3070800-26 - Honeywell Aerospace Turbine Engine Main Fuel Control Assembly

3070800-26 – Honeywell Aerospace Main Fuel Control (MFC) Assembly

Available now through the AirParts Aero marketplace.

Overview Section

The Honeywell 3070800-26 is a mission-critical Main Fuel Control (MFC) unit, often referred to as a Fuel Control Unit (FCU).1 It is a sophisticated hydro-mechanical computing device designed for the Honeywell TFE731 turbofan engine series. This specific dash number (-26) represents an evolved configuration of the fuel control architecture, often featuring refined internal porting and updated bellows to improve throttle linearity and cold-start reliability.

The 3070800-26 acts as the "analog computer" for the engine. It monitors various parameters—including Pilot Command (Power Lever Angle), Compressor Discharge Pressure ($P_3$), and Engine Intake Temperature ($T_2$)—to meter precisely the fuel flow required for combustion. By managing these variables, the MFC prevents catastrophic engine conditions such as compressor surges, over-speeds, and turbine over-temperature events.

Key Features Section

• Precision Hydro-Mechanical Governing: Uses a series of flyweight governors and pneumatic bellows to maintain stable engine speeds ($N_2$) regardless of changes in altitude or ambient temperature.

• Optimized Acceleration Schedule: Controls the rate of fuel increase during power transients to ensure the engine accelerates as quickly as possible without exceeding the surge limit of the compressor.

• Altitude Compensation: Automatically adjusts the fuel-to-air ratio as the aircraft climbs into thinner air, maintaining optimal combustion efficiency and preventing "rich blowout."

• Environmental Resilience: Housed in a rugged, cast-aluminum body designed to operate in the high-vibration and high-heat environment of the engine accessory gearbox (AGB).

• Internal Fail-Safes: Includes a mechanical "ultimate overspeed" shut-off to protect the engine core in the event of a primary governor failure.

• Service Bulletin (SB) Compliance: As a later dash-number in the 3070800 family, the -26 often incorporates hardware improvements that address the "stiction" or hysteresis issues found in earlier -21 or -22 models.

Technical Specifications Section

Aircraft Compatibility Section

The 3070800-26 is utilized on several iconic business jets that rely on the TFE731's geared turbofan performance:

• Dassault Falcon 50 / 900: Critical for the three-engine reliability and synchronized thrust management of these long-range jets.

• Hawker 800XP / 850XP: Often found in the TFE731-5BR configurations used for transcontinental missions.

• Learjet 31A / 35A / 45: Provides the rapid throttle response required for the Learjet’s high-performance flight envelope.

• Cessna Citation VII: Core fuel metering component for the TFE731-4R engines.

Technical Applications & Safety

The MFC is a "closed-loop" system. It senses the actual work the engine is doing (via $P_3$ pressure) and compares it to what the pilot is asking for (via the Power Lever). If the $P_3$ signal is lost due to a leaking sensing line, the MFC will default to a "minimum flow" or "flat" schedule, which may prevent the engine from reaching takeoff power.

Safety Warning: FCU Rigging is a safety-of-flight procedure. If the 3070800-26 is replaced, the technician must perform a "Part Power Trim" check to ensure the engine's ITT (Interstage Turbine Temperature) and RPM stay within limits. Failure to correctly rig the FCU can result in an engine that "overshoots" its temperature limits during a rapid throttle advance, leading to permanent turbine blade damage.

Technical Engineering Insight

The engineering value of the 3070800-26 lies in its Torsional Drive Shaft and Dampening. The unit is driven by a splined shaft from the accessory gearbox. To prevent the high-frequency vibration of the engine from "chattering" the internal metering valves, the -26 utilizes a fluid-dampened drive interface. This ensures that the fuel flow remains smooth and steady, preventing the "hunting" or RPM oscillations that can occur with lower-quality or worn-out fuel controls.

By choosing a certified 3070800-26 through AirParts Aero, you ensure the unit has been bench-tested on a specialized fuel stand to verify its Acceleration Schedule. This test ensures that when you "firewalled" the throttles, the engine will respond linearly and safely every time.

Installation & Maintenance Addendum

1. $P_3$ Filter Replacement: Always replace the $P_3$ (Compressor Discharge) air filter whenever the MFC is replaced. A clogged filter will cause the engine to be "lazy" during acceleration.3

   
   

   
   

2. Seal Integrity: Use new O-rings for the fuel inlet and outlet ports. Even a minor weep can lead to high-pressure fuel spraying on hot engine components.

3. Preservation: If the 3070800-26 is removed for storage, it must be filled with preserving oil (MIL-PRF-6081) to prevent the internal steel bellows from corroding or losing their calibration.