Date: 201311
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Weather elements / visibility: Thunderstorm; Turbulence
Light: Night
Aircraft: B747-800 Advanced
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: UPDATED FLIGHT CREW REPORT - additional information.  Filed altitude was FL380 with forecasted weather along the route as a wide perpendicular band of convective activity with embedded CBs topping at FL500 over a large area.  The weather band ran roughly diagonal across our course; and we encountered the weather which lasted for about an hour.  The weather radar was set to auto; and the radar painted a wide line of green and yellow extending well beyond the flight path to the left and right.  However; with a off course deviation to the south of up to 30 miles; we were able to transition the band perpendicularly before paralleling it to resume our filed route.  The area we transitioned through (I was the Pilot Flying) was mostly clear of returns with only occasional scattered green; with most of the CB activity off to the sides.  We used the procedure as outlined in the FCOM Boeing Bulletin to scan for CB activity below us.  We used both the manual and auto function of the weather radar; tilting the beam up and down between 0 and +/- 5 to evaluate our route.  Gain was adjusted to assure the clearest path through the line of weather.  Both radar displays were constantly in different modes and ranges to give us the best possible situational awareness.  The course we chose kept us clear of any yellow or red returns; as well as any solid green returns.  We did not transition over any CBs we could detect.  As we transitioned; we experienced continuous light to moderate turbulence and a climb to FL400 was requested and executed.  The TAT was consistent at -27C; SAT around -45C.  No visible moisture; no convective activity directly below us but there was lightning off to the sides in the vicinity of the larger cells.  The only indication we had that is stated in the FCOM Boeing Bulletin was St Elmo's fire - which in itself is not unusual in those kinds of conditions.  There was no indication of rain or visible moisture or any visible airframe icing.  The weather was in no way unusual or out of the ordinary - nothing different to what we operate in; through; or around on a daily basis.  The only difference was the time spent in the weather conditions we encountered.  On 2 different occasions; about 20 minutes apart; we experienced thrust reduction on one engine at a time; first on Engine 2 then later on Engine 4.  On both occasions; lasting approximately 15 seconds; the thrust slowly spooled back to around 60% N1; the EICAS message 'ENGINE 2 (4) THRUST' appeared and then the engines resumed normal thrust.  Both engines performed as expected after the event. The QRH was consulted - but because of the short duration and the quick return to normal operating conditions no pilot action was required.  I personally feel this is an issue that deserves immediate attention. The problem of ice crystal formation and resulting engine thrust reduction (and in some cases resulting in engine shutdown) is well known and documented by Boeing; GE; and the aircraft operators.  This known problem requires an immediate technical fix by the engine/aircraft manufacturer - asking pilots to exit weather conditions that are causal to this is simply unacceptable.  Basically the blame and dealing with the situation is left to the pilot.  As is; this aircraft should not be dispatched into known icing conditions; convective activity; or winter weather systems.  The weather we experienced on this flight were NOTHING unusual or out of the ordinary.  Just the weather any pilot has to deal with while fulfilling his duties.  If we did not fly in conditions as described; we would have to cancel a large proportion of our flights.




Synopsis: During cruise flight at FL380 in the vicinity of convective activity; the crew of a Boeing 747-8F experienced the loss of engine power to about 60% N1 on two separate occasions; on different engines; about 20 minutes apart.