Credit to Peter MacPherson:
Why is the Musketeer not approved for autofuel (auto fuel) and lots of other aircraft are? Including a lot of other Beech planes. Check out the link below:
http://www.autofuelstc.com/autofuelstc/pa/ApprovedAirframes.html
This statement is at the bottom of the link:
The airframes we have flight tested which failed the test include the Navion, Musketeer, Piper Apache PA-23-235, 7KCAB, Mooney M-20-C, Piper PA-24 250 Comanche & the Avcon converted 180 hp Cessna 172. Generally speaking, any pump fed airplane not already on the approved list is incapable of passing the flight tests unless substantial modifications are made.
Curious why the Musketeer failed….
Credit to Carl Foster:
Peter, I called last year and asked about the Musketeer line and was told that the Sundowner they tried had such severe vapor lock that the airplane had to be towed from the runway after a dead-stick landing. The particular test that it failed was the one where the fuel is heated to 120 F and the aircraft flown to around 10,000 feet at the maximum rate of climb. The combination of fuel line routing, tubing diameter, and the fact that the fuel system requires a pump, all make an autogas STC a distant dream.
I am not sure what modifications would be needed to get an autogas STC, but it would not be cheap to get it approved. The vapor pressure of autogas is not controlled by specification as is avgas. It is really easy to get 120F fuel temperatures in Southern Arizona, but Alaska may be a different story.
Editor’s note:
Carl nailed it. The vast majority of the autogas/mogas approved airframes are either high-wing, have high-dihedral wings with outboard tanks, or are equipped with feed pumps in or near the tanks. The problem lies in the aromatics that readily separate from autogas, with none of the tight specs that apply to avgas. In our planes, whether carbureted or fuel injected, the engine driven pump has to “suck” the fuel from the tank, through the boost pump and strainer-gascolator. Our low inboard tanks have a lot of advantages, but they don’t provide much “gravity head” to the higher engine-driven pump. If vapor readily separates from the heavier fuel, bubbles form; neither bubbles nor vapor are effectively moved by the engine driven pump, nor do they enable the check valves in the pump to operate properly. In order to work reliably, autogas-mogas has to have a bit of “gravity feed” on it, so that it gets “pushed” up to the engine driven pump. Some airframes are naturally better or worse than others, due to fuel line diameter, line routing (more or less direct, close to or away from heat, etc.), and the number and type of bends.
One of the design goals in a dependable homebuilt aircraft fuel system is to have few or no 90-degree bends in the fuel system, on the suction side of the engine driven pump, unless there will be a full time low pressure boost pump in the system near the tank. In turn, that can create a hazard unless there is a provision to automatically turn it off if the engine isn’t running.
This situation is not unique to airplanes. Older cars had vapor lock problems all the time in hot climates, because the tank was lower than the engine-driven pump, and the system had the restriction of a suction finger screen or filter in the tank. On newer fuel injected cars, nearly all have an electric fuel pump located inside the fuel tank, so that the fuel line always remains pressurized when the engine is running. For that reason, vapor lock problems are rare on a modern car.
Credit to Bob Steward, A&P-IA:
You mean other than the LACK of an STC, to make it legal?
The word from those that were doing auto fuel STC research in the 80’s is
that the low wing planes had some problems with vapor pressure, and would not pass the FAA “hot fuel” test. The test simulates taking off on a heat-soaked 100 degree ramp, and climbing to altitude.
Because the pressure drops as the plane climbs, and the fuel pumps SUCK the fuel from the tanks, there seemed to be problems with bubbles forming in the fuel lines if the fuel was warm.
The Piper STCs that I have seen required that additional electric pumps be
mounted in the wings, to pressurize the fuel from the tank to the
engine. So the mod can become quite costly.
There are SOME aircraft with low wings and unmodified fuel systems, which
have STCs for auto fuel. I am familiar with the Grumman American line, and have installed the EAA and Petersen auto fuel STCs on several of
them. They are limited to 160 HP, though. There is no STC for the Grumman Tiger with the 180 HP O-360-A4K.
The good news is that the EPA has mandated lower evaporative emissions from cars over the last 20 years, and that has caused the fuel to be made LESS volatile than when it was tested in the 80’s; so it MIGHT be possible to get the test passed today. The bad news is that it is getting harder to find auto fuel without any alcohol in it. In some states all auto fuel is
diluted with alcohol. No alcohol content is allowed under the terms of the STC.
Those who are interested should contact Todd Petersen at Petersen Aviation in Nebraska. It probably means giving up your plane for test flights over a period of time. http://www.webworksltd.com/autofuelstc/pa/intro.html
EAA never did the paperwork for the engines that cannot run on 80
octane. Petersen is the only source for the ENGINE STC that allows the use
of 91+ octane auto gas in aircraft engines, like the O-360 that was
certified on 91/96 avgas back in the 60’s. (Everyone raise your hand, who remembers the old 91/96 avgas.)