Coolant quandry

[FastEddieB]

The other day as part of my annual I started the engine to let it warm up for the compression check.

 

To get it really warm, I let the water temp get right up barely into the yellow, which you can see starts at about 125ºC:

 

 

When I shut down and got out of the plane, the coolant bottle, which had been about at the "FULL" line, was steaming and empty. Remember that in my installation the engine is not visible from the cockpit or I might have caught it in progress. Coolant was sprayed all over the empennage and vertical fin. After things cooled down, I pulled off the pressure cap and found the metal coolant tank maybe half way down - took about 2 cups to fill it.

 

I'm using 50/50 standard antifreeze/distilled water.

 

One time before I noticed the coolant bottle was nearly empty, for no apparent reason. No obvious leaks going on.

 

Any thoughts? Does 125ºC exceed the boiling point enough that this is normal behavior? Or might I have an issue elsewhere? Normal flying always results in cool temperatures enroute - sometimes a little too cool - the above image is in flight in winter.

 

Thanks!

 

[FlyingMonkey]

Were your compressions better warm?  I know a couple of your cylinders were on the low side when cold.

[FastEddieB]

Most were a tiny bit higher.

 

Exact figures posted to the other thread.

[Anticept]

Make sure there are no kinked lines around the cylinders, or the restricted flow will cause boiling.

 

Are you using 50/50 dexcool? Is it still a clear orange color, or has it turned cream colored? Flush the coolant system with water thoroughly and replace the coolant with fresh if it's anything but that transparent orange color.

 

Make sure that the coolant assembly (the "spider") is also completely full with no air. Otherwise as the coolant cools down, it can't easily pull coolant from the overflow bottle.

 

As a natural consequence of liquid cooling systems, there will be a small amount of boiling around hose bends, and that's why we use overflow bottles. It's to take up the expanding coolant.

 

It also might be worth having your coolant radiator (or thermostat if you have one) checked or replaced if this isn't normal.

 

EDIT: You're a pusher prop, I forgot. Your configuration needs ram air to function, whereas tractors get some of the ram air from the prop on the ground. Taxi around a bit during long warmups to keep air flowing through the engine bay if you need supplemental cooling.

[FredG]

Check my numbers.

 

125 deg C = 257 deg F.

 

According to the internet, a 50-50 solution of standard ethylene glycol antifreeze and water will boil at 265 deg F in a closed system equipped with a 15 psi pressure cap.  Note that as your elevation increases, the boiling point will decrease (even with the closed system and a 15 psi cap). 

 

I don't recall the cap rating on the 912.  Looks to me like you may have been close to the boiling point.  It might be hotter than indicated temperature in some places in the cooling system so there may have been local boiling.  Also, you gauges may not be perfect.

 

EDIT:  1.2 bar (listed on CPS site as cap pressure) = 17.4 psi.  Boiling point at 17.4 psi is slightly greater than at 15 psi. 

 

Check my numbers.

[FastEddieB]

Wow, guys, thanks!

 

Roger switched me from Evans, to Prestone 50/50.

 

I will take a close look at all the hoses.

 

I'm at close to 2,000' if that makes a small difference.

 

The Sky Arrow has two small switch-controlled fans to blow air back into the radiator and oil cooler as needed for ground operations.

 

Like I said, the "spider" was a couple cups low. Remember that on the Sky Arrow checking this sort of thing is a bigger deal, involving both de-cowling and a ladder. I can view the overflow bottle clearly from the rear, however, and that was full when I started the runup.

 

My cap is stamped "1.2".

 

In any case, I should, in the future, be able to keep the temps from getting that high. I was just surprised that a brief excursion into the yellow could have that effect.

[Anticept]

Are those fans really enough though? Are you just idling up the temp, or running it? Was it a hot day?

 

It's also a big deal on CTs, we too have to decowl. There really should have been an access door for those things, or maybe a sensor!

[FastEddieB]

Are those fans really enough though? Are you just idling up the temp, or running it? Was it a hot day?

 

 

Not hot at all - maybe a 45º morning.

 

Key here is I ran it at a higher-than-idle setting with the intent to get it hot. Probably too hot.

 

Normal operations I'm OK even on hot days. If I have to sit for a long time for any reason, it makes sense to shut down and then restart if the temps get too high, fans notwithstanding.

 

Here are those fans:

 

[Anticept]

I've seen you post that before.

 

Still, I've sat around idling and never had temps go that high, but again, I'm a tractor, not a pusher. Just don't let it go so high next time

 

EDIT: Also, does the fans only blow over the cooler? A lot of heat is also in the hoses and engine bay too...

[Rich]

Did you get air in the system?

If so, it may not work itself out and may cause the problem your speaking about.

[FastEddieB]

Did you get air in the system?

Maybe?

 

Like I said, the metal "spider" tank took a couple cups of coolant.

 

Should I have "burped" it by turning the prop over a few times?

[Anticept]

Crank the starter. you won't be able to move coolant by hand, it's a centrifugal pump.

[Roger Lee]

The answer is yes you were over temp and boiled it away. Depending on your coolant ratio the boil point is around 260F-270F for 50/50. If the coolant is old it will add to your problems because over time coolant will start to loose some of its properties.

 

248F is the Rotax max with standard coolant. Opening the coolant tank to check the level is on the Rotax inspection check list, but that may not have even been the problem. More likely over heated boil over..

 

See if that A&P guy that did your plane had read and learned the bulletins and operator's manual he would have known that.   

Just giving you a hard time.   It can happen to anyone, but what great timing. 

 

When doing the compression test there is no real need to really go over 50c-60c.

[Roger Lee]

If you drain the coolant system on the Rotax engine it should fill quite well since it's an open system top to bottom unless you have a thermostat, but the Rotax engine by itself does not have a thermostat. Rotating the prop by hand won't help much. You would need to start the engine for a few minutes (unless you have a thermostat then it's longer) and then check the level. It really shouldn't be down more than a few ounces.

 

p.s.

The Evans wouldn't have boiled over (boil temp is 370F), but Evans carries a 20F-30F higher temp penalty. No water means less cooling. water absorbs and releases heat better than pure Evans with no water. With Evans you could have used the 266F max temp, but you would run 20F-30F higher cruise temps. Fully open air engines don't have this issue. Cowled engines do.

[FastEddieB]

On my subsequent test flight I climbed to 9,500', baffle closed, albeit at relatively low temps.

 

Everything seemed as usual, with med to low water and oil temps.

 

So,

 

1) No harm done.

 

2) I know now to stay away from the yellow, and,

 

3) Every opportunity I will check the coolant tank.

 

As an aside, no thermostats on the Sky Arrow - winter warmups take a loooong time!

[FastEddieB]

BTW, if I neglected to metal "spider" coolant tank, it was likely because I thought that like most cars and bikes, a low level there would suck coolant in from the overflow tank.

[Anticept]

Cars don't suck coolant from the overflow either if there is enough air in them. That's why they too have radiator caps. Remember that air is very, very compressible, and as a consequence, it sometimes won't generate enough vacuum.

[Roger Lee]

Expansion Tank.  No bugs (spiders) on the Rotax.  

The Rotax check list specifically has you open and check the level.

 

 

This demonstrates how even simple items can be missed or left out for whatever reason and your check list for the plane that you used didn't have this.

 

 

This is exactly why I have been so adamant about using these list. If using a pre-flight check list is important to a pilot then this is just as important. It adds a positive target to inspect, gives the manual sections if you don't understand, keeps people from mis-interpreting what is supposed to be done versus what they think should be done. If you wrote IAW on your inspection page then it would have been wrong. You didn't do it IAW and that's why so many people miss things and set themselves up for problems if someone ask what IAW meant in your logbook. Then if the check list is used and signed off then the next inspection you know for sure it was done, the next mechanic knows it was done or the next owner. This was a minor thing, but it can be just as easily a major thing.

 

The last 2007 SW i just inspected was way over due for the gearbox inspection at 1000 hrs. If the other mechanic had used the check list he should have caught this, not to mention the other 2.5 pages of issues.

[FastEddieB]

I will include the ROTAX checklist for my annuals from now on. And I will check the metal tank* every time I have the cowling off.

 

Realistically, is it not possible the metal tank was full at the beginning of my fiasco, and that the cup or two of coolant boiled off? After all, the cooling had been operating fine prior.

 

 

*What is the correct name for that thing? I started calling it the "spider" after someone else did.

[Jacques]

Expansion tank is the name

 

the 'so called' spider is the hose arrangement coming to it.

912 cooling system.pdf

[FastEddieB]

Thanks.

 

Expansion tank it is!

 

Don't know why I didn't just look it up myself - just lazy, I guess!

 

To add to the confusion...

 

I'm pretty sure my expansion tank is not configured like the one in the schematic. Unless I'm misremembering, mine just has the hose coming off the bottom.

[Roger Lee]

Hi Eddie,

 

Believe it or not I have known you long enough to have great faith in your Knowledge and abilities not to mention your contributions to our forum. I for one have learned from you over the years.

 

Rotax sends out engines in one configuration and with limited hoses. Many owners or MFG's opt to change the top coolant hose configuration and tank arrangement due to mounting issues with each aircraft. In truth it doesn't make any difference how it gets turned. So many of our individual aircraft do have some differences.

[Anticept]

I call it a spider because expansion tank should have been the overflow bottle.

[Roger Lee]

When our engine coolant heats up, it expands. The coolant expansion tank provides additional storage for the expanding coolant and is commonly the location where coolant is added to the engine. It is more important on small systems like ours.

Our cars only have the radiator and the overflow tank. Our Rotax engine has a small radiator, the expansion tank and the overflow bottle. Unlike our car radiators the expansion tank is a distribution point for our engine so as the coolant comes in from a single hose it gets distributed to multiple points.

[Doug G.]

Roger, it also says we are to take top and bottom ceiling of each day. Do you do this?????