Originally Posted by
Juggler25
CPDLC is faster than voice with regards to physically issuing the instruction (there's no typing involved). However, it is slower than voice with regards to the actual aircraft physically performing the instruction. This is due to the time lag in the instruction being sent and received by the aircraft, the pilot acknowledging the message, then the pilot initiating the clearance. Voice is more or less instant. There's also a massive variance in the amount of time it takes for a message to be acknowledged. Sometimes it can be more or less instant, other times several seconds and sometimes the message will time out and not reach the aircraft at all. Voice is always instant - assuming the pilot is paying attention of course! :o) - so most controllers (in the UK at least) just stick to that. Or only use CPDLC for non time critical instructions, e.g. frequency changes/asking for preferred cruising levels instead of headings or level changes.
In the UK there are also lots of restrictions for it's use plus the huge difference in reliability between ATN and FANS equipped aircraft. FANS is so unreliable that controllers will mostly only use ATN.
Also, if there are any pilots reading this, please stop announcing on the frequency when you call on that you're CPDLC equipped. It's a pointless waste of RT when it's busy. We can see when you're logged and will use it accordingly if need be. Thanking you in advance!
There's lots of things to consider in this discussion. First, CPDLC capability is largely limited to western built civil heavy and super aircraft. Most military and the overwhelming number of narrowbodies do not carry it (Easyjet exception noted). Yes, there is some mouse work required by ATC to compose and send a CPDLC message that is by its nature slower than voice. The next factor is PBC. ATC separation standards are based on lots of things, one of which is the communications capabilities and speed between ATC and flights. While CPDLC is considered to be a form of DCPC, for the purposes of PBC and determination of separation standards it ranks lower than VHF voice because it is slower (as is SATPHONE) - has anyone tried vectoring by CPDLC or SATPHONE, for example? Having said that, it ranks higher than third party HF comms. CPDLCs greatest advantage lies in those regions that are outside VHF coverage - predominantly in oceanic areas. I can see less use of voice in the future, but our systems are not ready for it in a global sense yet.