SELCAL CODES?

[B737NG_Pilot]

Hi Everyone...

Have been wondering, any specific reason as to why the letters I,N,O are not used as SELCAL codes.

[Alex Whittingham]

A jolly good question. ICAO Annex 10 says "In accordance with the application principles developed by the Sixth Session of the Communications Division, the only codes at present used internationally are selected from the red group", that is RED A to RED S, omitting the letters you quote. That's as far as I have got as the sixth session sat in 1957 and records of their 'principles' seem hard to come by - maybe they hold the answer.

FYI after September 2016 RED codes T to Z and 1 to 9 may be assigned, but still not I, N and O.

[B737NG_Pilot]

Thanks Alex... Answers my questions