We have run out of numbers....kind of. Many simmilar ones 3899,3988..etc. ATC asked HQ to figure out a solution and the solution was the letter added to the last two number "88F" of "99c"

Both repos-revenue-all flights, DELTA and UA...but only if there is a similar flight number expected to be in the same airspace at the same time.

These call signs are used to avoid confusion with ATC and Company A/C. Many flight #'s sound similar. (6592, 6593, 6493, etc).

In the SLC area especially (Training tower for FAA), ATC has given instructions to wrong A/C and flights have also responded to clearances NOT for them.

So in order to help eliminate mistakes and radio congestion ATC asked if SKYW, and ASA would be opposed to changing call signs in order to help eliminate misreadbacks, radio congestion and promote safety. (eg. SKYW 6592 could be called 92A or 92N, etc.

There were a few times coming out of PHX where SKYW had the same flight number as us. We were ASH6529 and they were SKW6529...both departing around the same time. Now we use the 2000 number series and 6900's for repo's

Thanks for the insight. After looking a little more closely, it appears the number-letter combo is used almost exclusivley on Skywest flights out of SLC. Nearly all the flights into SLC are all numeric. I didn't realize SLC was a training tower, it all makes sense now.

Check out my example post above about the BOS - ORD - MIA flight. That is the reason why AWAC has alpha-numeric call signs now.