Fuel Temp Pred -- Why does the checklist fix work?

[eslader 1,774]

Got a Fuel Temp Pred warning last night at FL400. Went through the checklist. It recommended I increase speed. I did, and the message went away.

Cool! Neat to see this stuff simulated - I learned something today because of it. But why does that work? I only increased from M0.84 to M0.85 which at that altitude is only a few knots faster. Surely skin heating doesn't increase enough because of that to make a difference in the fuel temps, or does it?

 

 

[Headley 26]

Well, i had the same warning today, very first time i saw it.  Was a flight above Russia, not my everyday destination.

Fuel Temp was at that moment -7° C.

So increased speed from M0.84 to M0.847, and descended from FL390 to FL350.

The message went away, but not sure if it was because of my actions.

And during flight, the Fuel temp decreases to -23° C.

I was waiting for the engines to flame out, but no no, i could end my flight without problem...

Edited February 16, 2019 by Headley

[eslader 1,774]

The Pred warning means the plane is predicting that if you don't make changes, the fuel will drop below minimum temperature, not that it's below temp now. Your changes worked, I'm sure, because you did what the checklist told you to. I didn't even have to descend - just sped up a little. When I got the message I was over France, and the OAT was -61C. 

[scandinavian13 5,192]

On 2/16/2019 at 7:22 AM, eslader said:

Surely skin heating doesn't increase enough because of that to make a difference in the fuel temps, or does it?

Concorde was speed limited because the nose temps reached a point that could damage the material if exceeded. It also expanded quite significantly at altitude (at least for a metal object).

It was going much faster, sure, but the concept is the same.

[downscc 1,327]

16 hours ago, scandinavian13 said:

It was going much faster, sure, but the concept is the same.

Pushing the concept another step, the SR-71 wings literally dripped fuel until the aircraft got up to it's normal cruise 3+M and the titanium metal heated and closed the gaps.  Their first order of business was to hook up with a tanker after takeoff.

Edited February 18, 2019 by downscc

[eslader 1,774]

I knew that, and I knew the SR71 actually used its fuel as a heat sink because of the friction-heating. But what surprised me was the idea that accelerating a subsonic plane by 3 or 4 kts can mean the difference between fuel freezing and not, reliably enough to be included in the checklist. It made me wonder if it was purely a "lets increase skin heating" solution or if perhaps it was more related to higher fuel flow = less time for the fuel to  freeze in the lines.

I'm also assuming that we're talking about fuel freezing in the lines, not the tanks, or am I off-base there as well?

 

 

Edited February 18, 2019 by eslader

[downscc 1,327]

1 hour ago, eslader said:

It made me wonder if it was purely a "lets increase skin heating" solution or if perhaps it was more related to higher fuel flow = less time for the fuel to  freeze in the lines.

Not at all, the predictive warning is based on trend.  If you add a few degrees to the TAT then the fuel cools at a slightly slower rate and may eliminate the problem from occurring in the future.  The important thing about this warning is that it is a predictive warning based on current conditions may become a problem in the future.  The fuel is not freezing in the lines, more likely the problem starts as a formation of wax in the coldest regions of the tank.

[Iceman2 110]

This is the stuff that brings PMDG to the top of the leaderboard when it comes to aircraft simulation.

I’ve been flying around the US a lot in the winter months and haven’t seen any of the bug splatter that everyone’s been talking about. I flew from the US to Australia this week in the 744 and on short final into Sydney I had quite a lot of bug splattering. It’s summer downunder and the ambient temperature was over 30 degrees C. Bugs are more prominent in warmer months and in the VR headset this was a welcomed feature that looked great.

Its a pitty the bugs just disappeared over time as in RL our engineers clean the windows post flight when needed. (They should add a clean windows in the maintenance page of the FMC)

Im impressed with the little easter eggs that pop up from time to time.

Nice one PMDG 👍

IM

[PopsBellNC 21]

1 hour ago, Iceman2 said:

I'm impressed with the little easter eggs that pop up from time to time.

Nice one PMDG 👍

 

Chewie is my Co-Pilot! 😎

[Iceman2 110]

1 hour ago, PopsBellNC said:

Chewie is my Co-Pilot! 😎

I haven’t seen that one yet.. I prefer the the old girl, (747-400) so not sure if Chewie is only in the - 8.

Actual since the recent updates I don’t see the pens or the soft drink cans much anymore.

IM

[Boeing or not going 578]

The pre saved fuel temp limit in the fmc is -37. What I see is -42 set for regular flying and -45 over polar areas for one major carrier. - David Lee

Edited February 21, 2019 by Boeing or not going

[JRBarrett 3,034]

21 minutes ago, Boeing or not going said:

The pre saved fuel temp limit in the fmc is -37. What I see is -42 set for regular flying and -45 over polar areas for one major carrier. - David Lee

Is it a European carrier? Jet-A1 fuel is readily available in Europe, and has a lower freezing point than regular Jet-A.