Boeing and General Electric have flight tested an engine control software upgrade in a 747-8 which is designed to prevent the same form of ice crystal build-up that damaged three GEnx-2B engines on a Russian-operated freighter last month.
The July 31 incident, which hit an AirBridge Cargo 747-8F enroute from Moscow to Hong Kong, is the latest encounter of a high flying aircraft with the poorly-understood phenomenon of core engine icing. In this situation engines can surge and suffer power ‘roll-backs’ strike with little or virtually no warning because ice crystal clouds do not show up on weather radar. The problem is unusual because it generally occurs at high altitudes where atmospheric moisture levels are normally very low, and because it impacts the high pressure core of turbofans which were previously thought to be virtually immune from significant icing.
The AirBridge Cargo 747-8F was in darkness at 41,000-ft over China, near Chengdu, when it deviated to avoid a thunderstorm. According to Russian federal air transport authority Rosaviatsia, the aircraft entered an unseen area of ice crystal cloud not shown on the weather radar. Air temperature rose by 20 deg C to minus 34 deg C for a period of 86 seconds, and the crew switched the engine ice protection system from automatic to manual for around 10 minutes.
Around 22 minutes after flying through the warmer sector the aircraft’s No.2 (inboard left) engine surged and automatically restarted. The No.1 engine then experienced a speed reduction of 70% of N1. After landing at Hong Kong inspections revealed damage to the high-pressure compressor blades of the No.1 and 2 engines as well as the No.4.
Boeing says the flight test effort is focused on “verifying operational elements” of a change to the engine control software. The testing included monitoring the development of ice crystals on the GEnx-2Bs powering RC021, one of the company’s test airframes that has recently been used to evaluate fuel system upgrades and other performance improvements. The fully-instrumented aircraft was originally designated for 747-8I launch customer Lufthansa, but was retained as a test asset after the German carrier opted not to take the modified airframe.
The software changes to the GEnx-2B full authority digital engine control unit are designed to help the engine itself detect the presence of ice crystals when the aircraft is flying through a convective weather system. If detected, the new algorithms will schedule variable bleed valves to open and eject ice crystals that may have built up in the area aft of the fan, or in the flowpath to the core. The modification to the GEnx control logic leverages similar changes made to improve the ability of the CF6 to operate in similar icing conditions.
The ABC event is the latest in a growing number of engine icing incidents which have triggered recent changes in international certification requirements. 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 relatively recently is has been assumed ice particles would bounce off structures and pass harmlessly through bypass ducts, or melt inside the engine. Now there is evidence that there is an environment where there is a combination of water, ice and airflow which is susceptible to accreting ice. Like many of the other known core icing events, the ABC 747-8 incident occurred near convective clouds.
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. Most events were recorded above 22,000-ft, which is considered the upper limit for clouds containing supercooled liquid water. However, pilots reported that even though they were in cloud at the time, there was no evidence of the usual indications of trouble, including significant icing on the airframe or any other remarkable aspect to the weather.