What does a missing yellow disc on the side of the fuselage indicate?

The presence of a yellow disc on the side of the fuselage is a critical safety feature, particularly concerning the aircraft's fire suppression system. When this yellow disc is missing, it serves as a clear indication that the fire extinguishing system may have been activated or discharged intentionally. This is a crucial element for maintenance and safety personnel, as it alerts them to potential fire safety issues that need immediate investigation.

Understanding the implications of a missing yellow disc is essential for safety protocols. The missing disc suggests that the fire suppression resources in that specific area may have been utilized, which could mean there was an actual fire or a test of the system, necessitating further checks and possibly recharging the system.

The other options do not relate directly to the indication provided by a missing yellow disc. A malfunction in the turbojet engine, a failure in the emergency lighting system, or a lack of fuel in the engine would not typically involve the status or presence of a yellow disc and therefore do not provide relevant information in this context. Essentially, the missing yellow disc specifically signifies firewall discharge, making it the correct answer in this scenario.