Oh Boy, you could write half a Bible on CPDLC,. In a nutshell, 2 systems one based on an obsolete technology and never meant for ATC ( ACARS ) pompously labelled FANS 1 ot 1A and another one (designed by bright enginers who never flew an aircraft or controlled aircraft in their life) , called ATN which has many shortcomings and does not deliver what was expected.
2 users groups, the oceanic/low density where FANS1/A is enough and helps as it replaces HF, and the Continental high density where it does not work well.
Add the fact that upgrading to ATN cost a lot of money per air frame but gives no operational advantage , and has some shortcomings , so no wonder airlines a reluctant to equip. and here you go.
Add also 2 commercial entities ( ARINC and SITA delivering the comms parts ) not really interested in ATC .
A last point, some groups in FAA and Eurocontrol pushed in the beginning to data link replace all R/T comms ( to justify the investment) and we lost very good ideas then :
Like for instance having Data link and VHF COMM boxes connected , Controller click on an aircraft label : Contact next sector " the D/L send the message with correct frequency embedded. Pilot receive message : Contact LONDON WEST 123.425 , has just to press " ACCEPT-WILCO" and the VHF box automatically tunes in the next frequency . and send a message back. When pilot press the PTT ( and TX is noted) another message is sent and both controllers in both sectors sees/knows that the aircraft has been transferred and on which frequency he is .
How about that for reducing R/T ? Pilot worklosd ( no frequency to tune, no read back errors, etc.. science fiction ? no, it was technically possible 15 years ago. Still is by the way . One day maybe, but unfortunately not in my lifetime. If it does remember me.
