Cross-Engine ITT Spread in Cruise
A twin-engine turboprop is reported by the crew: at the same torque setting, the left engine shows ITT 22 °C higher and N1 1.2 % lower than the right, with fuel flow about 40 kg/h higher. The discrepancy appeared gradually over the last 60 h and is most noticeable in cruise at 20,000 ft. Both engines start normally, accelerate smoothly, and have no chip lights. The left engine's compressor wash was last done 300 h ago; the right engine 40 h ago. Bleed air is used for pressurisation and engine anti-ice. The aircraft is at a maintenance base with a borescope and a wash rig available. The operator wants to continue the next two sectors because 'the engine is still within limits'. The maintenance manual allows a maximum ITT difference of 15 °C between engines at matched torque, and the left engine is currently 22 °C higher.
1. Analyse the likely cause of the ITT, N1 and fuel-flow discrepancy. Explain which causes are more and less likely.
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The combination of higher ITT, lower N1 and higher fuel flow at matched torque means the left engine is less efficient: it needs more fuel and runs hotter to produce the same power. Gradual onset over 60 h favours compressor fouling or a bleed leak rather than a sudden sensor failure. The left engine's wash is 300 h old versus 40 h on the right, so fouling is the first suspect. A leaking bleed valve or damaged seal can also reduce compressor delivery and raise ITT.
An EGT probe error would normally affect indication only, not fuel flow, while a fuel-nozzle problem would usually show a larger spread at start. The 22 °C spread exceeds the manual's 15 °C limit, so this is not merely a monitoring item.
2. Decide whether to dispatch for the next two sectors. Describe the troubleshooting and release criteria you would apply.
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Do not dispatch for two sectors on the basis that the engine is 'within limits'. A 22 °C cross-engine spread exceeds the 15 °C allowed, so the aircraft is outside the maintenance manual's dispatch criteria. At the base, perform a compressor wash on the left engine, inspect bleed valves and ducts, and repeat a matched-torque ground run.
If the spread falls within 15 °C and fuel flow aligns, release with a trend check. If it persists, borescope the compressor and turbine and inspect the EGT harness and probe. Record the wash and the before-and-after figures. If the spread remains, the engine requires further maintenance before flight.