Intermittenly I have a high FF alarm, 20 plus GPH during T&Gs. I have a RV-9A, carbed O-360-A4M with fixed pitched prop and a D180. I have the alarm set at 20.1 GPH to keep alarms to a minimum. I add takeoff power smoothly, apprximately 3/4 power, stablize then full power. One out of four times the FF will exceed 20 GPH and alarm will soumd. Shortly after lifoff, the FF decreases and alarm resets. What do you think is the cause? Dan
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Erratic Fuel Flow (FF) reading with alarm
- Thread starter 65xx
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- Mar 23, 2005
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Re: Erratic Fuel Flow (FF) reading with alarm
Fuel flow accuracy can be affected by a fairly wide variety of things, including vibration, in-line pulsations, bends in the lines, and so on. In some low wing aircraft (some customers experience this, others don't), having the fuel pump on can cause pulsations to be counted as extra fuel flow. Is it perhaps correlated to your fuel pump usage during takeoffs and touch and goes? Turning on the fuel pump in flight to experiment might be revealing.
Fuel flow accuracy can be affected by a fairly wide variety of things, including vibration, in-line pulsations, bends in the lines, and so on. In some low wing aircraft (some customers experience this, others don't), having the fuel pump on can cause pulsations to be counted as extra fuel flow. Is it perhaps correlated to your fuel pump usage during takeoffs and touch and goes? Turning on the fuel pump in flight to experiment might be revealing.
Re: Erratic Fuel Flow (FF) reading with alarm
I use the Facet pump provided by Vans. However, I don't use it for t&Gs, in fact I don't use it at all except to provide pressure for the electric primer. You are correct, with the fuel pump on, the FF indicates .5 GPH before engine start. The FF meter is downstream, on the floor, from the fuel selector half way between the selector and the boost pump. There is approximately 12" straight run of tubing upstream and downstream from the FF meter and it is secured by Adel clamps. Any other ideas or techniques that might solve this nuisance? Dan
I use the Facet pump provided by Vans. However, I don't use it for t&Gs, in fact I don't use it at all except to provide pressure for the electric primer. You are correct, with the fuel pump on, the FF indicates .5 GPH before engine start. The FF meter is downstream, on the floor, from the fuel selector half way between the selector and the boost pump. There is approximately 12" straight run of tubing upstream and downstream from the FF meter and it is secured by Adel clamps. Any other ideas or techniques that might solve this nuisance? Dan
airguy
Active Member
Re: Erratic Fuel Flow (FF) reading with alarm
What if it's not a nuisance alarm, but an indicator of a bigger issue?
Is it possible you're seeing some vapor bubbles forming in the line during low power descent (classic vapor lock danger zone)? Under low power, the engine driven pump would still keep enough fuel in the carb bowl for operation even with vapor bubbles, until the situation became extreme. As soon as you increase power the fuel level in the carb bowl will drop and pump is now not only refilling the fuel bowl, but also displacing the vapor in the lines. This could possibly exceed 20 gph for a few seconds, triggering the alarm. As soon as the lines are full of cool fuel again, flow stabilizes and the alarm will shut off, just like you're seeing. An injected engine would stumble from the fuel pressure fluctuations, but a carb'ed engine will be able to take it for a few seconds with no indication to the pilot due to the fuel bowl quantity.
Do you have a cooling shroud on your engine-driven fuel pump? What about heat insulation on the fuel lines forward of the firewall? This may not be a fuel flow sensor problem.
What if it's not a nuisance alarm, but an indicator of a bigger issue?
Is it possible you're seeing some vapor bubbles forming in the line during low power descent (classic vapor lock danger zone)? Under low power, the engine driven pump would still keep enough fuel in the carb bowl for operation even with vapor bubbles, until the situation became extreme. As soon as you increase power the fuel level in the carb bowl will drop and pump is now not only refilling the fuel bowl, but also displacing the vapor in the lines. This could possibly exceed 20 gph for a few seconds, triggering the alarm. As soon as the lines are full of cool fuel again, flow stabilizes and the alarm will shut off, just like you're seeing. An injected engine would stumble from the fuel pressure fluctuations, but a carb'ed engine will be able to take it for a few seconds with no indication to the pilot due to the fuel bowl quantity.
Do you have a cooling shroud on your engine-driven fuel pump? What about heat insulation on the fuel lines forward of the firewall? This may not be a fuel flow sensor problem.
Re: Erratic Fuel Flow (FF) reading with alarm
I don't have a shroud but I do have a cooling air line on the fuel pump. The fuel lines are fire sleeved. I have not considered the possiblility of vapor lock. I will try to replicate the high FF in the pattern and then use the boost pump to see if it still occurs. Thanks for the info, Dan.
I don't have a shroud but I do have a cooling air line on the fuel pump. The fuel lines are fire sleeved. I have not considered the possiblility of vapor lock. I will try to replicate the high FF in the pattern and then use the boost pump to see if it still occurs. Thanks for the info, Dan.