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D1-112-5023 – Mitchell Aircraft Products 3 1/8 Inch Mechanical Tachometer (98480-23) ⚙️
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Overview
The Mitchell Aircraft Products D1-112-5023 is a certified, 3 1/8 inch Mechanical Tachometer designed for installation in various single-engine piston aircraft, primarily those utilizing Lycoming or Continental engines with the standard flexible cable drive system. This instrument is a legacy component, relying on rotational input from the engine's accessory case. It provides the pilot with a precise, analog display of engine speed, measured in revolutions per minute (RPM), with a full-scale reading specifically limited to 2300 RPM. The internal designation 98480-23 refers to the specific internal gearing ratio required for compatible engine models.
The primary function of the D1-112-5023 is to provide engine performance monitoring essential for setting cruise power, conducting magneto drop checks, and maintaining safe operating limits, particularly on propeller installations requiring a lower RPM ceiling. Its Clockwise (CW) rotation is critical for mating correctly with the engine accessory drive. The $3 \text{ 1/8}$ inch size is standard for primary instrument placement, ensuring clear visibility for the pilot.
Key Features
Mechanical (Cable-Driven) Operation: Utilizes a precision gear train and calibrated magnetic drag cup assembly, providing a reliable RPM reading that is completely independent of the aircraft’s electrical system.
Standard 3 1/8 Inch Size: Designed to fit the standard large instrument panel cutout (80 mm), ensuring high visibility for primary engine monitoring.
2300 RPM Full Scale: Features a dial specifically calibrated to indicate a maximum reading of 2300 RPM, appropriate for fixed-pitch propeller installations or specific engine models with lower operational limits.
Clockwise (CW) Rotation: The internal mechanism is calibrated for a clockwise rotating drive cable, which must match the output direction of the engine accessory pad.
Accurate Gearing Ratio: Specifically calibrated to the 98480-23 drive ratio to ensure the dial accurately reflects the true crankshaft RPM as dictated by the engine accessory gear train.
Corrosion-Resistant Housing: Constructed with a durable, sealed housing to protect the sensitive internal mechanisms from moisture, dust, and vibration.
Technical Specifications
Part Number: D1-112-5023
Internal P/N: 98480-23
Component Type: Mechanical Engine Tachometer (RPM Indicator)
OEM Manufacturer: Mitchell Aircraft Products (Designation used for cross-reference purposes)
Mounting Standard: Standard 3 1/8 inch (80 mm) Instrument Panel Mount.
Input: Flexible Tachometer Drive Cable (Standard Square Drive).
Direction of Rotation: Clockwise (CW).
Display Range (Full Scale): 0 to 2300 RPM (Must be verified against engine limit).
Gearing Ratio: 98480-23 (Ratio must be verified by engine manufacturer’s IPC).
Certification Status: FAA-PMA or TSO (Technical Standard Order) approved for appropriate applications.
Aircraft Compatibility
Primary Usage: Engines equipped with a standard mechanical tachometer drive, and specifically requiring the 98480-23 drive ratio and CW rotation. This often includes older, lower-powered Continental or Lycoming engines.
Engine Models: Continental and Lycoming Piston Engines (Exact model range must be confirmed against the engine's IPC for the 98480-23 ratio).
Verification Note: Engine and Propeller compatibility are critical. The 2300 RPM red line must match the maximum operational speed limit imposed by the propeller and/or the engine's Type Certificate Data Sheet (TCDS).
Applications Section
The D1-112-5023 Tachometer is indispensable for engine management:
Engine Control: Provides the primary reference for setting power output across all flight regimes, ensuring compliance with TCDS-defined power settings.
Propeller Limit Monitoring: Specifically designed for installations where the propeller's maximum continuous speed is 2300 RPM.
Maintenance Checks: Essential for accurately performing pre-takeoff checks, such as magneto drop tests and propeller governor checks (if applicable).
Why Choose the D1-112-5023 Mechanical Tachometer?
Selecting the correct tachometer is a matter of engine preservation and safety. The Mitchell D1-112-5023 ensures certified, precise RPM sensing. Failure to use the correct 98480-23 gearing ratio will result in persistently inaccurate RPM readings, leading to illegal engine over-speeding or reduced power output. Furthermore, the mandatory CW rotation must match the engine's drive output; using a counter-clockwise unit will result in immediate non-functionality or damage to the instrument. This FAA-PMA/TSO unit guarantees that the internal components are properly calibrated and utilize the robust magnetic drag cup mechanism for reliable readings, supporting the safe operation and longevity of the engine.
The D1-112-5023's mechanical precision is critical due to the relationship between engine speed and accessory drive speed. The 98480-23 gear ratio is an internal reduction system designed to accept the rotational input from the accessory case (which often rotates at a speed proportional to the crankshaft) and scale it correctly for the analog dial. The instrument operates on the magnetic drag cup principle, where the eddy currents induced by a rotating magnet create a torque on the indicating needle. This induced torque is precisely linear with the rotational speed, allowing the instrument to accurately display engine RPM independent of battery voltage or temperature, provided the mechanical input via the cable is free of binding and the internal spring resistance is correctly calibrated.
Note: The D1-112-5023 is a primary flight instrument. Before installation, the cable drive ratio (98480-23) and the required Clockwise (CW) rotation must be verified against the engine manufacturer's specifications. Always replace the tachometer drive cable if it is stiff or damaged, and ensure proper cable lubrication and routing to prevent severe needle oscillation. The Red Line (2300 RPM) must be clearly marked or visible. Installation must be performed and certified by a licensed A&P mechanic in strict accordance with the instrument's ICA and the aircraft's Maintenance Manual.
Documentation
No public-facing OEM or regulatory documentation was located for this specific part number. Consult the Engine Manufacturer's Illustrated Parts Catalog (IPC) for the specific engine model to verify the required drive ratio (98480-23) and rotation direction.
