Troubleshooting · 2 min read
Air waybill rejected: invalid check digit (FNA)
If the last digit of the AWB serial does not match the mod-7 check digit, the AWB number is invalid, and the airline's system can answer your FWB with an FNA. Here is how the check digit works and how to correct it.
Why an invalid check digit leads to an FNA
An FNA is the IATA Cargo-IMP Error message. When an airline or cargo community system cannot process a message you sent, it replies with an FNA that quotes your message and gives a short free-text reason, such as "ACK/INVALID AWB NO". An FMA (Message Acknowledgement) only confirms that a message was received; it does not mean the shipment is accepted.
A wrong check digit makes the AWB number invalid. The Cargo-IMP manual's own sample FNA rejects a booking request for AWB 125-92345675 with "INVALID AWB NO". The check digit confirms it: 9234567 leaves remainder 6, so the valid number is 125-92345676.
A wrong check digit can come from a typing mistake, such as two swapped digits, or from a digit carried over from another waybill.
The mod-7 rule
The check digit is the remainder when you divide the first 7 digits of the 8-digit serial by 7. Use the remainder as it is, including 0. The airline prefix is not part of the calculation.
| Step | Value |
|---|---|
| Serial without the check digit | 1234567 |
| Divide by 7 | 1234567 ÷ 7 = 176366, remainder 5 |
| Check the remainder | 176366 × 7 = 1234562; 1234567 − 1234562 = 5 |
| Check digit | 5 |
| Full AWB number | 020-12345675 |
A remainder of 0 gives check digit 0: 1234562 divides by 7 exactly, so the full number is 020-12345620. The check digit can never be 7, 8 or 9.
For example, 020-12345674 fails the check. The correct number is 020-12345675.
How to fix it in three steps
- Find the AWB number. It starts the line after FWB/17, the AWB consignment details (for example 020-12345675FRAJFK/T2K150.0). Take the 7 serial digits before the last digit.
- Recompute the check digit with the AWB check-digit calculator, or divide by 7 yourself.
- Correct the number at the source, in your shipment record or AWB stock, and in the message. Then check the corrected number again with the AWB check-digit calculator before you hand the message off. The free FWB validator, which will check the whole message, opens at public launch: see what it will check.
If the number came from the airline and still fails the check, ask the airline for the correct number. A valid check digit does not show that the airline allocated the number to you.
Preventing check-digit errors
In AWB Studio you type the prefix and the first 7 digits of the serial, and the check digit is completed and checked as you type. Loading your number ranges as AWB stock is optional.
Updated 4 November 2026. AWB Studio is not affiliated with IATA.