one of which a pilot was killed, did not help matters.

The safety board has used computer simulations to study many recent crashes.

The most recent crash prior to this occurred on January 19, 1986.

"We have not found any similarities in the three most recent crashes to date," the official said.

Politicians, diplomats and prominent business people have been among the dead in recent crashes.

Better data might have tipped off safety officials to problems before two other recent crashes, experts said.

Some recent crashes also did not conform to broad trends.

Although the circumstances in each case differed, the three recent crashes raised questions of crew training and safety standards, he said.