76575-4 - Textron Aviation OEM Aerospace Hobbs Hour Meter Pressure Switch

76575-4 – Textron Aviation OEM Aerospace Hobbs Hour Meter Pressure Switch

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Overview

The Textron Aviation 76575-4 is a high-reliability, OEM pressure switch that serves as the automatic trigger for the aircraft's flight time recorder, commonly known as the Hobbs meter. This is not a generic industrial switch; it is a flight-certified component specifically engineered for the demanding environment of an aircraft engine compartment. Its sole function is to sense the rise in engine oil pressure upon startup and automatically close an electrical circuit, starting the Hobbs meter. Conversely, upon engine shutdown, it senses the loss of oil pressure and opens the circuit, stopping the meter. This provides an accurate and reliable method for logging flight time, which is a critical requirement for tracking maintenance intervals, component life limits, and adhering to FAA regulations.

Constructed with a robust brass or steel body and an environmentally sealed electrical housing, the 76575-4 is designed to withstand the constant vibration, temperature extremes, and fluid exposure of its operational environment. The internal diaphragm and contact points are precision-calibrated to actuate at a specific pressure setpoint, ensuring that the Hobbs meter records time only when the engine is running and developing normal oil pressure.

Key Features

• OEM Certification: A genuine Textron Aviation part, ensuring the correct actuation pressure, thread fitment, and electrical specifications for their aircraft.

• Precise Actuation Point: Factory-calibrated to close the electrical circuit only when engine oil pressure exceeds a specific, predetermined PSI, preventing false readings.

• High-Reliability Design: Features a durable, oil-resistant internal diaphragm and high-quality electrical contacts designed for tens of thousands of cycles.

• Environmentally Sealed: The electrical portion of the switch is sealed to prevent contamination from engine oil, moisture, or cleaning solvents, ensuring long-term reliability.

• Vibration Resistant: Engineered and tested to perform reliably under the constant high-frequency vibration present on a reciprocating aircraft engine.

Technical Specifications

• Part Number: 76575-4

• Component Type: Electromechanical Oil Pressure Switch

• OEM Manufacturer: Textron Aviation (Cessna)

• Application: Hobbs Meter (Flight Time Recorder) Activation

• Switch Type: Normally Open (NO), Single Pole, Single Throw (SPST)

• Actuation Pressure: Calibrated to actuate at a specific setpoint, typically in the 20-35 PSI range. The exact pressure setpoint is proprietary to the OEM specification.

• Thread Size: 1/8"-27 NPT (National Pipe Taper)

• Electrical Connector: #8-32 UNC Stud Terminal

• Voltage Rating: Compatible with 12V and 24V DC aircraft electrical systems.

Aircraft Compatibility

This pressure switch is widely used across the Textron Aviation single-engine piston aircraft line, including but not limited to:

• Cessna: 172 Skyhawk, 182 Skylane, 206 Stationair, 210 Centurion, and their turbocharged variants.

Note: This compatibility list is for general reference. The aircraft's specific Illustrated Parts Catalog (IPC) is the sole and final authority for determining part applicability for a given serial number.

Applications

• Flight Time Recording: Provides the automatic switching logic for the Hobbs meter based on engine operation (presence of oil pressure).

• Maintenance Tracking: Ensures accurate timekeeping, which is the basis for all scheduled inspections, component overhauls, and AD compliance.

Why Choose 76575-4?

Accurate flight time logging is a legal and safety requirement. Using a non-certified, automotive-grade pressure switch is a significant risk. Such components are not tested for aviation environments, and their actuation pressures are often unknown or inconsistent. A faulty switch can fail in two critical ways: it can fail to close, resulting in un-logged flight hours and potentially over-flying critical maintenance inspections; or it can fail to open (or stick closed), causing the Hobbs meter to run continuously, leading to premature and expensive maintenance actions. The genuine Textron Aviation 76575-4 is a certified, tested, and reliable component designed to provide the accuracy required for airworthy operations.

The engineering principle governing the 76575-4 switch is that of a precision electromechanical transducer. Inside the switch, a calibrated spring exerts a predetermined force on a diaphragm of a specific effective area. The switch remains in its normally open (NO) state as long as the force from the spring is greater than the opposing force generated by the oil pressure (Force = Pressure × Area). Once the engine's oil pressure rises above the switch's setpoint, the force on the diaphragm overcomes the spring preload. This causes the diaphragm to move a precise distance, actuating a "snap-action" contact mechanism. This snap-action is critical, as it ensures a rapid and positive closure of the electrical contacts, preventing the slow-moving "teasing" that would cause arcing, pitting, and premature failure of the contact points. This design ensures a clean electrical signal and a long service life in a high-cycle application.

Note: The 76575-4 is a critical component for aircraft maintenance and airworthiness tracking. Installation must be performed by a certified A&P mechanic. Use an appropriate, aviation-grade thread sealant on the NPT threads to ensure a leak-free seal. Do not overtighten. After installation, perform an engine run-up to verify the proper operation of the Hobbs meter and inspect for oil leaks.

Documentation

No public-facing OEM or regulatory specification sheet was located for this proprietary part number. All authoritative installation and application data are contained within the official Textron Aviation Maintenance Manual and Illustrated Parts Catalog (IPC) for the applicable aircraft models.