aerosiam
Joined: 26 May 2008
Posts: 59
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 Posted: Wed Nov 12, 2008 9:53 am Post subject: Water in wing tanks |
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Allegro 2000 two years old, 150 hours based in Thailand fitted with
wing tanks used about once every three months.
We found about a pint of water in the right wing tank. I will do my best to summarise the story which is a little complicated.
We removed the wings in order to paint the fuselage. The hoses were taped off and the wings sat for about four weeks.
Before refitting the wings, we tilted the wings briefly about 30° to remove any fuel. No fluid came out. With the wings refitted, I was in the process of connecting the right wing outlet hose to the valve when about a quarter of a pint of water drained out of the hose.
We removed the wings and tilted them nearly vertical. Another quarter of a pint of water drained out of the right wing, but no fluid was found in the left wing.
A fuel test is done on the belly tank as part of the pre flight. The wing tanks do not have the means to drain or test.
I think what happened is as follows: water accumulates in the tanks due to condensation. The tanks are long and the outlet is slightly above the bottom of the tank. My aircraft sits slightly right wing down and by habit I always fill the right tank first. Also the Allegro has a slight dihedral. Water gravitated from the left to the right tank. In addition, water that collected at the lowest point of the system in a slioght loop (where fuel from the wing tanks joins the main fuel system near the fuel pump) would have been pumped back up the hose into the right tank when filling it.
I believe the reason the water was not discovered when tilting the wings prior to refitting them was due to the baffle in the tank. I am guessing that the battle is quite restrictive to prevent fuel sloshing end to end in the long tanks. It was sufficiently restrictive to hold the water between the baffle and the farther end of the tank when we tilted the wing to check.
I have only once found water when fuel testing the belly tank. This was during a 2000 mile trip where we were refuelled at air strips with jerry cans. I assumed the water was due to poor fuel or procedures. In retrospect it was more likely due to water from the right tank flowing into the system in turbulence or a series of left turns in the pattern on the long trip.
My first thought is concern if there is another Allegro with a similar pattern in terms of age and usage of the tanks especially if used in a training environment where it may frequently be doing left-hand turns or 360° turns.
I strongly recommend that anyone with these wing tanks checks for water contamination. With the wing tanks empty, remove the fuel outlet hose, tilt the aircraft as far as possible by jacking one main wheel and perhaps also removing air from the tire on the other side. Presumably this would be sufficient to drain any fluid from the tank. Water could have accumulated in the transfer hose between the two tanks and the lowest point of the system where it joins the main tank feed. It will be necessary to remove this hose and also inspect the small plastic gascolator between the tank and the pump. This in turn means the main tank must be trained but this is not a bad idea anyway.
My suggestion to resolve this problem is to replace the 2 way on-off valve above the doors with a three-way valve similar to the fuel selector valve on the console. The extract outlet would have a short flexible hose so that the tank can be drained at the valve into a container. I think it would still be necessary to jack the air craft up on one side to do this.
When the wing tanks are full, the fuel flows thru and fills the clear vent tubing and recirculates back to the main tank. I think this prevents the vent from working effectively as a vent until the fuel has dribbled out of it and been replaced by air. For this reason I suggest that the wing tank is half filled then some of the fuel is drained out to check for water before the vent tube fills with fuel.
A sure way of preventing condensation is to keep the wing tanks full but that means hauling around an extra 60 lb of fuel.
We did a test on the tanks and found that the nominal 40 litre capacity was about 38.5 litres less half a litre unuseable.
I am no mechanical expert and I welcome any suggestions about this problem. I am surprised that with such a large amount of water, none was found on pre flight checks of the main tank. This may be partly explained by the fact that when the wing tanks valves are opened in flight, some fuel flows into the main tank and some direct to the engine. I have not yet checked the float bowls for water.
Regards
Paul
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