Dry Motoring a PT6
When it comes to turboprop engines, a hot start is a really bad thing. For you piston drivers out there thinking, “What’s the big deal, you are just starting a hot engine,” then here’s a little education for you.
In a PT6 turboprop engine, there is a very important temperature gauge that a pilot monitors very closely during each and every start. It is called the Inter-Turbine Temperature gauge, or ITT. This temperature is a measurement of the exhaust gases between the compressor turbine and the power turbine (s). In the picture below, the probe is located where the blue and red colors meet.
In a turboprop engine, specifically the Pratt & Whitney PT-6 in all it’s different sizes and variations, there will always be a specific temperature that the pilot will want to keep the ITT below. This article will deal specifically with a Piper Meridian.
A Piper Meridian starts hotter than almost any other PT6 engine because of the way it’s air intake is designed. Unlike other turboprops, the Meridian has a permanently open inertial separator. This means that not all the intake air makes it to the engine during start because some of it goes out the inertial separator opening. So, coming to a Meridian from operating other turboprop engines can lead to a little bit of a surprise on the ITT temperature being higher than what a pilot is used to when starting.
As a rule of thumb, when starting a Meridian, never let a start continue when the ITT hits 875 degrees. Based on the chart below, you are still in the safe zone at 875 and have about a 50 degree buffer before you have to start getting worried.
On cold starts with a good battery or a GPU, 875 is typically not an issue. Most starts when cold are going to be in the high 700s or low 800s. On a cold start, if you are seeing starts in the mid to upper 800s, try starting with a GPU and see if that lowers the start temperature. If it does, then that means your battery is weak and needs to be replaced. Another tell-tale sign of a weak batter is the Ng doesn’t spool up properly (meaning it settles around 12-13%) or takes a really long time to spool up. Also, never start on the battery with less than 24 volts.
When there are multiple flights in one day, the pilot has to take into consideration the warm engine prior to starting. If the ITT, prior to the start sequence, is above 150 degrees, it is time to do some motoring of the engine.
What is motoring? It is simply using the starter to turn the engine, which leads to air being sucked into the engine allowing the engine to cool off prior to start. The theory is, the cooler your engine prior to start, the cooler the ITT peaks at during start.
Here’s the steps on how to dry motor a Piper Meridian:
- Battery on
- Strobes on
- Fuel Pumps and Ignition off
- Throttle idle
- Condition Lever feather/cutoff
- Push the start button
- Monitor the ITT temperature
- Reaching 150 degrees, if less than 30 seconds have elapsed:
- Fuel Pumps on
- Ignition On
- Condition Lever run
- Reaching 150 degrees, if 30 seconds have elapsed:
- Push Manual/Stop button to stop the start
- Let starter rest for 30 seconds
The starter has a 30 second limit on the Meridian, followed by a 30 second rest period. You can do the sequence twice, then, after the 3rd start, there is a 30 minute rest period. Typically, if the ITT won’t cool down to 150 after the 3rd time, there is probably something wrong.
The most important thing a pilot can remember is never, ever push the condition lever forward if the ITT is above 150 degrees. You’ll be well on your way to avoiding hot starts that way.
One thing that I learned this summer was this. Sometimes, with an area of closely grouped storms, the cloud base is actually quite high (some storms I went underneath in New Mexico had cloud bases of 15,000 MSL). The thing to do here is just stay below the bases of the clouds and go around the areas of rain. These will be visible and you’ll be able to navigate around them. With bases even as low 9-10 thousand feet, it’s a much better idea to stay below the clouds, sacrifice some speed, and dodge the rain shafts.
