PT6 Fuel Nozzle Maintenance
The Pratt & Whitney PT6 engine has proven itself to be one of most reliable and cost efficient engines ever developed. Operators, and those of us in the repair and overhaul business, have become keenly aware that the ever-reliable PT6 is not immune to increasing maintenance costs. The challenge of maintaining high levels of quality while holding the line on costs is always present. Maintenance and repair organizations, which are capable of meeting the ever-increasing quality demands of its customers, and at the same time controlling costs, will surely be in demand.
This article will address one aspect of hot section maintenance, those dealing with fuel nozzles and their proper servicing.
Design and Installation
The fuel nozzle design and installation in the PT6 engine is simple and efficient. The installation consists of 14 adapter assemblies consisting of, primary, secondary, and inlet adapters of either simplex or duplex design. Unfortunately there exists some misunderstanding regarding the ongoing maintenance, recertification, and required overhaul of the fuel nozzles. We will make every effort to clear up some of the confusion on this topic and outline some of the choices available to you.
The Pratt & Whitney maintenance manual addresses recommended and approved fuel nozzle servicing procedures. The manual outlines recommended time intervals, inspection tasks, fits and clearances, cleaning procedures, leak testing, and flow checks.
Time intervals between fuel nozzle servicing should initially follow the Pratt & Whitney maintenance manual recommendations. Once an operator gains a history or trend on the condition of their fuel nozzles, adjustments such as time extensions or contractions might be warranted.
The initial incoming inspection for damage to the tip, adapter, and sheath should be completed first.
It's very important to carefully check each nozzle tip for the correct part number. Fuel nozzle sets with incorrect part numbers installed can not only be dangerous but cause extensive hot section damage.
The installation of an incorrect tip on an adapter is possible since a variety of part numbers have the same physical and outward appearance. This problem is more prevalent with fleet operators using various engine models. The ultrasonic cleaning procedures for fuel nozzles are outlined in the P&W maintenance manual. The use of a smaller, dedicated ultrasonic cleaner, works best. Cleaning one set of nozzles at a time avoids the possibility of co-mingling customer or engine sets. The proper cleaning of fuel nozzles takes time and is a procedure that should not be rushed. Improper cleaning procedures often result in unnecessary nozzle tip replacements.
The P&W maintenance manual outlines that if nozzles fail to clean up, the procedure may be repeated. The repeat cleaning is an important consideration when it comes to holding the line on fuel nozzle expenditures. In fact, the second cleaning often results in the return of a normal flow pattern. Replacement nozzle tips are expensive and should only be required when multiple efforts to clean them fail.
The next step in the process is leak testing. The leak test reveals any problem between the nozzle tip, lock tab, and adapter assembly at the sealing areas. Also important to note at this time is external leakage at welds, and areas between primary and secondary ports.
Flow checking is next, making careful records of any irregularities in the spray pattern, and identifying its location. It's common to encounter two to four nozzles with streaking or flow problems at this time. The experience of the technician is key here. The technician must be able to recognize any irregularities in the flow pattern. Any streaking, dripping, or irregular flow are grounds for rejection of the tip. If there is any question about the flow pattern, it's best to margin on the safe side and reject the tip. On average, we encounter one or two nozzle tips that ultimately fail (after the second attempt to clean them).
The last area to discuss about fuel nozzle servicing is sheaths. Sheaths accompany almost all fuel nozzle sets we receive. There are three primary areas of concern with regard to sheaths: dome thickness, the area where the combustion liner comes in contact with the sheath, and the nozzle port diameter measurement.
Most sheaths require a minimum dome thickness of .040 to be considered serviceable. The measurement of the dome is 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 correct size. This measurement is variable with each engine model. Sheath replacements take place on the average of one or two per servicing.
Poor fuel nozzle flows are one of the primary causes of reduced hot section life. That's why it is critical to provide the operator with a condition report. Using the report, the operator can identify and inspect their engine for damage.
The typical time frame required to complete a fuel nozzle service should be around four shop hours.
When is it wise to overhaul fuel nozzles?
Fuel nozzle overhaul is expensive and sometimes confused with recertification. The overhaul of fuel nozzles requires each tip to be overhauled or replaced with an overhauled tip. The overhaul process for the nozzle tip includes checking the flow pattern. Also required during the overhaul is the measuring of the flow rate and cone angle. The P&W overhaul manual is used for a guideline during this procedure. The cost to overhaul a set of fuel nozzles (14) can reach well over $1,000. When is it wise to overhaul fuel nozzle sets? Some reasons for suggesting overhaul include: fuel nozzles with a questionable history, i.e. purchase of a serviceable or core set of nozzles, fuel that turned out to be from a questionable source, a hot section with a problem history, MORE program requirements and engine overhaul.
Operators that are interested in obtaining optimum performance for their engines should seek a fuel nozzle vendor that specializes in that service. The training, equipment, and work environment are all-important aspects of fuel nozzle servicing. The fuel nozzle condition report goes a long way to stopping minor hot section problems before they become too serious. Be sure your fuel nozzle vendor supplies you with a written report on how your nozzles performed.
Adjustments to fuel nozzle service times can be effectively dealt with after reviewing condition reports. If your condition reports indicate a higher than normal rejection rate for nozzles perhaps it would be wise to reduce the time interval for servicing.
Each fuel nozzle recertification should be accompanied by the proper documentation. The documentation should include either a serviceable tag or a FAA form 8130-3 for return to service.
The development of a comprehensive fuel nozzle maintenance program can produce noticeable savings in hot section costs. Those operators with on-condition hot section maintenance programs will see a longer life. Operators using a hard time hot section life will notice the savings when the bill arrives.
When selecting a fuel nozzle vendor be sure to inquire about turn times, availability of exchange kits, and what type of a condition report you will be receiving outlining the condition of your fuel nozzles. This is also a good time to order the fuel nozzle gasket kit that will be required for the re-installation of the fuel nozzles.
The shipping of fuel nozzle sets should be planned carefully as the delicate nature of the nozzle tips warrants careful protection. Each fuel nozzle should be packaged individually and their position as installed on the engine noted.