Fuel icing (avgas)

[IO540]

There appears to be a huge variation in the susceptability of different aircraft types to this problem.

The Aztec has been widely reported as very like to do it, so pilots flying in low temps add Prist or some other stuff.

However I have never heard of a fuel icing incident on the TB20/21 and they use the same engine (IO-540) crucially with the same fuel distribution spider on top which one would logically think must be the weakest point as the very thin steel fuel pipes (leading to the injectors) are exposed to the full high speed flow of outside air.

Can anybody supply any input on this, and also indicate whether the engine(s) failed smoothly, or failed as if with some cylinders still operating for a while.

[Sandy Hutton]

I have vague recollections of Isopropyl alcohol being used as an additive but as for quantities, you'll have to look that up. :8

[gordon field]

You need to refer to the POH for each aircraft and also to the SBs issued as to whether or not you can add isopropyl or Prist (be very careful it is toxic).

I am more familiar with the Continental IO-520, TSIO-520 and GTSIO-520 engines installed in Cessna aircraft and on certain models you should add ISOP in accordance with % stated in POH.

My 'rough' rule of thumb in the 70s when flying in Western Europe, as per hangar heating, use it from 1st Oct to end March and that should solve the problem.

This problem arose in the early 70s, mainly in Europe and then further information was forthcoming from other countries such as Canada. Operators were reluctant to write about it as there was no published solution.

Trials were carried out using T310 / 340 aircraft with some tanks having ISOP and others not. When the problem was really bad a change to a tanks with 100L and more than plenty of ISOP cured the problem.

One of the first indications was a reduction in EGT with a rough imbalance on one or more engines. The problem was in the fuel manifold spider on top of the engine in that ice would restrict the flow to certain cylinders so they would run lean of peak. The larger centre avionics panel on a C402 could vibrate in sympathy, 310s didn't as the panel was more rigid. Eventually the flow to several cylinders could be restricted and the engine would cease to function sufficiently, time to go home.

Many pilots found that the only way to solve the problem was by descending to lower (warmer) altitudes, some had no option as they were unable to maintain assigned level.

The engines in later C421B and 421C aircraft had an oil feed pipe routed through the spider so this kept the temperature high enough to prevent this problem. Why other similar engines did have this feature is a question that Cessna / Continental would not answer.

If there are known problems with the IO-540 in the Aztec then perhaps others will respond.

[IO540]

Very interesting...

You seem to have evidence (rough running) that the distribution spider (or more likely the six or so thin and exposed pipes coming out of it) really is a problem.

The spider is mounted on the crankcase, which is pretty hot. It could be that there are different mounting methods in use, with differing thermal conductivity.

A simple cure would be to put fire sleeves over the pipes...

[Fuji Abound]

IO540

I suspect you will recall Timothy had exactly this problem over Germany I seem to recall with Iceman.

I also supect the Aztec would be more susceptible given the engines are on the wing and because it is easy to under manage the cowl flaps - which you dont have on the TB20.

As you know the amount of water in the fuel is surely a significant factor and I guess if it was a serious concern testing the fuel for dissolved water would be worth while.

I would imagine in a single with much shorter pipe runs and tightly cowled installations there is a risk but perhaps you know otherwise?

[IO540]

I don't know why the Aztec should be so much more prone to fuel icing than say a TB20. The fuel pipes are much longer, yes, but those pipes are hidden inside the wings, out of the airflow.

Unless some are exposed in the wheel wells?

[SNS3Guppy]

Fuel icing is more a problem at the tank, plumbing, and up to and in the fuel filter than at the fuel flow divider or injection lines/injectors.

Same reason that in very cold weather, one can't drain accumulated water from the tanks: it's already frozen.

[IO540]

Fuel icing is more a problem at the tank, plumbing, and up to and in the fuel filter than at the fuel flow divider or injection lines/injectors.Do you have experimental data for this?

Same reason that in very cold weather, one can't drain accumulated water from the tanks: it's already frozen.That's a different issue. A pool of water has collected at the bottom of the tank, where it will freeze when below 0C.

Fuel icing is the freezing of water dissolved in the fuel. As British Airways know only too well, post-BA038 ;) this happens at temperatures well below 0C.

My TB20 handbook doesn' give a figure for the temp at which the additive (they give two options) should be used; they just say if flying in cold temps. But it is thought to be about -30C or lower. My coldest flight had -29C for about 2 hours (no additive).

Avgas itself freezes at about -55C but that's irrelevant. The point is that with water dissolved in it, it starts to go solid somewhere around -30C.

The question is where in the fuel system does the fuel temp fall to that level?

"Logically" it should be at the very end, because the fuel pipes are not heated anywhere so the coldest bit should be at the very end. Especially as, at the very end, you have a ~ 2ft length of a very thin (1/8" OD) steel pipe which is fully exposed to the air intake airflow.

But it could be happening further back. The flow velocity in the thin pipe will be high so the fuel will spend very little time in that pipe.

[gordon field]

fire sleeves are not the answer as these could then cover a leak or weep. They would also alter and possibly restrict the cooling air to the cylinders. Approval ? Major - minor Mod?

The engine manufacturer overcame the problem by routing the oil feed pipe through the spider.

Check the POH and use ISOP if approved.

It doesn't need to be particularly cold the incident in the C402B was at FL090.

[IO540]

fire sleeves are not the answer as these could then cover a leak or weep

True but they are used all over the rest of the fuel system forward of the firewall, so why not "up top" ?

[SNS3Guppy]

Fuel icing is the freezing of water dissolved in the fuel. As British Airways know only too well, post-BA038 this happens at temperatures well below 0C.

First of all, fuel icing is any ice found in the fuel system, period. In aircraft using avgas (not turbine kerosine jet fuel as in BA038), the most common location of fuel icing is in the tank itself, and the second most common is in the gascolator or strainer, at the filter.

Fuel icing at the injectors is seldom a problem, though injectors, like spark plugs, can ice up.

Avgas doesn't suspecnd moisture like kerosine does.

You're making a ridiculous comparison when you attempt to equate any aircraft burning kerosine with any aircraft burning avgas, but more so when you attempt to equate a light piston aircraft with a large transport category airplane. The fuel systems are entirely different, as are the storage, capacity, anti-ice capabilities, and operating altitudes and subsequent temperatures attained by the fuel.

British Airways BA038 experienced ice suspended in the fuel, as turbine fuel tends to suspend moisture very well, and the most common source of fuel icing in turbine equipment is on the fuel filter. In the case of BA038, ample icing caused blockage of the fuel heaters exchangers as well, which ultimately caused the loss of power. Certain turbine engines are also very susceptible to icing of the fuel control unit, which can cause power rollbacks, runaway power, or frozen power, as well as disable certain safety features of the engine such as overspeed protection.

In a small piston engine, the fuel system is very different, and ice isn't typically suspended in the fuel. It tends to fall out in suspension, and while filter clogging does occur, icing beyond the filter into the fuel flow divider and nozzles is not at all common.

Products such as HEAT and PRIST use alcohol and other chemicals to "absorb" water as well as serve a two fold purpose to prevent ice, and to prevent microbial formation in the tanks. They can also be damaging to fuel bladders, fuel lines, gaskets, etc...and must be used very carefully and in strict accordance with the manufacturer recommendations.

[gordon field]

SNS3Guppy says icing beyond the filter into the fuel flow divider and nozzles is not at all common.

Oh yes it is!

Common enough for Cessna to include instructions for its use in the GAMA format POH of T310, 340A, 402C, 404, 414A and 421C aircraft. If you have ever experienced it and realised that you didn't add ISOP it concentrates the mind, well maybe not yours but it did mine!

[SNS3Guppy]

Common enough for Cessna to include instructions for its use in the GAMA format POH of T310, 340A, 402C, 404, 414A and 421C aircraft.

Ironically, as it happens, I'm current in each of those aircraft, and an instructor, and check airman in them (with the exception of the 402 and 404, which I haven't flown lately, but in which I do have experience). All my experience in type has been in mountain locations, frequently in cold weather and freezing conditions.

Where in the flight manual are you finding an emergency procedure or abnormal procedure for ice clogging the fuel flow divider or injector nozzles? You're not? Didn't think so, as it's not there.

Where in the flight manual are you finding a requirement for flight in cold weather, or flight into known icing conditions, to use a fuel additive? You're not finding that requirement? Didn't think so, as it's not a requirement.

Now, Cessna does provide direction on concentration of additives in certain cases, if you intend to use them. The use is not mandatory, nor is icing at the injector nozzles a big problem.

In turbine equipment, suspended ice and moisture in the fuel is a problem, but then turbine fuel suspends moisture very well. Avgas does not.

The icing potential in your piston airplane is certainly there if you allow moisture in the fuel by not draining the tanks. The best way to eliminate this is by preheating the entire aircraft; best done by the use of a heated hangar overnight. This enables you to drain moisture before it freezes in the tanks. You can also wait at least a half hour after fueling to allow moisture to settle, if it's there, and drain the tank during that time and after that time.

The use of anti-icing additives is also an option, though not often used, with avgas.

If you've developed moisture in your fuel and it doesn't settle out and get drained or frozen on the bottom of the tank, it's going to be a problem with blockage of the fuel drain ports in the tank (where fuel drains into the fuel system on the way to the engine), at the selector valve, or at the gascolator/fuel filter. The fuel filter generally does not admit moisture and certainly doesn't pass ice...which means that the ice has no means in most cases of becoming an issue at the fuel flow divider or fuel nozzles. If the ice has formed in the system somewhere, it seldom has any opportunity to pass beyond the filter, to the engine fuel system.

Ice, therefore, is a much bigger problem to the airframe fuel system, than the engine fuel system.

If you have ever experienced it and realised that you didn't add ISOP it concentrates the mind, well maybe not yours but it did mine!


I have flown piston airplanes operating on avgas in very cold climates for many years now, from single engine airplanes through four engine transport category piston powered airplanes...without having to worry about fuel freezing. I've had frequent use of fuel heaters and fuel anti-ice in turbine equipment over the years, but it's just not an issue most of the time in piston avgas equipment.

Where anti-icing additive is useful is in equipment left out overnight, or allowed to cold soak, or flown for extended periods in very cold climates...in times when there is never an opportunity to heat the airplane and drain water. The hope in adding the prist, heat, or other chemical is to absorb water and allow it to pass through filters and be burned, rather than icing in the tank and causing blockage of drain ports, fuel lines, selector valves, or the filter.

Icing beyond the filter is seldom, if ever, an issue.

[Timothy]

Sorry, just had my attention called to this.

I fly to very cold places in my Aztec, as some of you may know. My experience is that if you are going to expose the Aztec to -15C for more than 4 hours or -20C for more than 3 hours you must use 1% IPA to prevent engine failure.

IPA can be bought of eBay for approximately the same price as Avgas, so there is no huge problem with expense.

The flight that Peter was referring to was from Stockholm to Biggin. We had already been to Svalbard, and I had religiously used IPA, but I thought, wrongly as it turned out, that Bromma/Biggin in April would be OK.

One engine failed over northern Jutland and I made an immediate emergency landing on the grounds that if one engine had failed the other would follow suit. Luckily we were only 8 miles from a major airfield (I forget which now) so I did s-bends, and one orbit, down the ILS from F100. The failed engine picked up at the freezing level, which was about 3000' from memory.

The issue is ice crystals in the fuel filters. I have no idea why the Aztec is peculiarly susceptible, presumably the location of the filters or pipes that lead to them.

If you are a member of PPL/IR you can read an article (http://www.pplir.org/index.php?option=com_content&task=view&id=125) I wrote on the subject. If you are not a member, it is worth joining for many such articles! :}

[gordon field]

Guppy you seem to take pleasure in denegrating the experience of others and quote:

Where in the flight manual are you finding an emergency procedure or abnormal procedure for ice clogging the fuel flow divider or injector nozzles? You're not? Didn't think so, as it's not there. AND ON and ON.

Guppy. The problems encountered by others are quite clearly stated in following manuals.

C414A POH D1584-13 Section 8 pages 8-9,8-10 and 8-11
1980 model NO oil heated fuel manifold.

C4121C POH D1618-13 Section 8 pages 8-14,8-15,8-15
1985 model WITH oil heated fuel manifold.

As it is in other Cessna POHs.

Whilst Section 8 Airplane handling, servicing and maintenance is NOT mandatory and everything doesn't warrant inclusion in the Emergency Procedures Section 3.

Cessna do state " It is wise to follow a planned schedule of lubrication and preventative maintenance based on CLIMATIC AND FLYING CONDITIONS ENCOUNTERED IN YOUR LOCALITY" my caps. As you state ALL of your flying has been based in a mountain area where it is cold and freezing. Others who read the forum do live in other parts of the world and fly in different climatic conditions.

Cessna wouldn't have added it to the POH if there wasn't a problem. TCM wouldn't have incorporated the heated manifold on the 421C if there wasn't customer feedback.

Where in the flight manual are you finding a requirement for flight in cold weather, or flight into known icing conditions, to use a fuel additive? You're not finding that requirement? Didn't think so, as it's not a requirement.

Now, Cessna does provide direction on concentration of additives in certain cases, if you intend to use them. The use is not mandatory, nor is icing at the injector nozzles a big problem.

Guppy, READ THE MANUAL who mentioned flight in cold and known icing?

The manual states: One exception to this can be encountered conditions of High Humidity on the ground followed by flight to high altitude where low temperatures will exist.

It is the combination of Humidity and Cold.

If you have never experienced the incident then it has not been a problem for YOU but IT HAS been a problem for others and IT COULD be a problem for your students if they then fly where they may encounter the combination of Humidity and Cold. Before their next check ride they MAY encounter the problem.

The owners of the T310 flying at night from UK to Belgium DID have a problem with both engines but fortunately descended over The Channel. Their feedback started the ball rolling and OTHERS have encountered the problem that is why Cessna responded quickly, carried out the testing and incorporated the recommended advice in the POH.

Guppy
If you have just uplifted fuel prior to departure then you cannot drain the sumps and there is the possibility of an unknown amount of water in the system, Now how are you going to get rid of that as it can exacerbate the problem?

You are obviously a highly experienced instructor and 'check airman' whatever that is on the type with mega millions of hours, however I would have thought that, rather than denigrate what others say, a more professional and reasoned approach would have been for you to read the POH with an 'Open Mind', review ALL of the POH with your students and point out that, in your decades of worldwide experience on Cessna 400 series aircraft, you have NEVER experienced such a problem but, as stated in the manual others may have. If you choose not to carryout a complete review of the POH with your students then that a decision for you to make, perhaps you're too busy flying.

You've been there, done that and got the tea shirt, I've been there, done the testing and like many others got icing.

Guppy come down off your mountain, appreciate and understand that there are others throughout the world that may have had experiences that you may not yet had, and you may never encounter.

[SNS3Guppy]

Interesting. I didn't see you include anything from the manual which confirms your statement.

Guppy. The problems encountered by others are quite clearly stated in following manuals.



Quote it, then.

Cessna do state " It is wise to follow a planned schedule of lubrication and preventative maintenance based on CLIMATIC AND FLYING CONDITIONS ENCOUNTERED IN YOUR LOCALITY" my caps. As you state ALL of your flying has been based in a mountain area where it is cold and freezing. Others who read the forum do live in other parts of the world and fly in different climatic conditions.


Actually, no. I stated that all my experience has been in mountainous locations...not necessarily all of it in freezing conditions. I spent several hundred hours this spring in the aircraft you identified providing instruction and acting as check airman in the desert in very hot climates, as well as in the winter in cold climates. All mountainous terrain, of course, but a variety of climactic conditions. Of course, whereas we're talking about ice, climactic conditions other than cold, freezing weather are really irrelevant...which is why I specified...cold, freezing conditions. Go figure.

You're perhaps aware that pursuing a "schedule of lubrication and preventative maintenance" has nothing to do with icing at the fuel flow divider or at the fuel nozzles.

Cessna wouldn't have added it to the POH if there wasn't a problem. TCM wouldn't have incorporated the heated manifold on the 421C if there wasn't customer feedback.



Very well. Quote the flight manual, then. As far as what Cessna *would* or would not have done, supposition does nothing to address the issue.

The issue is ice crystals in the fuel filters.


Filters, yes. Why? Because they're intended to stop debris which might plug injectors (or carb jets, in the case of carbureted equipment). This includes ice; ice forming in the fuel system that makes it as far as the filter, usually goes no further. It may plug the filter and stop the engine, but by the time frozen moisture reaches the filter, it's usually unable to proceed to the engine. Hence my previous explanation(s) that most fuel system ice in aircraft using avgas is found in the tanks, lines, potentially at the selector, and finally at the gascolator/filter...but rarely if ever beyond that in the engine fuel system.

Guppy
If you have just uplifted fuel prior to departure then you cannot drain the sumps and there is the possibility of an unknown amount of water in the system, Now how are you going to get rid of that as it can exacerbate the problem?


Which is precisely why one should wait a half hour after fueling to sump and sample the fuel...per authority and manufacturer recommendations.

[gordon field]

Very well. Quote the flight manual, then. As far as what Cessna *would* or would not have done, supposition does nothing to address the issue.

Guppy: It ain't no supposition its a fact. I was flying for Cessna at the time, Cessna customers reported a problem and as part of the team we were asked to evaluate the problem, we replicated it, reported back to Cessna and the result is the recommendation in the POH. If you choose to ignore their words of wisdom so be it.

You are again confusing the cause of the problem
It is the combination of Humidity and Cold.

with cold and icing.

Just take care if you ever fly in the Benelux between October and March, meanwhile safe flying.

[IO540]

My experience is that if you are going to expose the Aztec to -15C for more than 4 hours or -20C for more than 3 hours you must use 1% IPA to prevent engine failure.

That is actually quite a long time. Looking at how many pilots seem to avoid flights exceeding their bladder duration, this issue could remain undetected in quite a lot of other airframes.

The issue is ice crystals in the fuel filters.

How did you establish this? Did you dismantle one, in ambient temp below 0C, and found it blocked with ice? I can't readily see any other way to prove it.

If it is the fuel filter, the way it is mounted could play a part. My fuel filter (TB20) is right underneath the fuel selector (it is all one item) and is in the belly. It is mounted against a metal surface which at a guess is 2mm thick and whose other side is the cockpit, which will be heated to say +20C. There will thus be a significant heat transfer from the cockpit into the fuel selector assembly.

Whether this will be significant to the icing issue would depend on how fast the fuel carries the heat away. Any liquid is pretty damn efficient at doing that, and at say 10GPH (typical fuel flow at high altitude) the flow velocity is about 0.4m/sec (6mm ID tube) which is a LOT.

OTOH if the Aztec (whose fuel system I know nothing about) had its fuel filter(s) mounted against a non-heated part of the hull, that could make all the difference.

The only way to settle this would be a complex temperature collection measurement in flight, which is fiddly (multiple PT100 sensors etc) and awfully hard to do legally...

[SNS3Guppy]

You are again confusing the cause of the problem
It is the combination of Humidity and Cold.

with cold and icing.

No, I'm really not. Given that I operate globally and in every climate and location save for Antarctica, including very humid, cold locations...it's really not something I've missed.

What I have missed is someone showing where the flight manual dictates that fuel flow divider and fuel filter icing is a big problem, as some here have suggested.

Several have contested the assertion that the lions share of icing in avgas powered airplanes takes place in the tank, lines, selector, and gascolator...not in the engine fuel system. Let them, therefore, show. Specific models of aircraft have been called out, and I reviewed the flight manuals for those aircaft before posting...and would like those who suggest Cessna shows a requirement for anti-icing fluid to post the quote, show the page, do something other than allude to preventative maintenance and lubrication. This requirement won't be found in aircraft limitations, because it isn't one. It won't be found in procedures, because there aren't procedures developed for iced fuel flow dividers or injectors, and it's not even an abnormal procedure.

Perhaps the best the posters can do is allude to an authorization by the manufacturer allowing the use of certain fuel additives, and specifying the minimum and maximum concentrations at which those additives are allowed.

This overlooks, then, the important issue of where the fuel contamination is to be found, it's effects, and the need to ensure it's removal...to say nothing of the mechanism or import of fuel additives themselves.

To overlook these things is to miss a big part of the picture needed to operate safetly, and if one operates while overlooking the important aspects of fuel icing, then one does so dangerously, and in ignorance.

What "Cessna would do" or "Cessna wouldn't do" isn't really the direction one should lean when addressing an operational issue. Only what IS.

For those citing their AFM's and POH's, then...why not site what IS...and show the text to which you refer?

[gordon field]

Guppy from your postings one can surmise that you assume that if, in your mega millions of hours flying around the world, you have never experienced the event that it has never happened to others. It is written in the manuals and one day an insurance guy may just come knocking on your door and deny a claim as you failed to adhere the manufacturer's data published in the POH. It's all in there to be read and taken into consideration.

Safe flying but make sure you have enough payload remaining for your ego.

[Timothy]

It's such a shame when seriously important flight safety matters get turned (yet again) into playground squabbles.

I can say for absolute certainty that it is not safe to fly an Aztec at colder than about -10C for any length of time without 1% IPA. If you do you are risking a double engine failure. I know this through painful experience.

IPA is a pretty unobtrusive chemical. It is the same stuff you stick in your ear if you have swimmer's ear (ie water trapped behind wax). It is kind to hoses, seals and bladders.

It is also pretty much exactly the same price as Avgas, if bought sensibly, so there is no financial loss.

The only disadvantage is that it is not easy to source locally so you have to carry it around, which is a fire risk.

But given the benefits of 1% IPA I just cannot see why people would take the risk of not using it.

And that has nothing to do with ego, willy size or whatever else you kids are chiding each other with.

[Timothy]

eBay listing today (http://cgi.ebay.co.uk/IPA-Isopropyl-Alcohol-Isopropanol-4-x-5-LITRE-20-LTRS_W0QQitemZ250542572300QQcmdZViewItemQQptZUK_HomeGarden_C LV_Cleaning_CA?hash=item3a5580430c) or just search "Isopropyl alcohol 5 litres".

[IO540]

That's good value.

Does anybody know about the Aztec fuel system and what kind of fuel filters it has and how they are mounted?

[SNS3Guppy]

It is written in the manuals and one day an insurance guy may just come knocking on your door and deny a claim as you failed to adhere the manufacturer's data published in the POH.

Then cite the reference.

I said nothing about failing to adhere to manufacturer requirements. Show the requirement. Can you do that?

The Cessna manual cites a very specific, very unusual operating condition, and states as much...but identifies flight from a very humid location to a flight above 20,000'. Do you do this in your Cessna 414? I certainly wouldn't. I don't even generally operate a King Air 200 above FL200, let alone piston equipment. They don't do well up there, turbocharger or not.

You see, you keep on about "the manuals," but can't seem to quote them or cite the reference, and the only one we've been provided in this thread thus far relates to the use of lubricants and preventative maintenance in cold weather...which is, of course, irrelevant.

As you're unable, I shall, this from a Cessna 414:

Isopropyl alcohol may be added to the fuel supply in quantities not to exceed 1% of the total. Refer to the Fuel Additive paragraphs in this section for additional information.

Fuel Additive
Strict adherence to recommended preflight draining instructions as called for in Section 4 will eliminate any free water accumulations from the tank sumps. While small amounts of water may still remain in solution in the gasoline, it will nomally be consumed and go unnoticed in the operation of the engine.

One exception to this can be encountered when operating under the combined effect of: 1) use of certain fuels, with 2) high humidity conditions on the ground 3) followed by flight at high altitudes and low temperature (flightlevels of 20,000' or above and temperatures of -28.9 C or below). Under these unusual conditions small amounts of water in solution can precipitate from the fuel stream and freeze in sufficient quantities to induce partial icing of the engine fuel injection system.

While these conditions are quite rare and will not normally pose a problem to owners and operators, they do exist in certain areas of the world and consequently must be dealt with when encountered.

therefore, to alleviate the possibility of fuel icing occurring under these unusual conditions it is permissible to add isopropyl alcohol or ethelyne glycol monomethyl ether (EGME) compound to the fuel supply.

The introduction of alcohol or EGME compound into the fuel provides two distinct effects: 1) it absorbs the dissolved water from the gasoline and 2) alcohol has a freezing temperature depressant effect.

Alcohol, if used, is to be blended with the fuel in a concentration of 1% by volume. Concentrations greater than 1% are not recommended since they can be detrimental to fuel tank materials.

The manner in which the alcohol is added to the tank is significant because alcohol is most effective when it is completely dissolved in the fuel. To insure proper mixing the following is recommended...

This is, of course, what I stated at the outset. Fuel icing takes place in the tanks, lines, selector, and gascolator, filter, but generally not beyond that point in the engine fuel system. Such occurrences are rare and unusual, and not a problem in normal operations.

If one elects to use EGME instead of alcohol, then on should also pay heed to the caution regarding concentration and mixing, also found in the manual:

Mixing of the EGME compound with the fuel is extremely important because concentration in excess of that recommended (0.15 percent by volume maximum) will result in detrimental effects to the fuel tanks, such as deterioration of protective primer and sealants and damage to O-rings and seals in the fuel system and engine components. Use only blending equipment that is recommended by the manufacturer to obtain proper proportioning.

Do not allow the concentrated EGME compound to come in contact wit hthe airplane finish or fuel cell as damage can result.

[Timothy]

EGME is unpleasant stuff and I wouldn't let it near my aeroplane.

It's all very well to talk about dealing with it carefully, but try doing that with a 40kt wind and a wind chill of -40. If IPA blows around it just evaporates, if EGME does it can write off the aircraft, not to mention the pilot.

[SNS3Guppy]

EGME (Prist) isn't that hard to mix. It's typically either tank batched by the fuelin vendor, or sold in 20 ounce cans. These spray like any aerosol can. A plastic delivery line clips on to the end of the fuel nozzle, the EGME is delivered into the fuel as the aircraft is refueled.

Anyone who deals with turbine equipment in the winter is familiar with the use of Prist.

I had several cases of EGME in cans several years ago, which lost their charge. We were in a part of the world in which obtaining the chemical was difficult, and we had to take what we could get. I ended up puncturing each can with a screwdriver, and pooling all the chemical in one dispenser for more efficient use. We attempted several garden spray devices to help deliver the chemical, but the seals and containers themselves became badly deteriorated in short order. In the end, it was all by hand.

This isn't common, however, and in most cases when using Prist, you'll never touch the chemical. I've had occasions over the years when it's sprayed back on me, and ended up on the finish and even in my face and mouth. It's never presented an issue so long as it's cleaned up, mouth washed out, etc. These occasions are rare. With the clip on the fuel nozzle, one simply depresses the button while fueling until the can is empty. The concentration is the same in avgas and turbine fuels, and is easy to calculate.

One can order it nearly anywhere that turbine fuels are sold, so most FBO's and fueling services will have access, and in most cases the fuelers will apply the chemical for you. Just not a big deal.

[IO540]

Why doesn't everybody use IPA when it works and is less hazardous?

Is there a difference between avtur and avgas usage?

Does any of the people here who seem to know everything about everything know anything about the Aztec fuel system?

[SNS3Guppy]

Isopropanol isn't available everywhere, and often Prist is easier to get.

[IO540]

Looking for compatible bottle materials for IPA.

This (http://www.borealisgroup.com/pdf/chemical-resistance/chemtab_PP.pdf) (page 7) suggests that polypropylene should be fine. These (http://www.camlab.co.uk/item.asp?itemid=27881&categoryid=683&browsecategoryid=307) look ok.

It would be somewhat embarrassing, or worse, if the container dissolved inside the aircraft :)

[SNS3Guppy]

Alcohol shouldn't be much of a problem in any containers such as you suggest. One should remember that it's a hazardous material (dangerous goods) and approach it accordingly when considering how to carry the chemical in the aircraft...especially in a passenger area or the flight deck/cabin.

If you're going to carry in a poly/plastic container, don't forget static and bonding issues. Much like one wouldn't want to carry or use a plastic fuel container on carpet or on a plastic truck/lorry liner, alcohol and other volatile chemicals must be taken into account as well.

EGME does considerably worse in many chemical-resistant containers, as we found out. We didn't carry it on board, but tried initially to store it that way for use during refueling. It didn't last long enough to complete the refueling, and even with a poly tank and a metal applicator, it still destroyed the seals inside the sprayer tank, fittings, and nozzle.

It's for these reasons that Prist in the can is a safer and easier means of carriage, than toting around a poly can full of IPA or other additive. The sealed cans of Prist discharge in the tank into the fuel stream at a metered rate, are fully self-contained and disposable, and take up very little room.

[Timothy]

Prist is OK when delivered at the Avtur hose, I agree, but that is hard to organise for Avgas delivery. It may be safe to get in the mouth, but that's not what the HazChem descriptor (http://www.indogulfgroup.com/MSDS/Ethylene%20glycol%20monomethyl%20ether.pdf) says!

The answer to Peter's question is that EGME is required in much smaller quantities than IPA. For my trip over the Pole I had to carry 50 litres of IPA (enough for 5000 litres of fuel) because there was no chance of getting it in Svalbard, Eureka, Greenland etc. 50 litres is a lot of inflammable chemical to carry around, not to mention storage and weight.

However, the fact that I decided to carry 50 litres of IPA rather than 5 litres of EGME must say something!

[IO540]

OK, interesting. I can see carrying 1% of IPA could be a lot. I have been looking for containers (high density polypropylene - common as muck - is compatible) and it is slightly tricky given that no "plastic" container will be anti-static, and the pump man may be unhappy about you pouring the stuff in right under his nose, after all the earthing precautions which he is supposed to be doing himself. I asked this morning and would have to get permission from the head fireman. So obviously one would pour in the stuff before taxiing to the pumps, and then as the fuel is filled, that should stir up the tank sufficiently (or not?).

I would have preferred stainless steel containers but they are not easy to get with the right o-ring material, and chasing down o-ring chemical compatibility is more hours on the internet. It is a very specialist area. PTFE o-rings should be OK.

I have not even tried to look for compatible containers for the other stuff. The TB20 POH lists two options one of which is IPA; the other I can't remember.

But I am still waiting for details of the Aztec fuel system :) Having seen the underside of my TB20 I know what that looks like. Has anybody seen the underside of an Aztec? Where is the fuel filter, and what is it mounted onto?

[SNS3Guppy]

It may be safe to get in the mouth,

It's not, and nobody suggested that it is.

However, the fact that I decided to carry 50 litres of IPA rather than 5 litres of EGME must say something!


Yes, it does. It says you went about it the hard way and unnecessarily carried extra bulk and weight. That's your choice, of course.

Prist is OK when delivered at the Avtur hose, I agree, but that is hard to organise for Avgas delivery.


It's no more difficult to mix in avgas than isopropanol. I have no idea what an avtur hose is, but mixing Prist with an avgas hose is no different than mixing it when delivering kerosine. Clips onto the fuel nozzle, and done.

When delivering other methods, such as jerry cans or a wobble pump from cans, even slowly through a chamois cloth, delivery is still not a complicated process, and can be mixed in while putting the fuel into the tank, or tank batched in the cans prior to pouring; either way works.

[Timothy]

I carry a common or (more literally) garden watering can to mix the IPA with avgas before load into the tanks, and I am sure that the same method would work with EGME. The watering can has handy markings for volume, such that I can easily put two litres into a 200 litre tank.

Peter, I have been pouring IPA into fuel tanks from polythene 5 litre containers (as seen in the eBay link) into the watering can, thence into the filler for many, many years, in all circumstances from major airports to wobble pumps in the deep Arctic and no-one has ever even raised an eye-brow.

I suggest that just looking authoritative and just doing it is probably the answer.

[IO540]

Epaulettes?

[Timothy]

Nah, steely eyes.

[IO540]

FWIW I did a test flight today with 0.6% IPA in one tank and 0 in the other.

No measurable performance difference (measurement resolution ~ 0.5%).

[AndiKunzi]

Old thread but vital infos in here and a couple of misleading informations.

Out of the AFM / POH of Cessna 300 and 400 series twins:
"... and freeze in sufficient quantities to induce partial icing of the engine fuel injection system"

Continental and Cessna were aware of cases of fuel icing in high flying piston twins. They found out that fuel icing occurs in the fuel injection system, not in tanks or previous filters / screens. The finest filter, which is the last filter, is the one at the fuel flow divider = fuel manifold = fuel spider.

Due to it's 5.0 psi cabin differential pressure and high power output, the C421C is operated the most likely of all Cessna piston twins at well above FL200 and below -30 dC. That's why all C421C / Continental GTSIO-520-N produced in 1980 and after (trailing link gear facelift) were equipped with a heated spider and a retrofit kit is available for previous aircraft.

Sweet spot for the C421C regarding speed and economics for trips of more than 400 NM is FL250 (depending on wind). CHTs below 400 dF even during high summer and solid 220 KTAS @ 36 GPH (60 % MCP) at mid cruise mass. Those aircraft were built to fly high.

[BoeingBoy]

Well that's just solved a 40 year mystery for me.

I was a rookie air taxi pilot in the seventies and I lost an Aztec engine for no explainable reason in very cold freezing conditions. It ran perfectly the next day following a diversion!

No one taught me about, or even mentioned that fuel icing was a risk.

[787chauffeur]

My personal experience with fuel icing in Avgas occurred many years ago in a Cessna 402. Departure was from KTTD (Troutdale, Oregon) during the winter months. Weather on departure was the typical for western Oregon, cool and wet. The airplane had been parked outside for weeks but no water was found in any fuel sumps during preflight. The fuel was mixed with, what at the time, was the recommended amount of isopropyl alcohol. (Cessna later gave approval for our operation to double that amount after other similar incidents of power loss due to what was assumed to be fuel icing).

On this day everything was normal until about 30 minutes after reaching cruise altitude, (FL 230). I don't recall the exact OAT now, but it was well below ISA. The first indication of problems was a slow but steady rise in EGT on both engines. Turning on boost pumps and readjusting mixtures a couple of times did not stop the trend. Eventually the mixtures were at full rich with boost pumps on high, but the EGT's kept rising. A divert was initiated and shortly thereafter the right engine quit and was ultimately feathered. A couple of minutes later the left engine quit and I elected not feather it, due to the likelihood that once feathered it would not be able to unfeather due to congealing of the oil in the prop dome. The divert airport, (KDLS) was reached with just enough altitude to maneuver for landing. In the pattern the windmilling engine began to show signs of life but never developed useable power.

After letting the airplane sit for an hour on the ground both engines started and ran normally. No water was detected in any of the fuel sumps. We had no means of testing the fuel for water content, so just assumed that we had moisture in the fuel and topped off the tanks, mixing in Prist as no isopropyl was available. After a normal runup the flight departed and flew across country to Oklahoma with no additional problems.

I could relate an almost identical experience in a Cessna 340 that occurred within a few months of this incident.

We could never "prove" that fuel ice was the culprit, but all the indicators pointed that way and after we started doubling the amount of isopropyl in our aircraft we never had another problem.

The common features of both of these incidents were that both airplanes had been fueled in damp locations, (western Oregon/Washington). No visible water was ever observed in any of the fuel sumps. Both were cruising in the Flight Levels (low 20's) when the engines quit. But manifested in a similar manner, (as described above). Both ran normally after sitting on the ground for about an hour. Neither had any further problems after the concentration of isopropyl in the fuel was increased.

I've been away from GA for many years now, but I am skeptical when I hear pilots saying that fuel icing is not a concern in AVGAS.

[Pilot DAR]

During the early phases of Mogas flight testing here in Canada, "fuel containing alcohol" was prohibited. Like a silly person, I stopped adding deicing alcohol. You'd think that after my first iced fuel forced landing I'd have figured out my error, but I actually had two (C 182, and C150). Luckily I got onto a suitable runway each time, so no "event". Transport Canada rapidly exempted deicing alcohol from the prohibition on alcohol in fuel when I made my second report.

It's a thing in some types. Be extra aware of the flush fuel caps (the GABB type, for example) common on many GA types. They have an O ring around the periphery, which will crack over time, and allow small amounts of water to weep into the tank, which may later freeze at altitude. Mogas/Avgas, same outcome. So consider deicing alcohol even on above freezing days, if you think you might be climbing for a prolonged time above the freezing level.