Boeing will temporarily ban airlines from flying 747-8s and GE Aviation-powered 787s within 50nm of high-altitude, convective weather systems to prevent thrust loss caused by ice crystals build-up.
The revision to the airplane flight manual has already prompted one 787 operator – Japan Airlines – to withdraw the aircraft on routes from Tokyo to New Delhi and Singapore.
“Boeing is disappointed in any impact this situation may have on our customers, as is the GE team,” Boeing says in a statement.
Though JAL has withdrawn the 787 on two routes, most GEnx operators will continue operating the aircraft as planned, Boeing says. Boeing has issued a multi-operator message to airlines, warning the carriers of the upcoming revision to the airplane flight manual banning flights by GEnx-powered aircraft near thunderstorms.
High-altitude icing conditions on such tropic routes are blamed for six events of temporary thrust loss on GEnx-powered aircraft between April and November, according to Boeing and GE. The ice crystals can form in rare conditions usually found in tropical zones involving high-altitude thunderstorms, GE says.
GE says it is now accelerating an effort to solve the icing problem by modifying software for the full authority digital engine control (FADEC) systems. The software change will add a tweak to the aircraft’s debris rejection system – originally intended for birds and rocks but not ice. The goal is to prevent ice from entering the compressor and causing a stall. Variable bleed valve doors stationed ahead of the compressor section will open at scheduled moments, allowing any ice crystals to be ejected before reaching the engine core, according to GE.
The schedule for opening the variable bleed valve doors will be developed from analyzing more than 12,000h of GEnx flight data. The FADEC software will be modified to detect the presence of ice crystals passing through the engine, GE says, then open the variable bleed valve doors to allow the ice to escape the airflow into the compressor.
GE has used similar techniques to allow the CF6 fleet to “minimise the adverse operational effects” of ice crystals forming in tropical conditions, the company says.
The software change for the GEnx engine family should be completed during the first quarter of 2014.