False FULL: why lav sensors keep failing (and usually aren't)
A FULL message on a tank you know is empty. A CAS message that clears with a power cycle and comes back on the next leg. A water quantity that reads zero with water coming out of the tap. These are some of the most common repeat write-ups in business aviation, and the usual response, replacing the sensor, is also one of the least successful.
Most removed waste-level sensors test No Fault Found. The sensor was reading exactly what was on it.
Why modern systems are sensitive
Many current waste and water systems use capacitance-based sensing with tight tolerance thresholds. They're accurate, but far less forgiving of servicing variation than older float-switch designs. A thin film of chemical or residue near the probe is enough to look like a full tank. The difference between "nearly empty" and "full" is small, tank shapes can let waste cling near the probe, and a warm fuselage dries residue in place.
What actually causes false FULL
- A thin residual film from lav chemicals.
- Incomplete rinsing during vacuum servicing (many carts stop early).
- Thick, gel-type or "heavy-duty" dark-dye chemicals, especially winter blends.
- Leaving waste in the tank overnight or longer.
- Cold soak causing condensation on the probe.
What fixes it
| Area | Practice |
|---|---|
| Servicing | Dump and rinse, never dump only. If there's no rinse, add fresh water and dump again. Make sure the tank is fully evacuated. |
| Chemicals | Low-viscosity, low-dye, non-gel products, in the right amount. |
| Discipline | Never store the aircraft with waste in the tank; service after the last leg if the lav was used. |
| Correction | Several short warm-water rinses work better than chemicals or one long soak. |
Potable water: the same story
Erratic or zero quantity readings with normal flow usually come from air trapped during servicing, hard water, or biofilm in partly filled tanks. After servicing, run the galley and lav faucets together until the flow is smooth. Store tanks fully drained or fully serviced, never partly filled, and keep to the sanitization interval. Normal flow with a wrong indication means contamination, not a wiring fault.
When a new sensor is the right call
Replace a component only after cleaning or sanitizing, confirming an air purge on the water system, and an AMM functional test that confirms the failure, or when there's physical damage or corrosion.
Track where the aircraft was serviced. Repeat write-ups tend to follow particular FBOs and service carts. Once you know where the contamination comes from, you can fix it at the source instead of at your parts budget.
General guidance for orientation only. Your MEL, approved manuals and the regulations govern; confirm against them before acting.