Fire detection and protection

Five procedures. Procedure 0 is not negotiable.

Procedure 0 — Safe the squib (R1)

Before any work on an extinguishing circuit.

1. De-energize the aircraft electrical system as the manual requires.
2. Disconnect the squib connector at the bottle.
3. Cap or short the connector per the manual, so no induced current can reach it.
4. Never apply an ohmmeter or any test set to a squib circuit unless the manual specifies a method and an instrument for it (GEN-K Concepts §4).

Check — say out loud that the squib is disconnected, and point at the capped connector.

Failure — a discharged bottle. At best expensive and messy; at worst it happens in your face (AF-M Concepts §1).

Procedure 1 — Inspect a fire detection system (S9, S12, S3, S4)

1. Trace the loop routing — security, support at the specified intervals, clearance from structure and from moving parts.
2. Inspect the sensing element for chafe, kinks, crushing and corrosion. Chafe is the commonest defect and it produces false warnings (AF-M Concepts §2).
3. Inspect connectors and end fittings.
4. Check thermal switches or thermocouples where fitted, and their security.
5. Inspect smoke and CO detectors — cleanliness, sensor life, security.

Check — a false fire warning is a loop fault until proven otherwise. Inspect the loop before condemning the control unit.

Procedure 2 — Operational check of a detection system (S10, S13)

1. Do the press-to-test first, and understand what it proved: the lamp and the warning circuit (AF-M Concepts §4).
2. Then perform the system operational check by the manual's method, which exercises the detection function itself.
3. Confirm the warning appears, and that it clears when the test condition is removed.
4. Check any fault or short monitoring function.
5. Record which check you performed — press-to-test and an operational check are different entries.

Check — write down which test you did. "Fire system checked" is ambiguous, and ambiguity here means a dead loop signed off as serviceable.

Procedure 3 — Inspect an extinguisher bottle (S8, S2, S11)

1. Procedure 0.
2. Check the hydrostatic test date against the manufacturer's interval. Out of date is unairworthy regardless of condition.
3. Check the discharge cartridge date — cartridges have their own life limit.
4. Read the pressure gauge and the ambient temperature, and compare against the pressure-temperature chart. Neither number means anything alone (AF-M Concepts §3).
5. Inspect the bottle: corrosion, dents, mounting security, discharge line condition.
6. Check blowout plugs — a missing thermal plug means the bottle discharged from overheat, a missing discharge indicator means it was fired.

Check — read the chart, not the gauge. A bottle low on a cold morning may be correctly charged.

Procedure 4 — Inspect a discharge circuit (S8, with R1)

1. Procedure 0 — squib disconnected and capped.
2. Inspect wiring to the bottle for security, chafe and connector condition.
3. Check the discharge switch and its guard.
4. Check continuity of the circuit only by the manual's specified method and instrument — never with an ordinary ohmmeter.
5. Reconnect the squib last, after all other work is complete, and confirm the guard is in place.

Check — the squib goes back on last, and nothing is done to the circuit afterwards. If something else needs doing, disconnect it again.

The checks, collected

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