SUPPLEMENTARY INFORMATION: Discussion
Over the past decade, we have been aware of temporary engine thrust loss, and other engine- related events that occurred in ice crystal icing (ICI) conditions at high altitudes. These events have prompted the release of ADs on various airplane models equipped with General Electric (GE) CF6- 80 series engines. Each event was in or near convective weather conditions that included ice crystal icing.
This type of icing does not appear on radar due to its low reflectivity, and neither the airplane ice detector nor visual indications reliably indicate the presence of icing conditions. Therefore, it is often undetected by the flight crew. Flight in these conditions can cause ice crystals to accumulate in the core gas flow path of the engine. In the events leading to those prior ADs, the ice has shed during throttle transients and in the descent phase of flight, causing temporary thrust loss.
Since the beginning of 2013, similar events have now occurred on Model 747-8 and 747-8F series airplanes powered by GEnx-2B engines and Model 787-8 airplanes powered by GEnx-1B engines. The new events that prompted this AD, however, have occurred during the cruise phase of flight and caused permanent damage (beyond maintenance manual limits) to the engine compressor. In all thrust loss events, data indicate that ice crystals entered and collected in the initial stages of the compressor. Engine temperature data indicate small ice accretions were shed through the core of the engine.
All of these ICI events occurred during cruise at 33,000 feet or above, either within or after the airplane traversed a large Mesoscale Convective System (MCS). MCSs are areas where several thunderstorms have merged, with a continuous cloud larger than 100 kilometers (62 miles) across.
Within or near MCSs, ICI events have occurred where convective activity has driven a significant quantity of moisture, in the form of ice crystals, to altitudes at or above the tropopause. ICI events tend to occur in warm geographic locations.
As of the date of this AD, there have been nine events on Model 747-8 airplanes and Model 787- 8 airplanes.
During two events on Model 747-8F airplanes, two engines experienced thrust losses during the cruise phase of flight. In one of these events, one of the engines recovered to idle but would not accelerate and was left at idle for the rest of the flight. The other engine recovered and operated normally for the rest of the flight. In both airplane events, subsequent inspections of all four engines revealed compressor damage on both of the event engines as well as damage to a third engine that had not experienced a thrust loss.
In four other events–one on a Model 787-8 airplane and three on Model 747-8 airplanes– uncommanded engine decelerations (i.e., thrust losses) of approximately 20 seconds in duration occurred. All engines automatically recovered commanded thrust without crew action and operated normally for the rest of the flight.