14 CFR § 135.87

It is carried in an approved cargo rack, bin, or compartment installed in or on the aircraft;

It is secured by an approved means; or

It is carried in accordance with each of the following:

For cargo, it is properly secured by a safety belt or other tie-down having enough strength to eliminate the possibility of shifting under all normally anticipated flight and ground conditions, or for carry-on baggage, it is restrained so as to prevent its movement during air turbulence.

It is packaged or covered to avoid possible injury to occupants.

It does not impose any load on seats or on the floor structure that exceeds the load limitation for those components.

It is not located in a position that obstructs the access to, or use of, any required emergency or regular exit, or the use of the aisle between the crew and the passenger compartment, or located in a position that obscures any passenger's view of the “seat belt” sign, “no smoking” sign, or any required exit sign, unless an auxiliary sign or other approved means for proper notification of the passengers is provided.

It is not carried directly above seated occupants.

It is stowed in compliance with this section for takeoff and landing.

For cargo only operations, paragraph (c)(4) of this section does not apply if the cargo is loaded so that at least one emergency or regular exit is available to provide all occupants of the aircraft a means of unobstructed exit from the aircraft if an emergency occurs.

Each passenger seat under which baggage is stowed shall be fitted with a means to prevent articles of baggage stowed under it from sliding under crash impacts severe enough to induce the ultimate inertia forces specified in the emergency landing condition regulations under which the aircraft was type certificated.

When cargo is carried in cargo compartments that are designed to require the physical entry of a crewmember to extinguish any fire that may occur during flight, the cargo must be loaded so as to allow a crewmember to effectively reach all parts of the compartment with the contents of a hand fire extinguisher.