MCD – MAGNECTIC CHIP DETECTORS

 

The Olympus has three magnetic Chip Detectors and a specialist group to analyse them. The three MCD’s were strategically placed within the engine as follows:

 

In the left-hand gearbox scavenge filter trapping debris from the thrust bearings (2 and 3) and the left-hand gearbox.

In the right-hand gearbox scavenge filter trapping debris within the right-hand gearbox.

 

In the main external return-flow pipe. This one designed ‘Master MCD’; it attracted debris from the total system.

Every service checks – 175 flying hours – all three MCDs were removed for deposit analysis. ‘Big lumps’ went under the microscope; a rolled flaky appearance was indicative of a bearing problem, while more granular particle suggested gear of shaft frottage. But it’s the sludge that was most fascinating. To put it another way, reading message was in the ring around the rim.

 

The sludge sample was bombarded with X-rays. Each chemical element in the amalgam would emit a different radiation. Using a computer, programmed with radiation signature of elements used in engine metals, the analyst would be presented with readout of the detailed composition of the sludge. Moving on a step further, knowing exactly what each engine alloy was made of, and introducing graphics, the sample’s footprint could be compared with known footprints: eg No.1 bearing housing was made from an iron-based alloy, MSRR 6544 – in addition to its base material it had 25% carbon, 14% chrome, 1% manganese, 1% nickel and 0.8% silicon. Having matched the footprint, one could point fairly precisely to the assemble under suspicion.

 

It was well known that a newly installed engine would generate a relatively high level of debris as shafts, gears and bearings bedded-in and grease, used in engine build, washed down-grease raises the silicon content.

 

Nevertheless, the checks remained. An engine must demonstrate three consecutive good checks before removed from the ‘alert’ file.

 

EXHAUST GAS TEMPERATURE (EGT)

 

The combustion chamber, even in its latest incarnation, still took a battering from the high energy gas flow. Deterioration, before becoming damaged would still upset temperature patterns, and to catch a trend before it became a problem had huge benefits.

In 1986 Rolls-Royce devised an EGT trend analysis program to be loaded into a programmable Sharp 1248 hand-held calculator. This became a routine part of the flight engineer’s in-flight tasks. Using real-time data, even the smallest divergence of EGT was apparent; recording and plotting created the trend. Its ‘raison d’ etre’, a raising trend was indicative of a problem within the combustion chamber/turbine area-cooling holes unzipping, vaporizer cracking, hot streaking – 20C up would result to base. A dynamically increasing trend would require a shut-down judgment. A minor reducing trend would result from compressor deterioration (nicks, scratches, erosion etc). Cruise EGT, incidentally would be about 650c

 

OIL CONSUMPTION

 

At each station, oil uplift and flight time were assessed to produce a consumption rate; once again recorded and plotted so the trends were immediately apparent.

 

BORESCOPE

 

The medical people call it an endoscope. It’s the same minaturised viewing technology. Engines are designed with inspection points in key areas, blanked-off in service, but readily available for either routine internal inspections or ‘alert level’ extras when SOAP, MCD or EGT require.