NAT Ops Bulletin #2018-02 goes into effect 24MAY2018. This speaks directly to the issue of CPDLC message latency. Here is a link to download a copy. Paragraph 3 in this bulletin contains the expected pilots actions if you receive a CPDLC message to “SET MAX UPLINK TIMER VALUE TO XXX SECONDS”
Honeywell, Universal, Rockwell-Collins and other OEM’s have designed different ways to adjust this latency timer. The timer is one method to prevent pilots from reacting to messages that are too old and are no longer valid. Some OEMs will give pilot the abilty to set this timer (Honeywell is an example of this) others will make this an automatic function transparent to the pilots, (Rockwell-Collins is an example of this).
Take a look inside your AFM, supplements and bulletins for your specific aircraft. You will want to know what exactly your FMC uses in this situation of setting a CPDLC message latency timer. This is so that you will better able to determine your response to the “SET MAX UPLINK TIMER VALUE TO XXX SECONDS” message.
It helps to remember from your initial FANS training that CPDLC messages are either “Open” or “Closed”. Open means that there has not been a response to the message and Closed means that the message has had a response. It is important that you respond to the preformatted message with an “ACCEPT” or “ROGER” that is presented with the “SET MAX UPLINK TIMER VALUE TO XXX SECONDS” message.
Recall that responding with a FREETEXT to a preformatted message does not resolve the CPDLC message into a closed exchange. FREETEXT should only be use to ampilfy or explain a response in greater detail.
Once you have sent an “ACCEPT” or “ROGER”, then explain in a FREETEXT what your avionics are doing with the latency timer. Examples might be that “NO LATENCY TIMER ONBOARD” or “LATENCY TIMER AUTOMATICALLY SET” or “LATENCY TIMER MANUALLY SET TO XXX SECONDS”.
Research what your avionics is designed to do with the “SET MAX UPLINK TIMER VALUE TO XXX SECONDS” CPDLC message
Respond to this message with the “ACCEPT” or “ROGER”
Explain to ATC what your avionics is doing with the latency timer via FREETEXT
Any question as to the validity of CPDLC messages, execute the abnormal procedure of “Reverting to voice communication”, either HF or SATCOM VOICE.