Here's a question: how often does the same booking reference combination get recycled, or do different airlines use the same reference simultaneously (as the reference is also further qualified by airline and passenger name?).

I did the maths, booking references are 6 characters, quite easy to remember, alphanumeric. So 36^6 = 2.18bn combinations.

Now according to ICAO in 2012 there were around 2.9bn people using air transport. Now they don't specify if this is 2.9bn individuals or 2.9bn including multiple count for repeat traveller (i.e. if I travel twice I am counted twice).

Assuming an average of say 1.5 pax per PNR that would mean that per year in 2012 there would have been just under 2bn PNRs/booking references. We can safely assume this would be about 15-20% higher in 2015.

Now a PNR generally can have a maximum validity of 12 months as you can't purchase a ticket to flights beyond 12 months mostly, so are the references recycled once they're used up/cancelled? How does it work, does anyone know?

If we reach 6bn pax annually as ICAO predicts that would mean potentially 4bn PNRs, i.e. booking references would need to be recycled on average every 6 months. Not sure what the average duration from booking to final sector of a PNR is, but at some point if that average exceeds the recycling average requirement basically the world will run out of booking references...

Sorry for the complicated technical subject, but I think it's quite interested. Might we see 7-character or even 8-characters PNRs in the next decade?

EDIT: ICAO Ref: http://www.icao.int/Newsroom/Pages/a...t-results.aspx