Ensure the aircraft is secure with the brakes set, the wheels chocked, and start the engine. Focus your attention on the aircraft engine gages.
With the engine's oil pressure stabilized, advance the power to 1,500 to 1,800 rpm and wait for the oil temperatures to come up to normal operating temperatures. With the mixture full rich, reduce engine power to the specified idle rpm.
It is critical to accurately document all engine test parameters on the fuel system operational test form. This is the form SB 97-3 on which the manufacturer's specs were previously recorded.
Check the unmetered fuel pump pressure at idle rpm and record this reading on the operational test form. Next, check the idle fuel air mixture by slowly moving the mixture control from full rich to idle cutoff. A rise of 25 to 50 rpm should occur as the mixture control is slowly moved from full rich to idle cutoff. Return the mixture control to full rich and record the actual rpm rise on the operational test form.
With the mixture control full rich, advance the throttle to full power. On turbocharged engines, verify the manifold pressure is at the value specified by the aircraft manufacturer's maintenance manual. Record the rpm, metered fuel pressure, unmetered fuel pressure, and manifold pressure on the operational test form.
Using the operational test form, compare the actual readings with the required readings for the following areas:
- Idle fuel pressure
- Fuel/Air mixture rise
- Full power fuel pressures
- Fuel flow
- RPM install the
- Manifold pressure readings.
Boosting Your Knowlege of Turbocharging
Part II -Valves and Controllers
By Randy Knuteson
Raytheon Aircraft's turbocharged Barons, Bonanzas, and Dukes depend on a Variable Absolute Pressure Controller. The VAPC has a bellows that extends into the upper deck air stream, sensing deck pressure and comparing it to a referenced absolute pressure. What makes this controller "variable" is that it is linked directly to the engine throttle. Through a system of cams and followers, it adjusts a moveable poppet seat and accordingly achieves the optimum deck pressure for a given throttle setting.
Continental's turbocharged engines rely on absolute, variable absolute, pressure ratio, dual, or slope controllers. Dual controllers are a hybrid combination of absolute controllers coupled with a rate or ratio controller. These units are housed in a common body and were most frequently used in the older Cessna 200-300-, and 400-series engines. TCM equipped Piper Malibus, the TSIO-550 equipped experimental Lancair IV, Piper's new PA-32 and Cessna's new 206's all incorporate yet another style of controller — the sloped controller. The sloped controller incorporates the functions of an absolute pressure controller and a differential controller in a single housing. Acting much like the density and differential arrangement in the Navajo, the sloped controller maintains the rated deck pressure at wide-open throttle, and a reduced deck pressure at part throttle, and looks for discrepancies in the differential between deck and manifold pressures. If either pressures rise above a pre-determined value for a given throttle setting, the sloped controller opens the exhaust bypass valve, thereby lowering compressor speed and output. However, unlike the density controller, it does not correct for temperature excursions.
Manual control systems
Not all turbocharging systems are automatically actuated. Instead, systems like those found in the Turbo Arrow and the Mooney 231 have a simple, built-in exhaust leak upstream of the turbo that allows a fraction of the exhaust gases to be dumped overboard and the remainder to be routed through the turbine. This "fixed" wastegate arrangement has also been successfully used in the Seneca II-IV. Other systems rely on manual inputs from the pilot. As power begins to decrease at higher altitudes with the throttle in the wide-open position, the pilot then incrementally adjusts the wastegate toward closure utilizing a separate vernier wastegate control cable. Or, as in the case of Cessna's Turbo Skylane and Piper's Turbo Saratoga, some installations have the wastegate linked directly to the throttle. As the wastegate valve closes, backpressure forces additional exhaust gases through the turbine wheel assembly and speeds up the compressor. This series of events results in an increase in engine manifold air pressure and a resulting increase in power. The pilot becomes the "controller" of these systems as he systematically monitors and regulates manifold pressures while ascending/descending in altitude. These simplified manual systems reduce both cost and complexity, albeit at the expense of increased pilot workload. This type of arrangement is often used in aftermarket turbo-normalizing kits.
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