From Aviation Week. It really is worth the subscription price. I have quoted some interesting reader comments after the article.
GE-powered 787s Getting Relief Icing Limitations | Ice Flight Envelope | Commercial Aviation content from Aviation Week
"GE-powered 787s Getting Relief From Icing Limitations
Software update brings full icing flight envelope clearance into view for GE-powered 787s
Boeing and General Electric (GE) have begun delivery of modified engine control software that is expected to free the GEnx-powered Boeing 787 from all operating restrictions in high altitude ice crystal icing (ICI) conditions by year-end.
The move, if sanctioned by the FAA, will end limitations originally imposed on the GE-powered 787 fleet in 2013, when Airworthiness Directives issued by the agency prohibited operation in moderate and severe icing. A final load of new software is also being installed on GEnx-2B powered Boeing 747-8s, which have also been subjected to mandatory operational limits in ICI environments. The FAA took action after several incidents in which engines suffered unexpected thrust loss, and even damage in one case involving a GEnx-2B powered 747-8, while flying through or close to convective weather systems.
Under the original directive, in the presence of known ICI conditions crews were directed to detour 50 nm around convective cells or reduce altitude to a maximum of 30,000 ft. These limits were subsequently eased with the introduction of revised electronic engine control (EEC) software that allowed operators to increase operating altitudes in ICI conditions to 35,000 ft. for 747-8s and 37,500 ft. for 787s. GE and Boeing believe the latest software will enable all restrictions to be lifted.
The ICI phenomenon, sometimes referred to as core icing, chiefly affects engines powering aircraft at altitudes above 38,000 ft.—much higher than the conventionally understood form of airframe/engine icing, which affects aircraft flying through moisture-laden clouds at low and mid-altitudes. The ice particles, which form when convective weather lifts huge concentrations of moisture to high altitudes, measure only around 40μ in diameter and have a reflectivity of just 5% of that of a raindrop, making them hard to detect using standard weather radar. The particles enter the core, impinging on static surfaces such as vanes behind the core stage, and eventually form ice. The ice slabs then break off, entering the compressor and causing thrust loss.
To counter the problem new EEC software has been introduced to control an ice crystal anti-ice (ICA) system. This detects the presence of icing conditions and automatically cycles the variable bypass valves (VBV), situated in the GEnx engine between the booster and high pressure compressor, ejecting ice into the bypass duct. Although the fleet has not been affected by ICI events since initial versions of the software were introduced in 2014, GE has continued to develop refined software logic to cover for “operational nuances,” and pave the way for an anti-ice system that would ultimately allow the 747-8 and 787 to be cleared to fly throughout their entire flight envelopes in ICI conditions.
The system is designed so that each engine has independent, automatic anti-ice detection and reaction functions. The 787 ICA activates only above 30,000 ft. and in the latest version will activate for at least 30 min., more than double the time of the original software load. The system, which turns off when the aircraft descends below 28,500 ft., cycles the VBV every 35 sec. During each cycle the valve doors are open for 30 sec. and closed for 5 sec. Operator sources say that even when the ICA system is active the impact on fuel burn is insignificant.
Activation of the system, which is independent of the standard engine anti-ice system, is indicated on the 787 flight deck display by the symbol “ICA,” which appears above the N1 (fan speed) indicator of the relevant engine. Because the system logic for the activation of the bleed valves requires specific airflow and energy, the latest software also requires crews to maintain a higher Mach speed or thrust level when operating at lighter weights. For example, when approaching or flying through clouds or visible moisture at 37,500 ft., crews must maintain a minimum of Mach 0.87 if the aircraft’s gross weight is less than 320,000 lb. In contrast, maximum required speed for the same weight at 30,000 ft. is only Mach 0.79.
Boeing and GE expect that the latest version of the software for the GEnx-1B will be installed in all GEnx-powered 787s by the end of August. Boeing is meanwhile mapping out a follow-up program with the FAA to enable the lifting of all operational limits. The company adds that “extensive testing and analysis has been conducted and we’re working closely with GE, our customers and the FAA to permanently remove the remaining flight restrictions.” Operator sources indicate this is expected by year-end. However, the time line for the GEnx-2B powered 747-8 is likely to lag that of the 787; GE says the software was rolled out to the fleet more recently."
COMMENTS BELOW
"well, seems that in that case the GEnx engine seems to have a lower temp in the first part of the compressor…other engine models may have a higher temp in that front part of the core which may help that such conditions cannot develop that easily..
but in all fairness, ist seems to have happened in other engine / airframe combinations as well…maybe not that often..."
"the GEnx also uses a lean burn combustor (TAPS) which can be less stable under certain conditions and thus more prone to flameout."
"I think the 3 spool RR engines dont 'have the problem' due to their architecture. The compressor stages are shorter and run faster as the front fan is its own stage."
"Engine core ice accretion. The first noteworthy incidence, I know of, was a PWJT15D powered Beechjet that deadsticked a landing in Florida after a dual flameout. Personally, I am witness to numerous major shop level turbine engine events; it is not all that uncommon to see minor, unexplained, FOD indications in mid-stage compressor sections w/ no upstream indications. I believe core ice accretion common phenomenon across all turbine engine types/makes."