Strange B206 start issue

[flyingsniffer]

Hi, first post so be gentle.

My 206 BIII has a strange start problem that has my engineers flumoxed. If it's left for any amount of time (24 hours, sometime less), the fuel drains back through the pipework and it just doesn't start - oh and the problem is intermittent, sometimes it starts fine! When the start fails, the system needs to be bled, usually back to the pipe that leads into the FCU. Once this is done, it's fine. It starts normally and runs sweet as a nut.

So far they have changed the in-line non-return valve, had my PT governor overhauled, tried another engine driven fuel pump, non return valves on the two electric fuel pumps.Closing the fuel valve after shutdown has no effect. None of this has made any discernible difference to the start.

Anyone come across anything similar? This problem has been ongoing for a few months now.

[nodrama]

The only component you haven't mentioned is the FCU. Have they looked at that? Fuel passes through this component before and after the PT governor.

[ramblingrotors]

Unless you have a CECO system no fuel goes to PT governor, can't think of ever seen a B3 with a CECO system.
Think you should be looking for external leaks from fcu and pump.

[flyingsniffer]

My bad! I meant FCU not PT Governor. Although, the PT governor was overhauled as part of the diagnosis showed it was out of limits although not due for OH by time.I'll edit the OP.

BTW it's a Bendix system.

FCU has been swapped for another one, as has engine driven fuel pump. Issue still persists. There is NO evidence of external leakage of fuel, in flight or during the start process.

To bleed it, the nut on the FCU end of the line from the fuel pump has to be undone, the electric fuel pumps switched on until fuel runs out (being careful not to let fuel flow into the Starter/generator), then the nut is done back up. All this of course with the fuel line into the burner can removed and wrapped up in a plastic bag.

[MLHeliwrench]

been a while, but the pilot controlled valve shouldn't matter, the two pump check valves and the one after the FCU should be enough.

You have checked out the inline check valve after the fcu and before the nozzle right?

Also, as a final check, make sure all connections of flex tubes in the fuel cell are tight and that none of the pressure lines have chafed or have any small leaks.

[vfr440]

Intermittent snags like this have contributed to my loss of hair - and an increase in the daily intake of wine (it just hurts less after a few :O!) But simplistically it is either fuel draining back to the tank (impossible I would have thought if it still happens with the fuel valve 'shut') OR it is air getting in. And the prime area would be on the upstream (suction) side of the EDP. Could the hose be old-ish, sharp bend and possibly porous when so bent? A Px check would prove or disprove this hypothosis.

Erratic starting in my experience has historically been an FCU malfuntion, but this would not be the case here if it has been changed. And anyway it doesn't always happen............. :confused::confused:

Would be VERY interested to find out what the fault is when it is divined ~ VFR

[vfr440]

Great minds and exactly coincident timing (19.25)!! Could be the cause and good diagnostics. ~ VFR

[MLHeliwrench]

HAhaaa. been two years since I have deal with a fuel issue on a 206. I know some pilots just leave the fuel valve open all the time. Shouldn't matter.

But, it is a good bit of troubleshooting because assuming the electronic valve is solid, and you have it off right after shutdown, the problem must be below it.

I would focus in on the lines themselves that are submerged in the fuel. Could be a small leak in one of them, or it could be rotten. You would not notice because the pump would pressurize it and just piss the fuel back into the tank.

One way to test would be to drain the fuel until just at the top of the pumps, close the electronic fuel valve and turn on the boost pumps. Peek into the tank with a flashlight or just listen for spray or dripping.

This would explain the intermittent nature of the problem too. The leak would only matter if it was below the fuel level. So, you could even experiment in the mean time and see if it happens on a full tank vs minimum fuel.

[flyingsniffer]

Thanks all, good intel. I can see why it's causing the engineers so much hassle.

I'm no engineer (you can probably tell!), but if the fuel valve isn't stopping it draining back, isn't the problem on the engine side of it rather than in the tank? I'm trying to do things that will isolate where the problem is occurring.

Having said that, I've not correlated if the problem ties with fuel levels. One to try for sure.

I'm sure all the flexible hoses have been pressure checked. And yes, check valve between FCU and burner has been changed (I remember the bill!)

[yellowbird135]

It's been at least 8 years since I did any work on RR250 series, but I seem to remember that it is possible to assemble the poppet in the Inline check valve incorrectly (or something like that...)

[unstable load]

I'll put my money on the lines and fittings inside the tank.
You say it's intermittent, but how does that relate to the fuel level in the tank?
I think you have a drainback below the shutoff valve and when the shutoff is opened prior to start, the rest of the head of fuel then runs back down to the lowest point in the line, leaving the engine pump sucking air.

Check the in-tank check valves and all connections from the pumps to the electric shutoff valve and you may find a loose connection or a faulty check valve is the cause.

It's been a while since I had a tank out or fiddled around in one, but that is my guess for the starting point of your snag.

[PEASACAKE]

Flying Sniffer

First of all you should bleed at 2 places, at the top nut on the fuel pump filter housing as per the manual and then at the fuel nozzle with throttle open and cranking.

Secondly I have had the same problem for 2 years on a 206, after replacing the FCU (A free loan item so no cost) and all other pipework and pump it was traced to the fuel line in the fuel tank, we changed the pipe, problem gone.

It was intermittent and always failed away from base, hopefully its just the pipe

REMEMBER TO BLEED AT THE CORRECT PLACE ON THE FUEL PUMP OR YOU COULD HAVE A BUBBLE OF AIR WAITING TO CAUSE YOU A PROBLEM WHEN YOU DEMAND HIGH FUEL FLOW..........

Good luck

[flyingsniffer]

Thanks all: great intel. Peasacake, interesting that you had a similar problem.

Unstable load: we've just (literally) changed the check valves on the fuel pumps. But the fuel line in the tank could be a possibility. It hadn't occured to me that the fuel could be draining back when you open the valve prior to start!

[RVDT]

Obviously not many LAME's here!

1/ Don't tell us you are still flying it like this!

2/ Look out for a possible leaking bypass switch in the AF Fuel filter.

3/ Be aware that with the issue you have, without boost pumps operating it will probably STOP!

4/ What happens when you run the aircraft on the ground with the boost pumps OFF?

5/ When was ASB 206-05-103 complied with? Did it become apparent after this?

6/ Beware the 206 Fuel system it is not as simple as you think and even trickier on an L.