Starting a PT6 with bleeds open?

[looproll]

What are the consequences of starting a PT6 with bleeds open?

[tired of the ground]

hotter starts

[looproll]

how much hotter are we talking about? No other concerns?

[snowbear]

It depends entirely on what variant of PT6 and what airframe it is hung on. They are all plumbed differently. In general hotter starts can be anticipated but how hot is an unkown.

[twinpratts]

hmmm... maybe a smell of exhaust? (I'm just guessing here...)

[ahramin]

-21 can be started with bleeds on without trouble. I would imagine with the -28 you wouldn't even notice a difference.

[Captain X]

Why would you do it?

This all stems from a you tube video where some guy starts a King Air 90 with the bleeds open.

http://www.youtube.com/watch?v=z7hnOn6qggA

[desertduster]

we always started the King Air 300 with the bleeds on low.

[Captain X]

we always started the King Air 300 with the bleeds on low.

What is the Benefit from doing this?

[desertduster]

Well, this is what I was told to do. But the reason was that as different turbines are more efficient at different speeds, it released some of the back pressure, and made for an easier start. I guess it kind of helped the bleed valve out. Never tried to start it with the bleeds off to compare.

[Pile-it]

Cheyenne bleeds were rigged to be open at all times, ie no switches for us monkeys...

[Pile-it]

The first stages of the compressor section are axial flow, meaning they compress the air by pushing it together. They are more efficient at lower speed than the last compressor stage which is centrofugal. This stage further compresses air by "flinging" it outward. At low speed power settings, a valve between the two compressor regions allows air to escape preventing a "traffic jam". Bleed air comes from this source to provide cabin heat among other useful things. On start-up if the bleed valves are open, it is possible that (depending on the configuration), not enough air is making it to the hot section, resulting in higher than necessary temperatures. Follow the manufacturer's recommendation and you're covered...

[fogghorn]

Why would someone start a turbine with the bleeds open? It is not like they are controllable as they are preset, so not sure of what the purpose of this thread is.

[Pratt Punk]

The model B200 and B300 require engines to be started with Enviro off during starts to prevent possible damage to flow packs. It is acceptable to start with Pneumatics on with no risk of warmers starts.

[TheCheez]

Our checks for the King Air C90B have bleeds open, though I'm not sure of the logic behind it.

[trey kule]

Pile it wrote:

Follow the manufacturer's recommendation and you're covered...

100% in agreement with this. I will never understand rational like "because I was told to do it this way", or, as some mentioned, why the original question......unless.

someone, who was not suppossed to, started with the bleeds on......and is hoping that it did no harm......it is always interesting to wonder why anyone would ask such a question here, is it not?

[looproll]

Why would someone start a turbine with the bleeds open? It is not like they are controllable as they are preset, so not sure of what the purpose of this thread is.

fogghorn I'm talking about a PT6-21 or PT6-28. You can have them closed or open during start, but the C90 checklist calls for them ON after start. I'm not talking about the bleed between the axial and centrifugal. I'm talking the p3 bleed air for environmental and such which is controlled from the cockpit. As seen in the youtube video posted above, the guy opens them prior to start. Maybe it's in the checklist for the newer models?

[trey kule]

loop roll, I must apologize. I see now where the origianl question came from. and truthfully, I dont know the answer. If you follow manufactures recommendations should not be a problem. You never know. The a/c might have had engine mods of some other mods that make starting this particular aircraft different than usual.

Some years ago there was a procedure for arming the ignitors on the 100 series, and many many pilots simply ignored the proper procedure. Well, there was a reason for it which unfortunately some pilots (some of which did follow procedure) found out when the ignitors failed. As an aside there was an AD put out. the point here is that many pilots simply made assumptions that, using perfect logic brought them to the totally wrong conclusion.
To re-iterate another poster again. follow the manufactureres recommendation for the particular aircraft you are flying.

[just curious]

I believe if you check with your friendly AME, you'll discover that on -27 & 28 PT-6's the Bleed valve opens at 70 percent NG; therefore their position on start is moot.

[carholme]

just curious;

The position of a PT6 engine bleed valve at start is definitely not a moot point. The bleed valve is not a unit which can be switched on or off, it is speed (Ng) sensitive.
In normal operation the valve remains open as the engine is spooling up and does not fully close until the engine is stabilized at approx 70-80%Ng. It remains open on start to unload the compressor and prevent surging/stalling. This is because the axial compressor is far more efficient than the centrifugal compressor at low Ng speeds and if the bleed valve were closed, the more efficient axial stage would choke the centrifugal stage and cause a reversal of the airflow; that is, surge or stall.

At high power settings, above 70-80%Ng, should the bleed valve open, you would have a dramatic stall/surge and because the airflow through the compressor is reversed, the combustor senses this loss of air and the Ng wheel will blow the burn cycle out the exhaust as can be seen in this link. If it is surging because of a weak bleed valve or incorrect closing point adjustment, the flame out the exhaust will continue to cycle as will the NG, ITT and Q readings. A full stall may flame the engine out completely.

http://www.youtube.com/watch?v=RPkZBR89y_M

P3 air is just that, air taken from position 3 in the engine and can be used for a variety of purposes, heating, pneumatics, packs, FCU sense, etc., and as such will be identified in the flight manual.
If you wish to verify the ITT change in operation, just look at the ITT gauge and shut the heater off if you are using it and you will see the resultant change, which is quite small by comparison to a bleed valve failure at high power settings.
One of the most common causes of surge/stall, other than a defective or incorrect valve setting is FOD damage, dirt or erosion of the axaial compressor blades. Good ECTM analysis combined with effective compressor washes will go a long way to ensuring that you don't encounter stall/surge.

The main point is please do not @#$! around with aircraft switches and procedures based on these forums. Sit down with the flight manual, your training/Check pilot/Chief Pilot/Maintenace staff and learn the correct procedures.

My firm belief is that all line pilots should be involved in ECTM analysis so that you understand the history of the engine in the present overhaul cycle and how you can become better engine managers. It is how you operate it, that shows up on the ECTM.

Regards

carholme

[bezerker]

Christ, can there really be this much confusion between whether we are discussing the COMPRESSOR BLEED VALVES or the ENVIRONMENTAL BLEED AIR VALVES?

My understanding is that the only issue with starting with ENVR ON, asides from slightly warmer starts, is (as was already mentioned) damaging the electronic flow control units. I believe most newer aircraft (B1900, B300/350, newer/modified 200) have the electronic units while older ones (100, old 200's) don't. Supposedly the "abnormal" airflow recieved by the electronic unit during a start can cause it to go a little bonkers, opening and closing valves rapidly and things of that nature. This will apparently shorten its already pathetic life expectancy.

I think the question to be asked should be, "What benefit is there to starting with ENVR ON?" (asides from having 2 more switches in the cockpit that would never need be touched by lazy pilots.)

[1000 HP]

I've accidentally started a PT6-114A with the bleeds air open a few times(against POH procedure) and noticed no increase in start temperature. It's sometimes hard to go through a huge checklist when the waves are trying to bash you to bits on the rocks...

[trey kule]

1000 hp wrote

I've accidentally started a PT6-114A with the bleeds air open a few times(against POH procedure

Ah, the old......I never make the same mistake three times!

Perhaps that was when you were new to type or had not learned the importance of checklists...but...
take it from an old guy if that is not the case....have a little chat with yourself. the next thing you may "accidently" do is forget to lower the gear, or take out the engine plugs, turn off the master....all things that have happened...
this is not a rant at you per se, but it should be a lesson to all pilots who forget stuff.. Once and you learn.
the second time you need to have a self attitude adjustment before something worse happens.

sorry to hijack the thread.

[looproll]

Christ, can there really be this much confusion between whether we are discussing the COMPRESSOR BLEED VALVES or the ENVIRONMENTAL BLEED AIR VALVES?

My understanding is that the only issue with starting with ENVR ON, asides from slightly warmer starts, is (as was already mentioned) damaging the electronic flow control units. I believe most newer aircraft (B1900, B300/350, newer/modified 200) have the electronic units while older ones (100, old 200's) don't. Supposedly the "abnormal" airflow recieved by the electronic unit during a start can cause it to go a little bonkers, opening and closing valves rapidly and things of that nature. This will apparently shorten its already pathetic life expectancy.

that's what I was looking for

I had heard that it might damage the flow control, but I couldn't remember

and yes the bleeds (instrument and environmental) that are controlled from the cockpit are the ones I'm speaking of, of course

[rigpiggy]

Chapter/verse. Beech 1900 manufacturers checklist before start 18c. Bleeds-open. Offload the compressor to allow faster start easier on the battery/starter generator