The TBM and meridian both use big block (highly derated) PT6s

This was a major consideration for me.

Many Garrett powered aircraft operators I am sure will plan for GPU availability or carry a portable power pac. There are Hawker Enersys 40 Ahr batteries available for some of these aircraft and they make GPU's a thing of the past.

Where I fly, King Airs-TBM-Pilatus all operate on batteries which is a great operational advantage or it can be.

Nothing wrong with GPU use. Hope I did not come across as condemning it.

Outside of the U.S. they are just not as readily available and you have to know how you will get by without them.

For me, no. The only exception would be if I was going to really cold place and wanted to be sure we had a GPU available OR a heated hangar. The heated hangar is more beneficial than the GPU, but also generally way more expensive.

I would say 95% of the airports I go to have a GPU, so it wouldn't be a great limitation if I required one.

I use GPU if engine driven or AC powered and it is free. Otherwise, I don't. I figure it saves me about $10 per start in battery costs.

Using a GPU is generally routine, costs no time, and usually costs no money. It does good things to your batteries and engines.


A GPU is always helpful, even for the small PT6, but the big block PT6 and TPE331 benefit the most.

Note that the TBM is a big block PT6, PT6A-6x series, the Meridian isn't, PT6A-42A.

Mike C.

For me, no. The only exception would be if I was going to really cold place and wanted to be sure we had a GPU available OR a heated hangar. The heated hangar is more beneficial than the GPU, but also generally way more expensive.

I would say 95% of the airports I go to have a GPU, so it wouldn't be a great limitation if I required one.

I use GPU if engine driven or AC powered and it is free. Otherwise, I don't. I figure it saves me about $10 per start in battery costs.

Using a GPU is generally routine, costs no time, and usually costs no money. It does good things to your batteries and engines.


A GPU is always helpful, even for the small PT6, but the big block PT6 and TPE331 benefit the most.

Note that the TBM is a big block PT6, PT6A-6x series, the Meridian isn't, PT6A-42A.

Mike C.

Of course not.

Look, this GPU issue is no big deal. If they have one I use it. If they don't, I don't. It plays 0% into my planning. I don't care one way or the other. However, MOST places I visit have them even the little airports. It's funny, it's the little airports that usually have a brand spanking new GPU sitting in the corner.

Don,

I've never picked a location because it had a GPU available. The planes are designed and will start perfectly fine on just the batteries. Issue is whether you use a GPU when available so that you don't put the load on the Batteries.

The biggest difference between Garrett and P&W PT-6 engines is the connection to the prop. At the risk of telling you something you already know, the propellor on a Garrett is directly connected to the turbine shaft via a reduction gear. If you turn the prop, the engine turns and vice versa. On a PT-6, they are not physically connected. The turbine has the compressor and turbine (which spins the compressor) one a shaft. Immediately behind the turbine is ANOTHER turbine which is connected to the propeller. The two shafts are NOT connected. so if you spin a PT-6 prop, the turbine will not spin. Now when you start a turbine, the key thing is to get it up to "self sustaining speed" which is the rpm at which the combustion exhaust is producing enough pressure to spin the turbine to keep the compressor spinning at that rpm. Below that speed, you get "bog down" where not enough air is being brought in to keep the thing spinning (yes, this is a simplification of the phenomenon). So on a PT-6, all the starter has to spin is the compressor and turbine, on a Garrett the starter is also spinning the propeller as well (with all the load that puts on the starter as it tries to accelerate this mass of metal). This is why, when shut down, Garrett props are "on the locks" and in a flat pitch while PT-6 props are in the feather position due to loss of oil pressure...

Now whether spinning up a PT-6, a Garrett or a jet engine, the electrical load is much greater than starting a piston engine. As a result, the batteries are bigger and more expensive (think $2-4K each). Every time you start engines on the battery, it stresses them a bit and wears them out a bit more. No battery will last forever. On average, on an MU-2 (Garretts), batteries last 2-3 years. Things which can make them last longer are keeping them on battery minders when not in use and using GPU for starts when able. In my case, my last set of batteries lasted over 5 years. Cant' tell you if it was the battery minders, use of GPU, or (most likely) a combination of the two.

Bottom line is that a GPU is a bit of a pain (delay in getting it, need the lineman to disconnect, may have to pay for it), but there are benefits to using one (especially if it is free). In my case, I always use a GPU at my home base (free). If going somewhere else, I choose to go there because that field makes the most sense to me (generally, selecting based on location, price of fuel, landing/ramp/overnight fees, etc.). Whether or not a GPU is available isn't on my personal list. Even in really cold weather, I can get the engines started just fine with the batteries...just may have to wait around 2-3 minutes to recharge them before starting the next engine (in the MU-2 with Garretts, at low temps we do a series vice parallel battery start which jacks up the voltage to the starter to get you started but at the expense of really dragging down the batteries).

Hope this helps.

Actually the -42 on the Meridian would be considered mid block as its smaller than the -64-66/67 in TBMs and PC12.
I will use a GPU when its available and free but not many airports I go to have some or they want big $$ to use it. But if its really cold or really warm and high you need it as you will get much faster and cooler starts in the TBM.

FWIW, here's a couple of paragraphs out of "The King Air Book" about starting:

"Whenever an EPU is conveniently available, the crew should use it for starting. Doing so allows both left and right engines to benefit from a faster, cooler, start. It eliminates starting wear and tear on either generator.

The airplane's battery needs to be exercised regularly to remain strong and hence using an EPU for every start can be detrimental to the battery. However, since EPUs are not "conveniently available" at many locations King Airs frequent, plenty of internal starts -- those using the airplane's own battery and generator -- are accomplished during routine aircraft operation. For that reason, conducting an EPU start when you can makes a lot of sense."

(EPU = GPU; External Power Unit; Ground Power Unit.)

I would just like to know where the Pilatus shoes have gone...

I would just like to know where the Pilatus shoes have gone...

Gills or Concorde?

My first two sets were flooded Gills, lasted 2 years each. Now on third year with Concorde AGM sealed batteries and they seem strong.

Mike C.

A GPU increases brush currents due to higher voltage, thus the S/G will see slightly INCREASED wear from GPU use from the extra arcing and heat generated. S/G wear is mostly the brushes and commutator. Bearings and windings are relatively unaffected by current.

If the GPU is used in place of a cross generator start, then the operating generator sees less wear during the other engine's start since it isn't on.


That's bad guidance, particularly for lead acid batteries. Lead acid batteries prefer to be fully charged and to have the least high drain cycles as possible. There is no medicinal value to an occasional battery start. You may do that to TEST the batteries are good, but that is NOT making them healthier.

Mike C.

I appreciate your input and mostly agree, Mike. Yes, I was speaking of less wear on the Starter/Generator since it wasn't being used for a cross-start or generator-assisted start. And as for the cycling of the battery, this was written back when NiCads were the norm. Would they be different than Lead-Acids in this regard? I know that the head of the electrical engineering wing at Beech sure convinced me that the occasional deep discharge was good.

High start currents are not healthy for any battery. It stresses the battery physically and chemically.


Yes, but that's a slow discharge to remove cell imbalance, an entirely different thing and vital for NiCad health. If you don't do this periodically (and it is a maintenance function, not something the pilot does), then you develop diverging cell charges which can eventually lead to a battery fire from thermal runaway.

I have experienced a thermal runaway in a NiCad aircraft battery, no fun at all, but fortunately it didn't progress to an actual fire.

A start does NOT level cell charges, so it is of no use in maintaining cell balance in a NiCad battery pack. Indeed, it might make the situation worse.

Lead acid batteries do not need deep discharges. A deep discharge causes significant damage to lead acid batteries.

Mike C.

Gills or Concorde?
Mike C.