What type of agents are preferred when combating engine or APU/EPU fires?

Clean agents are preferred when combating engine or Auxiliary Power Unit (APU) and Emergency Power Unit (EPU) fires due to their effective fire suppression capabilities without causing collateral damage to sensitive electronic equipment. Clean agents, such as halon or other halon alternatives, are chemically designed to extinguish fires through mechanisms that interrupt the combustion process while leaving no residue. This is particularly important in aviation environments where delicate components and systems are prevalent.

Using clean agents minimizes the risk of damage to critical instruments and electronics that could occur with other types of fire-fighting agents, which might leave residues or require extensive cleanup. While foam agents can be effective against flammable liquids, they might not be suitable for use on electronic fires due to potential damage and the challenges of cleanup. Dry chemical agents can suppress fires, but the residue may also harm electronic components. Water-based agents, while effective for many types of fires, are not ideal for engine and APU fires as they could lead to short-circuiting and further complications.

Thus, clean agents provide a safe and efficient means of addressing these specific fire incidents in an aviation context.