The second and third digit of a TCTO number are used to identify what?

• A. The revision level
• B. The unit identification
• C. The ATA chapter that applies
• D. The serial number

Answer: (C)

The second and third digits identify the ATA chapter that applies to the modification. This follows the ATA 100 organization, where each two-digit code points to a specific aircraft system or area. Knowing these digits lets you quickly locate the relevant section of the maintenance data and procedures for the change, so you can understand which system is affected and what procedures apply. The other attributes—such as revision level, unit identification, or serial number—are not conveyed by these digits and are handled separately in the documentation.