Aircraft Turbine Engine Fuel Control Maintenance

The field repair of the turbine engine fuel control is very limited. The only repairs permitted in the field are the replacement of the control and subsequent adjustments.

These adjustments are limited to the idle rpm and the maximum speed adjustment, commonly called trimming the engine. Both adjustments are made in the normal range of operation. During engine trimming, the fuel control is checked for idle rpm, maximum rpm, acceleration, and deceleration. The procedures used to check the fuel control vary depending on the aircraft and engine installation.

The Engine Trimming Procedure

The engine is trimmed in accordance with the procedures in the maintenance or overhaul manual for a particular engine. In general, the procedure consists of obtaining the ambient air temperature and the field barometric pressure (not sea level) immediately preceding the trimming of the engine.

Care must be taken to obtain a true temperature reading comparable to that of the air that enters the engine. Using these readings, the desired turbine discharge pressure or EPR (engine pressure ratio) reading is computed from charts published in the maintenance manual.

Stabilization and Data Comparison

The engine is operated at full throttle (or at the part power control trim stop) for a sufficient period of time to ensure that it has completely stabilized. Five minutes is the usual recommended stabilization period.

A check should be made to ensure that the compressor air-bleed valves have fully closed and that all accessory drive air bleed for which the trim curve has not been corrected (such as a cabin air-conditioning unit) has been turned off.

When the engine has stabilized, a comparison is made of the observed and the computed turbine discharge pressure Pt7 (or EPR) to determine the approximate amount of trimming required. If a trim is necessary, the engine fuel control is then adjusted to obtain the target turbine discharge pressure Pt7 or EPR on the gauge. Immediately following the fuel control adjustment, the tachometer reading is observed and recorded. Fuel flow and exhaust gas temperature readings should also be taken.

Dual-Spool Compressor Calculations

On Pratt & Whitney engines using a dual-spool compressor, the observed $N_2$ tachometer reading is next corrected for speed bias by means of a temperature/rpm curve. The observed tachometer reading is divided by the percent trim speed obtained from the curve.

The result is the new engine trim speed in percent, corrected to standard day (59 °F or 15 °C) temperature. The new trim speed in rpm may be calculated when the rpm at which the tachometer reads 100 percent is known. This value may be obtained from the appropriate engine manual. If all these procedures have been performed satisfactorily, the engine has been properly trimmed.

Environmental and Operational Safety Restrictions

Engine trimming should always be carried out under precisely controlled conditions with the aircraft headed into the wind. Precise control is necessary to ensure maintenance of a minimum thrust level upon which the aircraft performance is based.

In addition, precise control of engine trimming contributes to better engine life in terms of both maximum time between overhaul and minimum out-of-commission time due to engine maintenance requirements.

WARNING: Engines should never be trimmed if icing conditions exist.

Modern Electronic Control Maintenance

Most electronic control fuel control systems do not require trimming or mechanical adjustments. Changes to the EEC in a FADEC system are normally accomplished through software updates or replacing the EEC unit itself.

Quick Review: Engine Trimming and Fuel Control Maintenance

What is engine trimming and what parameter adjustments are permitted in the field?

Field maintenance on turbine fuel control units is extremely restrictive. Technicians are only authorized to perform complete component replacements or complete specific adjustments known as engine trimming. These adjustments are strictly limited to resetting the idle RPM and the maximum speed adjustment to ensure the engine achieves its certified minimum thrust ratings.

Why must accessory bleed air systems be turned off before a technician adjusts the fuel control?

Before collecting trim metrics, the engine must be stabilized (typically for 5 minutes), and all secondary cabin environmental or accessory bleed air systems must be switched off. If bleed air lines are open, they siphon away compressor energy, which artificially lowers the Engine Pressure Ratio (EPR) or turbine discharge pressure readings. Trimming under these conditions would result in an inaccurate fuel schedule calibration.

How is speed bias calculated when trimming a dual-spool compressor engine?

On dual-spool engines, the observed high-pressure compressor speed (N₂) must be mathematically corrected to standard day conditions (59°F or 15°C). The technician reads the observed N₂ tachometer percentage, consults a manufacturer-provided temperature/RPM curve to find the speed bias percentage, and evaluates the standard day trim speed using the following formula:

Corrected Trim Speed (%) = Observed N₂ Tachometer Reading ÷ Percent Trim Speed from Curve

What environmental safety restrictions apply to trimming, and how does FADEC eliminate this procedure?

Engine trimming must always be conducted with the aircraft headed directly into the wind to guarantee steady airflow, and it must never be performed if icing conditions exist due to the risk of severe aerodynamic and internal engine data distortions. On modern aircraft utilizing Full Authority Digital Engine Control (FADEC), manual trimming is entirely obsolete; the Electronic Engine Control (EEC) monitors environmental parameters digitally, meaning updates are done via software configurations.

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