November 24, 2013
Credit: Guy Norris, Aviation Week
General Electric and Boeing are accelerating efforts to introduce new engine control software to counter core icing following further incidents of power loss in GEnx-powered 747-8s and 787s.
Although the two companies remain on track to roll out corrective software to operators in the first quarter of 2014, the increasing number of events affecting the GEnx-powered fleet has prompted Boeing to issue an advisory to operators warning them to avoid conditions where the rare phenomenon has occurred.
The advisory, which warns operators of GEnx-1B powered 787-8s and GEnx-2B powered 747-8s to stay at least 50 naut. mi. away from large convective thunderstorms which can generate high altitude ice particles, is expected to be followed shortly by an FAA airworthiness directive. Although five of the six events that have been reported so far involve 747-8 freighters, one recent incident occurred on a 787.
In the wake of the advisory Japan Airlines has replaced 787s with 767s on routes from Tokyo to New Delhi and Singapore, and has opted to suspend plans for using 787s between Tokyo and Sydney. All three routes are in sub-tropical regions where core icing has been encountered with increasing frequency since the 1990s in lockstep with the growing air traffic in the area. The unrecognized form of icing inside engines causes surges, thrust loss, or power ‘roll-backs,’ with little or virtually no warning.
The full authority digital engine control (Fadec) changes are designed to detect the presence of ice crystals passing through the engine. If ice is detected the engine-control software will open variable bleed valve doors behind the fan and low pressure compressor to eject ice crystal build-up before it enters the core. Unlike traditional engine icing, in which supercooled liquid droplets freeze on impact with exposed outer parts of the engine as the aircraft flies through clouds, engine core ice accretion involves a complex process in which ice particles stick to a warm metal surface. These act as a heat sink until the metal surface temperature drops below freezing, thereby forming a location for ice and water (mixed phase) accretion. The accumulated ice can either block flow into the core, or shed into the downstream compressor stages and combustor, causing a surge, roll-back or other malfunction.
Until recently it had been assumed ice particles would bounce off structures and pass harmlessly through bypass ducts, or melt inside the engine. Now the growing number of core ice events proves various combinations of water, ice and airflow can be susceptible to accreting ice. When incidents were first reported, investigators initially assumed supercooled liquid water, hail or rain was responsible because it had been lifted to high altitudes by updrafts. Yet most events have been recorded above 22,000 ft., which is considered the upper limit for clouds containing supercooled liquid water.