CPDLC Timers, Max Uplink Delay and Position Report

Article 3 covered how CPDLC sends and answers; this article covers its safety valves — the message-timing system and position reporting. Why a separate article? Because, as article 1 said, datalink is not instantaneous and is delayed: a message crossing the air/ground link may be slow, lost, or obsolete. CPDLC uses a set of timers and a "maximum uplink delay" to prevent acting on an obsolete or unanswered message — the heart of datalink safety (of a piece with article 1's ±1 s clock accuracy).

1. Technical timer — SEND FAILED

When a downlink is sent, the technical timer watches delivery. Per FCOM DSC-46-10-30-20:

TECHNICAL TIMER — When the flight crew sends a downlink message, a timer is started to ensure the successful delivery of the message. If the ground does not confirm the good reception of the message in time or the transmission of the message takes too long, the message delivery is considered failed and SEND FAILED is displayed on the message information area.

2. Operational response timer — no reply in 7 min 30 s, revert to voice

The technical timer watches "did it arrive"; the operational response timer watches "did they answer". Per FCOM DSC-46-10-30-20:

OPERATIONAL RESPONSE TIMER — Operational response timers are used in order to avoid datalink dialogues to remain open for too long. When the flight crew sends a downlink message that requires an answer from the ATC, the answer from the ATC must be received within 7 min 30 s. If the ATC does not answer in time, the dialogue is closed and NO ATC REPLY is displayed on the message information area. The flight crew should revert to voice communication to clarify the message status.

[!note]- Two timeouts: SEND FAILED (did not arrive) vs NO ATC REPLY (no answer) SEND FAILED = your message did not reach the ground (technical, a link problem); NO ATC REPLY = it arrived but no one answered within 7 min 30 s (operational, the controller did not reply). Both point to the same action — revert to voice (one of article 1's four conditions, "an operational timer times out"). Remember: for a downlink requiring an answer, no reply in seven and a half minutes → make the call.

3. MAX UPLINK DELAY — rejecting obsolete uplink messages

The uplink-side safety valve, against acting on a clearance that lingered too long in transit. Per FCOM DSC-46-10-30-20:

The MAX Uplink Delay function enables the aircraft to reject (without any notification to the flight crew) any obsolete CPDLC uplink message. Messages are considered obsolete if the delay between the message ground emission and the message onboard reception exceeds the time specified on the MAX UPLINK DELAY field of the MAX UPLINK DELAY page on the MCDU.

Its default value and a CAUTION. Per FCOM DSC-46-10-30-20:

By default, the value of the MAX UPLINK DELAY is set to NONE. This setting means that datalink messages are not rejected due to a too long transmission time. [...] CAUTION Do not enter any value in the MAX UPLINK DELAY entry field unless requested by the air traffic controller.

[!warning]- Do not set MAX UPLINK DELAY on your own initiative Setting a delay limit feels safer, but the value is set by ATC, not by you. Enter a small value yourself and you may wrongly reject a valid uplink — and the rejection is not notified, so you never know a message was dropped. Hence default NONE and the CAUTION "do not enter unless requested by ATC". Related handling: the value resets to NONE when the connection with the active centre ends; if you receive CONFIRM MESSAGE LATENCY TIMER OFF, check the value is NONE and, if not, clear it.

4. Position reporting — manual/automatic, and the monitoring boundary

The REPORT function gives ATC position and trends. Per FCOM DSC-46-10-30-20:

The REPORT function provides ATC with information about the aircraft position, trends (climb, speed, descent, heading, track) and other requested parameters. [...] When the flight crew positively answers report request, the FMGEC monitors associated parameters. As a result, the DCDU displays a reminder message, when monitored parameter satisfies the condition.

Position report is manual or automatic. Per FCOM DSC-46-10-30-20:

The flight crew can report the aircraft position to the active ATC center manually, or automatically: MANUAL POS REPORT [...] on the POSITION REPORT page. [...] AUTO POS REPORT — Position report message appears on the DCDU, if waypoints are sequenced correctly in the FMGES. [...] Note: Independently from the CPDLC position reporting, the ADS-C also reports the aircraft position automatically.

One boundary. Per FCOM DSC-46-10-30-20:

BACK ON ROUTE [...] Note: The FMGEC cannot monitor the back-on-route situation.

[!note]- Three legs of position reporting, and the back-on-route boundary Position is reported three ways: CPDLC manual (you fill and send on the POSITION REPORT page), CPDLC automatic (waypoints sequence correctly, the DCDU produces it), and ADS-C independent (article 5, fully background). All three coexist. But back-on-route (returning to the original track after an offset or weather deviation) cannot be FMGEC-monitored — the crew must send a back-on-route message to tell ATC "we are back". Remember this boundary: the FMS helps you watch most clearance conditions, but returning to route you must report by hand.

Self-test

[!note]- Q1. What do the technical timer and operational response timer each guard, what do they display on timeout, and what is the response-timer limit here? Technical: delivery — SEND FAILED. Operational response: an answer — NO ATC REPLY. Here the limit is 7 min 30 s.

[!note]- Q2. Both timeouts point to which action? Revert to voice.

[!note]- Q3. What does MAX UPLINK DELAY do, what is its default, and why must it not be set unless ATC requests? It rejects obsolete uplinks (without notice). Default NONE. Setting it yourself may wrongly drop valid messages unnoticed; ATC defines the value.

[!note]- Q4. What do you do on CONFIRM MESSAGE LATENCY TIMER OFF? Check MAX UPLINK DELAY is NONE; if not, clear it.

[!note]- Q5. What are the three legs of position reporting, and which situation cannot the FMGEC monitor? CPDLC manual, CPDLC automatic, and ADS-C independent. The FMGEC cannot monitor the back-on-route situation.

Key takeaways

References

• FCOM DSC-46-10-30-20 — technical timer SEND FAILED, operational response timer 7 min 30 s NO ATC REPLY, MAX UPLINK DELAY (NONE / do not set / reset / LATENCY TIMER OFF), position report manual/automatic, FMGEC monitoring and back-on-route, ADS-C independent reporting.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.