View Full Version : turbo prop warm up

23rd Apr 2002, 10:41
was asked this question the other day by a colleague and not knowing much about prop driven birds couldnt give a good answer. help please:confused:

when warming up a prop driven a/c which of the following conditions would u set -
1) prop feathered, idle mix?

2) prop fine pitch, idle mix?

3) prop fine pitch, rich mix?

any answer is more than i could give, but i would opt for 2??

23rd Apr 2002, 19:42
Never flew a turboprop aircraft with idle or rich mixture. Up in Alaska, when it got cold (< -40) and we didn’t have a Herman Nelson, we would tie the prop down and start the engine (you can do that on the PT-6 series) and let the oil temps creep up before introducing the oil into the propeller governor. More for limiting blown gaskets, otherwise keep the engine in feather until temps have stabilized.

Yup, flying around in bunny boots and a down parka, don't miss it at all.

23rd Apr 2002, 23:12
Often wondered how much force is needed to prevent a PT6 prop from turning during start.

I had heard - but can't verify - it's possible to hold onto a blade during start. Wouldn't think it's advisable though...

I've also heard of an operator that uses a prop. brake to stop rotation during ground loading operations, allowing them to leave the gas generator running. Wonder if it's true?

23rd Apr 2002, 23:28
Yes indeed, it is possible to hold the propellor blade with only two fingers while the PT-6 is started....PT6-27 anyway.
Suggest that the oil is truly warm and toasty before the prop is unfeathered and taxi commenced. For those that have paid the overhaul cost can only testify as to the .....BIG bucks to do otherwise. Forwarned is forearmed.
And yes, using a prop brake has been tried with good success on certain models.

24th Apr 2002, 02:46
This is the first sensible post I have seen you produce 411A. Wish the other 99% were the same!

24th Apr 2002, 09:29
CityFlyer (or whatever they are called these days) are a UK operator who use the prop brake as standard on their ATRs. This allows them to shut down the left engine completely and stop the prop turning on the right engine while keeping the generator running (as there is no APU) so that the GPU can be plugged in by the ground crew. The engine can then be shut down with a seamless transition of electrical supply from engine generator to GPU without risking chopping off the ground crew's head.
Use of the prop brake on turboprops without APUs is a deeply debateable point due to safety issues and more carriers don't use them than do use them. By the way, on the Saab 340 the prop brake meachanism weighs more tham a male passenger with baggage so when you can only take 34 people anyway you can get a significant percentage improvement in payload capability by ditching it.

24th Apr 2002, 12:07
Thanx for reply's - I do know that when we used to operate ATP's the guys would often turn over the gas gen with someone holding the prop stationary! I think they stopped when one of the lads was launched across the hangar one night.:)

24th Apr 2002, 12:37
Only flown pt6 and garret on ag work pt6 can be started and hold the prop no worries wont hurt the engine so p&w say (never tried it).... always start pt6 in gnd idle no mixture all done by the fcu leave in feater till min oil temp then into fine for taxi book says cycle at least once to get warm oil into the prop hub and governor.... seen airline jocks in pc12 and king air having props going into fine as the secondarys ar coming in...no idea why only loads the starter for no reason and makes the start longer. The garret dont try the prop thingy will throw you round a bit.... start it as per the book low rpm and flight idle when self sustaining then into gnd idle till oil at min temp have had good results 6000hrs on pt6 b4 overhaul using this method. Operating down to 0c in the winter and 40 c in summer :) :eek:

24th Apr 2002, 17:49
The reason some put the prop into fine while starting is for starter/generator and battery cooling, getting some airflow into the NACA vent and the battery wing root vent. It shouldn’t prolong the start any as the –6 is a split free turbine and the gas generator section doesn’t really care what the power section is doing - open loop design.

25th Apr 2002, 12:25
Thanks mac for the info there ...... realize there is no connection to the gas generator thought that the oil required to drive the prop to fine would require some hp too ??? is the temp that critical on the batteries ?? and the starter gen has its own fan so that as soon as you start cranking it has air going through it. When we start always leave in feather till oil temp in the green starters always make overhaul no problems. Just wondering not being smart....... thanks again byee :)

25th Apr 2002, 14:35
Our starting SOP is to first consider the apron surface under and in the immediate vicinity of the prop blades. When the surface is covered by lose stones, the props are set to full increase for the start to minimise the chances of stone damage to the blades.

Other than that, always start with the props in feather. This also has a practical application on a busy apron as there's invariably another aircraft - usually another Kingair - parked directly behind.

25th Apr 2002, 21:11
Hey Burner, you’re playing with me.

The last post was to try to answer the question of why some operators choose to use ground idle as opposed to feather when starting, for all persons reading the thread – not only PT-6 drivers. It is with sincere doubt that I could provide you with any edification regarding the power-plant which you are obviously very familiar. When referring to the split engine design, I was trying to solidify the point that at low power settings the gas turbine doesn’t really know what the power turbine is doing, as long as they are turning in opposite directions. ;)

“Oil required to drive the prop to fine would require some hp too”
You are correct that an additional amount of energy is required to provide the prop with high pressure oil, in the –65 it was 375psi and in the –67d it was around 750psi. In the smaller Kingairs and the –99 it is less. Sorry to say, other than adhering to starter limits, I never compared the duration of the different types of starts. Is it materially different, and do the advantages out way the disadvantages? Guess that would be another thread.

“Is the temp that critical on the batteries ??”
In the Kingair series one cannot do cross-generator starts and the NiCd batteries are taxed starting two engines in quick succession. An already warm NiCd that is depleted and is trying to absorb as much juice as it can from the generators may get very hot, resulting in a possible thermal runaway.

“The starter gen has its own fan so that as soon as you start cranking it has air going through it.”
In the BE-02 cross generator starts are the norm with min N1 of 72% and this zaps the supplying generator of 400 amps. The maximum continuous load on these gens is 300 amps, hence they tend to be overworked in the start. The generator has its own NACA vent located on the inboard nacelle of each engine, a fabric conduit then runs to the generator section to provide outside air. When in feather there is no airflow to the gen from the outside (in the 02) and overheating may ensue, causing premature failure. Then again, it may not.

“When we start always leave in feather till oil temp in the green starters always make overhaul no problems.”
The green arc for oil temp on the –65B starts at 0 degrees C and at 10 degrees C for the –67D, if you’re in any part of the world that I would inhabit, those would be already below ambient temperatures, so there should be no harm to the prop governor seals if starting out of feather.

As OzExpat pointed out, on unimproved surfaces the large suction vortex that is formed below the props at feather tends to transport FOD in a vertical plane. When the props are at ground idle some amount of horizontal initiative is given to the FOD, sending it to the rear of the aircraft rather than against the fuselage or into the engine. Obviously aircraft such as the Twotter and AgCat don’t suffer from these problems.

So those are my thoughts on why and when to start out of feather, or more accurately why some people do it that way. Oh one last thing Burner, you’re quite smart in my book, you manipulated me into spending an hour in front of the computer typing – something my professors could never do.

Fly Safe - Man

26th Apr 2002, 12:17
Hey mac thanks for the info makes sense now wasnt aware of all the twin stuff cross starts etc etc and the oil pressure thing came outa the garret part of my brain...i think.... ;) the green arc on the airtractor -15 starts arond 10 c i think cant remember the 65 ag but it was similar i seem to remember but one further thing at the risk of being podantic i have never seen a pt6 pick anything up in feather and have been on a lot of dirt strips have seen dash 8 started at tanami same thing in feather doesnt pick anything up but as the prop comes out of feather into fine ....well thats a different story stones everywhere so ozexpat cant understand your theory????.... :)

26th Apr 2002, 12:39
burner... this is just a guess, but I suspect that the DHC8 that you observed had its props standing over a specially cleared area, or a rubber mat. Such things are becoming more common here. If I get to park over a mat, I can happily modify my starting technique.

If you've seen the gouges that I've had to have filed out of props over the years, you might understand more about why one starts engines with props feathered on, as mac so delightfully phrased it, "unimproved surfaces". But, hey, feel free to do things the way you see fit - that's what command is all about.

27th Apr 2002, 20:25
A reply to the original post: I was under the impression (having operated the DHC8 for 2 years) that a turboprop is driven by a gas turbine engine! Surely this is completely different from piston driven A/C?
Therefore why need to worry about how fine the prop is? Once the engine has stabilised what's the problem, except possible ice ingestion?

Also the idea of a prop brake sounds great!! No real need to worry about when that dreaded GPU will arrive!!!

28th Apr 2002, 22:08
Dav - you are quite right in what you've noticed. In the title I've said "turboprop" but my original question was actually directed towards piston engine/prop aircraft !! Thank's - I hope this clarifies the issue.

Interesting answers and info so far - but I do love my trusty and reliable CFM56-3's!!!!!