High fuel flow alert

[jay507]

Second flight of the day, right around take-off speed, an engine alert was shown and disappeared immediately. Could be reproduced several times and cause shown was HIGH FUEL FLOW. This turned out to coincide with a value between 13 and 18 gal/h, for a short while only. It could then even reproduced with simple run-up, not driving, when above 3500 rpm, the alert came up again, and again just for a moment. 

Doesn't look like a sensor failure to me but a temporary leak in the carburetor. But I am not a mechanic in any way, shape or form. Any experience and/or recommendation?

Thanks to everybody here. Great place. 

 

Jay507

 

[Skunkworks85]

11 minutes ago, jay507 said:

Second flight of the day, right around take-off speed, an engine alert was shown and disappeared immediately. Could be reproduced several times and cause shown was HIGH FUEL FLOW. This turned out to coincide with a value between 13 and 18 gal/h, for a short while only. It could then even reproduced with simple run-up, not driving, when above 3500 rpm, the alert came up again, and again just for a moment. 

Doesn't look like a sensor failure to me but a temporary leak in the carburetor. But I am not a mechanic in any way, shape or form. Any experience and/or recommendation?

Thanks to everybody here. Great place. 

 

Jay507

 

What Fuel are you running? Did this happen after a long taxi, What was the temp outside?

[jay507]

Flying MoGas 95, have flown 30 mins before without any hickups. Then the plane stood for an hour in the tropical sun.

[Anticept]

It's vapor getting purged. Vapor doesn't like to move unless a lot of fuel is flowing, and the moment it hits the carbs, they're suddenly going to vent at velocity. Your pressure will drop and your fuel flow will rise, then all will go back to normal.

The vapor sits in the fuel pump, I suspect, since that is where the heat will get to the fuel most easily.

[jay507]

Thank you. Sounds reasonable and comforting.

However,  we have been taxiing back and forth before doing 3 more attempted take-offs. But standing in the hot environment may certainly be a contributing factor. 

[Anticept]

Throw in a gallon of avgas for every 10 or so gallons if it annoys you. It will raise the vapor pressure enough to eliminate the problem.

There is a rotax service bulletin on this stating that the fluxuations are allowable for around 5 seconds; with the software smoothing on dynons, i find around 10 seconds are allowable.

[Jim Meade]

No mechanical fuel pump?  What airplane and engine do you have?

[jay507]

3 hours ago, Anticept said:

Throw in a gallon of avgas for every 10 or so gallons if it annoys you. It will raise the vapor pressure enough to eliminate the problem.

There is a rotax service bulletin on this stating that the fluxuations are allowable for around 5 seconds; with the software smoothing on dynons, i find around 10 seconds are allowable.

Why would a red engine alert during take-off annoy me? Just kidding and never had that in more than 300 hours in that aircraft. 

Seriously, thanks for your hint. Will look into the Rotax docs again. 

[jay507]

1 hour ago, Jim Meade said:

No mechanical fuel pump?  What airplane and engine do you have?

Sorry, I mixed things up. Of course there s in my CTLS.

[Roger Lee]

I used to get these alerts every 5 minutes at one time. Trouble shot everything. Then I finally squeezed the connectors on the sender with a pair of pliers even though they felt firm. Problem solved.

[jay507]

interesting. Thank you.

[jay507]

Another day to check things out. 

Now the plane comes out of the hangar, no sun heating it up. 

Doing a number of high-speed taxiing along the runway and each time, right around take-off speed, the fuel flow goes way up for a second, causing red alert and goes back to normal. Not influencing manifold pressure. 

Never seen before, just now, after transferring the plane from annual inspection. 

Will have the connections checked and will be grateful for more input. Could pumps play a role?

Thank you all.

 

 

 

[Roger Lee]

Pump?  Very doughtful. I'm still leaning on a connection then the sender. Do the cheap and easy things first.

Squeeze the sender connectors with pliers.

[airhound]

https://legacy.rotaxowner.com/si_tb_info/serviceb/sb-915i-010.pdf

maybe a different topic, but deals with FP. See para 1.3. Where it refers to 912is. Does that mean to raise the upper limit to 46 psi, or is it an error? 

[Tom Baker]

3 hours ago, jay507 said:

Another day to check things out. 

Now the plane comes out of the hangar, no sun heating it up. 

Doing a number of high-speed taxiing along the runway and each time, right around take-off speed, the fuel flow goes way up for a second, causing red alert and goes back to normal. Not influencing manifold pressure. 

Never seen before, just now, after transferring the plane from annual inspection. 

Will have the connections checked and will be grateful for more input. Could pumps play a role?

Thank you all.

 

 

 

I have seen this after maintenance before, and it normally rectified itself without having to do anything, My guess is that there are some air bubbles in the system.

[Tom Baker]

4 hours ago, Roger Lee said:

Pump?  Very doughtful. I'm still leaning on a connection then the sender. Do the cheap and easy things first.

Squeeze the sender connectors with pliers.

The fuel flow uses bullet connectors, I don't think squeezing them is the way to go.

[Roger Lee]

The  VDO fuel sender has two bolt on spade connectors.

[Tom Baker]

18 minutes ago, Roger Lee said:

The  VDO fuel sender has two bolt on spade connectors.

That's fuel pressure. Fuel flow would be Flow Scan, or Red Cube with three wires and bullet connectors.

[FlyingMonkey]

I don't have fuel flow in my airplane so I could be off base here...but is it possible the K factor for the red cube is set incorrectly and causing this?  Seems like if there is a K factor error it would be magnified when fuel flow is highest -- on the takeoff roll, where you are at full throttle and low altitude.

But I suppose if that were the case then this would not be a new problem and would be happening all along, unless the K factor changes over time and needs periodic recalibration.

I think the vapor bubble explanation is the right one, but this just occurred to me.

 

[Tom Baker]

K factor would come more into play if the fuel flow was reading 6 GPH, and actual gallons divided by hours showed that it was only burning 5.5 GPH. The only way to get those real high readings is for the little impeller to spin real fast. This could be from vapor or it could be from an actual fuel leak. My bet is vapor, especially after maintenance if the fuel filter was cleaned,  the wings removed, or better yet a hose change. The times I have seen it it was always vapor, and the problem went away in a short time.

[Roger Lee]

Hi Tom,

Sorry my mistake. I re-read the original post. I screwed up and thought it was fuel pressure. I agree it could just be bubbles in the line especilly if there has been any maint. upstream or the red cube.

[jay507]

Thank you for all the hints.

Have to discuss the points made with my mechanics.

Bubbles seemed likely when the plane after a first flight stood in the hot sun for an hour but that's not the case anymore. Whenever, at high-speed taxiing take-off speed approaches, the light and message goes on for a second. Self-clearing alert. Fuel flow indication goes up to 17 gal/h for this moment, Manifold pressure not dropping. 

Strange. Will keep you posted. 

Jay

 

  

 

[FlyingMonkey]

If it hasn't been mentioned, you might want to also check all the grounds carefully too.  Ground issues cause all sorts of electrical gremlins on a CT, and I have had engine instruments that act weird and bounce to obviously incorrect indications until the wheels lift off, then indications are normal...kind of the reverse of what you're seeing.

[Tom Baker]

1 hour ago, jay507 said:

Thank you for all the hints.

Have to discuss the points made with my mechanics.

Bubbles seemed likely when the plane after a first flight stood in the hot sun for an hour but that's not the case anymore. Whenever, at high-speed taxiing take-off speed approaches, the light and message goes on for a second. Self-clearing alert. Fuel flow indication goes up to 17 gal/h for this moment, Manifold pressure not dropping. 

Strange. Will keep you posted. 

Jay

 

  

 

I am not saying to ignore it, but if you have confirmed that there are no fuel leaks, and all other indications (fuel pressure, manifold pressure, RPM) are normal I would fly it. I have made quite a few post maintenance flights on CT's, and have never aborted a take off just because of high fuel flow, and I have seen them before. I would have if there was some other indication that something was wrong. The issue has always went away after a short period of time, and didn't resurface until the next maintenance.

[Tom Baker]

Just a thought. get a 5 gallon jug, and drain some gas through the gascolator with the master switch on. Check the fuel flow readings. I see normal readings of 18-20 GPH for both feeding, and slightly less with one clamped off. Clamp the fuel line on each side one at a time and check the readings. Drain at least a couple gallons through. Then try a take off again.