Avgas Freezing

[AlanA]

This week it has been mighty cold throughout the nation!  Temperatures subzero in many places.  It made me curious as to whether or not anyone has ever had a problem with Avgas freezing (gelling)  in their Mooney?  

 

According to what I read Avgas freezes at about -54 degrees F.

Lately at my airport  it has been -10 at field elevation in the early morning.  Because of the mountains or clouds  I often have to fly 10,000 feet higher.  

Lapse rates are about 3 degrees per 1000 (in high humidity) to about 5.5 degrees in dry weather.

 

So theoretically if I start at  - 10 degrees at field elevation and go up 10K on a clear dry night the temp could be -65 degrees.

 

I assume inside the cowling it is warmer but exactly by how much and where it's I don't know.  My question is, at what point do you start worrying about your fuel freezing?  And has anyone had any real life experience with this happening?  If you did have freezing gas, how did you correct the problem and fast was the problem resolved?

[jetdriven]

The standard lapse rate doesn't really apply to subzero temps. I can say I only see -65C in the jet at FL350 very rarely, and usually north of 55 degrees latitude. Usually it stops at -56C. Check this latest temps aloft chart. -30 in your area. It's alright. not only that bit it will take 2-3 hours for the fuel to cool to the ambient temp.

[carusoam]

Byron,

I think the fuel lines would be the focus. The tanks would take a while to cool, but fuel in the metal lines would be more at risk.

Definitely would need to have a good read on OAT. Relying on std lapse rate won't work for such a critical situation.

I've not ever heard of a concern for this in GA before. Anyone else?

Best regards,

-a-

[aviatoreb]

I am not worried about my actual fuel freezing. I am worried about water in the fuel freezing.  I always sump, and I have never found water in this airplane.  Tanks seem tight, knock on wood.  But if you ever do have water in your fuel, it is supposed to go to the bottom of the tank so you can find it and sump it out during your preflight.  If it is well below freezing, that water will instead freeze.  So you can sump and find no problem.  It can freeze and cover over your sump port as well as the lines that lead to your engine and cause fuel starvation.  About 6 months ago I did a lot of reading about this and it is a not common, but not trivial problem with piston airplanes that operate in extreme cold.  So that covers airplanes that operate in frigid cold winter conditions and or airplanes that go very high. 

 

You are like me - my airport can get to well below 0F sometimes in the winter.  Even if you do not fly on those coldest of days and wait for a nice "warm" 10F to fly, if that water is frozen at the bottom of your tank then the danger is already there.  It will not unfreeze until the airplane is warmed up.  So a heated hangar would be nice - I have a very nice, but unheated hangar.  So any water that would freeze in my fuel system would stay there for the duration of the winter.

 

So, just last month, I decided to start running prist in my fuel.  It is a standard aviation product.  It is usually used and common in jet fuel (they have a different problem with water because the water does not separate), but it is certified and allowed for avgas as well.  I talked to the company rep on the phone before I proceeded.  Long story short - not so short anymore I guess - I am now adding PRIST with each fill up.  It comes in a metered flow-rate can, but other than it now takes me two hands to fill-up, its not hard.

 

I am not sure if it is really necessary, but I am not convinced it is not necessary, and except for a slight nuisance of filling with an extra product, and a slight cost (~$.15 extra per gallon to include the product), it won't hurt.  You can buy PRIST at aircraftspruce or any such outlet.

[Jamie]

According to what I read Avgas freezes at about -54 degrees F.

 

I can say I only see -65C in the jet at FL350 very rarely,

 

According to the EAA, Avgas freezes at -58 -celsius- not farenheit (avgas specs). -58C == -72.4F.

So, you still have some distance to go from -10F on the ground to -72F in the air.

(aviatoreb is right, but the inconsistent units bugged me.)

[Bennett]

I started using Prist many years ago when flying my 261 above FL230 (certified to FL 280). Several Mooney pilots who operate between FL 210 and FL 270 have had the suspended water in their fuel freeze and block the injectors causing either an engine stoppage, or major power loss. This was well reported at one of the Mooney Homecomings. Prist is relatively inexpensive for the peace of mind if operating at high altitudes, and low ambient temperatures. My cross country flights in winter were generally at FL250 or FL270 eastbound to take advantage of the prevailing winds, and occasional a dip in the lower edge of the winter jet steam. Damn cold up there. Fly safe

[aviatoreb]

I started using Prist many years ago when flying my 261 above FL230 (certified to FL 280). Several Mooney pilots who operate between FL 210 and FL 270 have had the suspended water in their fuel freeze and block the injectors causing either an engine stoppage, or major power loss. This was well reported at one of the Mooney Homecomings. Prist is relatively inexpensive for the peace of mind if operating at high altitudes, and low ambient temperatures. My cross country flights in winter were generally at FL250 or FL270 eastbound to take advantage of the prevailing winds, and occasional a dip in the lower edge of the winter jet steam. Damn cold up there. Fly safe

 

Hi Bennet,

 

Actually it was in chatting with you about this very thing which was the straw that broke the camel's back and I decided to start running prist.  I remember you told me about the above.

 

But how did those Mooney pilots know that it was suspended water freezing at the injectors which was their injectors?  I would have guessed it was not-suspended water freezing in the lines or perhaps at the uptake ports at the bottom of the tanks.

[yvesg]

The problem we encounter here is when you have some water, like Erik says, it freezes and you can end-up with a drain letting fuel out and stuck this way. it has happened to me. i guess PRIST would be a good solution for this.

yves

[N601RX]

The older Mooneys have a service bulletin about the routing of the fuel hose that connects to the bulkhead fitting on the firewall and connects to the mechanical pump.  The fuel line should connect to the fitting at an upward angle so there is no low spot for water to get trapped and freeze. The SB had never been done on my plane when we purchased it.  We complied with the SB when we overhauled the motor last year.

[aviatoreb]

The problem we encounter here is when you have some water, like Erik says, it freezes and you can end-up with a drain letting fuel out and stuck this way. it has happened to me. i guess PRIST would be a good solution for this.

yves

 

Or simply blocking the port entirely.

[jlunseth]

I just use isopropyl alcohol.  The POH allows up to 3% and Bruce Jaeger used to teach that at the MAPA PPP's (he doesn't teach them any longer).  3% is quite a lot though, I typically just add one container per side, I believe the containers are about a pint.  I do not do it much anymore though, I have many flight hours at altitude where the temps are always low, and fuel gelling or freezing has never been an issue.  I do sump every time though.  I will still occasionally add isopropyl in the winter, particularly if I plan to go high.

[Piloto]

On my M20J I am more concern about freezing myself than on the fuel. At those temps I still need to wear my parka because the heat is not enough to keep me warm, specially for those on the back seats. Stuff in the baggage area will freeze. Best battery to have for subfreezing is an AGM battery like the sealed Concorde. Some avionics may stop working at subfreezing temps. I always pull out the avionics cooling fan CB. Beware that unlike the old steam gauges LCD displays may not show any reading after engine start until the LCD warms up. Besides engine preheating is a good idea to preheat the cabin.  

 

José

[Seth]

Hi Bennet,

 

Actually it was in chatting with you about this very thing which was the straw that broke the camel's back and I decided to start running prist.  I remember you told me about the above.

 

But how did those Mooney pilots know that it was suspended water freezing at the injectors which was their injectors?  I would have guessed it was not-suspended water freezing in the lines or perhaps at the uptake ports at the bottom of the tanks.

Bruce Jager is a big supporter of Prist.  I don't fly way up high in the 20s, but if I did, I'd probably use Prist as well.

 

-Seth

[Seth]

I just use isopropyl alcohol.  The POH allows up to 3% and Bruce Jaeger used to teach that at the MAPA PPP's (he doesn't teach them any longer).  3% is quite a lot though, I typically just add one container per side, I believe the containers are about a pint.  I do not do it much anymore though, I have many flight hours at altitude where the temps are always low, and fuel gelling or freezing has never been an issue.  I do sump every time though.  I will still occasionally add isopropyl in the winter, particularly if I plan to go high.

Never mind - this is what Bruce Jager says to use. 

 

This issue tends to come from when you fill up in a humid warm area like New Orleans and then fly high quckily into cooler weather (like flying accross a front up high and landing in Minnesota).  The suspended humid drops of water can freeze during flight and then block the fuel drains and lines and thus, stop your engine. 

 

Don't use too much prist or isopropyl alcohol or it can hurt the seals - there was one pilot who overused it and his seals started having issues - he swears it was a direct relationship but there is no proof.

 

-Seth

[Bennett]

Hard to independently know if it was water freezing in the injectors (that was the theory put forward in the discussions at Kerrville) or elsewhere, but both pilots reported problems that self corrected at much lower altitudes. The pilot with the engine stoppage was very relieved when the engine fired up around 15,000', running roughly at first.  As I said, I used Prist, and never had a problem with freezing water in my fuel. I concur with Jose. Very cold up there, and I always wore a leather flight jacket, lined with thick fur, to try and keep warm.  The side windows frosted over badly, while my feet were burning from the heater. Really never liked being up that high, but the winds were the inducement. I always had a second (portable) oxygen bottle hooked up with masks in case of failure of the built in system. I consider Prist as cheap insurance, and it is really not that difficult to use (as directed). Adding isopropyl alcohol is easier, but I believe that spraying Prist into the fuel while fueling is a better way.    

[BorealOne]

-32 on the ground this morning in CYFS. I'll be flying today to CZVL at 9000 AGL. Temps enroute will vary from -35 to -15 to -35. I'll be doing flights like this all winter long, and have been for years.

 

I run a little isopropyl as per Bruce Jaegers' recommendations. Avgas won't gel, but you don't want ice crystals in the fuel line.

[aviatoreb]

My airplane has fabulous heat.  I think it is part of the rocket mod, because I have been in other Mooneys which are very cold.  I was flying last weekend in a polar fleece vest and a cotton short sleeve shirt - starting out at 2F on the ground and getting colder as I went up - only went to 11k.  Generally I wear a puffy down jacket until I am done climbing when about the cabin has heated up nicely from the heater.

[flyboy]

It got down to 57F here today in upstate SC with a high of 72F. If it gets much colder I may go to pants instead of shorts. If its really cold out we wear shoes, sometimes even socks in the dead of winter. Perhaps a blanket thrown over the shoulders would help keep you less frozen. I really feel for you guys who fly in such frigid temps. Dress warm and stay safe.

[Shadrach]

My airplane has fabulous heat.  I think it is part of the rocket mod, because I have been in other Mooneys which are very cold.  I was flying last weekend in a polar fleece vest and a cotton short sleeve shirt - starting out at 2F on the ground and getting colder as I went up - only went to 11k.  Generally I wear a puffy down jacket until I am done climbing when about the cabin has heated up nicely from the heater.

 

My old F is a cooker as well. I just wish it had more outlets for uniform heating. The "blast furnace" that sits on top the nose gear well gets hot enough to make it uncomfortable for exposed skin in the vicinity... 

[Bob - S50]

 

Lately at my airport  it has been -10 at field elevation in the early morning.  Because of the mountains or clouds  I often have to fly 10,000 feet higher.  

Why would you go outside when it is -10?  Stay inside, start a fire, play with Mom.

[neilpilot]

I use HEET, one bottle per side, if it goes below 20F and/or I'm concerned about residual moisture.  Can be found in the PMN section of many auto parts stores.

 

Be sure to use HEET in the red bottle, since the other formulation is methanol which is not good for the Mooney.  I once met a pilot who would use rubbing alcohol.  That is isopropanol in water and does not go well in the fuel system.

 

BTW I make isopropanol at work, which we burn as a supplemental fuel in our steam boiler.  Sometimes tempted to "borrow" some, but that's not worth the hassle.

[yvesg]

Why would you go outside when it is -10?  Stay inside, start a fire, play with Mom.

If I would follow your advise, I would not go outside for several months here! :-)

Yves

[jlunseth]

The coldest we have seen was -54 F on a trip from Williston to Minneapolis at FL230.  We were up there because there was a huge wind aloft on the tail and we made the trip in about 1:45 as I recall.  No problem with the fuel gelling or icing up.  I have not been that cold very often but certainly in the -20 range on many trips with no issues, whether I have put isopropyl in the tanks or not, so I am not very diligent about doing it.  I put some in every now and then on the theory that I may collect very small amounts of water that don't show up in the sump cup and I want to get that mixed into the fuel and out of the aircraft.

[BorealOne]

If I might share some cold weather flying advice - don't put frozen headphones over your ears. Wear a toque underneath, and wait until the ear cups warm up... 

[aviatoreb]

If I might share some cold weather flying advice - don't put frozen headphones over your ears. Wear a toque underneath, and wait until the ear cups warm up... 

 

HAha - even worse - if you use one of those halo in the ear style headsets - I can tell you from personal experience that it hurts putting a -10F ear piece into your ear.  I now use a Bose X in the winter.