Chemical contamination (tolerance & permitted time frames)
| Property | In-service limit | Tolerance | Permitted time frame |
|---|
| Acid number (TAN) | max 1.0 mg KOH/g | 1.0 – 3.5 mg KOH/g | see acidity table → |
| Density (at 23 ±3 °C) | 970 – 1066 kg/m³ | not applicable | <600 FH or 750 FC |
| Water content | 0.5% recommended
max 0.8% | 0.8% – 1.0% | <600 FH or 750 FC |
| 1.0% – 1.5% | 7 calendar days |
| Viscosity (at 38 °C) | 6.0 – 12.5 cSt | 5.5 – 6.0 cSt | 10 cal. days or 150 FH |
| Chlorine content | max 200 ppm | 200 – 300 ppm | 10 cal. days or 150 FH |
| Electrical conductivity | min 0.30 µS/cm | below 0.30 µS/cm | <600 FH or 750 FC |
Rectification actions: Water 0.8–1.3% → clean fluid with hydraulic purifier (HSPA181-380J) or WSS4, inspect LP/HP filter elements for corrosion. Water 1.3–2.5% → additionally do internal-leakage check, WTB/POB operational tests, inspect HP-manifold bobbins for corrosion. Water >2.5% → additionally RAT operational check, aileron/elevator damping checks, rudder hydraulic actuation test. Chlorine ≥200 ppm → purifier/WSS4 cleaning or fluid change. Density / viscosity / conductivity out of limits → fluid change. Always: new fluid sample 1–5 flight cycles after corrective action (30-day return-to-service while waiting for results).
Acid contamination (TAN grace periods & procedure)
| Acidity level | Grace period | Procedure |
|---|
| TAN ≤ 1.0 | n/a | No action — next MPD check |
| 1.0 < TAN ≤ 1.2 | 30 days | Replace reservoir fluid 2× |
| 1.2 < TAN ≤ 1.5 | 30 days | Replace reservoir fluid 3× |
| 1.5 < TAN ≤ 1.8 | 14 days | Replace reservoir fluid 4× |
| 1.8 < TAN ≤ 2.2 | 14 days | Replace reservoir fluid 5× |
| 2.2 < TAN ≤ 2.5 | 14 days | Replace reservoir fluid 6× |
| 2.5 < TAN ≤ 3.5 | 3 flight cycles | Change hydraulic-system fluid (grace only if a single system is contaminated) |
| TAN > 3.5 | no grace | Change hydraulic-system fluid immediately |
Between reservoir replacements operate the flight controls (Green: + eng-1 reverser, MLG/NLG door mechanisms; Yellow: + eng-2 reverser, cargo doors; Blue: flight controls) or fly ≥1 flight cycle; new sample after ≥1 flight cycle within 7 days; result within 30 days. Particles > Class 9: single system → recovery per AMM 12-32-29-281-802, temporary release max 2 days / 36 FH (if flight-control tests done); multiple systems → fluid change, cleaning or reclaim + replace all HP/LP/case-drain filters; resample 1–5 FC; max 30 days release while waiting for lab results.