Date: 201809
Location: ZZZZ.Airport, FO
Flight conditions: VMC
Light: Daylight
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
ATC / advisory: Center ZZZ
Aircraft component: Air/Ground Communication
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying; Relief Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Multiengine
Human factors: Troubleshooting; Communication Breakdown
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Air Traffic Control
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While traveling [internationally]; and while logged on to CPDLC; we received a message to contact the next sector. This closest fix I recollect on our route would be ZZZZZ. The pilot not flying dialed in the new frequency and contacted that controller. ATC acknowledged our transmission. After some period of time the pilot not flying started that he hadn't heard anyone on frequency for a while and tried contacting [ATC] on the VHF. Having no luck we found a few alternate frequencies; and was able to contact someone. That controller gave us the frequency that we should be on. The pilot not flying contacted that person and checked in. Several minutes went by when the controller came on and advised us we had been NORDO for 45 minutes and were close to entering US airspace and was about 18 miles from having fighter aircraft scrambled to intercept us. We were unaware that anything was wrong until we realized the radio had been silent for a while. We both had been diligently listening the entire time. I am confident that we didn't miss a call. We also were monitoring guard on 121.5. Nothing came through there. Additionally we were logged on to CPDLC and nothing came across there either. Shortly after this event was over; the PIC returned from his rest. I moved to the right seat; and became the pilot not flying. I immediately noticed after my first transmission that the mic got stuck. We eventually got an EICAS message that said RADIO TRANSMIT stating we had a stuck mic. This was intermittent for the next 15 minutes. We had now crossed into US airspace; and the controller there broadcast that someone has a stuck mic. We suspected it was us; and at some point the controller somehow knew that it was us and advised us to try to fix it. After thinking that this happened immediately after I took the seat; I realized that the mic had been sticking prior to this point; and was related (perhaps) to why we didn't hear the [foreign] controllers giving us a frequency change. The EICAS message RADIO TRANSMIT didn't come on consistently since the mic has to be on for a predetermined time to generate the message. So therefore; it may very well have been sticking when we were in [foreign] airspace; but since they were using a series of ground repeaters; perhaps they couldn't tell that an aircraft on frequency had a stuck mic. In our case it would have prevented us from hearing. These are mostly theories I have to try to explain what happened. The thing I know for sure however is that both of us in the cockpit were listening. The only thing I can think of that we could have done better at was to have noticed earlier that we weren't hearing voices on the radio any longer.




Synopsis: Air Carrier First Officer reported communication difficulties due to an undiscovered stuck microphone.