PT6 Hot Section Inspections
Some additional helpful tips from the field
By Kelly M. McCallum
In July’s issue of AMT, the Turbine Technology article on PT6 hot section inspections provided several helpful tips on performing the PT6A engine hot section inspection. Following are a few more recommendations/tips from Mr. Kelly M. McCallum, Manager, PT6A Engineering and Service Centre at StandardAero VOF in Tilburg, The Netherlands, that should prove to be beneficial to technicians who service this engine model. As a result, component life may be extended, meaning lower direct operating cost to the end user.
Unscheduled hot sections
If the hot section inspection you are performing is unscheduled due to performance related concerns, please ensure that the airframe systems such as the ITT indication system and the torque meter system are calibrated properly prior to splitting the engine. You may also want to confirm by borescope inspection of the hot section that there is damage that would warrant the unscheduled inspection. As indicated in the July article, bleed valves should be checked for correct operation if suspected to be faulty. Approximately 75-percent of the time, a sudden shift in a trend cannot be attributed to hot section deterioration. A faulty accessory or airframe system can often be quite easy to repair, and can save several labor dollars, which do not need to be invested in splitting an engine, just to find that there is nothing wrong internally.
Thorough turbine rinses
Compressor washes are carried out by most operators on a regular basis, using the solution specified in the related maintenance manual. There is, however, a tendency for some operators to not perform a turbine rinse after the compressor wash. Try to get into the habit of always rinsing the turbine after a wash with solution. If you don’t perform this rinse, the corrosive solution that is pulled through the compressor to the hot section area during the wash will sit and corrode the turbine blades. (Don’t assume that the water rinse through the compressor will remove all of the solution from the turbine area.)
If the compressor turbine (CT) blades show evidence of sulfidation (in particular, stage 2) during the hot section inspection, and are deemed to be acceptable for reinstallation, ensure that turbine rinses are subsequently performed on a regular basis. Advanced sulfidation (attack of the blade coating) can be attributed to several factors. If operating your aircraft in an area with high amounts of atmospheric pollutants, or close to salt water, then turbine washes may even be performed on a daily basis to slow any attack.
Know your service bulletins. In particular, all technicians should be aware of which SB’s affect the engine’s life-limited components, and which cover the engine time between overhaul (TBO) considerations.
Cycle / life limits
When re-installing the compressor turbine (CT) or power turbine (PT) after the individual component inspection, ensure that there are enough cycles remaining on the component to make it to the next scheduled inspection interval. If there aren’t enough cycles remaining, then this should be flagged in the engine logbook so that the turbine will be removed when it reaches its life limit.
The small series PT6A engine (such as –20, 27/28, 34, 114, 135) has a life limit (TBO consideration) on the compressor turbine blades. Please ensure that the CT blades are not due for overhaul (which includes fluorescent penetrant inspection, blade stretch and coating inspection etc.) at the time the hot section inspection is being performed. Per the current and applicable Pratt & Whitney Canada (P&WC) instruction, small series engine CT blades can be run for 3,000 hours from the time of full overhaul, or 5,000 hours from the time installed new. With some operators running extended TBO’s on their engines, this should be monitored closely for compliance.
CT vane ring burning / material loss
The P&WC maintenance manual states that burning/material loss at the trailing edge location of the CT vane ring is unacceptable, and that the vane ring will require replacement due to this condition. Please note that if this condition exists, it is recommended that a designated overhaul/repair facility be contacted for evaluation assistance. This is important, as a set of turbine blades that have been run behind a vane with material loss beyond P&WC Overhaul Manual limitations will require replacement due to excessive blade cyclic loading. The hot section may, therefore, have to be shipped to an authorized repair facility to perform this blade replacement.
Oil filter and strainer
Try to get into the habit of checking the engine oil filter along with the RGB strainer. My recommendation is to perform these two checks at the beginning of the inspection. Performing a full hot section inspection on an engine that may be making metal would not prove to be cost-effective. This is also a good time to see if your engine oil filter requires replacement due to life limitations.
Compressor blade damage
When checking your first stage compressor blades for erosion, etc., please ensure that there is no significant foreign object damage (FOD) that may not be allowable per the Maintenance Manual criteria. Try to get into the habit of making this inspection one of your initial checks as well. Once again, you may waste money and time if you start the hot section inspection, only to find out later that the compressor blades are damaged beyond limits and the engine needs to be removed from the airframe.
During the inspection, check your ignition and/or glow plugs for wear. Worn plugs may cause your engine to not light off correctly during the starting procedure, and subsequent damage to the hot section may result.
#3 bearing cover
Check that the #3 bearing cover, if installed, is not showing any tear indications and that the lugs are not worn. You may have to remove the power turbine disk on small series engines to confirm this. This is not a requirement on post S/B-1430 engines.
The July article indicated that a similar class of PT vane should be installed if replacing this item in the field. Note that this is also the manner in which a CT vane replacement should be handled. Ensure that the class value is observed and maintained within the specified limitations.
Checking for proper fit
Ensure that the fit of the CT shroud housing to the small exit duct (SED) is tight. This should be observed on all engine models. A good check, one can perform on the –28 series engine, for instance, is to place the two parts together as if assembled, but not to secure with bolts. Then, grip the shroud housing at the sealing ring location and lift the two parts together, slightly off of the work surface to see if the two parts remain together. If they do not, chances are that the fit is not sufficient.
If the CT vane ring to SED clearance is not sufficient, it is recommended that the SED lugs be stoned (ground), rather than the vane slots. This is equally as important with the fit between the vane ring and the #2 bearing cover. Get into the habit of stoning the bearing cover and not the vane ring. Why? If you stone too far, and take a part outside its acceptable tolerance in that particular area, it is much cheaper to replace either the SED or the #2 bearing cover in comparison to the vane ring.
Even though the maintenance manual recommendations give minimum and maximum clearances, always try to aim for the minimum size. If the clearance for instance, on your CT vane ring slots to the SED lugs is at max when performing the hot section inspection, where is it going to be in 1,750 hrs? If installed at the maximum limitations and operated for the next required interval, the clearance at the next scheduled inspection may be well beyond limitations. This is fine if the hot section stays centered, but due to the increase in looseness, there is room for movement and a chance for significant damage to occur to the compressor turbine blade tips.
Remove the sealing ring(s) from the engine and install on either the inside of the PT stator housing, or around the CT shroud housing depending on the engine model. You can then check the fit with a .002-inch feeler blade around the circumference of the two mating parts. If you find that this feeler blade will fit between the sealing ring and the shroud/stator housing for an area of 60 degrees or more, then re-index the ring and re-check for a gap. Typically, a gap of 60 degrees or less is acceptable, providing engine performance is satisfactory.
Always ensure that the sealing ring be installed with the "pressure side" facing the correct direction in the hot section. Be sure that when installing rings in engines with the two-ring configuration that the gaps are staggered 180 degrees for the best seal.
As mentioned in the July article, the "ZERO" grind is a significant time saver when reinstalling a repaired hot section in the field. Minimizing the need to touch-up grind "tight spots" will certainly help you to get your hot section inspection completed more quickly. Here are a couple of additional tips to follow that may allow you to not have to perform a touch-up grind:
1. Install your hot section and hold in place with four bolts as opposed to the required 16 for an initial clearance check. Do not secure the washers.
2. Load your hot section as applicable, with four rubber blocks, to simulate the loading that the hot section will see while the engine is running.
3. Install the CT disk per the manufacturer’s instruction, and measure the tip clearances. Do not lock the bolt. Tip clearance is typically measured at the trailing edge of the blade over two to three locations per segment. For example, measure at two locations when 16 segments are installed and three locations on each segment when nine segments are installed. Remember that you may have to load the disk in a particular direction while measuring the clearance as per specific instruction.
4. If you find that the tip clearance is slightly tight in one location, and loose in another location, you can then try to "index" the hot section. This includes removing the disk assembly, then the four hot section bolts, and then the hot section. You can then rotate the hot section 90 or 180 degrees (as necessary) and repeat steps 1 to 3. Repeat the procedure a couple times as necessary to try to achieve the optimum fit.
Once the optimum fit is achieved, remove the CT disk assembly, install and secure all hot section bolts, and then re-install and secure the CT disk assembly. You can then remove the four rubber blocks and complete the installation of the remaining hot section components. Final assembly of the engine can then be finalized.
It may take you a little more time than normal to perform this check, yet it may be worth it. Having a uniform average tip clearance around the entire circumference of the disk to shroud, and preventing "tight-spots," can significantly reduce the risk of having a turbine tip rub.
Care when marking components
Remember to always use a P&WC approved marker or silver pencil when identifying or index marking components. Failure to do so could result in a "hot spot" forming on the marked component and a possible failure could then occur.
These are some of the tips I have learned that should prove beneficial. As with any maintenance action, when performing a hot section inspection, be sure to refer to the manufacturer’s maintenance manual and all applicable data. Happy Inspecting!
About the Author
Kelly M. McCallum is Manager, PT6 Engineering and Service Centre at StandardAero VOF in Tilburg, The Netherlands. He has worked as a repair technician and field service representative on the PT6A series engines and has given several technical presentations on PT6A hot
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