AVSIG: JetA freeze concern? wwswsigarch.jpg (7236 bytes)

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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
JetA freeze concern?
      #389251 - 04/09/14 11:41 AM

Can you tell, I just returned from recurrent training (g). We always discuss a lot of things and sometimes don't really settle all of them.
So, in the course of events, we discuss the systems on the KA and it looks as if fuel isn't heated until it enters the Fuel Control Unit area where engine oil heats the fuel. So, I wonder why Jet A in the tanks (bladders) or making it's way to the fuel control unit doesn't freeze or turn to thick goop too dense to fly. The general freeze point for Jet A is -40 which is where C and F cross. Obviously, turbine and jet aircraft at altitude often get below -40. Any thoughts on why this isn't an issue? We don't have a fuel temp probe on my plane. I know some do.

Best,

Dave

Edited by Dave Siciliano (ADS) (04/09/14 12:07 PM)


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Ralph Jones
Top Gun


Reged: 08/31/01
Posts: 21214
Loc: 4CO2
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389252 - 04/09/14 01:21 PM

That's a challenging problem in heat transfer. The rate of heat removal will depend on the ambient temperature, the amount of fuel in the tank, and the amount of empty space above the fuel (which serves as an insulator that gets thicker as fuel is consumed). I'm guessing that at the operational altitudes of your type, and the mission durations, the fuel just doesn't have time to get fully cooled.

Might be interesting to pull a fuel sample immediately after a landing and stick a thermometer in it.

--------------------
Ralph Jones
LS-4a N49LS 6R


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Russell Holton
AVSIG Member


Reged: 07/07/05
Posts: 14136
Re: JetA freeze concern? [Re: Ralph Jones]
      #389253 - 04/09/14 01:31 PM

Quote:

I'm guessing that at the operational altitudes of your type, and the mission durations, the fuel just doesn't have time to get fully cooled.



It would also depend on the starting temperature. It would be interesting to know, since at some point that assumption may not work - like flying out after overnighting in a very cold place.

Are there any restrictions on "starting temperature"?

Edited by Russell Holton (04/09/14 01:34 PM)


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Scott Dyer [HPN/NY]
Top Gun


Reged: 01/11/03
Posts: 20065
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389254 - 04/09/14 01:33 PM

Dave -- Interestingly, Shem recently posted an article on this subject:

http://airlinesafety.wordpress.com/2014/04/03/cold-fuel/


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Randy Sohn
Gradn Exlated Ordre of teh Fyling Fingres


Reged: 08/31/01
Posts: 23187
Loc: Savage, MN - U.S.A.
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389255 - 04/09/14 02:08 PM

Quote:

and it looks as if fuel isn't heated until it enters the Fuel Control Unit area where engine oil heats the fuel. So, I wonder why Jet A in the tanks (bladders) or making it's way to the fuel control unit doesn't freeze or turn to thick goop too dense to fly




Dave, it definitely WAS a concern, just one more thing we had to think about when accepting or not accepting some routes, generally to Asia.

best, randy


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Bruce Gorrell [EQY]
Top Gun


Reged: 04/29/04
Posts: 7864
Loc: Charlotte, NC
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389269 - 04/09/14 09:47 PM

Quote:

Can you tell, I just returned from recurrent training (g). We always discuss a lot of things and sometimes don't really settle all of them.
So, in the course of events, we discuss the systems on the KA and it looks as if fuel isn't heated until it enters the Fuel Control Unit area where engine oil heats the fuel. So, I wonder why Jet A in the tanks (bladders) or making it's way to the fuel control unit doesn't freeze or turn to thick goop too dense to fly. The general freeze point for Jet A is -40 which is where C and F cross. Obviously, turbine and jet aircraft at altitude often get below -40. Any thoughts on why this isn't an issue? We don't have a fuel temp probe on my plane. I know some do.

Best,

Dave




See http://en.wikipedia.org/wiki/Fuel_system_icing_inhibitor


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Mase Taylor
Top Gun


Reged: 04/29/04
Posts: 9446
Loc: SOCAL
Re: JetA freeze concern? [Re: Bruce Gorrell [EQY]]
      #389271 - 04/09/14 10:11 PM

Prist?

--------------------
Fly The Airplane As Far Into The Crash As Possible. - Bob Hoover 1922-2016 R.I.P.


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Russell Holton
AVSIG Member


Reged: 07/07/05
Posts: 14136
Re: JetA freeze concern? [Re: Mase Taylor]
      #389272 - 04/09/14 11:14 PM

If you need your paranoia levels raised:

The Price of No Prist

Also, google "Beechjet double flameout"


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Mac Tichenor (DAL)
Top Gun


Reged: 06/04/04
Posts: 1857
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389273 - 04/09/14 11:39 PM

Dave - if the KA PT6 installation is like the Socata TBM's, the unused heated fuel is rerouted from the FCU back to the tanks. I don't know exactly how much fuel that is, but I think one can make a reasonable estimate.

(All of this is based on the TBM's numbers) Fuel flow for max power at sea level is about 90 GPH, and at FL310 (its highest and coldest altitude) is about 45 GPH. The amount of fuel delivered to the FCU is related (proportional?) to Ng (gas generator turbine RPM) which, at cruise power, only varies about 5% between sea level and FL310. So I think a reasonable guess is that around 40 - 45 GPH, or 3/4 gallon per minute of heated fuel is being returned to the tanks at FL310. (The two tanks hold 146 gallons each.)

I don't know how much the oil/fuel heat exchanger raises the fuel temperature. The oil temp at cruise is about 67 degrees C.

I've had a number of flights this winter with the OAT below -50C, and this has been on my mind. But, other than the BA accident where frozen fuel was implicated, I'm not aware of any reported problems attributable to this. Are you? The TBM fleet has over 1 million hours and the KA fleet, I'm sure, many times that. So it seems to me, if it occurred with any frequency, it would be in the lore.

I found a thermometer on Amazon that will measure down to -40C, so I ordered it and will report some readings once I get it.

Mac

p.s., I believe that Prist is useful in keeping suspended water in Jet A from freezing but does not serve to reduce the freezing point of the fuel itself.


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Mase Taylor
Top Gun


Reged: 04/29/04
Posts: 9446
Loc: SOCAL
Re: JetA freeze concern? [Re: Russell Holton]
      #389274 - 04/10/14 02:59 AM

Quote:

If you need your paranoia levels




Aargh.

--------------------
Fly The Airplane As Far Into The Crash As Possible. - Bob Hoover 1922-2016 R.I.P.


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Mase Taylor]
      #389278 - 04/10/14 10:33 AM

Of course, Prist doesn't affect the freeze temp of Jet A. It would inhibit any water in the tanks (bladders) from freezing.
Jet A1 freezes at -47 and is used by some airlines when flight through colder air is planned.

Best,

Dave


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Mac Tichenor (DAL)]
      #389279 - 04/10/14 10:39 AM

Thanks Mac! I didn't think about the heated fuel being recirculated and and that didn't come up in systems discussions. Our fuel is heated at the fuel filter going into the FCU. Makes sense that unused fuel is recirculated, but I think that would go into the center tank or nacelle on my plane, not the bladders in the wings. Anyway, I'll look into that. Kinna wondering why we don't have fuel temp gauges. I normally fly low flight levels, so, not as much of a concern, but I could see leaving up north with cold soaked fuel on a -20C day and it being colder higher up.

Best,

Dave


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
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Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389280 - 04/10/14 11:09 AM

Well, I called my shop and their thoughts are fuel isn't recirculated after being heated. It comes into the fuel/oil heater, goes into the transducer which is your fuel flow meter and then into the Fuel Control Unit. If it was recirculated, the transducer wouldn't be an accurate indication of fuel flow. Scratching my head.

Best,

Dave


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Richard Duxbury (Dux)
Top Gun


Reged: 05/15/04
Posts: 5468
Loc: Minneapolis/Tucson
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389282 - 04/10/14 11:25 AM

It's been a long time -but like Randy noted, I think fuel cell temps were sometimes a concern on the long winter flights to Narita.

I also seem to remember from my military flying, that JP-4/5/6 had some different freeze points as well as some other stuff?

All the Boeing birds had various forms of fuel heaters as I remember (not fuel cell heaters).

Regards,

Dux, no frozen fuel in Tucson today at 95 degrees again.


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Geoff Sjostrom - Chicago
Top Gun


Reged: 04/29/04
Posts: 4312
JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389283 - 04/10/14 11:32 AM

I'd sure like to see Paul Millner get into this thread. I'll bet the fuel
producers are on top of the issue, or at least would have some good,
practical advice for pilots.

Geoff


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Bill Bridges - 9S1
Top Gun


Reged: 05/17/04
Posts: 6008
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Re: JetA freeze concern? [Re: Mac Tichenor (DAL)]
      #389291 - 04/10/14 01:54 PM

Quote:

the unused heated fuel is rerouted from the FCU back to the tanks




Mac,

I have a coffee drinking buddy that flies the BD-700 (?) who said that it recirculates the fuel similar to what you are describing (my understanding, anyway) once the temp gets below -25C. He indicated he had seen -70C with out any problems.

the other bill


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Randy Sohn
Gradn Exlated Ordre of teh Fyling Fingres


Reged: 08/31/01
Posts: 23187
Loc: Savage, MN - U.S.A.
Re: JetA freeze concern? [Re: Richard Duxbury (Dux)]
      #389295 - 04/10/14 04:04 PM

Quote:

It's been a long time -but like Randy noted, I think fuel cell temps were sometimes a concern on the long winter flights to Narita




Yup, been a l-o-n-g time ago but I seem to recall casting a nervous eye at the FE's/SO's (whatever you want to call him) panel and worrying about the fuel's "jelling point". And, on-loading around 400,000# of fuel for departure didn't include any Prist, although I think I do recall helping Clay do some of it on his Learjet at HRL once. Only had to divert to a more southerly Pacific track once for it IIRC but also once had to do some fast monkey-motion thinking stuff when one of my outboard tank valves acted up and needed to be thinking about our zero fuel weight and the structural considerations. Guess some think it's all "fun and games" for 12-15 hours and drinking coffee (which, thank God it usually is). At any rate, "it's good to be retired".

(Which reminds me, it's time now to go down to Caribou for afternoon coffee.)

best, randy


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Mac Tichenor (DAL)
Top Gun


Reged: 06/04/04
Posts: 1857
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389310 - 04/10/14 09:49 PM Attachment (103 downloads)

Dave -

>>It comes into the fuel/oil heater, goes into the transducer which is your fuel flow meter and then into the Fuel Control Unit.<<

It's in a different order on the TBM...



This is reminding me that I'm due for recurrent soon, too. Normally that's the only time I dig this stuff out.

Mac

Edited by Mac Tichenor (DAL) (04/10/14 09:50 PM)


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Jeff Hartmann CIC
Top Gun


Reged: 05/18/04
Posts: 7323
Loc: Chico,CA
Re: JetA freeze concern? [Re: Mac Tichenor (DAL)]
      #389316 - 04/11/14 07:19 AM

And that is how it is on later King Airs.

--------------------
Jeff

nothing clever to say right now...


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Dave Siciliano (ADS)
Top Gun


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Re: JetA freeze concern? [Re: Jeff Hartmann CIC]
      #389328 - 04/11/14 10:59 AM

Thanks. I'll look again this weekend.

Dave


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389339 - 04/11/14 12:29 PM Attachment (166 downloads)

Here's the fuel flow schematic I have. I don't see where fuel is recirculated right off.

Sorry, don't seem to be able to post the image directly.

Best,

Dave

Edited by Dave Siciliano (ADS) (04/11/14 12:31 PM)


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Mase Taylor
Top Gun


Reged: 04/29/04
Posts: 9446
Loc: SOCAL
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389340 - 04/11/14 12:43 PM Attachment (118 downloads)

Here ya go.



--------------------
Fly The Airplane As Far Into The Crash As Possible. - Bob Hoover 1922-2016 R.I.P.

Edited by Mase Taylor (04/11/14 12:44 PM)


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Jeff Hartmann CIC
Top Gun


Reged: 05/18/04
Posts: 7323
Loc: Chico,CA
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389343 - 04/11/14 01:10 PM

Dave,

Recirculation is a poor choice of words. From the 300/350 manual.

"A fuel-purge line is positioned at the output side of the high pressure pump, which constantly directs a small amount of fuel back to the gravity-feed line between the wing and the nacelle tank, to ensure the FCU stays clear of vapor and bubbles."

On the 300/350 if you have fuel in the aux and full wings, when you start it will automatically draw from the aux tanks, and the purge fuel returns to the wings. So, on a short hop the wings may overfill and vent overboard.

( you have read Tom Clements speak of pulling the aux breakers until you have flown a half hour or so, to prevent this. )

The medium blue line on your system. It goes through the purge valve and back to where it started from. On later planes they pick off the Fuel Flow just before the flow divider after the FCU.

Edited by Jeff Hartmann (EQY/SVH) (04/11/14 01:14 PM)


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
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Re: JetA freeze concern? [Re: Jeff Hartmann CIC]
      #389351 - 04/11/14 02:44 PM

Very helpful Jeff. Purge was what I was seeing. I'll take some time this weekend to look.

Best,

Dave


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
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Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389427 - 04/13/14 11:01 AM

It turn out the blue line from my FCU is in fact a purge line and really doesn't return heated fuel to my tanks. Tom Clements was nice enough to provide this insight:

... In most 90s and 100s, it is only the start switch that activates this solenoid valve to open. Since the purpose of the purge is to make sure that the nozzle discharge of fuel is not interrupted with "air" bubbles -- more typically, fuel vapor -- during the start, and since someone realized that we can start a PT6 without using the start switch -- i.e., a windmilling airstart in which Auto-Ignition is used to activate the ignitors -- when the 200-series got designed Beech wisely also sent power to the purge valve when ignition OR starter was activated.

Best,

Dave


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389428 - 04/13/14 11:03 AM

Since I have cold tanks, I was still puzzled as to why Jet A doesn't freeze at that temp. An engineer on Beechtalk added this:

The Latent Heat of Fusion is why it doesn't get that much colder, in any appreciable time.

Put a cup of water in the freezer. It will chill to liquid at 32F very quickly.
It takes 10x the heat removal to get the water from 32F liquid to 32F solid (ice).

Same idea at work, but the specific latent heat of fusion, in terms of kiljoules per kilogram (or BTUs per pound) is different than water, but the same general behavior is followed.

Best,

Dave


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Russell Holton
AVSIG Member


Reged: 07/07/05
Posts: 14136
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389432 - 04/13/14 02:43 PM

Yes, but percentage wise, how much of the fuel can you freeze before you run into trouble? The fact you've hit the freezing point says it's no longer a question of "if" but "when" before it gets ugly.

If it's like ice, what happens is the ice will form bit by bit as it's able to give up it's heat. It won't wait until you hit some jackpot and the whole thing flash freezes.

In the case of a fuel tank, I'd think it would start forming "ice" at the bottom (largest surface area connected to the outside air). But that's where the fuel intake is.


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Mac Tichenor (DAL)
Top Gun


Reged: 06/04/04
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Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389436 - 04/13/14 05:58 PM

>>Latent Heat of Fusion<< good point.

Mac


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Mac Tichenor (DAL)
Top Gun


Reged: 06/04/04
Posts: 1857
Re: JetA freeze concern? [Re: Russell Holton]
      #389437 - 04/13/14 06:01 PM

Russell -

I've been told that Jet A thickens starting a few degrees above its freezing point, theoretically giving a little advanced warning via fuel pressure readings.

Mac


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Randy Sohn
Gradn Exlated Ordre of teh Fyling Fingres


Reged: 08/31/01
Posts: 23187
Loc: Savage, MN - U.S.A.
Re: JetA freeze concern? [Re: Mac Tichenor (DAL)]
      #389440 - 04/13/14 06:41 PM

Quote:


I've been told that Jet A thickens




Mac, that's what I'd written above, the "gell point". Diesel truckers also encounter it often.

best, randy


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