Japan Airlines is well-known as a cautious, safety-conscious carrier, but its decision to take the Boeing 787 off particular routes in response to a Boeing advisory about special engine icing conditions seems to have taken the aircraft maker and General Electric unawares.

Boeing's notice warns operators that pilots of GEnx-1B-powered 787-8s and GEnx-2B-powered 747-8s should stay at least 50 nm away from large convective thunderstorms, which can generate high-altitude ice particles. The action, seen as an interim step until the introduction of improved engine-control software early next year, was followed by an FAA airworthiness directive issued Nov. 27. Although five of the six such icing events to date involved Boeing 747-8 freighters, one occurred on a 787.

Boeing and GE see JAL's move to suspend 787 operations from subtropical zones susceptible to the unusual core icing issue as a mixed signal. Though neither would comment directly last week, some within the companies consider the type's withdrawal to be an overly cautious step, while others view it as symptomatic of the increasingly difficult relationship between the airline and Boeing. The lengthy delays to the 787 and its subsequent service-entry problems have strained the JAL-Boeing rapport, which took a turn for the worse in October with the carrier's historic decision to acquire the Airbus A350-900 and -1000.

In the wake of Boeing's advisory, JAL is withdrawing 787s from service between Tokyo, New Delhi and Singapore and is suspending plans to use the type between Tokyo and Sydney. All three routes are in subtropical regions where core icing has been encountered with increasing frequency since the 1990s, in lockstep with the growing air traffic there. The unrecognized form of icing inside engines causes surges, thrust loss or power “roll-backs” with little or virtually no warning.

GE and Boeing say they are meanwhile accelerating efforts to introduce new engine-control software to counter the problem. Although the two remain on track to roll out corrective software to operators in the first quarter of 2014, Boeing's advisory was likely prompted by the increasing number of events hitting the GEnx-powered fleet.

The engine-control changes, test-flown on a GEnx-2B-powered 747-8 in the summer (AW&ST Sept. 2, p. 20), 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 buildup 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 it into the downstream compressor stages and combustor, causing a surge, roll-back or other malfunction.

Until recently, it had been assumed that 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 accretion ice.

When incidents were first reported, investigators initially thought they were caused by supercooled liquid water, hail or rain that 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.