Testing Pt6 Fuel Nozzles: Managing Optimal Performance

Pratt & Whitney's PT6 engine is a highly reliable and cost-effective aircraft engine. However, like any other engine, it requires regular maintenance to ensure its longevity. One crucial aspect of PT6 engine maintenance is the inspection and servicing of its fuel nozzles, which can have a significant impact on engine health. During operation, the fuel pump and fuel control system generate a fuel pressure of 450 to 600 psi, which highlights the importance of inspecting the fuel nozzles and ensuring they are in optimal condition.

The initial inspection of the sheath, adapter, and nozzle tip

Firstly, it is important to check the part numbers on these components before installation. Fuel nozzles attached to incorrect parts can be dangerous and cause extensive damage to the engine. As different part numbers may have similar physical appearances, always verify the part numbers to avoid mistakes.

Next, perform a visual inspection of the sheath, adapter, and nozzle tip for any visible damage. This includes checking for signs of wear, corrosion, or physical deformation. Pay close attention to the mating surface between the flat of the sheath and the flat of the nozzle assembly, ensuring there are no voids or gaps.

Then, measure the dome thickness of the fuel nozzle sheath using a calibrated micrometer. The minimum acceptable dome thickness is typically .040 inches, but this may vary depending on the specific engine model.

Additionally, inspect the area where the combustion liner comes into contact with the sheath. The wear limit for this area is typically .010 inches. Again, this value may differ depending on the engine model, so refer to the appropriate maintenance manual for the specific measurements.

Finally, measure the nozzle port diameter using a dial caliper or a calibrated pin of the correct size. This measurement varies with each engine model, so ensure you are using the correct tool and following the specifications provided in the maintenance manual.

By following these steps, you can ensure the sheath, adapter, and nozzle tip are in proper condition and compatible with the engine, helping to maintain the overall health and safe operation of the PT6 fuel nozzles.

Ultrasonic cleaning of the nozzles

Ultrasonic cleaning is a vital step in the maintenance of PT6 fuel nozzles. This process involves using ultrasonic waves to create microscopic bubbles that implode and remove carbon deposits and other contaminants from the nozzle surfaces. The procedure is outlined in the P&W maintenance manual and should be performed by trained professionals.

The ultrasonic cleaning process for PT6 fuel nozzles typically involves the following steps:

• Disassembly and part preparation: Before cleaning, the fuel nozzle assembly units are disassembled and prepared according to the P&W Maintenance Manual.
• Cleaning solution preparation: The cleaning solution is prepared by mixing a specific cleaner with water. For example, Ardrox 185L is recommended for removing carbon deposits and should be diluted with water to a concentration of 24-30%.
• Ultrasonic cleaning: The fuel nozzle assemblies are placed in a mesh basket and immersed in the cleaning solution inside an ultrasonic cleaning tank. The ultrasound is activated, creating microscopic bubbles that implode and remove contaminants from the nozzle surfaces.
• Rinsing and drying: After cleaning, the assemblies are thoroughly rinsed in very hot water and then dried using clean, filtered compressed air.
• Leak and function test: Following the cleaning process, a leak and function test is performed to ensure the nozzles are in proper working condition.

It is important to note that different cleaning solutions are specified for simplex and duplex nozzles, and using the wrong solution can damage the nozzles. Simplex nozzles have stainless steel components, while duplex nozzles have silver-coated lock tabs. The cleaning solution for simplex nozzles can damage the silver coating on duplex nozzles, leading to leaks. Therefore, it is crucial to refer to the manufacturer's manual and select the appropriate cleaning solution for the specific type of nozzle being cleaned.

Ultrasonic cleaning is a highly effective method for restoring jet engine fuel nozzle assemblies to a like-new condition. It can remove even the most stubborn carbon deposits and reach tiny internal orifices that other cleaning methods cannot access. Additionally, ultrasonic cleaning is a safer option as it avoids the use of manual cleaning methods that may damage the precision-machined surfaces of the nozzles.

Leak testing

Initial Inspection:

Before performing a leak test, it is essential to conduct an initial inspection of the fuel nozzles. This includes checking for possible damage to the sheath, adapter, and nozzle tip. It is crucial to verify the part numbers on these components before installation, as attaching fuel nozzles to incorrect parts can be hazardous. Due to the similar appearance of parts, mix-ups are common, so always refer to the manufacturer's maintenance manual for the correct part numbers.

Ultrasonic Cleaning:

Prior to leak testing, the nozzles must be ultrasonically cleaned according to the maintenance manual. It is recommended to use a small-sized ultrasonic cleaner and focus on cleaning one nozzle at a time to ensure thorough cleaning. The cleaning process can be time-consuming, so avoid rushing it. If the nozzles are not adequately cleaned, they may require more frequent replacement.

Leak Test Procedure:

The leak test helps identify leaks between the nozzle tip, lock tab, and adapter assembly at the sealing areas. It also checks for external leaks at welds and areas between primary and secondary ports. The maintenance manual provides instructions on performing the leak test and manufacturing a blanking tube. This test is crucial to ensure the safe operation of the engine and prevent fuel leaks that could lead to safety hazards.

Flow Checking:

After the leak test, flow checking is performed to identify any irregularities in the spray pattern and their location. Common issues include streaking, spitting, drooling, voids, and skewness in the spray pattern. The experience of the technician is vital in identifying these irregularities. Any issues identified during flow checking may require further cleaning or the installation of new nozzle tips.

Final Inspections:

After flow checking, final inspections are conducted on the sheaths. This includes checking the sheaths for proper dimensions, ensuring the tip is centered in the sheath opening, and verifying clearances between the tip and sheath opening. Additionally, the mating surface between the flat of the sheath and the flat of the nozzle assembly should be inspected for any voids.

Re-installation and Leak Check:

Once you have a good set of nozzles and sheaths, it is time for re-installation. Follow the manufacturer's maintenance manual for proper installation procedures. After re-installation, perform a final leak check to ensure there are no fuel or air leaks. This step is crucial to verify the integrity of the fuel system and ensure the safe operation of the engine.

Spray checking

Initial Inspection:

Before performing a spray check, it is crucial to conduct an initial inspection of the fuel nozzles. This includes checking for possible damage to the sheath, adapter, and nozzle tip. Ensuring the correct part numbers are installed is vital, as attaching fuel nozzles to incorrect parts can be dangerous. Due to the similar appearance of various parts, mistakes are common, so always verify the part numbers before proceeding.

Cleaning the Nozzles:

The next step is to clean the fuel nozzles. Refer to the P&W maintenance manual for the recommended cleaning procedure. It is advisable to use a small ultrasonic cleaner and focus on cleaning one nozzle at a time. Proper cleaning takes time and should not be rushed. If nozzles are not adequately cleaned, they may require more frequent replacement. The maintenance manual suggests repeating the cleaning process until the nozzles are thoroughly cleaned. Typically, a second cleaning is sufficient to restore a normal flow pattern.

Leak Testing:

After cleaning, it is essential to perform a leak test. This test helps identify any issues between the nozzle tip, lock tab, and adapter assembly at the sealing areas. Pay close attention to external leakage at welds and areas between primary and secondary ports. The leak test is crucial to ensure the integrity of the fuel nozzle assembly.

Now, it's time for the spray check. Refer to the maintenance manual for the specific procedure. Ensure you have a good light source and perform the inspection in a controlled environment. Remember that the fluid used in this check is flammable, so adhere to proper safety protocols.

During the spray check, fuel nozzles are tested at two different pressures. At the lower pressure, you are primarily concerned with observing the basic shape of the spray pattern. At the higher pressure, you can thoroughly inspect for defects in the spray pattern, such as streakiness, spitting, drooling, voids, and skewness. These defects can cause severe damage to the hot section if left undetected.

Final Inspections and Re-installation:

After successfully performing the spray check, refrain from touching the nozzle tip, as the oils from your skin can negatively affect the spray pattern. Proceed to conduct final inspections on the sheaths, ensuring they meet the proper dimensions. Mate the sheaths and nozzle assemblies, verifying that the tip is centred in the sheath opening and that clearances between the tip and sheath opening are correct.

Once you have a good set of nozzles and sheaths, you can proceed with re-installation, following the instructions provided in the maintenance manual. This includes installing the metal gasket between the sheath and the gas generator case, as well as the transfer tubes with new o-rings. Torque all connections according to the specified torque values.

Overall Engine Health:

Remember, fuel nozzle inspection and maintenance are crucial, but they are just one part of maintaining engine health. It is essential to consider the health of the entire fuel system. Regular fuel filter inspections, sumping your tanks, and maintaining the overall fuel system are vital to ensuring the safe and efficient operation of your engine.

Final inspections

After you have flow-checked your nozzles, it is time for some final inspections. Inspect the sheaths for proper dimensions. Mate the sheaths and the nozzle assemblies and check that the tip is centred in the sheath opening and for proper clearances between the tip and sheath opening. Also, check to ensure the mating surface between the flat of the sheath and the flat of the nozzle assembly doesn't have any voids.

The dome thickness of the fuel nozzle sheath should not be less than .040 inches. This measurement should be taken with a calibrated micrometer. When the sheath comes in contact with the combustion liner, the wear is limited to .010. Nozzle port diameter dimensions are taken with a dial caliper or a calibrated pin of the correct size. This measurement varies with each engine model.

Now that you have a good set of nozzles and sheaths, it is time to reinstall the nozzles. Install them according to the maintenance manual. Make sure you install the metal gasket between the sheath and the gas generator case. It is somewhat difficult to inspect for missing gaskets after the nozzles are installed. Install the transfer tubes with new O-rings, and torque all connections to the specified torque. Too little torque and you have a fuel or air leak. Too much, and you can snap the bolt.

After a leak check, do one final torque and safety the hardware.

Frequently asked questions