Two Indication Problems on the Same Airplane
A twin with fuel-injected engines came out of a shop after replacement of the left engine's #3 cylinder. On the first flight the pilot reports that #3 exhaust gas temperature reads about 80 °F below its stable historical value at the same power setting and fuel flow, while the other five cylinders are unchanged. The shop had already replaced the #3 probe with a new one of the same part number, with no change. On the same flight the electronic fuel totalizer showed 18 gallons remaining in the left main after 3.2 hours, while the dipstick showed 26 gallons. Both engine fuel flow indications agree within 2% at cruise. The totalizer's fuel flow transducer was replaced 40 hours earlier and the unit has not been recalibrated since.
1. Lay out a step-by-step isolation for the #3 EGT discrepancy.
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Isolate by substitution at a single junction, then work outward.
- Swap leads at the indicator or engine monitor connector between #3 and a known-good channel, such as #2. If the low reading moves to the #2 channel, the fault is in the probe circuit: probe, extension wire, junctions, or connectors. If the low reading stays on the #3 channel, the indicator input is at fault.
- Since a new probe of the same part number changed nothing, the fault is almost certainly in the extension wiring or the junctions. The most common cause after cylinder work is the wrong extension wire alloy or a reversed polarity connection - a type K extension on a type J probe, or a chromel/alumel pair swapped at a terminal, produces a fixed bias of exactly this magnitude.
- Check immersion. Verify the probe is fully seated in the cylinder well and not backed out or fitted with a stray washer; a probe recessed from the gas path reads low and the error grows with power.
- Confirm configuration. Verify the monitor is set for the thermocouple type actually installed.
Finish with a millivolt check at a known reference temperature, so the fix is proven rather than assumed.
2. Why does an 80 °F low EGT matter when 80 °F is inside the normal cylinder-to-cylinder spread? Tie the answer to how the pilot uses EGT.
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Because EGT is not just a monitoring indication on this airplane - it is a control input.
- Leaning. Pilots lean to a target EGT or a fixed number of degrees rich or lean of peak. A channel biased 80 °F low makes the pilot lean that cylinder 80 °F further than intended, and the cylinder he is leaning hardest is the one whose reading lies to him.
- Fault detection. A plugged or partially restricted injector shows up as a falling EGT on one cylinder. An 80 °F bias sits in the same range as the early stage of that failure, masking it.
- Diagnostics. The same bias will make a real ignition or compression problem on that cylinder look smaller than it is.
The correct response is to fix the indication, not to mentally add 80 °F to every reading or to change the leaning target. A biased indication that the crew compensates for by memory survives the flight and then bites the next pilot, who has no memory of it.
3. Evaluate the fuel totalizer discrepancy. Which indication do you trust, and what do you check first?
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Trust the dipstick first, with conditions: the airplane level, the chart correct for this tank, and the sump drained the same way each time. It is a direct physical measurement. The totalizer is an inference - a flow signal integrated over time and subtracted from a starting quantity.
Work through the error sources in order of likelihood and cost:
- Starting quantity entry. The most common totalizer error by far is a wrong fuel-on-board figure keyed in at startup. Verify what was entered against the actual upline.
- K-factor. The transducer was replaced 40 hours ago and the unit has not been recalibrated. A K-factor error scales with fuel burned - 8 gallons over 3.2 hours is about a 2.5 gph bias, entirely plausible.
- The agreement at cruise proves little. Both flow indications matched within 2% at one power setting; that does not validate the totalizer across the range or at low flow, because the two systems are derived differently.
- Rule out reality. Check for leaks, crossfeed or transfer valve positions, and stains, before concluding it is a box problem.
Recalibrate the K-factor per the manufacturer's procedure, then verify the totalizer against the dipstick over two or three full tanks before using it for fuel planning.