Why the airline rejected your FWB for an invalid check digit
When you transmit an FWB (Freight Waybill) Cargo-IMP message, the airline's system validates the AWB number the moment it arrives. The final digit of the seven-digit serial — the 8th character after the prefix — must be the correct mod-7 check digit. If it is not, the airline immediately issues an FNA (Freight Not Accepted) response. Cargo does not depart until you correct and retransmit.
This is one of the most common FNA triggers, and it almost always comes from a manual keying error: a transposition (1234756 instead of 1234567) or a digit carried over from a previous waybill.
The mod-7 rule explained
The check digit is the unweighted modulus-7 remainder when you divide the seven-digit serial by 7. Use the remainder directly, including 0.
Worked example (the IATA golden reference): Serial: 1234567 1234567 ÷ 7 = 176366 remainder 5 Check digit: 5 Full AWB number: 020-12345675
Zero-remainder example: Serial: 1234562 Remainder: 0 Full AWB number: 020-12345620
Common mistake: writing 020-12345674 (check digit 4) produces an FNA. The correct number is 020-12345675.
How to fix a check-digit FNA in three steps
Step 1 — Identify the serial in your FWB. Look for the AWB line (usually starts with "AWB" or sits in the header block). Pull out the seven-digit portion before the check digit.
Step 2 — Recompute the check digit. Use the free calculator at /tools/awb-check-digit: enter the seven-digit serial and the tool returns the correct digit and the full formatted number.
Step 3 — Correct your FWB and revalidate. Update the AWB number in your message, then paste the revised FWB into the validator at /fwb-validator. Confirm no other errors appear, then retransmit.
Preventing check-digit errors before they happen
If you use AWB Studio to create and send your FWB, the AWB number field validates the check digit in real time — you cannot save a waybill with an invalid number. For forwarders migrating from legacy systems where staff hand-type AWB numbers, the check-digit calculator at /tools/awb-check-digit is a fast sanity check before you build the FWB message.
Repeat check-digit FNAs are a signal that your team is copying numbers from PDFs or emails rather than pulling them from a system. Consider importing the AWB number block directly from the airline allocation to eliminate transcription errors entirely.