Date: 201606
Local time of day: 0601-1200
Location: EGGX.ARTCC, FO
Flight conditions: VMC
Light: Daylight
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center EGGX
Aircraft component: Data Transmission and Automatic Calling
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: B757 eastbound via the oceanic Reduced Lateral separation track 'U' at flight level 370. I returned from break as we were approximately 120 NM east of position N5430W03000. I was briefed there was a problem with the CPDLC Logon and that HF voice contact was intermittent. I checked the setting through the Captain's CDU and could find no discrepancies. Further inquiries of the relief pilot he informed me Gander Oceanic had rejected all logon attempts after coast out. They had also had difficulties establishing HF voice comms on two other frequencies; however they had good contact with Gander on 8891.Crossing 30W we established comms with Shanwick Oceanic initially and were told to establish CPDLC data link with them. We again were unable to logon; each attempt was rejected. We began to have difficulty communicating with the oceanic controller; we could receive all broadcasts but were unable to transmit. I called another aircraft on 123.45 and requested they relay our position report to Shanwick. A different aircraft called on 123.45 and passed a message from Shanwick on the position of aircraft to the north and south of our track in our general vicinity.I had sent an ACARS message to Maintenance Control and Dispatch explaining our problem and asking for advice. I did not receive an immediate reply so initiated SATCOM voice contact with Dispatch. Once the call went through I asked that Maintenance Control be conferenced in to the call. After explaining the difficulties we were having Maintenance Control explained they had seen this problem before and that the failure was in the left HF coupler. That prevented the CPDLC from logging on and that it also blocked the right HF voice from transmitting.With that information we terminated all attempts to establish CPDLC with Shanwick and maintain solid voice contact with the controlling authority. At this point they had become very concerned about maintaining safe separation of aircraft on the tracks and had rerouted the aircraft in our immediate area for greater separation. We then rerouted off of the RLAT track to present position to N55W020 direct RESNO direct NETKI; we were also given the VHF contact for Shannon at RESNO. The remainder of the flight proceeded without incident.




Synopsis: B757 Captain reported difficulties logging on with Shanwick Oceanic and with HF communication. Maintenance Control was contacted and indicated that the left HF coupler was likely causing both problems. Communication was maintained with Shanwick via other aircraft in the area.