If the LiftFan clutch rubs or "chatters" during normal flight, the "clutch sludge" is likely not just ordinary age-related grime; it is a symptom of a chronic functional defect. The system generates its own contaminants: wear from the carbon-carbon discs, carbon dust, and synthetic oil degraded by heat, resulting in hardened deposits within the cooling circuit.
And this is nothing new. A Navy SBIR document from December 2011 had already called for innovative clutch plate concepts for the F-35B, noting that while the carbon-carbon plates met the target of approximately 1,500 engagements, this was at least half the desired service life, and issues regarding friction coefficients and wear persisted. It is also worth recalling that Pratt & Whitney revealed in 2011 that the LiftFan clutch discs were making unexpected contact during normal (non-vertical) flight, generating more heat than anticipated.
They aren't addressing the root cause; they are addressing the byproduct of the root cause.
If the sludge has hardened, it is no longer a simple degreasing issue. Emulsifying the oil is no longer sufficient, because the oil likely no longer exists in a free state; it has transformed into a polymerized, oxidized, and coked matrix laden with carbon dust.
In this scenario, the residue resembles a carbonaceous varnish—a type of composite deposit—more than it does "dirty oil": thermally degraded synthetic oil, carbon particles, composite dust, and potentially metallic residues or oxidized additives. This is far more difficult to remove.
Hot water with a surfactant may work only if the deposit remains soft or friable. However, if the deposit has hardened, it must first be fractured, softened, or detached. The actual process therefore becomes:
- controlled chemical or thermal softening;
- gentle yet targeted mechanical action;
- suspending the particles;
- rinsing;
- recovery;
- complete drying.
There is a risk that the hardened deposit might adhere to duct walls and screens. Simply increasing water pressure could tear off chunks, push them further into the system, clog an inaccessible area, or damage sensitive components. Consequently, simple pressure washing is likely hazardous.
More suitable fluids would be semi-aqueous cleaners or specialized mild solvents capable of swelling or softening baked-on oil residues without attacking seals, composites, paints, surface treatments, or sensors. In the aerospace industry, the aim is typically to strike a balance between overly aggressive organic solvents and ineffective water-based solutions—using options such as microemulsions, mild oxygenated solvents, moderate alkaline detergents, or material-qualified decoking agents.
However, there is a complication: if the matrix is truly carbonized, even a solvent will not "dissolve" the carbon. While it may dissolve or soften the organic binder, the carbon particles themselves must be mechanically detached and then vacuumed or filtered out.
Therefore, I would envision a solution along these lines:localized pre-soaking with an aerospace-compatible softening fluid (potentially heated);
- hydraulic pulses or targeted low-pressure jets, rather than a high-force stream;
- brushing, swabbing, or use of a specialized tool, provided the geometry allows access;
- simultaneous vacuuming to prevent fragments from migrating through the system;
- rinsing with deionized water and a dispersing agent;
- drying with dry air or nitrogen;
- endoscopic inspection before returning the unit to service.
This explains why the Navy requires a system specifically tailored to the F-35B clutch assembly. The issue is not just “which liquid to use?”; it is the combination of chemistry, geometry, recovery, and the avoidance of contamination.
If the deposit has hardened, it can break apart. If the process pushes fragments inward rather than extracting them, a cooling issue can turn into a mechanical one—leading to abrasive particles, scratches, contamination of friction surfaces, local imbalance, increased heat buildup, or even accelerated clutch wear.
Therefore, an acceptable cleaning method must strictly follow a reverse-flow approach: suction before injection. The cleaning adapter needs to create a vacuum or controlled suction right at the deposit site, then inject the fluid in a way that dislodges the residue toward the outlet rather than toward the clutch discs. Otherwise, it poses a risk.
Annual reviews should be a part of any business or military program. perhaps annual reviews would have found some previously missed aspects on the Rafale.
