In Safety Spot in the February issue of Light Aviation magazine I feel there is a difference in thinking about fuel return lines.
For the Rans S6 the fix was to move the return line to the top of the tank, above the fuel level and in air, to prevent fuel being able to pass through the firewall.
However for the Europa the return line was attached to the bottom of the tank but if air was allowed into the return line the engine would stop. Strange when the electric pump should have kept the lines full of fuel.
Any thoughts on a better design?
Rgerads, Alan.
Fuel return line
Moderators: John Dean, Moderator
-
Ian Melville
- Posts: 1001
- Joined: Mon Feb 11, 2008 7:21 pm
Hi Alan,
I was just about to pen an email to engineering as I thought the two were at odds as well.
I always thought the fuel return line should be from the carb(after engine pump) and return to the top of the tank. I'm not a Europa builder, but I cannot see it in the manual I have. I thought that info was supplied by Europa, but may have been a Europa Club member.
Still could be an issue if the Europa is low on fuel and flying on the reserve, then holds a sustained bank away from the reserve tank. Return fuel and slosh may end up in the main tank section. Hence point of return to the tank is important and should be as far outboard as possible.
Another point not mentioned is that the Europa's electric pump may be turned off in cruise. Thus in the drawing from the AAIB, the engine pump will attempt to draw unfiltered fuel through the restriction. It won't get much, but debris may block the restriction.
The point of Malcolm’s article still stands, that all changes should be thought through fully and where appropriate checked by an independent engineer.
I was just about to pen an email to engineering as I thought the two were at odds as well.
I always thought the fuel return line should be from the carb(after engine pump) and return to the top of the tank. I'm not a Europa builder, but I cannot see it in the manual I have. I thought that info was supplied by Europa, but may have been a Europa Club member.
Still could be an issue if the Europa is low on fuel and flying on the reserve, then holds a sustained bank away from the reserve tank. Return fuel and slosh may end up in the main tank section. Hence point of return to the tank is important and should be as far outboard as possible.
Another point not mentioned is that the Europa's electric pump may be turned off in cruise. Thus in the drawing from the AAIB, the engine pump will attempt to draw unfiltered fuel through the restriction. It won't get much, but debris may block the restriction.
The point of Malcolm’s article still stands, that all changes should be thought through fully and where appropriate checked by an independent engineer.
-
Joe Iszard
- Posts: 60
- Joined: Mon Jan 07, 2008 4:36 pm
- Location: suffolk
Glad a discussion's started as a result of the Europa article.
I looked at the schematic of the fuel line(s) and one thing that struck me is how the fuel flow meter is close to useless. If you look at it, yes it's measuring fuel flow, but the actual flow to the engine is the flow from the tank less the flow back to the tank from the return line, if we assume that the return works as was intended / hoped.
If you look at what has actually come to pass then:
if the electric fuel pump is on you'll get a fuel flow reading in excess of engine usage, as above.
if the electric fuel pump is off you'll get a fuel flow reading lower than the actual flow, as the engine is being fed from the return line as well as the main one. How much would be coming though the return line would depend on the (intended) restriction on the return line and the (unintended) restriction of the main line caused by the filters, inoperative electric pump and fuel selector valve.
Either way it renders the fuel flow metering useless. I would think the flow meter needs to be close to the carb, after the return line and well after all the pumps.
As regards the apparently conflicting advice from engineering regarding the positioning of return line connection back to the tank I think the reason is due to the position of the tank itself, and it's height relative to the engine/carb.
The Rans has a wing tank (if memory serves) well above the engine so we have an undesirable gravity flow situation happening in the return line that needs to be addressed.
The Europa has a fuselage tank, and a low one at that (again, from memory) so gravity flow is not a problem. Persuading the fuel to flow uphill to the engine is the issue here - hence the need for an electric pump to be able to start the aircraft.
Different problems requiring differing solutions.
As to whether I'm on the money with this last point... Over to Malcolm, I suppose.
I looked at the schematic of the fuel line(s) and one thing that struck me is how the fuel flow meter is close to useless. If you look at it, yes it's measuring fuel flow, but the actual flow to the engine is the flow from the tank less the flow back to the tank from the return line, if we assume that the return works as was intended / hoped.
If you look at what has actually come to pass then:
if the electric fuel pump is on you'll get a fuel flow reading in excess of engine usage, as above.
if the electric fuel pump is off you'll get a fuel flow reading lower than the actual flow, as the engine is being fed from the return line as well as the main one. How much would be coming though the return line would depend on the (intended) restriction on the return line and the (unintended) restriction of the main line caused by the filters, inoperative electric pump and fuel selector valve.
Either way it renders the fuel flow metering useless. I would think the flow meter needs to be close to the carb, after the return line and well after all the pumps.
As regards the apparently conflicting advice from engineering regarding the positioning of return line connection back to the tank I think the reason is due to the position of the tank itself, and it's height relative to the engine/carb.
The Rans has a wing tank (if memory serves) well above the engine so we have an undesirable gravity flow situation happening in the return line that needs to be addressed.
The Europa has a fuselage tank, and a low one at that (again, from memory) so gravity flow is not a problem. Persuading the fuel to flow uphill to the engine is the issue here - hence the need for an electric pump to be able to start the aircraft.
Different problems requiring differing solutions.
As to whether I'm on the money with this last point... Over to Malcolm, I suppose.
Rob Swain
If the good Lord had intended man to fly, He would have given him more money.
If the good Lord had intended man to fly, He would have given him more money.
-
Alan George
- Posts: 62
- Joined: Mon Jan 07, 2008 8:22 pm
- Location: Bristol
The key seems to be the non return valve that would prevent gravity flow through the firewall.
It would also stop the engine drawing fuel through the return line that would solve the unfiltered fuel problem. However it would also be solved if the return line was connected downstream of the engine pump, near to the carburettor, as the engine pump would keep this line pressurised.
Good point about the flowmeter. As drawn it saves a firewall penetration for the wiring to the flowmeter but since they are fitted for accuracy they need to be downstream of the return line.
The part of the disussion that exercises me is whether high or low wing I am not comfortable with taking the return line to the top of the tank. That could cause a spray of fuel and vapour in the tank. Surely it is better to go in at the bottom and diffuse the return flow into the fuel already in the tank.
It would also stop the engine drawing fuel through the return line that would solve the unfiltered fuel problem. However it would also be solved if the return line was connected downstream of the engine pump, near to the carburettor, as the engine pump would keep this line pressurised.
Good point about the flowmeter. As drawn it saves a firewall penetration for the wiring to the flowmeter but since they are fitted for accuracy they need to be downstream of the return line.
The part of the disussion that exercises me is whether high or low wing I am not comfortable with taking the return line to the top of the tank. That could cause a spray of fuel and vapour in the tank. Surely it is better to go in at the bottom and diffuse the return flow into the fuel already in the tank.