?unique=aa31f43)
51158-1 – Air Tractor OEM Stewart Warner Oil Cooler Assembly
Available now on the Airparts.aero Marketplace.
The Air Tractor 51158-1 is a high-efficiency, flight-critical oil cooler assembly manufactured by Stewart Warner (now Meggitt/Dunlop) specifically for the demanding operational envelope of aerial application and firefighting aircraft. Designed as an Original Equipment Manufacturer (OEM) component for the Air Tractor series, this oil cooler is responsible for maintaining the engine's lubrication system within a strict thermal range. In the high-torque, low-airspeed, and high-ambient-temperature environments typical of crop dusting and fire suppression, the 51158-1 is the primary defense against thermal degradation of the oil, which could otherwise lead to premature engine component wear or catastrophic failure.
Constructed using a furnace-brazed aluminum plate-fin architecture, the 51158-1 offers an exceptional heat-transfer-to-weight ratio. The internal oil passages and external air fins are precision-engineered to maximize the Reynolds number of the fluid flow, ensuring turbulent flow for optimal heat dissipation while maintaining a low pressure drop. This assembly is designed to withstand the significant pressure surges encountered during cold-weather starts and the constant vibration cycles inherent in high-performance turboprop and radial engine operations.
Key Features
Furnace-Brazed Aluminum Construction: Utilizes a high-strength aluminum alloy that is vacuum-brazed to create a monolithic structure, providing superior burst strength and leak resistance.
High-Density Plate-Fin Design: Engineered with a specific fin-per-inch (FPI) count to balance maximum surface area for cooling with the need for easy cleaning in environments prone to chemical or debris buildup.
Vibration-Damped Mounting Interface: Specifically designed for the Air Tractor’s cowling geometry, ensuring that structural loads are distributed away from the sensitive cooling core to prevent fatigue cracking.
Internal Turbulators: Features internal mechanical agitators within the oil passages to break up the boundary layer of the oil, significantly increasing cooling efficiency.
Corrosion-Resistant Coating: Finished with a protective treatment to resist the corrosive effects of agricultural chemicals (fertilizers/pesticides) and fire retardants.
Technical Specifications
Part Number: 51158-1
Manufacturer: Stewart Warner (Meggitt/Dunlop)
OEM Application: Air Tractor
Component Type: Oil-to-Air Heat Exchanger
Material: 6061 or 3003 series Aluminum Alloy
Operating Pressure: Rated for standard engine oil gallery pressures (Approx. 100–150 PSI proof)
Test Pressure: Factory tested to ensure integrity under cold-start surge conditions.
Mounting: Integral side-mount brackets for Air Tractor cowling frames.
Fluid Compatibility: MIL-L-23699 (Turbine Oil) or AD-grade Piston Engine Oil.
Aircraft Compatibility
Air Tractor AT-402 Series: (A and B variants).
Air Tractor AT-502 Series: Including AT-502B and AT-504.
Air Tractor AT-602: (Check specific serial number for dash-number variance).
Air Tractor AT-802 / 802A: Primary cooling for PT6A-67 series engines.
Note: Always consult the Air Tractor Illustrated Parts Catalog (IPC) to verify the exact oil cooler part number for your specific engine and airframe configuration.
Applications
The 51158-1 is utilized for Propulsion System Thermal Management. Its primary role is to reject heat from the engine oil—which carries thermal energy away from the bearings, gearbox, and turbine sections—into the ambient slipstream. In the Air Tractor, this cooler is often positioned in high-velocity ducting to ensure consistent cooling during high-power climbs and heavy-load departures.
Why Choose 51158-1?
Choosing a genuine Stewart Warner OEM 51158-1 cooler is vital for maintaining the Time Between Overhaul (TBO) of the engine. Non-OEM or "look-alike" coolers often lack the internal turbulator design and high-grade brazing of the 51158-1, leading to higher oil temperatures that thin the lubricant and compromise the hydrodynamic film on critical engine bearings. Furthermore, the 51158-1 is built to handle the chemical exposure unique to the aerial application industry, where inferior alloys would succumb to rapid intergranular corrosion.
The 51158-1 operates as a cross-flow heat exchanger, where the thermodynamics are governed by the Log Mean Temperature Difference (LMTD) between the hot engine oil and the ambient ram air. The furnace-brazing process is critical here; it creates a metallurgical bond at the molecular level between the fins and the plates, ensuring a high thermal conductivity path. The internal geometry is designed to manage the fluid's viscosity changes; as the oil cools and its viscosity increases, the internal turbulators ensure that the oil does not "stagnate" or form a laminar insulating layer against the walls. This design ensures that the heat transfer coefficient remains high even during lower airspeed operations (e.g., during the "pull-up" at the end of a spray run) when mass airflow across the core is reduced.
Note: This is a flight-critical engine component. Prior to installation, inspect the core for any fin damage or shipping debris. Ensure the oil system is flushed if the previous cooler failed due to metal contamination. Use only approved O-rings and torque fittings to the values specified in the Air Tractor Maintenance Manual. Installation must be performed by a certified A&P mechanic.
