Fuel Flow issue
 

Hi,

I am flying a Cessna U-206F used for skydive operations.
For 3 weeks now we are facing an issue with a fluctuating fuel flow.

Here are the aircraft info :
•Registration : FGSFZ
•Type : Cessna U-206F S/N : U20601982
•Engine manufacturer : Teledyne Continental
•Engine model : IO-550-F(20), S/N :1041847
•Engine driven fuel pump : Fuel pumpnaturallyaspirated engine (no altitude compensation)

Issue summary :

•SinceFebruary, 16th 2025 :
-Fluctuating and unstable fuel flow
-Fuel flow can strongly drops at any time
-High EGT when fuel flow isdropping

•Engine driven fuel pump and control valve replaced March,13th but fuel flow issue stillongoing
•Issue occurs on both fuel tanks
•CHT steady

Sometimes during takeoff the fuel flow can drop to low value (up to 20 GPH instead of 27GPH) resulting in high EGT and and the decision to land back.
Sometimes it can happen in flight at any time (it happened once when I switched the fuel selector, but no always)
Sometimes the fuel flow is steady on take off, but start to fluctuate and decrease slowly as I climb and without touching the mixture control.
When I switch the auxiliary fuel pump on, the FF is increasing, but decrease again when switched off.

We suspected the engine driven fuel pump and changed it but the issue is still ongoing.

We grounded the aircraft and did several checks :
- adjustment of engine driven fuel pumpperformed as per TCM OVH manual X30607 chap 73-00-00
- Check of the whole fuel system (pipe, connections, fuel bladers…)
- Cleaned fuel selector
- Checked check valves

After checked the whole fuel system I did several flight test but the issue is still ongoing.

We thought that maybe the electrical fuel pump can be involved as there is a relief valve inside to maintain pressure (even when the pump is off). Maybe this valve is starting to tire.
An other idea we have is about the fuel selector that may have a damaged seal?

If you have any ideas of what is going wrong please help us.

Thanks a lot for you help

I've not worked on C206 aircraft for many years and my memory's a little hazy! The first points I'd check would be the fuel filter in the nose gear bay and the Fuel Control Unit finger filter. I've forgotten if the flow divider (spider) valve has a screen but I know there can be shut-off diaphragm leakage problems, so it might be worth having it inspected for correct operation.
Sorry I can't be of more help at the moment.

Crossed post with EXDAC!

1) Other than the fuel flow and EGT indications, does the engine operation change when you notice the flow problem?
2) Have you checked accuracy and serviceability of the fuel flow sender/indicator?
3) When was the last time the injectors were cleaned?
4) Do you have TCM injectors or aftermarket injectors?
5) Have you performed the Fuel System Operational Check per TCM Publication M-0, Section 6-4.7.4.?
6) Have you performed a "bottle flow check" of all the injectors?
7) What STC was the IO-550 installed under?

Hi

Thanks a lot for your answers


We checked all the filters and they are in good conditions

We also checked the spider but are going to perform some more tests today.

1) Not apparent change in power

2) We have 2 indicators, one analogic based on pressure conversion. And one on EDM base on fuel flow measurement. They seems to be working correctly. Moreover each time the fuel flow decrease or is too low, EGT are high and incresing.

3) Few days ago

4) TCM injectors

5) Yes, and even after some adjustement of pressure, fuel flow in still unstable. Sometimes showing great value of FF, sometimes too low and fluctuating.

6) not yet

7) SA2830SO

A) Was there any maintenance or other work performed in the previous weeks on the aircraft prior to the start of this problem?
B) Is this the 1st time you’ve experience this specific problem on this aircraft?
C) Have you had any other engine related issues in the past 6 weeks or so?

So the RPM did not increase, even slightly, as the fuel flow went down and EGT went up?

So is this the unmetered fuel flow that is unstable? Or is it the metered fuel flow that is unstable? Or is it both? And if both, are they “unstable” in the same manner, ie., move up and down together?

A) No
B) Yes
C) No

Correct, RPM is stable as FF goes down and EGT up.

Both metered and unmetered FF are unstable in the same manner.

Okay, to verify. Per the diagram below when you ground ran the aircraft for the system check, both the unmetered fuel PSI/flow and the metered fuel PSI/flow had unstable (fluctuating) indications on the test gauges?

Was the indicated unmetered/metered fuel PSI within the required limits per the manual aside from the fluctuations?

And after you replaced the engine-driven fuel pump on the 13th, there was no change in the unmetered fuel PSI/flow?

When you checked the airframe fuel supply system, did you inspect the internal passages of the fuel lines/hoses from the electric fuel pump to the engine driven fuel pump?

Did you check the airframe fuel bladder venting system was properly working?

https://cimg9.ibsrv.net/gimg/pprune....ed983d519c.jpg

Yes both indicators have fluctuations.
Sometimes showing good pressure and FF (within the limits), but sometimes we face a sudden decrease in pressure and FF without any change in power settings or mixture. 99% of the time without any power loss (except yesterday, while performing a ground check at full power, FF decreased rapidly from 27 GPH to 12 GPH, with a sudden and noticable loss of power).

After remplacing the engine driven fuel pump nothing has changed (I mean that sometimes FF and pressure are good, sometimes fluctuating, and sometimes we encounter a sudden loss). It is an eratic and unpreductable behavior.
And we also tried to adjust many times the engine driven fuel pump as per TCM OVH manual X30607 chap 73-00-00. But after few minutes of engine running we start to get (again) FF and pressure fluctuations and sudden decrease.

We did not inspect the internal passages of the fuel lines/hoses from the electric fuel pump to the engine driven fuel pump yet. We plan to do it next week as everything else has been done.

The airframe fuel bladder venting system is properly working

Based on what you’ve posted, my guess its a supply/return problem with the airframe fuel system. Since you’ve changed the engine pump and the unmetered fuel is still fluctuating tells me you have more a supply obstruction, cracked fitting, loose connection, collapsed line, etc. restricting the engine pump pick up fuel supply. And given you can duplicate the problem on the ground, if it were me, I would connect a test fuel supply/return system at the engine driven fuel pump and run it to see if it makes a difference before I'd throw any more parts at it.

Hi,


Thanks a lot for your answers.

We may have found the root cause.
This week, we performed some ground tests with a bypass on the auxiliary fuel pump. The FF was looking great.

It seems that the relief valve of the auxiliary fuel pump is not working properly. So maybe the relief valve opens at a wrong pressure and doesn't close properly.

We ordered a new auxiliary pump and should receive it next week.

I will keep you posted.