206: Random flameouts at idle

[DigitalFudge]

Hey guys,

 

I was hoping to pick your brains on something strange that's been happening to me.

 

I have about 300 hours on the Bell 206 JetRanger. I work for a tour operator, and I do a lot of short flights, so I go through a lot of start cycles.

 

One of the machines has recently come out of an MPI, and was test flown and signed out with no problems.

 

So, next morning I rock up to work and start flying. First few flights go well - no problem. On my third flight that morning, I start up, everything looks normal, machine starts like a dream (TOT only goes to about 720C on second peak..), but suddenly at 55%N1, it just dies.

 

First thing through my mind - you idiot, you forgot the fuel valve off! I glance down - nope - fuel valve is on! I'm still cranking at this stage - there's some white smoke coming out of the exhaust.. So I close throttle, shut off valve, pull boost pumps etc.

 

Chief pilot rocks up, jumps in helicopter. Vents it, starts it.. some white smoke and then it starts normally.. goes up to flight, everything seems normal.

 

Engineer tells boss that I probably started with closed fuel valve and opened it again..

 

Chief doesn't believe me when I tell him fuel valve was on. So I think, ok sometimes the fuel valve actuator sticks a bit, maybe that was it.. Besides, I've started with fuel valve closed, and normally it sputters a bit before it dies. This time it just -died-.

 

So we continue flying.

 

Flew probably 5 times more during the day with no incidents.

 

Then after one flight, I roll throttle to idle and start my two minute cool down period.. 10 seconds later - engine flames out again!

 

Now I'm confused.. throttle is against the gate like normal.. don't get it!

 

Chief gets in again.. starts machine. Goes flight to idle a few times - of course nothing happens.

 

Engineer checks out the rigging on the throttle with relation to the FCU, and everything looks normal. He looks at the gate 'button', and that little pin that connects the box containing the starter switch to the collective - everything looks normal.

 

They all think I'm crazy.

 

Next day, I get to work. Chief does first flight. No problem. I do 3 or 4 flights in same machine. No issues. Last flight of the day, during 2 minute cool down - 45 seconds after closing to idle - flame out! My hand wasn't even on the throttle..

 

And this after being particularly careful - making sure that I'm not closing too hard against the gate etc..

 

Now the chief is fuming. He starts up again, flight to idle flight to idle. etc - of course now everything is normal again! He intentionally kills the engine with the fuel valve. Engine sputters and dies.. and says - ok you hear what that sounds like. Is that happened - did you close the fuel valve?

 

I tell him that there is no way that I would close the fuel valve during the two minute cooldown.. He thinks I'm doing something stupid..

 

So next day, another pilot flies that machine - as Murphy would have it no problem with machine..

 

I don't understand it. It doesn't happen when I fly any of our other jetrangers.

 

I've done over 200 starts on that particular machine, and more than 500 starts on type, and have never had any issues until after it came out of MPI. Then I have 3 flameouts at idle in two days over 12 flights.

 

Is it me? Is it the machine?

 

 

Crazy..

 

Any of you have any ideas what it could be?

 

 

All the best,

DF

Edited February 6, 2012 by DigitalFudge

[Wally]

I've never had that happen, a couple thousand hours and starts. Somethings wrong.

[Gomer Pylot]

My guess would be a fuel control problem. It's going to be hard to find, though, and probably hard to convince maint to troubleshoot it. The engine should not be flaming out. It's also possible that it's a fuel problem, perhaps water in the fuel, but that should show up during the start, not at the shutdown, since the fuel should be well stirred by the end of the flight with the boost pumps running.

[Hotdogs]

Bleed air valve?

[aeroscout]

Have you noticed any fluctuations in fuel pressure, or any related fuel display, flow, or any other weird indications ? It definitely sounds fuel related. I attempted a JetRanger II start with the fuel valve closed once, it lit off, but never made it to idle.

[akscott60]

My ass would write that aircraft up and not fly it again until someone looked at it. I wouldnt do any EPs in that either, since it wont hold a steady idle.

 

What Ng does Mx have the idle set to?

[gary-mike]

Bleed air valve?

 

Not if the engine reached idle speed prior to shutting down. The bleed air valve or strap, is for start up, and if it fails, you will have either a hung start or a hot start, but not reach idle RPM and just shut down.

[Wally]

My ass would write that aircraft up and not fly it again until someone looked at it. I wouldnt do any EPs in that either, since it wont hold a steady idle.

 

What Ng does Mx have the idle set to?

 

AMEN! MNT should sign it off each and every time an issue is addressed. As it's not a predictable issue, it may take progressive repairs to finally identify the cause of unintentional shutdowns.This situation is difficult for you and maintenance, but it should be addressed before it causes a bigger issue. Think how the both of you would feel if this is the precursor to some catastrophic event...

Edited February 7, 2012 by Wally

[Hotdogs]

Not if the engine reached idle speed prior to shutting down. The bleed air valve or strap, is for start up, and if it fails, you will have either a hung start or a hot start, but not reach idle RPM and just shut down.

 

I was under the impression the the aircraft never reached idle rpm.

[gary-mike]

I was under the impression the the aircraft never reached idle rpm.

 

I went back and re-read the original post. Sounds like you are right. The turbines I am experienced with are self sustaining around 50% and by 55% the starter has shut down. It is a possibility that a bleed valve could be the issue. It sounds like it is getting fuel ("there's some white smoke coming out of the exhaust").

 

I don't have experience with 206's, but I have 12 years turbine engine experience on GE F-110 and PW F-100 engines. I am a little rusty though, (got assigned a special duty, and now sit at a desk). I had to jump in, I love troubleshooting and miss the flight line. Wish I could get my hands dirty and help figure it out. Gives me something interesting to research today.

[Mikemv]

I would recommend that you write up the sequences for the DOM/A&P and ask him to contact the RR Tech Rep for input.

 

On the RR250 Series turbines, the bleed valve is open during start and idle and modulates toward fully closed at 94%Ng.

 

Check the static rigging on the fuel control with the pin in holes rigging check.

 

Best info is from RR Reps!

 

Mike

[Spike]

In my opinion…….

 

From the description of events (Chief’s actions), writing up the machine is more than appropriate. That would simply take the problem out of your hands….

 

As for the problem itself, seek out the Maintenance Manual and read. Troubleshooting starts with reading, not with tinkering… In any case, it appears you have an idle circuit fuel/air mixture problem (obviously). While I have my suspicions, I’ll keep them to myself. Let your mechanics figure it out……

[aeroscout]

Very interesting topic. I hope we see the discrepancy get successfully troubleshot without incident. It is almost like a mystery novel. In this case, I am sure the butler didn't do it.

[DigitalFudge]

Hey guys!

 

Thanks a lot for all the responses. After speaking to the engineer myself, he said that he'll take another look.

 

He took out the little 'arm' on the FCU that connects to the throttle linkages and found that the one hole had become slightly elongated.

 

It seems that occasionally it would move from the one end of this hole to the other, causing a larger or smaller movement due to the change in arm. Once in a while it would just be enough to kill the engine.

 

He replaced it, and now when you go from the idle gate and slowly roll it closed, it goes about 5mm before the engine actually dies. Before, it was more like 1mm.

 

Machine flew about 10 times today, everything seems good!

 

Hoping that this was it!

All the best, and thanks again!

 

DF

Edited February 7, 2012 by DigitalFudge

[aeroscout]

Hmmm, an elongated bolthole on a throttle control arm linkage. Here's hoping you have no further issues.

Edited February 7, 2012 by aeroscout

[DigitalFudge]

Yea..

 

You gotta wonder how that happens. I mean it's not like there is that much force on it. It must be very old.

 

I think they dodged a bullet on that post maintenance test flight - I believe they did an auto or two to check the rotor rigging..

 

I'll be a lot more strict about the amount of 'play' on things from now on..

 

DF

[Hotdogs]

Yea..

 

You gotta wonder how that happens. I mean it's not like there is that much force on it. It must be very old.

 

I think they dodged a bullet on that post maintenance test flight - I believe they did an auto or two to check the rotor rigging..

 

I'll be a lot more strict about the amount of 'play' on things from now on..

 

DF

 

Always amazes me how something that small can make your day goto hell in a hand basket real quick in aviation.

[rotormatic1]

You can check the N1 rigging during your preflight to help make sure this does not happen again.

The fuel control has a pointer that has 0, 30 and 90 on it. For 0 and 90, the fuel control arm should be hard on the stops. Pointer position is not important.

The 30 degree position is important for preventing flame outs.

During your preflight, open the throttle all the way, and do a hard snap to the idle position. Go back and look at the pointer. It should not be more than 5/64^th of an inch below the 30 degree mark.

This is usually not a problem on Jetrangers, unless you have excessive slop in the N1 linkage. More of a issue with 500’s.

If you have duals installed, snap the throttle from the co pilots stick. This is where you normally find the problem with slop between the co pilots stick and the pilots stick.

Also, on your preflight’s, be on the lookout for the bolt on the fuel control arm being installed the wrong way. If the nut is inboard, this can limit travel, and you won’t get max power.

[iChris]

Yea..

 

You gotta wonder how that happens. I mean it's not like there is that much force on it. It must be very old.

 

I'll be a lot more strict about the amount of 'play' on things from now on..

 

DF

 

 

Those levers normally had sleeve bushings installed (see figure) to prevent the bolt from making that type of wear and elongation. That must have been an older lever. I assume your new replacement lever has bushings.

 

 

 

[aeroscout]

Man, oh man, that's good stuff iChris. A picture is worth...

[Tenacious T]

DF, good job sticking to your guns, it can be hard when you know something is not right and people with more experience are looking at you like you are crazy. Been there.

[LifeApresMil]

Amen to what Tenacious T said. I really hope for DF's sake that that Chief Pilot realizes what a hero you are now. It's a shame that once you get a little mud on you, it's hard to shake that dirt off. It's human nature for what is remembered: easy to go from Hero to Zero, difficult to go from Zero to Hero. I could be mistaken, all in all, I think somebody owes DF some beers... DF is my new hero.

Best Regards,

Occasional Hero

[Gunner]

Hey DF... Good on you. Initially I feel guilty about calling the DOM in at 3:00 AM to address an issues but then instantly I get over it. He gets paid to come out and it's my butt and my crew's butt on the line.

 

Stick to your guns and if you think there is something wrong then their usually is!