Washington (CNN) -- Fire-protection systems on freight aircraft are inadequate, top U.S. aviation investigators say.
The National Transportation Safety Board recommended improvements and rule changes Wednesday based on investigations of three catastrophic cargo plane fires.
NTSB Chairwoman Deborah Hersman recommended that the Federal Aviation Administration require better early detection of fires inside cargo containers, development of fire-resistant containers and requiring active fire-suppression systems on all freight airlines.
An NTSB report focused on three cargo fire accidents since 2006. Two of those fires killed the flight crews and destroyed the aircraft, in incidents in Dubai and North Korea. In the third incident in Philadelphia, the crew escaped with minor smoke-inhalation difficulties, and the plane was significantly damaged.
"These fires quickly grew out of control, leaving the crew with little time to get the aircraft on the ground," Hersman said in a statement released with the report. "Detection, suppression and containment systems can give crews more time and more options. The current approach is not safe enough."
In all three cases, the fires started within the cargo containers aboard the planes, but by the time the plane's fire warning system alerted pilots to the dangers, there was little time for them to react.
Federal regulations require cargo airline fire detection systems to alert pilots within one minute of a fire starting, but the NTSB's investigation found current systems detected fire and smoke anywhere from two and half minutes to more than 18 minutes after the fire started.
The NTSB concluded cargo containers made of flammable materials significantly increase the intensity of the on-board fires because there's been little focus by manufacturers or regulators to develop fire-resistant cargo containers.
Additionally, the NTSB's report recommended improved fire suppression systems on cargo planes, a recommendation it originally made to the FAA in 2007. After the 2007 recommendation, the FAA did a cost-benefit analysis of upgrading fire suppression systems and found it to be too expensive, a fact the NTSB highlighted in its report.
"The two catastrophic cargo airplane fires that occurred in less than a year occurred after the FAA's cost-benefit analysis concluded that the installation of fire suppression systems was not cost-effective," the board said.
UPS briefed Hersman earlier this week on the shipping company's efforts to develop improved fire safety standards. Also, FedEx is in the process of installing a fire suppression system on its long-haul fleet, the NTSB said.
In an e-mail statement to CNN, FAA officials said the agency "has long supported improved fire protection on all cargo airplanes through research on new, non-Halon fire extinguishing agents, fire suppression systems directed at individual cargo containers or specific cargo compartment zones, and advanced cargo containers incorporating smoke detection, fire containment, and/or fire suppression systems."
FAA officials said they would carefully evaluate the NTSB's recommendations.