Protecting One of Your Largest Investments In Your Aircraft? Maintaining the PT6A

While this article is primarily all about what you do with your LARGE PT6 engine during the off season, the very first paragraph or two will cover a couple of other “Nice to Know” topics.  Large in this article relates to PT6A-60AG, -65AG, -67AG, -67F and any other model with a -60 series engine installed, like a -67R, -65B, -67D etc.   If there’s a 6 as the first number of your engine model…this article is for YOU.

From time to time, during a 100 hour inspection, your maintenance shop may find some shiny bits of metal in the oil filter, without knowing exactly what they are comprised of.

I used the PT6A-60AG Maintenance Manual for the data I gathered, but there will be very close similarity with any other PT6A Maintenance Manual.  In section 70-00-00, around page 225 thru 233 for the -60AG (it’ll be similar numbered pages for other PT6A engines, plus or minus a few pages), there’s a list of Materiel Specifications.  These lists, once an oil analysis facility has clearly identified the material, will tell you which parts in the engine are made from those materials.

A great picture with awesome clouds from @a.rowland12’s #AT602. This is his dad’s plane, but AJ is a mechanic to help out his dad! Thanks for sending AJ!

Following these specifications, there’s also several pages of P&WC qualified/recommended oil analysis facilities around the world.  This is where you would send your samples of metal contamination for identification.

In the Maintenance Manual, section 71-00-00, from page 501 thru approximately 535 to 540 is where you’ll find all the charts that relate to what to do if there’s an overtemp, or overtorque, along with a lot of neat, nice to know info on how your engine is built.  Info on drive pads, the ratios they spin at…a lot of neat info.

Proper Engine Preservation during the Off Season

The following information is especially relevant to ALL of the PT6A-60 series engines.. -60AG, -65AG, -67AG and -67F, primarily because all of these engines have FCU’s (Fuel Control Units) that are VERY different from the smaller Bendix, now Honeywell FCU’s found on all the smaller PT6A AG engines.  All of the -60 series of PT6A’s have FCU’s manufactured by Woodward, the same folks who make the propeller governors and overspeed governors.

What makes this FCU different?  While the smaller Bendix/Honeywell FCU has some fuel flowing through it, the primary material flowing through those smaller FCU’s is AIR; it’s a pneumatically controlled (P3) fuel control unit.   On the other hand, the Woodward FCU is FULL of fuel, and therein lies a potential storage issue.  While the Woodward FCU is also regulated by P3 (Compressor Discharge Air), the vast majority of moving parts within the Woodward FCU are working while submerged in fuel.

fuel control unit pt6a
Fuel Control Unit Diagram of the PT6

Here’s the IMPORTANT PART for all of you operators using ANY version of a PT6A-60 series engine; Storage during the off season.  P&WC, in section 72-00-00, around page 306 thru page 310 provides the guidelines. They are expecting ALL of you operators to follow their preservation guidelines, complete with LOG BOOK ENTRIES.  Failure to follow these guidelines gives P&WC all they need to walk away from you, with a very firm Warranty Denied if you failed to follow their guidelines.

With the large investment operators have in their turbine engines, it’s only smart to follow the recommended guidelines, as spelled out in those pages from 306 thru 310 (give or take a few pages, depending upon engine model).  P&WC do provide some easy options to keep your engine in good conditions, as well as some not so easy options, depending upon YOUR choice.

There are several preservation schedules; Engines inactive for 0 to 7 days, 8 to 28 days, 29 to 90 days, and engines expected to be inactive for more than 90 days.

The simplest solution; move the aircraft to an area where it can safely be run, start it up, and let it run until you have good, solid indication of oil temperature.  Cycle the prop several times, including some Beta and minor reverse checks, and just prior to shutdown, run the engine in feather at high idle for 3 to 5 minutes, to ensure all of the oil that was in the propeller dome was scavenged back to the oil tank, select low/ground idle and once the ITT is at the lowest stabilized temperature, shut the engine down, and make a logbook entry.

If this is not going to be possible, then you can follow the other options in the P&WC Maintenance Manuals (MM’s), but notice that for engines inactive for 29 to 90 days, the FCU will need to be drained of all fuel, and a P&WC approved preservation oil run thru the fuel system up to, but NOT including the fuel nozzles.  This procedure fills the FCU with a preservative oil in place of the fuel that was originally inside the FCU.  Caution must be exercised not to allow any of the preservation oil to reach the ITT probes, as this preservative oil will contaminate the probes, and render them unserviceable!

Engines to be preserved for more than 90 days require all of the previous maintenance checks to be accomplished, PLUS, draining the oil system, tagging the oil cap, and installing humidity indicators to notify you when the relative humidity in the engine has become moist enough to require replacement desiccant bags; these are what attracts all the moisture, and they can be purchased from several local vendors…as well as P&WC.

For engines that went into the 29 to 90 day period, or longer, the MM has specific procedures for depreservation of the engine prior to going back to work.

I’ve reviewed this specifically for the operators of the large PT6A’s (AT-602, AT-802, Thrush 550, Thrush 660, all Fire Bosses and any Dromeder with a PT6A-60AG or larger installed. This because I’m aware of an operator who did NOTHING and after almost 5 months of off season shutdown, noted that there was some fuel puddled on the hangar floor under his FCU.  Investigation after removing the FCU and sending it back to Woodward/P&WC for repair under warranty, it was discovered that moisture had collected on a daily basis inside the FCU, which is full of fuel, and the water that gathered at the bottom of the FCU corroded through the body, causing the small leak the operator found.  After an $ 18,000.00 repair, where warranty was DENIED, the operator was back in the air. That’s something we don’t want happening to anyone else.

By Fletcher Sharp  ,  PT6A Technical Support, Covington Turbines, a division of Covington Aircraft

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