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HeliTorque :: View topic - Bell 206 starting problem.
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HeliTorque Forum Index » Flight Dynamics

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Bell 206 starting problem. Goto page 1, 2  Next
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Thomas Coupling
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PostPosted: Wed Apr 21, 2010 3:52 pm    Post subject: Bell 206 starting problem. Reply with quote

This one has flummoxed the engineers – to the point, that they are now grasping at straws trying to alleviate the problem. I wondered if there’s anyone out there that may know the answer.

Cranking up the B206 has always been ‘not for the faint hearted’ and seasoned jockeys, after a spell away from the cockpit, will almost always refer to the start manual to refresh their memories before leaping ahead. In this case, all jockey’s concerned, are high houred, very experienced and know exactly what they are doing – so the bloke in the right seat is not the issue here (that gets that one out of the way Wink ).

Hit the start button, wait for the required N2 reading, open the throttle to flight idle and bingo! Off it goes, sounding like a tune from a Beethoven Symphony – lovely. Very Happy

Erm… Unfortunately not…. Evil or Very Mad

Hit the start button, wait for the required N2 reading, open the throttle to flight idle and then A BIG PAUSE of 1 – 1 ½ seconds, loads of fuel still going in of course - Igniter then cuts in, the big bang happens and the TOT shoots round to 850 – 875 degrees. At this point, everyone is counting: 1001, 1002,1003 – hoping that it doesn’t hit the 900 mark and above. The TOT then comes down to normal limits, and then all is well.

Maybe the next start will go just Dandy – and it does Confused . But the following day – it doesn’t Twisted Evil . No rhyme, nor reason for it. The engineers have changed the Governor, Igniter, HT lead and fuel components. Tried adjusting everything, scavenged parts from other machines – all to no avail.

It’s the pregnant pause that can’t be cured. It allows too much fuel into the chamber, before the igniter cuts in – hence the high temp. It has been muted that maybe adjust the igniter (in or out) to re-position the spark? I don’t know…Any idea’s gratefully received.

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PostPosted: Wed Apr 21, 2010 4:04 pm    Post subject: Reply with quote

Have you tried a start on external power?
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Thomas Coupling
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PostPosted: Wed Apr 21, 2010 4:08 pm    Post subject: Reply with quote

That was my thinking initially. Wink

The problem started a few Months back, and has got worse and worse. That is to say; the pause got longer & longer - then it didn't, then it did again! Twisted Evil
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PostPosted: Wed Apr 21, 2010 4:18 pm    Post subject: Reply with quote

TC

From the pilots point of view, I guess you mean N1 not N2 ?

And I know you probably know this already but, the Ignitors should start from the moment you hit the start button not when you open the throttle.

The governor has effectively (if not completely) nothing to do with the start cycle so why changing that would make a difference I cannot see (maybe because I am dumb). The governor effectively controls the amount of fuel that the fuel control unit is allowed to deliver once the engine N2 gets into the governed range. The fuel control unit (the bit you are controlling when you move the throttle) on the other hand is a different kettle of fish.

Think out aloud without any real rhyme or reason or in any particular order.

1. Whats the fuel flow rate like out the engine driven fuel pump ?
2. Is the N1 reading correct ?
3. Is the ignitor ticking as fast as you might expect on a normal machine (if the charge rate (my term for it, hopefully you can see what I mean) came down it might take longer to light off, from a batery thats been left overnight).
4. Whats the condition of the fuel Nozzle ?
5. Any restriction in the pipework from the fuel tank to the engine ?
6. If the ignitor really does only come in at the point you suggest has the igniton unit been changed or just the ignitor ?

Bendix or CECO ?

Is the TOT normal on the following start or does it get into the high 800s again.

Just for starters.
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Thomas Coupling
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PostPosted: Wed Apr 21, 2010 5:14 pm    Post subject: Reply with quote

Blimey Shocked

The 'crackers' are present throughout the start, from initial hit the button and throughout. Quite fast, as usual, I would say. it's a CECo system - not Bendix. We've changed the N1 and FCU (stolen from another machine) and the fuel flow is normal (apparently). Fuel Nozzel is okay. We don't however think it is a fuel issue - the overall opinion is that it's electrical? The following start is quite normal - maybe 750 - 800 degrees. 2nd surge is always around 800 degrees on a 'dodgy' start, and 700 degrees on a good one.

I did mean N1 Embarassed

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PostPosted: Wed Apr 21, 2010 5:41 pm    Post subject: Reply with quote

Stolen from another site, in response to a similar problem ("it sometimes 'hangs' for a second or so before firing. The resulting start is then hotter than normal, (still within limits)":

Quote:
Have your mech/engineer try shimming the fuel nozzle. The plume of fuel and air mix will sometimes not make adequate contact with the ignitors, resulting in the characteristics you have outlined. Moving the nozzle fore or aft will cure this in many instances, and it does not take a lot of shim to get a positive result.
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PostPosted: Thu Apr 22, 2010 5:37 pm    Post subject: Reply with quote

Personally I would junk the ceco system as they have always been crap as you have to modulate the fuel. Put a bendix system on and your problems will go away. Is sounds like the system ( pilot) is putting too much fuel in and swamping the ignitor with wet fuel, it then manages to dry itself out and then whoomp goes the xs fuel
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PostPosted: Thu Apr 22, 2010 5:45 pm    Post subject: Reply with quote

Drop the Pilot! (sorry - just been to see Joan Armatrading - she was fantastic!).

With over 5000 starts under the belt - the jockey knows what he is doing! Wink new igniter box going in tomorrow - let's see if that cures it.
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PostPosted: Thu Apr 22, 2010 8:42 pm    Post subject: Reply with quote

Are you using too much or not enough choke when its cold?! Wink Smile

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PostPosted: Thu Apr 22, 2010 9:37 pm    Post subject: Reply with quote

Try the kickstarter, when the electric start doesn't work i always use the kick starter.
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Thomas Coupling
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PostPosted: Fri Apr 23, 2010 6:23 am    Post subject: Reply with quote

There it was. Staring me square in the face all the time! Why didn't anyone think of it before? Shocked Use less choke, and a bump start. That should do it.

I wonder, if the Mrs spins up the tail rotor fast enough, and then throw open the throttle, it’ll fire up. No sweat. Very Happy

I'll mention it to the Engineer this morning - as he's clutching (no pun intended) at straws, he'll try anything I expect. Rolling Eyes
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PostPosted: Fri Apr 23, 2010 9:58 pm    Post subject: Reply with quote

OK this may sound really stupid.

Mrs animalsticks (Full B1) and i were chatting this over and if the powers good, FCU/ Can that was borrowed is OK, nozzles clean, fuel of quality and quantity, N1 checked and ignitors good...

... is it possible that the powers on and the fuel pump CBs are in when the pilots opening the throttle during 'controls free check?'

that would maybe account for the on off occurrence

OR

any deposit build up hindering gas producer or mixing? i.e position at start up again accounting for the random occurrence

forgive me if i'm missing or forgetting something
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PostPosted: Sat Apr 24, 2010 6:43 am    Post subject: Reply with quote

Animal

I may be mistaken but from memory it is not possible for fuel to get through the engine unless the engine is turning, the engine driven fuel pump takes the low pressure fuel from the tanks (provided by the electric pumps or drawn from the tanks when the engine turns without the pumps) and makes it into high velocity (and by means of a restriction, namely the FCU into high pressure) fuel, if the engine doesn't turn, the fuel doesn't even with the throttle fullly open.

Even if the fuel got to the FCU by going down the return line from the FCU to the pump, or if the pump allowed some fuel to leak through, that fuel has then got to get through the FCU which is not turning and is full of spring loaded valves and then it needs to reach the fuel Nozzle with sufficient pressure to open the nozzle's metering valve to at least primary flow to allow any fuel in.

If all of these things were present then I guess you might get some fuel into the engine just with Boost Pump pressure alone.

Any of the above couild be wrong , its just my musings on a Saturday morning.

Out of interest TC how long is the pregnant pause ? The Allison CECO Traning Manual says Lightoff should be within 3 seconds and you are within limits temperature wise so are you trying to fix a non problem ?

Might it be better to tweak the start temp down slightly and accept what it does ? Is the behaviour actually outside anything specified ?
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PostPosted: Sat Apr 24, 2010 6:59 am    Post subject: Reply with quote

Another wild stab in the dark, is the fuel check valve sticking ?

The one between the FCU and the Fuel Nozzle.
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PostPosted: Sat Apr 24, 2010 8:00 am    Post subject: Reply with quote

We've found a possible cause.

The machine is fiited with a PHi Mod, which prevents starting with the throttle open. We're going to disconnect that next week and have another go.

The new igniter box didn't work yesterday Sad In fact, it was worse. The 'hang' was so bad, we aborted the start all together Evil or Very Mad The nexy possibility would be to adjust the igniter, effectivily 'shimming' it out of the can, for better spark distribution.

The ship has had a history of hot starts over it's life-span. Beggars belief what the cause is. H&S want £1700 per day to send an engineer out, and £3000 to test the engine Shocked Yikes!
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