MRO USA: Engine diagnostics: GE opens the envelope

A bustling engine remote diagnostics business at GE Aviation is fuelling a new family of prognostics health management (PHM) tools to further boost safety while cutting disruptions, maintenance costs and shop visit durations.

The evolution - exemplified by the new engine diagnostics system for the GEnx family of engines for the Boeing 787 and 747-8 - is one aspect of a broader push within the industry to begin offering similar gains for a variety of subsystems, including flight software.

GE today is monitoring close to 22,000 engines in operation, nearly 90% of the total number of GE and CFM56-family engines in the field. CFM is a 50/50 joint venture between GE and Snecma, a Safran subsidiary.

Of the engines covered by the diagnostics programme, about 75% are in the "standard" diagnostics service category; the other 25% are in the for-fee "comprehensive" programme, says Lorenzo Escriche, manager of advanced technology operations and prognostics health management for GE Aviation. GE in 2006 made the basic diagnostic service free for all engine owners, greatly increasing the number of engines covered by the programme and statistical value of the database.

Escriche says the savings possible with engine diagnostics varies by platform, for example the CFM56-3 doesn't have the data available as later CFM56 models, but a good "ballpark" number is about $6.00 savings for per engine per hour for the standard no-fee service.

As many as six "trend points" are sent to GE, the operator or to both, using ACARS digital data messages from an aircraft via the Arinc or SITA networks, including snapshots for takeoff, cruise and when any preset limits or thresholds are exceeded. The engine parameters, collected by the aircraft's central aircraft condition monitoring system (ACMS) and packaged in a single 3.2kb ACARS message, include exhaust gas temperatures, fuel flow and core speed as a function of the fan speed, parameters that indicate the health of the engine's "gas path".

After "normalising" the data based on Mach number, altitude, pressure and inlet temperature, GE's trending analytical software compares the values to previously collected data that is considered to be "normal" data, culling parameters that are out of family for further investigation. Escriche says the normalisation process cuts data scattering, reducing the possibility of a false alert. "We can act on something or suppress it, based on if the trend looks real or not," says Escriche. "If we do detect a real issue, we begin the isolation process" to identify component issues.

At GE's five company-owned service centres, the trend data is also being used to better plan overhaul visits be estimating the state of the engine before it arrives. Escriche says the predictions in part can help shops better plan for their material needs, potentially trimming turn around time. The process is not currently available to third-party overhaul providers.

The next big step for engine diagnostics will soon go live with the GEnx-1B and -2B engines, with on-board "reasoning" algorithms that use physics-based models to evaluate performance real-time in addition to comparing values to historical data.

Rather than analysing data associated with thermodynamics of the gas path, as in present remote diagnostic systems, GE is dividing the engine into six major subsystems, each of which will have simple physics based models for on board monitoring and more comprehensive off-board monitoring. Along with engine starting subsystem, which includes sensors to monitor fuelling, igniter and current and voltage for electric starts, the GEnx diagnostic package includes data on the health of the gas path, fuel system, lubrication system, mechanical elements and controls.

"Today we're at the mercy of trend points," says Escriche. "If I can monitor the system real-time, I can better pick up on changes." He says the onboard monitoring function should be available at entry-into-service for GEnx-2B, slated for the late 4Q delivery of the first 747-8 to Cargolux. Along with the ACARS messages, aircraft will also connect to local wireless providers at the gate, sending a variety of additional diagnostics information for trending and storage.

Five years from now, Escriche says new aircraft designs will likely include centralised integrated vehicle health management (IVHM) systems that, in GE's case, will essentially be scaled-up versions of the GEnx engine diagnostics system that monitor aircraft systems like auxiliary power units, brake systems and avionics. "If you take what we have on our engines, the processing could be the same," he says. GE says it is readying such an IVHM system for a 2012 or 2013 entry into service on an as yet unannounced new aircraft.

Another critical subsystem to be diagnosed in the future is flight software, an increasingly common element in all aircraft subsystems. NASA is leading research aimed at the complex issue of how to devise software health management (SHM) systems that can diagnose, predict and potentially mitigate a software failure.

Researchers are currently building several "narrow" prototypes to study how SHM can be applied in a flight control system simulator. "We're exploring if there's an adverse event, for instance a hardware sensor goes out, how does it affect the software," says Ashok Srivastava, principal investigator for the NASA's Integrated Vehicle Health Management research project. "We're investigating the opposite as well - if there's an adverse event in the software, how does it affect the hardware."

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Source: Flight International