Date: 201511
Local time of day: 1201-1800
Location: MMID.ARTCC, FO
Flight conditions: VMC
Light: Daylight
Aircraft: Small Transport
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A MMID
ATC / advisory: Center MMID
Second aircraft: Commercial Fixed Wing
Reporter organization: Personal
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Second reporter function: Pilot Flying; Captain
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors; Environment - Non Weather Related
Primary problem: Human Factors

Narrative: We were traveling on airway at 35;000 feet. I was talking with ATC and Captain was flying. As we were in Mexican airspace occasionally controllers would try to talk with us in Spanish and when we spoke English they would respond in English. I was talking with Merida Center and asked for climb to 37;000 feet to get better radio reception. Controller said to standby. About ten minutes later controller came back and talked quickly that there was traffic ahead and said 36;000 feet. The transmission was partially unreadable so I responded to him climb and maintain 36;000 feet. There was no response back from him to me.As I had asked for 37;000 feet I assumed he had traffic ahead and that's why he only gave us 36;000 feet. When we started to climb we were at 35;400 feet when I saw another aircraft in opposite direction. I informed flying captain and he leveled off. ATC then pointed out aircraft and said maintain 35;000 feet. I told him we had a visual and descended to 35;000 feet.The other aircraft called and said they had a TCAS alert. ATC called me and said he would write a report. I told him that I had read back climb and maintain 36;000 feet and he should have advised me if that was not correct.There was no threat to safety as we had visual and separation was probably not less than 1;000 feet. Communication with Mexican ATC is spotty at best and I speak enough Spanish to understand. In the future I will use the phrase 'say again' as many times as is needed for Mexican ATC to slow down their transmission and speak clearly.


Second narrative: Was on a flight in the middle of the Gulf of Mexico northeast bound; we requested a climb from 35;000 feet to 37;000 feet with Merida Center and was told to stand by. A few minutes later we were told to climb to 36;000 feet. We acknowledged; climbing to 36;000 feet. At approximately 35;400 feet Merida advised maintain 35;000 feet and we immediately returned to 35;000 feet. We were told a [commercial] flight in the opposite direction got an RA and had to climb. The whole incident is caused by extremely poor communication quality in the middle of Gulf of Mexico. What Merida Center had said; was traffic at 36;000 feet.


Synopsis: An aircraft in Mexican airspace requested a climb from 35;000 feet to 37;000 feet. When the Controller replied the reporter states the transmission was garbled; but they heard 36;000 feet in the transmission. The pilot assumed the garbled transmission was permission to climb to 36;000 feet. They began their climb and observed opposite direction air carrier traffic at 36;000 feet. The aircraft stopped its climb at 35;400 feet and then was told by ATC to descend to 35;000 feet.