Starting a PT-6

[timeless]

On the Shorts 360 I've always started the engines (PT6-67R) with the prop levers in feather. That is the procedure I was taught and it is what the checklist calls for. Another pilot at my company is starting the engines with the prop levers out of feather. Can that cause any problems.

 

[CA1900]

For what it's worth, when I flew the Beech 1900D (PT6A-67D), we always took the props out of feather before starting the engines. (Having them in feather for took long would apparently cause the windows to melt from the exhaust!) Unless the 67R is radically different, I don't see how it would cause any trouble.

 

[avbug]

Timeless,

I've flown behind many of the PT-6 dash numbers, including the -67, and have worked on many of them. Operationally, most of them are the same, save for the specific numbers (torque, temp, etc). Internally they vary, and various propellers are hung on them, but operating a PT6 is really the same throughout the mix.

Where they differ is the time limits in which they can be feathered, the conditions in which they can be feathered (tailwind, for example), and ground operations ranges (harmonics and wind issues); these depend on the propeller and on the airframe.

Starting the engine in feather means that no oil is getting used for the propeller, which means that initial oil circulation stays with the engine bearings...which is where you want it. Move the propellers and you're dumping oil pressure when you need it most; at the engine start.

So far as the propeller itself, the engine doesn't know if it's turning or not; it's not attached. It's a free turbine engine, meaning the movement of the propeller with respect to rotation is strictly function of exhaust gas. In theory, you could hold on to that propeller during the engine start and prevent it from turning at all, though in reality the torque would toss you and likely hurt you, too. Point is, the exhaust gasses from the gas turbine generator (Ng) are turning that prop, after the engine is done with the gas flow...nothing mechanical between the engine and the prop but the case...and the oil.

This differs from other turbines you may fly, in which the turbine wheel or wheels are mechanically attached in some form to the propeller. In installations such as Allison or Garret engines, the turbine wheels are actually turning the propeller, too. In this case, the only function of the turbines in the engine is to turn the compressor. The propeller has it's own turbine, attached only to the gearing of the propeller, that isn't attached to the rest of the engine; it doesn't affect the compressor and isn't part of the turbine section either; it's just there to take advantage of the exhuast gas exiting the engine (more like a turbine in a river, making electricity).

Keeping this in mind, any oil that the prop is using isn't being used to make that engine run, lubricate the engine during start, keep things cool, etc...it's only there to actuate the propeller. You don't need to actuate the propeller during start, because you're not going anywhere. The propeller in feather isn't hindering anything, isn't putting a load on a starter, isn't boggng down the engine, isn't actually doing anything, because it's not attached to the engine.

Other engines such as the shaft-driven Garretts (honeywells now, I guess) and Allison engines are out of feather when they're stopped...because they need to be in a minimum drag position for the next start. When feathered, those engines do create a lot of drag for the starter and for the engine, and make for hotter starts. Not so with the PT6.

Starting out of feather means more airflow, as CA1900 indicated, past the exhaust. I've seen some BE1900 operators run the starts in feather, others with the props forward.

I generally start in feather and then advance the propeller once the engine is warming up.

Another reason, especially when it's cold, for not putting oil to the propeller too soon is taking care not to blow out a seal (a statement which has absolutely nothing to do with beastiality) with cold, high pressure oil. Start the engine, let it warm, then put it to the prop and let it work it's way through the propeller and prop controls.

To answer your question, yes it can cause problems, depending on the circumstances...ask the other pilot why he does it that way. I'd be curious to know.

 

[The_Russian]

Good post. Keeping what you said in mind, wouldn't the propeller spring prevent the oil from entering the hub until sufficient pressure is reached? This would keep the oil within the confines of the engine until it was at a relatively good speed to lubricate the engine. Thus, it would not matter the placement of the prop levers during the start process. Input?

We had the airframe limitation for start also, of course.

timeless, are you playing Guitar Hero on the radar? Where is your TSO? Just kidding!

 

[hawk16e]

In theory, you could hold on to that propeller during the engine start and prevent it from turning at all, though in reality the torque would toss you and likely hurt you, too. Quoted from Avbug

I saw a guy do that to a Twin Otter once...just once. He let go at about halfway through the start, I forget what % he said. He also was afraid the next blade was coming to get him.

 

[avbug]

Underspeed with the prop levers forward, oil will flow freely to the prop. It won't start governing off the high pitch stops until it senses overspeed. Until then, once the prop control has been moved out of feather, the prop will start taking oil.

 

[timeless]

Thanks for the info. Now I need to figure out how to ask an old timer freight dog to change his ways. At least I'll have some very good facts on my side.

 

[timeless]

To answer your question, yes it can cause problems, depending on the circumstances...ask the other pilot why he does it that way. I'd be curious to know.

His theory is that an unfeathered prop is easier for an engine to turn during start and that the engine starts cooler that way.

 

[JohnnyP]

.....

His theory is that an unfeathered prop is easier for an engine to turn during start and that the engine starts cooler that way.

I assume he is referring to the placement of the prop levers during the initial starting?.... as the actual blades will already be in the feathered position during start......Am I reading this wrong (damn white wine again)..?

 

[avbug]

Nope. Your white whine is just fine.

Timeless, if your friend were talking about a shaft driven engine, he would be correct, which is why shaft driven engines park the blades in a fine pitch position, on the high RPM stops, or in some cases, on the "locks." Those engines absolutely must be started out of feather, or should...they can be started in feather, but one must watch the temps, and it's harder on the battery and starter...more resistance at a time when it's not wanted.

As discussed before, the PT6 doesn't have that issue, as the propeller isn't mechanically connected to the engine. Holding the propeller still or allowing it to spin, in feather or out, doesn't affect the start, start temps, starting speed, starter load, current draw, etc. It's just not a critical issue.

Again, as others have noted, the operational concerns are a much bigger issue. Some engine installations have limitations as to when and where reverse and feather may be used on the ground, and these are more of a concern than anything. If you're starting in very cold weaher, a preheat is aways preferred but starting in feather does enhance engine life in that circumstance by some small margin.

The BE1900 mentioned earlier has limitations with respect to the airframe. The Piaggio has limits with respect to the propeller themelves, which are in the exhaust stream. The Air Tractor 802, which I flew this summer has various limitations with respect to ground time in feather, ground time with a tailwind, feather or reverse with a tailwind, and feather with a crosswind due to harmonics. That, and the gasses in the cockpit burn your eyes and choke you out.

What your friend is displaying is a gross misunderstanding of the function of the engine. I don't know that he's doing anything wrong, per se, but he's doing what he's doing out of ignorance, if that's his reasoning. What is learned here is that he could use a little more studying before he makes up his mind as to how to tackle that engine start.