My engine is running hot! My engine is not making power! My neighbor’s engine runs better than mine!
These are all comments that we get every year. Did you know that a tool is accessible to you to determine the health of your engine? Pratt & Whitney Canada has had airframe-specific ground run charts for most models and aircraft types in the engine maintenance manuals for years. However, with the growth of multiple engine models and platforms, each aircraft manufacturer has been moved to provide this ground performance chart. It is a beneficial tool when it comes to performance troubleshooting and tracking. I want to share with you how to use these charts. If you ever have questions or need help, take the best advice anyone can ever give you: ask questions.
For this article, I am using the chart for the PT6A-34, but all charts have the same basic properties. When we look at the performance charts, there are a few things we should pay attention to. The first is the listed propeller speed. The chart is configured at the maximum propeller speed. The second thing to notice is that the chart explicitly states that it should not be used to accept or reject an engine. This is a tool, if the engine doesn’t conform to the chart expectations it provides you with ideas as to why.
We also see several different “curves” on the chart. There is a curve for torque, Ng (gas generator speed), Wf (fuel flow) and ITT (internal turbine temperature). The chart also features outside air temperatures across one axis. As applicable, the chart will identify barometric pressures to help determine targets. For example, each line of the curve on torque has a barometric pressure listed. Temperature and barometric pressure are the mathematical inputs needed to allow the charts to provide the targets for each day and each location worldwide. This will enable us to help determine an engine’s health no matter where the engine is located. The basic curve was developed from “standard day.” Not many live or operate at standard day, 68 degrees F at sea-level.
The first thing we want to do with the chart is establish our targets. You will notice that I use the correct term of targets; these are not limits. We use our OAT (outside air temperature) and field barometric pressure. You can get the barometric pressure by adjusting your altimeter to field elevation, providing you with barometric pressure in the Kollsman window. I will not assume that you know these two critical pieces of data, but I will plot some targets using the chart.
I will use 26 degrees C and 29.92 as our barometric pressure for our example. With those numbers, our target torque is 1360 ft. lbs. If you need to translate that into PSI, divide 1360 by 30.57. One foot pound is equal to 30.57 psi. In our example, this would be 44.5 psi. This torque target will be the key number needed to utilize the chart correctly.
The next section of the chart is fuel flow. The chart is in pounds per hour. Not many airplanes have a fuel gauge that is in PPH. For our purposes, 1 PPH is equal to .162 gallons per hour. Our target for this exercise is 405 PPH or 72 gallons per hour. We multiply 405 by .162.
Our third target is gas generator speed or Ng. This is the first curve on the chart that only needs OAT to determine the target. Our OAT being 26 degrees Celsius puts our target Ng at 99%. Finally, our target temperature. This curve also only requires an OAT. Our example target is 740 degrees. We would then record our target data as follows:
OAT 26 Baro 29.92
Target Tq 1360
Target Wf 72gph
Target Ng 99%
Target ITT 740
I’m almost ready to run the engine to see how it is doing. We would want to verify the gauges and ensure our indication is correct. The other thing we at Covington do is secure the tail on taildragger aircraft. When doing performance runs, like in normal operations, respect the redlines. If you cannot hit a target number because you are hitting a redline, then stop, and let’s figure out the problem.
Run the engine to the target torque after a normal start and with the prop pushed forward. This is the number that we base everything on. Record the data. You can do this with a simple photo of the gauges or go old school with your pen and paper. This is the proper way to determine how your engine is doing. If any of the parameters are above the targets at the target torque, then we can decide if something needs to be done to address it. It would be great if any of the parameters were below the targets at the target torque. We call that cushion. For example, at a torque of 1360, you have an ITT of 715. That means you have 25 degrees of cushion. Suppose Ng is at 97%, a 2% cushion on Ng. The below target parameters are great when the engine is running at the target torque. That is what we hope to see.
When do I suggest doing a ground performance run? Anytime you feel the engine is not running the way you want, you can do a run. Please keep track of all your previous runs and use them to determine trends. Watch Ng, ITT or Wf for changes. This can help you with any preventative maintenance you might want to do. We do an incoming and outgoing performance run every time Covington works on an engine on wing. This helps make sure we get issues addressed. It is a great tool that I always encourage everyone to use. If you need help with it, all you have to do is ask.
Robert Craymer has worked on PT6A engines and PT6A-powered aircraft for the past three decades, including the last 25+ years at Covington Aircraft. As a licensed A&P mechanic, Robert has held every job in an engine overhaul shop and has been an instructor of PT6A Maintenance and Familiarization courses for pilots and mechanics. Robert has been elected to the NAAA board as the Allied-Propulsion Board Member. Robert can be reached at robertc@covingtonaircraft.com or 662-910-9899. Visit us at covingtonaircraft.com.
