Intermittent Engine Fire Warning in Heavy Rain
An operator's twin turboprop has continuous-element fire detection in each engine nacelle, with a red FIRE warning on the annunciator panel and a guarded bottle discharge switch. In cruise in heavy rain, the right engine FIRE warning illuminated for about six seconds and then extinguished; the crew did not discharge the bottle and landed normally. On the ground in dry conditions the warning could not be reproduced. The maintenance manual specifies a loop resistance check (2.4–3.0 kilohms at 20 °C) and an insulation test at 500 V DC. The mechanic measures 1.9 kilohms on the right loop and finds a chafed spot on the element's stainless sheath at an Adel clamp near the firewall, with green corrosion product. The operator wants the airplane released tonight for a sold-out route and suggests resetting the system and seeing whether the warning returns.
1. Interpret the indications. Why is this a genuine detection-circuit fault rather than a 'ghost' warning, and what are the safety consequences either way?
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What the numbers mean. The sensing element's resistance falls as its temperature rises, and the control unit trips when resistance drops through a threshold. A cold reading of 1.9 kilohms against a 2.4–3.0 kilohm band means the loop is already sitting below its normal cold value - the circuit is electrically closer to the trip point than it should be, and the chafed, corroded sheath gives a leakage path to structure that water can bridge.
Why the warning was intermittent. In dry air the leakage path is high resistance and the loop stays quiet. In heavy rain, moisture across the chafed clamp lowered the loop resistance past the trip point for a few seconds until airflow and heat dried it. The warning was the circuit behaving exactly as designed, given a degraded input.
Consequences either way
- A false warning in flight can lead to an unnecessary engine shutdown, an unnecessary bottle discharge, or a crew that learns to distrust a real warning.
- A degraded loop may also fail to warn when there is a real fire, because the circuit is already offset from its normal baseline.
Either failure mode is unacceptable in a fire-protection system. 'Reset it and see' is not a test - it is an experiment on a revenue flight.
2. Describe the repair and the post-repair tests you would require before approving the aircraft.
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Repair
- Rectify per the maintenance manual: replace the damaged clamp and its standoff, repair the element with the approved splice kit if the manual permits a section repair, or replace the affected loop if it does not. Remove corrosion from the structure and treat it; confirm the element cannot contact structure anywhere along its run.
- Inspect the remainder of the loop for further chafe points, correct clamping and heat damage, and inspect the left engine's loop at the same time, since both were installed and routed identically.
Post-repair tests
- Loop resistance at a recorded ambient temperature, within 2.4–3.0 kilohms, with before-and-after values written down.
- Insulation test at 500 V DC to the manual's minimum, confirming no path to structure.
- Functional response test using the manufacturer's test box, exercising the detector circuit from sensing element through control unit to the flight-deck warning, including the bell and the master warning.
- Extinguisher circuit checks as required: bottle circuit continuity and squib resistance within limits, discharge switch and arming circuit verified without firing.
- A moisture check if the manual requires it - for example a wet-down of the nacelle area - to confirm the original condition cannot be reproduced.
Only after all of these pass, with values recorded, would the system be considered airworthy.
3. The operator wants the airplane released tonight. How do you handle the dispatch decision, and could an MEL or deferral apply?
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The technical position comes first. The system has a confirmed fault with a measured out-of-tolerance value and physical damage. It is not a nuisance indication to be reset.
Deferral analysis
- Engine fire detection is normally on the operator's no-go list. Most MELs either prohibit dispatch with an inoperative engine fire detection system or, at most, allow restrictive conditions that an operator cannot meet on a normal revenue sector. The operator's own approved MEL must be consulted, not assumed.
- Even where a deferral might exist for a detection component, it would not cover a circuit that can produce a false warning in flight or mask a real one. Deferring a fire-protection item to save a schedule is exactly the kind of decision that regulators and insurers examine after an event.
- If the operator is not conducting revenue operations, the aircraft still cannot be flown unless it conforms to its approved type design and is in a condition for safe operation.
Practical handling The repair is small - a clamp, a section repair, and a test. Offer to do it tonight and test it, which is faster than the argument. If a part must be ordered, tell the operator the realistic timeline in writing and let them make a commercial decision with correct information. If they press, state clearly that you will not certify the aircraft as airworthy, and record the discrepancy and your refusal in the work order so the position is documented.