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Episode 100!

Posted: Fri Oct 03, 2008 9:54 pm
by Afterburner Al
There are at least two lines of defense against ice in the fuel for most high altitude planes. The first line of defense is as you discussed is the anti-ice additive which is branded as "Prist". I am not sure what the airline's policy is but depending on the operator on the corporate side prist is added or not added based on whether they are willing to pay the xtra price for Prist.

The built-in line of defense is the fuel-Oil heat exchanger where cool fuel straight from the tank is warmed by the oil returning from the engine..and of course the oil gets the desired treatment of being cooled before returning to the oil pan. To a lesser extent their is a fuel filter in most fuel systems and depending on the system if it the filter gets clogged there is or is not a bypass which may or may not affect the flow of fuel to the fuel control unit.

Depending on the aircraft there is a minimum fuel temp, but that is mostly for the GII there is no pump driven fuel transfer while in can only be done on the ground with pumps. In flight you can transfer fuel by opening a valve between the two collection hoppers in the wing and step on the rudder and bank the airplane slightly and transfer via gravity. This procedure is limited to fuel temps above 0 degress Celsius due to the potential for icing.

All other jet aircraft I have been in have in-flight fuel transfer with pumps and there is no minimum fuel temp transfer limit.

As for the 777 there must be some quirky design fault that when the planets align...the fuel can ice wonders if the fuel in china...just like their milk may have quality control issues.

Re: Episode 100!

Posted: Sat Oct 04, 2008 5:52 am
by Madmax
the other thing that prist does that heaters won't do is it works as an algaecide, because as hard as it is for me to grasp, the jet fuel will have organics growing in it these can foul up the fuel jets. some of the jets that I have flown (citations and lear) use a venturi system of pumps, to move fuel around, (transfer or supply) plus the nozzles in the burn box have really choked down the fuel to a very very small flow volume (high pressure) algae in these areas clog up the works.

The alge is not so much ‘growing’ in the tanks of the aircraft, but in the storage tanks at the airports, who knows how long it has been since those tanks were cleaned or purged…

Having lost an engine (more than once) in different aircraft, it is always (in my never humble opinion) to do whatever you can before you take off to minimize the risk. That is 'code' for adding fuel with an additive every time, and let dispatch or the chief pilot give me grief later on.

To paraphrase Bill Cosby’s line of “The coach must have benched you after that play… but that benching didn’t hurt nearly as much as what the other team was going to do to me..”

What was the company going to do? Fire me for being safe?

Re: Episode 100!

Posted: Sat Oct 04, 2008 10:35 pm
by Greg Bockelman
The 777 has a fuel oil heat exchanger such as Al described. Well, at least the Pratt powered ones do. I can't speak to the Rolls powered aircraft.

Whatever additives are in the fuel we use, they are there before it gets pumped into the airplane. Nothing is added after the fact.

When the OAT is expected to be below -65C for long periods of time, when we are on the polar routes, we need to watch the fuel temperature pretty closely. The fuel freeze point is in the neighborhood of -40C so if it approaches that value, we need to take corrective action.

Re: Episode 100!

Posted: Mon Oct 06, 2008 2:02 am
by Tracy
I would venture to guess that all transport-category aircraft have some sort of fuel-oil heat exchanger system.

The 747-200 systems book talks about the fuel-oil heat exchange system more in the area of an oil cooler. There is also a fuel heater system that uses bleed air for the heating with valve control switches located on the flight engineer's fuel panel. Some versions of the plane had manual control, some could be put into an automatic mode.

The 747-400 handles the fuel temperature problem automatically. The center systems screen continuously displays the fuel temperature with the display turning from white to amber when the temperature reaches -37 deg C or less. Fuel temperature is to be kept at least 3 deg C about the fuel freezing point, generally -40 deg C for Jet A, as Greg said.

After spending 10-12 hours flying in the FL300-FL400 range, it is not unusual for ice crystals to form in the fuel. The fuel heaters/heat exchangers are usually located just prior to the fuel filter and fuel control unit. The first indication of fuel icing is usually an illumination of the fuel filter bypass light, indicating that ice crystals are blocking the filter. If that is ignored, the effect of the ice on the operation of the fuel control will result in erratic engine operation.

Re: Episode 100!

Posted: Tue Oct 21, 2008 1:10 pm
by Biggles71
Prist is normally never added in modern long-haul airliners.

One just keeps an eye on the fuel temps in cruise when ISA deviation is - . Freezing temp for JET A (use in the US) fuel is -40C, JET A1 ( most of the rest of the world) is -47C.

Once these temp are reached, you can one of 2 things:
>fly faster, highter TAT
>Descend a couple of thousand feet

Both will cost you fuel!


Re: Episode 100!

Posted: Sun Oct 26, 2008 4:21 am
by Tracy
I realize that by now this is an old subject, but I thought I'd throw out the fuel temp numbers I watched on my last long-haul flight.
We started out in the eastern U.S. and flew to Japan. Total flight time was just under 13 hours.
Departure (1830Z) 333,000 lb of fuel in the tanks, with a main tank fuel temp of +13C.
1930Z - FL320 295,000 lb fuel at -8C.
2100Z - 256,000 lbs at -16C.
2300Z - 203,000 lb at -28C (FL320, OAT -61C)
0000Z - 175,000 lb at -31C (FL340, OAT -58C)
At that point I went on my break.
0600Z - 34,000 lbs at -32C (FL380, OAT -64C)
Our route that day had taken us as far as 72 degrees north latitude, north of Barrow, AK.

The lowest temperature our fuel reached was the -32C reading, still 5 degrees from our min allowable temperature.