21SN04-300B — Fuel Filter Differential Pressure Switch Repair & Overhaul

Component Overview

A fuel filter differential pressure switch monitors the pressure drop across an engine fuel filter and signals impending blockage as the element loads with contaminants, alerting the crew or annunciator before bypass or fuel-starvation conditions occur. As a differential pressure switch, it senses upstream and downstream pressure through a diaphragm or piston that trips an electrical contact at a calibrated differential. Operating in a fuel-wetted, vibrating environment, it degrades through diaphragm fatigue and rupture, spring relaxation that shifts the trip point, contact pitting and welding, seal hardening with external leakage, and contamination or corrosion that causes sticking. Failures appear as nuisance trips, late warnings, or no indication. Overhaul replaces the diaphragm, seals, and contacts, recalibrates the actuation and reset differential, and verifies setpoint, leakage, and electrical continuity.

Aircraft Application

Part number 21SN04-300B (Fuel Filter Differential Pressure Switch) is installed on the Pratt & Whitney PW2000/4000. QTC supports operators and MROs servicing this platform with certified repair and overhaul.

Our Repair Capability

QTC repairs and overhauls 21SN04-300B to the original manufacturer's specifications under our FAA and EASA Part 145 approvals. Our team — including former OEM sensors & switches engineers — performs teardown, inspection, cleaning, component replacement, functional testing, and certification, returning a serviceable unit with the appropriate airworthiness release. We also develop repairs for rejected and beyond-economic-repair (BER) hardware.

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Frequently Asked Questions