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HAWK21M
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Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 9:23 am

Is there any history of a leaking Hydraulic system Heat Exchanger contaminating the fuel , as most of the modern Aircraft have the Hyd HE immersed in the wing fuel tanks.A leaking HE could cause hyd fluid loss and contaminate the fuel, this could damage the seal of the related fuel system not supposed to be in contact with hyd fluid.

Just a thought.
Anyone experienced something similiar.
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rwessel
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 10:01 am

I'm sure it's happened, but the total volume of hydraulic fluid is very small compared to the volume of fuel.
 
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HAWK21M
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 10:13 am

True.....The contamination could be small, but if it enters the Fuel system, skydrol [Hyd fluid] could create further seal leaks.
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fr8mech
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 1:45 pm

We've seen it once or twice on B757/B767, but it's never caused a seal problem in the fuel system. Could it? Yes. But the volume of fuel vs. the volume of hydraulic fluid makes it very unlikely.
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planenut767
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 3:15 pm

I have seen it on a couple of 777's where the initial report was fuel migration into the center tank. Upon further troubleshooting they found that one of the hydraulic lines near the center tank was leaking into the fuel tank, so they had to do all kinds of filter changes and system flushes once the leak was repaired. As far as that happening via a heat exchanger I haven't seen that yet, but I'm sure it's happened some where.
 
tdscanuck
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 3:18 pm

Quoting HAWK21M (Thread starter):

Is there any history of a leaking Hydraulic system Heat Exchanger contaminating the fuel , as most of the modern Aircraft have the Hyd HE immersed in the wing fuel tanks.

Yes. It can happen on multiple types, typically more of an issue when it's cold and the seals don't function as well. Provided the aircraft doesn't sit for a very extended period with a very low level of fuel, the dilution into the fuel is so high that it doesn't seem to cause downstream effect on the fuel side.

Tom.
 
wingscrubber
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 6:54 pm

I think fuel in the hydraulic fluid is more of a problem than hydraulic fluid in the fuel, but the latter is probably more common, although there are contamination tolerances for both fluids.

FAQ from the skydrol website says:

Q: What happens when fuel is contaminated by hydraulic fluid?
A: In laboratory tests, concentrations of up to 5% Skydrol in jet fuel were cooled to -30oF. No gel formation or any other effect was observed. 5% is the maximum concentration possible in the worst case of a leak in a fuel tank heat exchanger.

http://www.skydrol.com/pages/faqs.asp
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HAWK21M
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 7:21 pm

Quoting wingscrubber (Reply 6):
5% is the maximum concentration possible in the worst case of a leak in a fuel tank heat exchanger.

Interesting stats.....Why do they state 5%, what about a leaking line, though rare, but not impossible.....

Quoting planenut767 (Reply 4):
they had to do all kinds of filter changes and system flushes once the leak was repaired.

This will def take time esp if the system requires flushing.
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wingscrubber
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 7:39 pm

Quoting HAWK21M (Reply 7):
Why do they state 5%, what about a leaking line, though rare, but not impossible.....

Presumably that's an estimate based on the loss of an entire system of fluid into a fuel tank, but depending on the aircraft type, fuel volume and hyd quantity I bet that number can be different.
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KC135Hydraulics
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RE: Hyd Fluid And Fuel Contamination

Mon Jan 21, 2013 9:36 pm

I have seen this occur on the KC-135 quite a bit. Our fuel pumps for the body tanks, which feed fuel to the A/R manifold, are hydraulically operated and submerged in the fuel tanks. Occasionally the motor and/or its hydraulic lines inside the tank develop a leak. It will lead to quantity loss on the affected hydraulic system but it can be difficult to detect. When we suspect this, we can sump the fuel tank but due to the shear volume of fuel in these body tanks (upwards of 40k) the fuel samples are visually inconclusive. They can do burn samples but even those can be inconclusive. Eventually when the problem becomes intolerable the last resort is going in tank and it is then that you will be able to find your leak. As far as I am aware after the source of the hydraulic leak in the tank is found and corrected, there is no additional corrective action taken as far as the fuel system is concerned. It is understood that the residual hydraulic fluid that was intermixed with the fuel will just burn off. Jet engines can burn just about anything. I've never heard of fuel flushes/filter changes/decontamination for hydraulic fluid-contaminated fuel.
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HAWK21M
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RE: Hyd Fluid And Fuel Contamination

Tue Jan 22, 2013 8:33 am

Quoting KC135Hydraulics (Reply 9):
Our fuel pumps for the body tanks, which feed fuel to the A/R manifold, are hydraulically operated and submerged in the fuel tanks

Any reason why hyd operated fuel pumps were opted for on this type.
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rwessel
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RE: Hyd Fluid And Fuel Contamination

Tue Jan 22, 2013 9:21 am

Quoting HAWK21M (Reply 10):
Any reason why hyd operated fuel pumps were opted for on this type.

One obvious advantage would be keeping an electric motor and the associated wiring and electricity out of the tank.
 
KC135Hydraulics
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RE: Hyd Fluid And Fuel Contamination

Tue Jan 22, 2013 9:52 am

Each HYD motor is flow regulated to 17 GPM at 3000 psi and there are 4 total A/R pumps. For an example I ran a mule today for a defuel and running one pump at a time was flowing 350 GPM of fuel into a truck. With 4 running that's 1400 gal/min. The size and wiring of an electrical motor submerged inside a fuel tank that could power a pump like that would be a major fire hazard. We don't have inerting systems for the body tanks. Hydraulic motors are safest in the fuel environment (and they sound great when they're screaming at max power)!
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strfyr51
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RE: Hyd Fluid And Fuel Contamination

Tue Jan 22, 2013 5:14 pm

Quoting HAWK21M (Thread starter):

I've seen this on the 737 and the Airbus 320 series airplanes. we now recognize what we're looking at pretty quickly because it will most certainly ruin an engine by clogging up the engine's start nozzle spray pattern and turning it into a steady stream rather than a spray . You have to look at the trend pattern for Hydraulic servicing or on the airbus, loafing roll spoilers. unless it's a massive leak you might be chasing it down for matter of weeks before you find it. checking the fuel low level sump drain for contamination after a long layover (6-8 hrs) seems to help verify the problem. I use that as part of my isolation procedure and I've narrowed it down on 4 airplanes. (over 13 years) so it is not common but it is certainly NOT improbable either. but you have to take your time and document what you're looking at.
 
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HAWK21M
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RE: Hyd Fluid And Fuel Contamination

Wed Mar 27, 2013 7:05 pm

Quoting strfyr51 (Reply 13):
it is not common but it is certainly NOT improbable either.

Anyone having stats on such a snag.
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