Hold the MOGAS!

[mnewbery]

This one came up during a BAK ground school class. Pop quiz:

 

1. Can I put MOGAS in a C150 and /or C152?

 

2. Why or Why not?

 

Its been bugging me for a while - in the ground theory class we argued about the C150 engine and whether or not it would or should run on mogas.

 

Not a simple answer, it turns out.

 

1. Engine STC, Airframe STC

 

2. Modifications to engine.

 

Engine STC, Airframe STC

 

To run mogas, both the airframe and the engine need to be certified that they are unmodified from original manufacture. Nothing is said about the longer runway required or lower MTOW but the A/C must be fuelled according to the POH without the STC. I wouldn't use mogas unless I had little choice. I've never seen a C150 POH long enough to look so if it says 80/87 you'd be right and I would be wrong. According to the ground school instructor, the C150M "as delivered" specifies minimum 80/87 fuel. One assumes that the take-off distances are calculated using this fuel. Next time I see the A150M (VH-WXR) I'll read the bit about fuel and take-off distances.

 

Modifications to engine

 

Keeping things simple, the engine normally found in the C150 is the O-200A (there's a B, C and D too but not relevant). This engine will safely run on 80/87 mogas without modification to anything (source EAA America) so I was wrong. The O-200D which is supposed to be in the C162 Skycatcher should not "be able to run on mogas" but I suspect it will have to, straight out of the box. It was going to be FADEC, therefore injected and possibly with knock sensors but Cessna wimped out in favour of sticker price and you get manual fuel mix control. MOGAS (80/87) OK, if it says so on the C150 POH which apparently it does. Otherwise the STC (for the high wing A/C) is often a few minor changes plus lots of new stickers and placards.

 

In contrast the Lycoming O-235 can run on mogas but only for models C,E and H. Then it gets weird. Models J,K,L,M were meant to run on 130 or 100LL, then the O-235 N2C came along at the same time as the C152 which was a low-lead only engine. Incidentally the "mogas capable" earlier plus N2C models are rated about 10% down on power from the units that were meant for 130 Leaded - models F and G. The C152 only shipped with the O-235 N2C so the POH values stand. Note also that CASA (CAO 2.7.4) requires the pilot to add 15% to the book take off distance for safety. No MOGAS for the C152 without the STC and some changes.

 

The Lycoming O-360 in the Cessna 172 might or might not run unmodified on mogas. Cessna models up to C172M should be able to run unmodified on mogas. Model N was introduced the same year as the C152 (1977) and used the O-320-H2AD - a 100LL only engine. This engine was rubbish. The engine in the C172P that replaced it was the O-320-D2J which was notable for having two impulse coupled magnetos and not being rubbish.

 

This also came up in the BAK ground school. Most aircraft engines from Continental and Lycoming have one impulse coupled magneto and another direct drive unit that starts working when the engine gains some RPMs. The O-320-D2J is an exception.

 

Finally, often the reason why low wing aircraft can't have a MOGAS STC is because the fuel lines are smaller than they could be which makes them prone to vapour lock when the fuel is pumped from the wing tank uphill to the engine on a hot day. Gravity feed tanks (therefore non aerobatic?) don't suffer as badly.

 

 

[TAA Student Pilot]

Ethanol can be a problem with older machines as well

 

 

[mnewbery]

Ethanol is hydrophillic. That means where there is a lot of water (e.g. a boat) you can't get the water out of the blended petrol once it gets in. Pure ethanol is only available in the lab. Everywhere you read ethanol, think "ethanol with varying amounts of water in it" Once its there some kwazy kwazy chemistry occurs. When you are driving a car every day and you have the right rubber parts its not a problem. Boats and planes that don't get used very much suffer accelerated deterioration where they are in constant contact with ethanol based fuels...

 

because the petrol with ethanol in it soaks up moisture from the air so much better than unleaded petrol and then it becomes weakly acidic. In time you end up with a tank full of acid. You'd be kinda sorta ok if you drained the fuel system every time but what a pain that would be!

 

Ethanol blended petrol also doesn't act like unleaded or aviation petrol during extreme hot or cold and planes can have both on the same mission

 

 

[TAA Student Pilot]

Yip all of that PLUS it buggers up old hoses

 

 

[mnewbery]

The C150 POH says that the minimum fuel is 80/87. I asked the Flying instructor who agreed that the performance data would be completed using this fuel (then slowly degrade as the prop and engine wears out). The C162 Flysmasher using the O-200D ... from the POH the minimum fuel quality is 100/130 (green) or 100LL (blue). No mention of premium ULP but it does suggest the use of DiEGME (fuel-anti freeze) when things get well and consistently below freezing.

 

 

[bsdunek]

Here in the U.S., mogas STC's don't allow any alcohol. That would be my biggest worry. Alcohol can damage hoses, aluminum fuel lines, o-rings and seals. I wouldn't allow it near my 170. Back in the 50's, Dad used to fuel the 170 from the farm tank - same one we fueled the Farmall's with. No problem without alcohol. Just IMHO.