The Invisible Danger That Left a Boeing 747-8 on the Edge of Disaster

Video from the channel Víctor Vic: Reconstrucción de Accidentes Aéreos and Victor Vic TV about Boeing 747, linked on its Above Ten Thousand entry. Duration: 20:09. Language: Spanish. Video type: Documentary.

The video reconstructs the incident on AirBridgeCargo flight RU349, a Boeing 747-8F freighter that on July 31, 2013, was flying the route between Sheremetyevo Airport in Moscow and Hong Kong. Over Chinese territory, at flight level 410 and about 80 minutes from the destination, engine number 2 suffered a compressor stall and shut down; the automatic system restarted it, but it remained limited to idle. Two minutes later, engine number 1 ran unstably and its speed dropped to around 70% N1 before recovering. The crew decided to continue and landed in Hong Kong without further incident.

The investigation attributed the event to ice crystal icing: tiny particles present around convective systems, which weather radar barely detects, partially melt inside the engine and end up building up on the compressor. When those chunks break off, they strike the blades and disrupt the airflow. The aircraft had avoided the most intense radar returns, there was no visible ice, and the detection system gave no alarm. Shortly before the failures, the outside temperature rose about 20 degrees in 80 seconds. The video explains that the phenomenon was already known in other engines, but that the GEnx engines on the 747-8 proved especially vulnerable to it.

On the ground, damage was found in the first stage of the high-pressure compressor of engines 1 and 2, and a deformed blade in engine 4, which had shown no symptoms: three of the four engines were affected. On August 19, 2013, the Russian authority Rosaviatsia issued safety information on the case. In November 2013, the FAA published an airworthiness directive for the 747-8, 747-8F, and 787 with GEnx engines that, above 30,000 feet, prohibited flying within 50 nautical miles of certain yellow or red radar returns located beneath the flight path. General Electric modified the control software to use the bleed valves differently, and in 2015 the FAA required the withdrawal of older versions of the FADEC software for the GEnx-1B and GEnx-2B. Later improvements made it possible to ease some of the initial restrictions.