Why did we stop using Mogas?

[Randy Johnson]

Ok. Know I'm opening up a can of worms here...especially for the purist....but I ask our old pros on the site the rhetorical question...Why? Is it octane, ethanol, gas quality?

According to my old notes, fifty years ago a lower octane and non aviation grade gas use was common. Anybody have thoughts on the issue?

I have recently owned a light sport that ran on mogas (non aviation grade fuel). Did have "trash in gas" issues but that was solvable with a funnel filter. Also had ethanol issues where the alcohol broke down the fiberglass fuel tanks and created fiber hairs caught in the fuel filter. oh yes...it was also a real hassle manually fueling the aircraft.

These are legitimate issues so I will put those on the table up front...what else?

[JIMICS2452]

Comanches were never approved for mogas. When an STC was applied for they were rejected because of a vapor lock problem.

[Randy Johnson]

Interesting. Thanks Jim.

[Bob Japundza]

I have almost 1200 hours flying my RV-6 on nothing but 87 mogas. No vapor lock problems even on 100 degree days since I have a vapor return line to the left tank. To put it bluntly, the Comanche fuel system is designed rather stupidly I suspect by a committee of Piper engineers back in the 50's. It is prone to vapor lock since hot fuel can recirculate with the boost pumps on and because the mechanical pump is bypassed. Then there's the fact that the boost pumps are firewall-forward, with many more places for fuel to vaporize. Since my Comanche is a restoration project I will be pursuing a field approval to redesign the fuel system make it less prone to vapor lock.

Thanks,
Bob