Date: 202004
Local time of day: 1801-2400
Location: VVTS.ARTCC, FO
Flight conditions: VMC
Light: Night
Aircraft: B767-300 and 300 ER
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class A VVTS
Second aircraft: Commercial Fixed Wing
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Communication Breakdown
Second reporter function: Pilot Not Flying; Captain
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Wake Vortex Encounter
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Contributing factors: Environment - Non Weather Related; Procedure
Primary problem: Environment - Non Weather Related

Narrative: We were on our second leg of the night flying from ZZZZ-ZZZZ1. We were east of Vietnam and in contact with HoChi Minh Control both with VHF and CPDLC. We were maintaining FL350 with another aircraft flying in the same direction (Eastbound) at FL360 on our same airway. We experienced two times light wake turbulence from this aircraft and we communicated to VVTS that we were requesting 2NM right of track due to that turbulence. Both times we received no reply from VVTS control. A few minutes later (about 20nm East of position Dudis) we experienced a more severe wake turbulence encounter. The aircraft was pitching and rolling abruptly at that time. The bank was going past 30 degrees and probably more at the point.I disconnected the autopilot and initiated control of the aircraft. At that point the wake turbulence got more severe and I came out of my seat repeatedly. In attempts to perform the upset recovery it was difficult to maintain control and manipulate the MCP (Mode Control Panel). At this time it was very difficult to even read the flight instruments due to the severe turbulence. In the recovery we lost about 800 ft. and our altitude was FL 342 until a recovery to altitude could be initiated with manipulation of the MCP.It has to be said that during the beginning of the more severe event we tried several times (more than 10 times) to call VVTS via VHF requesting a right of track deviation. To no avail. Only until the Captain made a more [forceful] request did we get a reply from VVTS control. We informed VVTS of our altitude and our recovery back to FL 350. There were no events after the recovery to altitude. Flight proceeded as normal.


Second narrative: ZZZZ-ZZZZ1 (Pilot Monitoring) While outbound of DUDIS at FL350 on airway M771; we made initial contact with HoChi Minh (after several attempts). We began to experience light but persistent wake turbulence from another same direction aircraft at FL360. Noting the quartering tailwind I thought it would be prudent to turn on the continuous ignition. I advised the First Officer of my intent to request a 2-mile offset to the right which would keep us out of the wake turbulence (which was increasing in intensity by the second). After two failed attempts to get HoChi Minh to answer our radio call; the wake turbulence intensified to what I would consider moderate to severe chop. As I was making my third attempt to get a response from HoChi Minh; it felt as if the bottom dropped out and the aircraft dropped 200 ft. The aircraft then violently rolled left between 30 and 45 degrees of bank and began a significant descent (I did not note the sink rate). I noticed [the] First Officer promptly disengaged the autopilot and rolled wings level. We arrested the descent at FL342. We started a VERY slow climb back to FL350; while I again attempted to contact HoChi Minh. On this last radio call I included our current altitude and how we got there (wake turbulence from the aircraft ahead of us); and included our request to offset. HoChi Minh finally responded; acknowledged our climb back to FL350 and approved our offset as requested. We re-engaged the automation and the flight continued without incident.


Synopsis: B767-300 flight crew on an international flight reported loss of control at FL350 from a wake turbulence encounter with an aircraft ahead of them at FL360.