Definition
PBN/C1-C4 are ICAO flight plan descriptors used in Item 18 of the flight plan to indicate that an aircraft is approved for RNAV 2 operations using specific navigation sensors. C1 indicates RNAV 2 capability using all permitted sensors (GNSS, DME/DME, DME/DME/IRU). C2 indicates RNAV 2 using GNSS. C3 indicates RNAV 2 using DME/DME. C4 indicates RNAV 2 using DME/DME/IRU. The codes are appended after the PBN/ prefix in Item 18 of the ICAO flight plan to declare which RNAV 2 navigation methods the aircraft is authorized to use.
Plain English
These are short codes a pilot writes on the flight plan to tell air traffic control that the aircraft can fly RNAV 2 routes, and which kinds of navigation equipment it will use to do so. C1 means it can use any approved equipment, C2 means satellite navigation, C3 means ground-based distance signals, and C4 means distance signals combined with onboard inertial navigation.
Context Anchor
Seen in the PBN/ part of an ICAO-style flight plan and in discussions of aircraft capability codes for instrument procedures and routes.
Derivation
PBN stands for Performance-Based Navigation, the ICAO framework that defines what an aircraft must be able to do navigationally rather than what equipment it must carry. The C-series codes are simply ICAO's lettered list for RNAV 2 capability — A-codes cover RNAV 10, B-codes cover RNAV 5, and C-codes cover RNAV 2. The numbers 1-4 distinguish the sensor combinations.
Why Pilots Care
Filing the correct code ensures ATC issues clearances the aircraft can actually fly and avoids routing restrictions or denied approvals.
Intuition Check
C1 through C4 are not increasing levels of accuracy. They all refer to RNAV 2; the number mainly tells what equipment source supports that capability.
Example Sentence 1
Because the aircraft was equipped with a certified GPS, the pilot entered PBN/C2 in Item 18 of the ICAO flight plan.
Example Sentence 2
ATC approved the direct routing after verifying the aircraft's PBN/C2 capability on the flight plan.