Safety of the fuel injected light sport aircraft engine fuel system




  • Dual external fuel pumps
  • External single fuel regulator
  • Fuel return commonly back to primary fuel tanks
  • 6 port duplex fuel selector valve


  • Dual in-tank pumps
  • In-tank dual fuel regulators
  • Header tank based system
  • No fuel valve needed


  • Dual external fuel pumps
  • External single fuel regulator
  • Fuel return commonly back to primary fuel tanks
  • 6 port duplex fuel selector valve



  • UL and Rotax use external fuel pumps for their injected engines.  They are a copy of what was used in cars prior to 1998.
  • When 2 pumps are used together, for takeoff and landing, the amperage draw can be 10-14.   This is a lot since these engines are not equipped with good alternator systems.   Even with a single pump running, the draw is scary high if either the generator or external voltage regulator were to fail.  The dual fuel injectors and sparkplugs cannot work without electricity.  
  • External pumps produce close to 2 x the required fuel pressure for the injectors.  The excess pressure is regulated back down, using an engine mounted fuel regulator.  Excess fuel is returned to the fuel tank.  
  • All this additional fuel, when returned, brings heated fuel to the tank.  The total fuel circulating between the main fuel tank and the engine, then returned, is about 30-35 GPH.  
  • Due to the excessive fuel circulating, the amount of fuel gravity fed to the firewall mounted pumps is also 30-35 GPH.  Hence the large hoses specified.
  • Rotax and UL each use a single fuel regulator at the end of the fuel rail, mounted to the hot, air cooled cylinders.  The Rotax brand is likely to be Bosh but the UL is a no-name copy.  Each is filled with rubber parts and will fail, sooner or later.
  • This is now an antiquated way to feed fuel to a fuel injected engine.  



What is the big deal about a fuel regulator?  Well, what is the big deal about a wing attachment bolt?

  • The external fuel regulator is the single point of failure in a UL or Rotax injection system. 
  • An external regulator require external fuel return hoses
  • The regulators used are automotive quality and have a definite life span.
  • The engine ECU (control computer) knows nothing about fuel pressure.  If the pressure is wrong, the engine will not operate correctly, no matter how many sparkplugs it has.
  • A single fuel pressure regulator is not consistent with any other dual capability.  The engine cannot run without it.
  • ASTM was never informed of this. 




  • The Viking fuel system does not use a 35 gph gravity drain system, a single fuel regulator or a complex 6 port fuel selector valve.  
  • The Viking system is based on dual, in- tank pumps, each equipped with its own fuel regulator.
  • Each pump draw less than 1.6A and have no external fuel return hoses.
  • Only the amount of fuel used by the engine is gravity fed from the main fuel tanks to the header tank
  • A fuel level gauge measure the exact fuel level of the header tank, providing accurate fuel gauging. 



  • A fuel injected engine MUST have a solid fuel pressure at all times, or the engine will stop
  • Having reliable gravity feed, all the way from the wing tanks to the firewall mounted pumps, of 30-35 gallons / hour is much harder than the 2-12 GPH that the engine is actually using.  When this system was used by cars, the pump was mounted right below the fuel tank.
  • There is an illusion that these systems work because fuel usually come out of the pump when turned on.  What is not so obvious is how small the margin is for the system to stop working. 
  • It is likely that even though the pump was never designed to pull fuel on the suction side, that in the airplane application it actually does this to keep up with demand.  Since there is suction, any air leak prior to the pump has the potential for pump cavitation.   An O-ring leak in a fuel drain, gascolator or selector valve would not be good. 
  • Since the fuel is at low pressure (suction) as it enters the hot engine compartment, any exposed metal fuel pump body or fuel system component could cause the fuel to boil and cavitate the pump, dropping the pressure from 43.5 psi to less than 10 psi.  Once the pressure has been dropped, the pump will not be able to re-gain pressure due to the 43 psi fuel regulator at the other end of the engine fuel rail.  (Here is where a small bleed bypass around the pressure regulator would be handy but this is not implemented by either Rotax or UL.  The bleed would allow some fuel to flow, re-priming the pump with fresh fuel and again be able to make pressure) 
  • A larger amount of unusable fuel.  Because more fuel is being returned to the tank, than is used by the engine, fuel is on the move in the tank and not always available at the pickup location.  To guard against this, more fuel must be kept in the tank, shortening the available range.  Also, as the fuel level decrease in the tank, the warm return fuel becomes more and more pronounced and pump cavitation is even more likely.
  • Serious concern of un-porting the fuel pickup location.  As mentioned above, one second without fuel pressure is one second the engine will not operate.  Carbureted engines have a fuel bowl from which the engine can draw fuel.   Not so with an injected engine.  There has to be fuel available to the pump at all times.  With low fuel, and in an extended descent, this is usually not the case.  Some installations add more complexity to the system by the installations of door-post mounted “sumps” in order to have some protection from this.  However, keep in mind that these fuel pockets only last for a few seconds when the system wants to pump 35 GPH in a circle.  The pump will draw the fuel from the pocket in no time flat, bringing back the un-porting issue.



  • The UL and Rotax systems usually use a duplex / 6 port fuel selector valve.  These are complex, costly and have 6 fuel line connections right inside the cabin.  Another style is from a pickup truck with dual fuel tanks.  These are of terrible quality and are electrically operated.  Cut one open and inspect the construction if considering using one.  Again, there is no backup so be sure it works. 
  • Viking does not use a fuel selector valve.  Fuel simply drains from the two wing tanks into a single header tank.  From there, no selection is required.  The popular C-150 also has no fuel selector. 



  • The Viking system consists of a header tank and two fuel pumps.  That’s it.  Fuel fills the header tank by gravity.  If the engine uses 6 GPH, the system only has to flow 3 gallons from each wing tank.  (only about 1 qt every 5 minutes)
  • The pumps are inside the tank and submerged in fuel, just like every modern fuel injected car.  The fuel regulators are right on the pump bodies, pre-filer screens are part of the system, etc. 
  • A precise fuel sender unit and gauge are available, allowing the main tanks to be used to a lower level with complete confidence.  The 30 min VFR daytime reserve is in the header. 



  • The short answer is – yes you can.  Viking has such a system and it is being tested on a UL engine.  The long answer is much more complex and you need to understand it if you are contemplating such a conversion.  Here are the details.
  • First it is important to understand how the original system works and the reason for why things are the way they are. 
  • Fuel pumps:  The fuel pumps are big, heavy and draw a lot of current for a reason.  In order to reliably produce 43 psi of pressure, additional capacity is used, and then regulated down.  Because the pumps are external to the fuel, they are only cooled by the fuel running through them and by convection.  If the pump slowly got hotter and hotter during operation, a vapor situation would surely occur.   
  • In order to use fuel for pump cooling, excess fuel needs to flow through the pump.  Most systems run 3-5 times more fuel through the pump than what is used by the engine.  The main reason the fuel is returned back to the fuel tank is to cool it.  The fuel also cool the fuel rail and purges air from the rail when first primed for starting.  
  • If we eliminate the large pumps, we also eliminate some of the heat put into the fuel.  Some fuel must still be returned to the header tank since these fuel rails are not designed to purge air without a return at the last injector.  A small amount of return also helps cooling the fuel injectors. 




  • The system operates exactly as if used on a Viking engine, with one exception.  There is a small amount of calculated fuel return from the fuel rail back to the header tank.  The existing fuel regulator is replaced with a bleed large enough to purge air from the system but small enough for the in-tank pumps / regulators to easily maintain 43 psi of rail pressure.  

Test setup with pump and fuel return

UL pressure regulator replaced with machined part to provide steady return along with pressure control from in tank dual fuel regulator.

Measuring return fuel amount.

keeping an eye on fuel pressure

Related VIDEO

Viking Fuel System 1

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Comment by Jan Eggenfellner on November 13, 2017 at 8:49pm
Hi, Dave
Each in-fuel pump has it’s own regulator. So no issue there.
Your engine has the same style regulator out on the engine. The difference is that there is just one of them. Dual everything but not the regulator. Your question also touch on a more complex issue regarding the vacuum compensation of the fuel regulator. Not sure if that is your real comment but to touch on that issue, keep in mind that the manifold vacuum is close to atmospheric since the throttle is usually open. Also, the fuel regulator has a very minimal control of the mixture above 75% power.
Comment by David J. Beaulieu on November 13, 2017 at 1:53pm

Jan, thanks for bringing up the discussion.  Interesting, informative and will help most of us with our considerations regarding fuel systems and fuel injected engines.  I'm running a UL350iS and can see the merit of a machined part to maintain pressure through the rail (eliminate a moving part with a limited useful life), but how do you compensate for altitude change and maintaining a three bar differential to atmospheric?  In your system, is that done via the pump's regulator?  Dave (happy user of the Viking Steel Bungee)

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