Some questions about turboprops

RobertL18C wrote:

Not sure how critical it is to be facing upwind on a strong wind day, but probably a sensible precaution.

On the PT6-equipped aircraft I fly, I was told (after I specifically asked this to the manufacturer) that there’s no real limitation in terms of wind during start, nor is there a need to point into the wind.

Blank shotgun cartridges were used to start RAF DH Chipmunk engines. I think there was a few cartridges in a carousel.

Gorgeous nonetheless!

@Mooney_Driver
Oh, none of these photos I had anything to do with – they’re just from the internet. For inspiration!

Sebastian_G wrote:

. I would not be astonished is old school turbine limits have sometimes been set thinking of analogue dials with acertain tolerance. Then you connect a digital system and still used the same limits to trigger expensive inspections most probably not required.

Yep. Also recall the overboost lamps on some turbo piston engines. There also are procedures what has to be done every time that thing comes on, but hardly anyone ever does anything else than reduce the throttles so the light goes out and then forgets all about it.

@ Adam how on earth did you get that last pic taken? Tailgunner stand in a Mitchell or something? But way cool.

[ there is an “Adam” but he has not visited for years, so that notification went to the wrong person ]

Those start up discussions make me wonder how effective the cartridge starters were on some older jets. You don’t need a battery, just a cartridge and a hammer in the worst case. It used to look pretty interesting in some instances how those old engines fired up. Not sure if there are turboprops who used those too.

You will find that a Garrett powered twin, will be on par, or even cheaper to operate than a modern SET (maybe with the exception of a Jetprop). I know that goes against every grain of collective knowledge, but it’s true when you actually run the numbers.

Ignoring the 20% less fuel burn on Garrets, you get 5400hrs (and in some cases 7000hrs TBO) TBO. An overhaul is about $250-300K. HSI is anywhere between $20K-100K. Let’s average it at $50K. That’s $64/hr per engine, or $128/hr for the MET. I know piston engines that almost cost that much to operate! And a big bore PT6, that you’ll find in any TBM, will be close to $500K to overhaul. Divide that by it’s TBO of 3600hrs and a HSI and you get $144/hr. Now add the extra fuel burn of 20% on top and you can see that a SET PT6 will take you to the cleaners compared to any Garrett twin. Plus no need for compressor washes and all that monkey motion that PT6’s require.

Here’s another saving nobody talk about: oil changes and plugs. I used to change oil on my Aerostar every 50hrs. It was a $500 job each time. Do that over the TBO of an engine and it comes to like $18000. That’ll pay for most of your HSI (Garrets need an oil change like every 900hrs or so).

But yes, they do require good batteries. Always a good idea to do GPU start first start of day. But I also know of plenty owners that never use a GPU – me included. But I do have BatteryMinders on mine so that she’s always “topped” up. I changed to Concorde batteries last year from Gill, and they’re a lot better. They’re not cheap, but when you keep after them, they should last.

MU2’s are great, but for the casual flier you should look at Turbo Commanders (or maybe Conquests) – big, benign airplanes that won’t bite. Plenty of them in Europe and I know at least 3 service centers that have worked a lot on TC’s for decades and have good TPE experience. You can get an older -5 powered 690A for not that much. And they’re great performers still. Will outrun most of the SET’s in both speed and range.

RobertL18C wrote:

concern on hung starts am suggesting the [single-] shaft turbine design has a more sensitive requirement for top batteries condition.

You are right there: a PT6 will be much more tolerant of low batteries during start, but with the cost delta, you can afford one full hot section rebuild between overhauls and still be cheaper on the 337.
OTOH, operating discipline should keep you away from hot starts. Worst case, if batteries are low, nothing 300EUR worth of GPU rental cannot resolve. You could even try this DC-6 trick

Sebastian_G wrote:

the police will eventually stop you to see what is wrong with you.

Mooney_Driver wrote:

Imagine the number of speeding tickets if cars would record exceedances in the same fashion ;-)

Well, quite some newer cars would be perfectly capable of this.

Very true but my point is that limits get set with a certain compliance policy in mind. I would not be astonished is old school turbine limits have sometimes been set thinking of analogue dials with acertain tolerance. Then you connect a digital system and still used the same limits to trigger expensive inspections most probably not required.

For cars speed limits are currently set knowing most drivers will drive a little faster and severe consequences will only result once you are quite a bit above. Imagine you would automatically loose your license and the car would be worthless if you exceed the speed limit for 1 second. Today a limit might be 50km/h. Under those circumstances the limit would probably have to be 70 to 75 km/h resulting in the same average actual speeds driven. Or if you treat a current 50 km/h limit like a trubine operting limitation you would dive 40 km/ and the police will eventually stop you to see what is wrong with you.