LOS ANGELES -- U.S. Sen. Dianne Feinstein is calling for federal rules that crash-warning systems be required on commercial helicopters like the one that crashed in foggy weather last month outside Los Angeles, killing Kobe Bryant and eight other people.

The helicopter carrying Bryant and the others did not have a device called the Terrain Awareness and Warning System that signals when an aircraft is in danger of hitting ground.

The National Transportation Safety Board had recommended the system be required for helicopters. But the Federal Aviation Administration only requires it for air ambulances.

"In order to ensure such a tragedy never happens again, I ask that the FAA commence a rulemaking process to require all commercial helicopters operating in the U.S. to have terrain awareness and warning systems,'' Feinstein, D-California, wrote in a letter Tuesday.

Last week, U.S. Rep. Brad Sherman, D-Los Angeles, introduced a bill requiring the FAA to mandate the devices as part of new safety standards.