Fuel system woes

Header tanks are a good return source like in the 912iS. Tecnam returns it into the left wing, but if you're not careful and don't keep the fuel level down it can over flow. 

Marc, surely shutting down the engine by closing the fuel valve will work. Besides, I do agree that a return line does not cure the problem you are experiencing. However, I am stunned that all measures did not help so far.

I've heard some operators open the oil check door on the top cowling.  This apparently let's the hot air escape.

Hi Mark

"I'm not sure though if a return line would really h"elp. The heat soaked carb filled with residue fuel would not get replaced by cooler fuel and it would not evaporate through the fuel line, right?

WRONG!

I fear you do not really understand the problem or its management.

When the fuel is heated to vapour, the carburettor can no longer control the air fuel mixture (stops working). In addition the fuel boost & mechanical pumps, designed to pump liquid, loose considerable efficiency.

The heating, of the above engine fuel lines, is unavoidable but can be minimised by using insulating sleeves (fire sleeves).

The heating of the float bowl & its contents, by the rear exhaust system, is also unavoidable but can be minimised by using some form of heat shielding between/on exhaust pipe & carburettor.

Pointing the aircraft into wind & opening inspection doors in top cowling (removing top cowling) will all increase cooling airflow to reduce the effect of the hot engine on the fuel system.

You could use AVGas as this fuel vaporises at a much higher temperature (read up on the management of  Rotax using AnGas) 

All of the above can be done - Insulating/shielding are usually permanent fixtures on Rotax 9's that have an ongoing benefit, apart from addressing fuel vaporisation.

Rotax (in line with a whole range of automotive manufactures) have found removing the hot fuel/vapour, is the most effective management tool.

Removing the bulk of the hot fuel & vapour, replacing it with cool liquid fuel, is the most convenient method of addressing this occasional (hot weather) phenomena.

The fuel is removed through a fuel return line, located as close to the carburettors as practical (minimising the amount of hot fuel remaining).

Combining the fuel return line, with a boost/auxiliary pump being turned on, forces the hot fuel/vapour into the return line, back to the tank AND replaces the hot fuel with cool, 

The incoming cool fuel, cools any remaining hot & the fuel supply system itself.

Liquid fuel enters carby float bowls, forcing out any remaining fuel vapour.

The carbys now have a chance of functioning, as designed and you can start your engine.

Fit a fuel return line & boost pump, to enable best management of vapour lock.

"I was thinking of opening the throttle fully after shutting down the engine, perhaps that will give the vapor from the carb an escape path?"

Certainly cranking the engine, may assist in removing vapour, from the inlet system but will have minimal effect on replacing vapour, in the hot fuel supply system, with liquid fuel. Net effect will be to flatten a weak battery - no start, possibly overheat/damage your starter - no start.

"Also, I could try closing the main fuel valve before shutting down the engine and wait until the fuel has been depleted from the float chambers. I'll give these options a go."

You persist in focusing on the carburettor float bowls, to the exclusion of the greater volume of hot fuel/vapour in the fuel supply lines.

The fuel lines must be flushed of hot fuel/vapour, only then will liquid fuel enter the float chambers.

The whole system - fuel supply & float chambers, must be managed together IF you are to have any hope of managing this problem effectively.

Fit a fuel return system. Any design will be better than none.

Hi friend Roger,

"Header tanks are a good return source like in the 912iS. Tecnam returns it into the left wing, but if you're not careful and don't keep the fuel level down it can over flow."

Little story in support of your advice;

My Sonex has Wing tanks & a large, in fuselage, Header tank .

In this context a define a "Header tank", as one that is filled from other tanks ie no external fill point.

My Wing tanks are 30L x 2 & the Header 40 L (37 L usable).

My fuel return is to the Header tank, at about 7L/hr.

The initial fuel plumbing arrangement, had the Boost pump (for Take-Off /Landing) plumbed into the Wing tanks.

This led to a problem on the first Test Flight.

The Test Pilot, to be extra safe, left the Boost on, for the whole if flight (fuel supplied by a Wing tank). As the aircraft  taxied back, after the 60 minute + flight, observes noticed a jet of fuel coming from the belly (Header tank  breather). Pilot was instructed to shut down immediately.

It didnt take long to work out what had happened - return fuel had taken up the small remaining capacity in the unused Header and the excess was being pumped overboard.

I have redesigned the fuel supply system - the Boost pump can now draw from any of the three tanks, however I have writen the POH to favour the Header, for all normally operations (including TO/landing). The fuel consumed by the engine is usually about x 2 + the infill from the Return line, so the Header fuel level slowly reduces. In normal ops the Header is replenished from either of the Wing tanks, using a Transfer pump - no more fuel spurting out of the belly.

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