Compressor fouling, a wash, and a surge on the post-wash run
A Bell 206B-3 powered by an Allison 250-C20J has been working for 150 hours from a coastal airfield and nearby sandy agricultural strips. At maximum continuous torque the pilot has watched Ng climb from 93.8% to 96.2% and TOT from 710 °C to 758 °C; fuel flow at the same torque is about 8 lb/h higher than the earlier baseline. Max continuous TOT is 810 °C. Maintenance performs the approved compressor wash, using the specified cleaning fluid and rinse, with the engine motored on the starter and then dried. On the first post-wash ground run the engine starts normally, but as it accelerates through 60% Ng there is a low-frequency rumble, Ng fluctuates ±2%, and TOT rises 40 °C in about three seconds. The engineer aborts the run. The operator wants the aircraft back on the line for the afternoon.
1. Explain the pre-wash trend. What does the rise in Ng and TOT at constant torque tell you about compressor condition, and why does the fuel control drive the engine this way?
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At a fixed torque the engine is producing the same shaft power, so the fuel control schedules fuel to keep Ng and torque at the demanded value. Fouling - salt, dust and oil baked onto the compressor blades and annulus - increases surface roughness and reduces the compressor's mass flow and isentropic efficiency. To pass the same air mass flow at the required pressure ratio, the compressor must run faster, so the control trims Ng upward. The same power then requires a higher fuel flow and a higher turbine entry temperature because less of the compressor work is recovered as useful cycle work; the same air mass is being heated with more fuel. The 2.4 percentage-point Ng rise, 48 °C TOT rise and 8 lb/h fuel-flow increase are classic fouling signatures. They do not by themselves prove damage, but they show a real loss of compressor performance and reduced surge margin, which is why the wash was appropriate. If the trend had continued unchecked, the engine would eventually have reached a TOT or Ng limit before producing rated torque.
2. The post-wash run shows a rumble and Ng oscillation at 60% Ng. What is the most likely phenomenon, and which components or wash-procedure errors would you investigate first? Justify each.
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The rumble, ±2% Ng oscillation and rapid TOT rise at 60% Ng are characteristic of compressor surge or rotating stall, not normal acceleration. Surge is an unstable breakdown of flow in the compressor: the compressor stalls, delivery pressure collapses, flow reverses momentarily, then re-establishes. On the 250 series a compressor bleed valve is open at low Ng to dump excess delivery air and keep the compressor away from the surge line; it is scheduled closed as Ng increases. If the valve is stuck closed, its sense line is blocked or water-filled after the wash, or the valve was refitted incorrectly, surge is likely at low speed. Residual cleaning fluid or rinse water pooling in the compressor or plenum can also disturb the flow. A wash nozzle directed wrongly, or motoring at the wrong speed, can leave deposits unevenly distributed. First checks should be bleed-valve free movement and rigging, sense-line condition, the compressor inlet for foreign objects, then a borescope of the compressor and a repeat wash and dry strictly to the manual.
3. What maintenance action and release decision do you recommend before the aircraft flies again? Include what evidence you would need.
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Release only after the cause is identified and corrected. The aircraft should not fly merely because it started normally afterwards. The engineer should inspect and functionally check the bleed valve and its sense line, borescope the compressor for blade damage or tip rub, confirm the wash was done to the manual with the correct fluid, nozzle and motoring speed, then carry out a full ground run with calibrated Ng and TOT indicators. Acceptable evidence: no surge during two accelerations through the previously affected range, Ng and TOT consistent with the expected post-wash improvement, bleed valve closing at the specified Ng, and a maintenance release signed by an appropriately rated engineer. If surge recurs, or if the borescope shows blade damage or increased tip clearance, the compressor or engine should be removed for repair. A trend check over the next few flights is also sensible to confirm the wash restored performance and that the pre-wash deterioration was fouling rather than erosion or tip rub.