What resource can be consulted if an unknown substance is discovered on the aircraft?

The FAM, or Flight Attendant Manual, is the most appropriate resource to consult in the event of discovering an unknown substance on an aircraft. This manual contains specific procedures and guidelines for flight attendants to follow during various in-flight situations, including handling hazardous materials or unknown substances. It outlines the appropriate safety protocols, which may include steps for assessing the situation, notifying the proper authorities, and taking preventive actions to ensure the safety and security of both passengers and crew.

In contrast, while the Flight Manual provides essential information regarding the aircraft's operational capabilities and limitations, it is not designed to address the immediate concerns flight attendants may face with an unknown substance. The Safety Handbook may also cover general safety principles but lacks the focused, actionable guidance tailored to the unique circumstances that flight attendants will encounter in-flight. The Operations Manual primarily targets operational procedures for managing the airline's day-to-day functions and would not specifically address issues related to unknown substances or emergencies on board. Thus, the FAM stands out as the most relevant resource for flight attendants in this scenario.