Date: 201910
Local time of day: 1201-1800
Location: MHLM.Airport, FO
Flight conditions: VMC
Light: Daylight
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Second reporter function: Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Aircraft Damaged; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: In cruise flight we lost all waypoint and route data on both CDU's with no route displayed on either navigation display. We had no EICAS or Status message. The scratchpad on both CDU's had a white 'Standby One' message. I tried to input waypoints on both the legs page and the route page. I tried to enter a new route in Route 2 page but was unable due to the standby one message. We both tried switching to alternate FMC but the Standby One message remained on both CDU's. Due to the fact that we had lost all route and waypoint data and were about to head out over open water and night was approaching; I asked ATC for a heading toward MHLM which was the closest airport displayed on the navigation display. We started a descent to FL300 to remain in VFR conditions. I tried to contact flight control with the SATCOM but it was inoperative. The FO (First Officer) and I discussed how it was raining heavily in ZZZZ before we departed and we could have water intrusion into the Electronics and Equipment compartment. I decided to [Request Priority Handling] and ask for vectors to ILS Runway 22 in MHLM was the safest course of action. I checked the QRH and only found the FMC MESSAGE checklist that was not applicable to the situation. After about 15 minutes the white 'Standby One' message disappeared and I was able to enter MHLM in the route page and LMS in the legs page. We navigated to LMS in LNAV. The [Company] Mechanic in MHLM found extreme corrosion which appears to be carbon buildup on the power connectors on both FMC units in the Electronics and Equipment compartment. Attached are photos of the power plugs the mechanic took. Carbon buildup can cause an electrical fire and this is a serious concern that needs to be addressed.


Second narrative: During Cruise flight; an EICAS caution sounded. The autopilot went into a degraded mode. Heading select and vertical speed. No EICAS caution messages; or status messages; just a 'STANDBY ONE' on both CA (Captain); and FO (First Officer) CDU scratchpad. [CA] pulled out the checklist and I flew the airplane. I notified ATC to deviate for some weather that was on our route. ALL CDU information on legs page; and route was no longer available. ALL NAV display waypoints were gone also. We also selected alternate on FMC. The CDU message remained 'standby one.' [CA] tried to call the company on SAT COM; it was also not working now. The MHLM airport was visible on our NAV display. [CA] contacted the company on ACARS. After we complete the checklists; while coordinating with company and ATC; we decided it would be the safest course of action to [Request Priority Handling] and divert to MHLM; as the weather was good; we had plenty of gas; and [CA] and I have been there many times and have good local knowledge of airfield. We reviewed all performance landing data; and completed all checklists and landed uneventfully. Approximately 15 minutes later the CDU messages went away on both CDU's and [CA] loaded approach into SAP. The CDU scratchpad message stated that FMC temporarily prevents further CDU inputs. On landing the Mechanic found 'Black soot' on both FMC male connectors. This aircraft also had an event several days before while on approach in LNAV and VNAV.


Synopsis: B757-200 flight crew reported Flight Management Computer failure inflight.