Avgas Or Unleaded?

[highwingloader]

What is the best advice on the use of AVGAS and/or unleaded. Thus far, the aircraft I fly has been using AVGAS. I understand that it may use unleaded but wonder if there are any potential poo traps in doing so. The engine is an ISI Subaru (118 hp) in a Europa and has done around 120 hours.

[2Donkeys]

The two common gotchas related to the tendency for MOGAS to be associated with Carb Icing to a greater extent than AVGAS, and the the tendecy for MOGAS to rot certain kinds of tank seal.

There is a CAA Safety Sense leaflet on this very subject available to download from their website.

2D

[Fly Stimulator]

Before you even get to the issue of any potential problems with MOGAS, you have the practical problem of getting it into your tanks. I could theoretically use it in my aircraft, but I don't fancy driving to the airfield with 120 litres of petrol in tins in the back of the car.

Once you're away from your home airfield you have the further problem of where to find anything but AVGAS. I've never seen an airfield with a MOGAS pump. Taking cans with you and getting a taxi to a petrol station seems a lot of messing around, and the taxi fare would probably wipe out any cost saving on the fuel itself.

[Flyin'Dutch']

Only certain airframe and engine combinations are allowed to use Mogas.

As the Europa is on a permit to fly the PFA is the club which can give you conclusive guidance on whether that is permitted for that combination.

Apart from this there are operative restrictions to the use of Mogas and guidance on how to handle the stuff safely. There is a safety leaflet issued by the CAA on this particular subject which is 'compulsory' reading material if you go down this route.

Have a look on the CAA website for it or in your local flying club.

FD

[Mike Cross]

One not mentioned so far is that Mogas is more volatile than Avgas, leading to a greater potential for vapour lock. Fuel temperature and altitude can both contribute.

The info you want is here (http://www.caa.co.uk/srg/general_aviation/default.asp?page=630)

Mike

[highwingloader]

Many thanks for the advice. Good stuff.

[MLS-12D]

A couple of links that may set your mind at ease re MOGAS:

(1) FAA position letter (http://autofuelstc.com/pa/faa.html);

(2) Transport Canada bulletin (http://www.tc.gc.ca/civilaviation/certification/engineering/powerplants/MOGAS.pdf).

If you do decide to go the MOGAS route, ensure that the manufacturer has approved its use, or purchase the appropriate STC (if no STC is available, personally I wouldn't even consider playing 'Mr. Test Pilot').

I hope that this helps.

MLS-12D :ok:

P.S. For what it may be worth, my gliding club has now been using MOGAS in our Citabria for one full season, without any problems (we do not fly in the winter, obviously, and that avoids potential vapour lock problems with "winter-blend" automobile gasoline).

P.P.S. While MOGAS in aviation is controversial, one thing that can be stated categorically is that most older light aircraft use piston engines that were never designed for 100LL avgas; so either way, you're using the 'wrong' fuel, with attendant potential problems. :( When the giant oil companies phase out 100LL (which they apparently plan to do within the next decade), we'll all probably be using MOGAS by default.

[shortstripper]

Vapour lock tends to be a hot summer problem here. Theoretically you can't fly if your tank temp goes over a set limit. Carb ice we all know about and is more of a problem with mogas than avgas but then one aeroplane can be more prone than another so nothing new there.

You can mix the two, so there's no problem with filling up with mogas at your home strip and topping up at your destination with avgas. You must also record when using mogas in your engine book.

Only certain engines are cleared to use mogas ... esp as here in the UK as leaded fuel is pretty unobtainable so unleaded is the only mogas you can get. Most low compression aviation engines are OK but valves need watching ... check first.

Aircraft cleared must be placarded and another condition is a resriction on altitude to no more than 5000 feet. I use mogas in a VW powered VP2 with no problems at all (including carb iceing). It's certainly cheaper and the conditions aren't that restrictive in most pleasure flying situations.

IM

[stiknruda]

Aircraft cleared must be placarded and another condition is a resriction on altitude to no more than 5000 feet

Short stripper, I'm sure the placard on my PFA C90 powered aeroplane says 6 000'!


Stik

[shortstripper]

It may well do Stik ... I was quoting off the top of my head this morning and finer details are pretty elusive for sieve brain like mine :ugh:

IM

[highwingloader]

Once again, deeply indebted to you all for your advice. The FAA and Canadian advice was particularly good. Thanks also for the heads up on the safety sense leaflet on the subject.

[cubflyer]

PFA engineering, or your PFA inspector will be able to tell you what you need to do to operate your aircraft on Mogas, there are a few things to check, but its all quite simple. Why not speak to the Europe factorty too. As you have a Subaru engine, surely that is designed to use Mogas and not Avgas, just like Rotax engines that are only really approved by Rotax for running on Mogas.
There have been problems with Avgas eating away at fuel pipes, tank seals etc, but this is mostly a problem for older aircraft, with approved "aviation standard" hoses. As your aircraft was built on a PFA system, most likely "aviation Standard" hozes werent used, but rather automotive ones, probably of a lot better quality and made for Mogas use. I am sure that Europa fuel tanks will be perfectly happy with Mogas.
Mogas and Avgas can be mixed freely, so no problem filling up at airfields with Avgas when you need to, but putting in Mogas from cans at your home base or whatever you like.
There are the restrictions on tank operating temperature and some aircraft need to have a fuel return line from the carb to the tank (Rotax 582, I know of) I think this helps in reducing vapour lock problems. Again PFA engineering will be able to tell you.
Ive operated my Cub on both and dont notice any difference, ecxept I get less lead deposits on the plug with Mogas. The worst thing about Mogas is the smell!

[ChrisVJ]

For my Rotax 912S Mogas is the recommended fuel with not more than 1 in 3 fills of 100LL.

Using more 100LL fouls the plugs and valves.

There is a caveat with Mogas. The manual suggests mogas with low levels (or preferably no, ) ethelene based components, definitely never more than 10%. In fact in Canada gas companies are restricted to 10% but I think in the USA they go up to 13%.

Here Shell and Esso do not used ethelene based additives, the rest use between 7 and 10%.

These additives can degrade the performance of some of the seals in the Rotax. (You would think, would you not, that knowing they are recommending mogas and that mogas has additives, they would use seals that were resistant.)

[Croqueteer]

One serious danger that has not been mentioned, and I have experienced it, and seen it happening with glider tugs, is that due to the much lower boiling point of mogas, if you land having been using mogas, and sit on the ground for about 15 mins, the carb body heat soaks from the engine, and then if you start up and taxi for take-off, you can get a rich cut due to the petrol boiling in the carb, and the only way out is through the main jet into the manifold. When flying the high fuel flow keeps the carb cool. I use mogas in my Jodel, but keep avgas in the front tank for take of and landing. The main reason most people have for mogas is the difficulty in getting avgas without a long flight or a high landing fee.

[shortstripper]

Yes Croqueteer .... it's known as vapour lock ;) It can happen anywhere in the iduction system including the carb, hence the placarded warning and hot tank restriction.

IM

[Croqueteer]

No need to be sarcastic, and what I have just described is not vapour lock. Read it again.

[shortstripper]

Vapour lock is where bubbles occur in the fuel lines or anywhere else in the induction system. The usual place is in the line between the fuel pump and carb but it can often occur in the carb itself. Like you say it's due to the slightly lower boiling point of mogas to avgas. A return line from the carb to the tank is sometimes used to help evacuate any vapour created this way. So, what you describe is classic vapour lock ... I used to get it a lot in an old Volvo I had too. I wasn't really being sarcastic ....... :rolleyes: OK maybe I was, but it was intended to be humourous ... I'm often the butt of a joke or two but I've learned to lighten up :ok:

IM

I'm sure someone more techie could give us a proper definition.

[Croqueteer]

The point is, that in the carb, when the fuel boils because of the considerably lower boiling point, neat fuel is forced into the manifold causing lots of black smoke and a rich cut.

[Bitberg7]

Hi there. I've got a Europa with an NSI Subaru 100 hp engine under the bonnet. I had been running it on unleaded for the last year until all my valves burnt out and the cylinder head cracked. Although the engine manual says that unleaded or AVGAS can be used, opinion from various sources is that unleaded is too low an octane for the engine and has caused all the problems. I've now fitted new valves and cylinder head and am running on AVGAS. So far, everything seems OK and I'm zooming along at 120 kt at about 16 litres / hour. I'm not going to risk unleaded again unless someone can convince me otherwise.

[ChrisVJ]

Bitberg7

What grade of unleaded are you using? I would have thought that unleaded was fine for what started out as an auto engine. The valves might be at risk if she was running too lean. Do you have an EGT gauge? That would be one way of making sure you were not exceeding the recommended temperatures.

I have a Rotax. 91OCT minimum auto gas without ewthylene is the rexcommended.

[WelshFlyer]

I seem to remember a similar discussion at my flying club about JetA and Diesel being used in compression ignition engines. The problem was sort of reversed because JetA is less viscose than diesel, but the answer we came up with is:

If in doubt phone the engine manufactures, and ask one of their engineers. If that draws a blank use JetA with no diesel.

It's better to pay a few more pence on the litre for fuel than to have a melt-down inflight!

WF.

[Bitberg7]

I was using normal unleaded from a car garage. The Subaru engine (EA81) is an old engine so I assume it was designed to run on the old 4 star. However, the engine has been somewhat modified to increase compression, amongst other things I don't understand. EGT temps seem pretty stable at 1400°F, which is similar to what I got with the unleaded. Max temp from the engine manual is 1650 °F, so it is well within that.