The following instructions cover some of the basic inspections and procedures for rigging and adjusting fuel controls, fuel selectors, and fuel shutoff valves.
- Inspect all bellcranks for looseness, cracks, or corrosion.
- Inspect rod ends for damaged threads and the number of threads remaining after final adjustment.
- Inspect cable drums for wear and cableguards for proper position and tension.
While rigging the fuel selector, power controls, and shutoff valve linkages, follow the manufacturer’s step-by-step procedure for the particular aircraft model being rigged. The cables should be rigged with the proper tension with the rigging pins installed. The pins should be free to be removed without any binding; if they are hard to remove, the cables are not rigged properly and should be rechecked. The power lever should have the proper cushion at the idle and full-power positions. The pointers, or indicators, on the fuel control should be within limits. The fuel selectors must be rigged so that they have the proper travel and do not restrict the fuel flow to the engines.
Rigging Power Controls
Many older conventional turbofan engines use various power lever control systems. One of the common types is the cable and rod system. This system uses bellcranks, push-pull rods, drums, fairleads, flexible cables, and pulleys. All of these components make up the control system and must be adjusted or rigged from time to time. On single-engine aircraft, the rigging of the power lever controls is not very difficult. The basic requirement is to have the desired travel on the power lever and correct travel at the fuel control. On multiengine turbojet aircraft, the power levers must be rigged so that they are aligned at all power settings.
Most computer controlled engines have an electronic connection from the flight deck to the engine. This eliminates the need for any type of cable or linkages. In the computer controlled system, the computer sends electronic information through wires or buses to the fuel control to command it to follow pilot inputs from the flight deck.
On older style aircraft the power lever control cables and push-pull rods in the airframe system to the pylon and nacelle are not usually disturbed at engine change time and usually no rigging is required, except when some component has been changed. The control system from the pylon to the engine must be rigged after each engine change and fuel control change. Figure 8-22 shows the control system from the bellcrank in the upper pylon to the fuel control.
Before adjusting the power controls at the engine, be sure that the power lever is free from binding and the controls have full throw on the console. If they do not have full throw or are binding, the airframe system should be checked and the discrepancies repaired. After all adjustments have been made, move the power levers through their complete range, carefully inspecting for adequate clearance between the various push-pull rods and tubes. Secure all locknuts, cotter pins, and safety as required.
Adjusting the Fuel Control
The fuel control unit of the typical turbofan on older aircraft can be a hydromechanical device that schedules the quantity of fuel flowing to the engine so that the desired amount of thrust can be obtained. The amount of thrust is dictated by the position of the power lever in the cockpit and the particular operation of the engine. Thus, the thrust of the engine and the consequent rpm of its turbine are scheduled by fuel flow.
The fuel control unit of the engine is adjusted to trim the engine to obtain maximum thrust output of the engine when desired. The engine must be retrimmed after a fuel control unit is replaced, the engine does not develop maximum thrust, engine change, or excessive throttle stagger.
After trimming the engine, the idle rpm can be adjusted. The idle rpm is adjusted by turning the INC. IDLE screw an eighth of a turn at a time, allowing sufficient time for the rpm to stabilize between adjustments. Retard the power lever to idle and recheck the idle rpm.
If wind velocity is a factor, the aircraft should be headed into the wind while trimming or checking the trim on an engine. Since trimming accuracy decreases as windspeed and moisture content increase, the most accurate trimming is obtained under conditions of no wind and clear, moisture-free air. Do not trim when there is a tailwind because hot exhaust gases may be reingested. As a practical matter, the engine should never be trimmed when icing conditions exist because of the adverse effects on trimming accuracy. To obtain the most accurate results, the aircraft should always be headed into the wind while the engine is being trimmed.
With the aircraft headed into the wind, verify that the exhaust area is clear. Install an engine trim gauge to the T-fitting in the turbine discharge pressure line. Start the engine and allow it to stabilize for 5 minutes before attempting to adjust the fuel control. Refer to the applicable manufacturer’s instructions for correct trim values. Compensate for temperature and pressure during the trimming process. If a hydromechanical fuel control is not within limits, turn the INC. MAX screw [Figure 8-23] about one-eighth turn in the appropriate direction. Repeat, if necessary, until the desired value is attained. If the aircraft is equipped with a pressure ratio gauge, set it to the correct value.
An example of a trim check using an electronic controlled fuel control must take into account temperature and pressure for each parameter measured. The parameters checked can include:
- Minimum idle (percent N2)
- Approach idle (percent N2)
- 2.5 bleed open (percent N1)
- 2.5 bleed closed (percent N1)
- Takeoff engine pressure ratio (EPR)
- 95 percent takeoff thrust (EPR)
- 90 percent thrust change decal (EPR)
The actual trim check would be done based on a temperature and pressure, such as the example in Figure 8-24. For these given temperature and pressures, the target parameter values can be derived from a chart in the manufacturer’s manual. The engine is run up, and these values are checked against the tolerances given in the manual.