In a four-RVR system, how many sensors must be operative?

• A. 4
• B. 2
• C. 1
• D. 3

Answer: (D)

Redundancy in an RVR system means the runway visibility readings remain available even if one sensor fails. In a four-RVR setup, there are four independent sensors placed at key points along the runway to provide coverage and cross-check readings. The minimum to declare the system serviceable is three sensors operative. With three working, you still have enough independent measurements to cover the critical segments and to verify readings against each other, preserving reliable visibility information for pilots. If only two or fewer sensors are functioning, the data isn’t sufficient to ensure accurate and reliable RVR reporting across the runway, so the system would not meet operational requirements.