PT6A-67D EGT Indication Split
An operator of a PT6A-67D-powered aircraft reports that the EGT indication on the #1 engine is 45°C higher than #2 at the same torque and altitude. Both engines start normally, produce normal torque and have no other indications. The engineer swaps the EGT indicator between engines and the high reading stays with the #1 engine. A resistance check at the firewall connector gives 3.9 ohms for the #1 EGT harness and 2.6 ohms for #2; the AMM states the thermocouple loop resistance must be within a specified band and that the harness and probe are matched. The #1 probe was replaced 20 hours ago after a known-good probe was fitted. The connector pins look clean. The aircraft is due for a 100-hour inspection in 10 hours; the operator wants to continue because 'EGT is only an indication'.
1. Interpret the fault and decide whether continued operation is acceptable. Explain the EGT system architecture and error sources.
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The indicator swap shows the high reading stays with the #1 engine, so the indicator itself is not the cause; the fault is in the #1 probe, harness or connectors. The resistance check confirms the #1 loop is out of band (3.9 ohms versus 2.6 ohms on #2), so the system is not calibrated. EGT is a thermocouple loop: probe, matched extension leads, connectors and indicator. A poor connection, wrong probe type, short to ground or to another conductor, or a mismatched harness changes the millivolt signal and therefore the indication. Note that high loop resistance often biases an indication low, not high; a high reading could come from a wrong probe, a short or a cold-junction error. That is exactly why you cannot assume the engine is hot or that the gauge is conservative. EGT is a limiting parameter, not only an indication: an inaccurate high reading can cause the crew to reduce power unnecessarily, while an inaccurate low reading can hide an over-temperature. Continued operation is not acceptable until the system is within the AMM resistance band and calibrated.
2. Detail the troubleshooting sequence, including resistance checks, probe type, cold junction and indicator calibration.
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First confirm the probe part number matches the engine model and the harness; the probe fitted 20 hours ago may be the wrong type or a rejected part. Disconnect the harness at the probe and at the indicator. Measure the resistance of each lead and the total loop, compare with the AMM band, and check insulation to ground. Wiggle the connectors and check for chafed or heat-damaged leads. Check thermocouple polarity and the cold-junction compensation at the indicator. Use a millivolt source to check indicator calibration at known points; if it is out, adjust or replace the indicator. Fit a known-good probe and compare readings. If the harness resistance is out of band, replace the harness. Do not use a generic multimeter to calibrate a thermocouple without the correct procedure. After repair, run the engine and compare #1 and #2 at the same torque and altitude; a small difference can be normal, but the loop must be within approved limits.
3. What release and record requirements apply, and what would you tell the operator?
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The aircraft cannot be released with an EGT system outside the AMM limits; there is no MEL relief for an inaccurate primary indication. Record the resistance readings, probe and harness part numbers, calibration results and the corrective action. If the probe was wrong, replace it and return the old part for investigation; if the harness is out of band, replace it. After repair, make a maintenance release referencing the approved data and the calibration check. Tell the operator that EGT is only an indication is incorrect: EGT is used to set power and protect the hot section, and a 45°C error can mask damage or cause unnecessary power loss. The 100-hour inspection is not a reason to defer; the system should be corrected before the next flight. If a probe or harness is not available, ground the aircraft.