Struggling passenger airlines can turn planes into cargo-only aircraft provided they follow a variety of safety requirements, U.S. aviation regulators said in new guidance.

With passenger demand plunging to nearly zero, some airlines have begun carrying more cargo to provide revenue. The Federal Aviation Administration’s guidance will allow them to carry it in what had been the passenger areas of the plane.

Transforming aircraft in that way is permitted, the FAA said in an alert dated Wednesday.

However, cargo compartments have specialized fire-detection systems that typically aren’t installed in passenger cabins, so airlines must take extra steps to ensure the crew can detect and fight fires, the FAA said. Airlines must also follow rules on properly securing cargo and the types of hazardous materials they can carry.

Pre-existing FAA rules also require that carriers perform a risk assessment of the new operations, the agency said.

All large airliners have holds beneath the cabin where cargo is currently permitted. The new guidance would allow expanding the cargo loads to the entire plane.