BACKGROUND:
Wet wings usually fail quite inconspicuously, starting with a slight smell of fuel around the aircraft or in the cabin and evidence of blue stains caused by leakage. This condition only worsens over time.
The main reason for failure appears to be the deterioration of the sealant caused by aging and, at least in Mooney aircraft, hard landings.
The landing gear is attached to a beam in the tank area, and the rubber shock absorbers on the Mooney landing gear legs do not provide sufficient damping. Years of landings, including a few hard ones, can bend the structure enough to loosen the old sealant, starting the leakage process. (You will sometimes see pieces of sealant when draining fuel).
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Mooney tanks also leak toward the front of the tanks, causing stains visible from inside the landing gear bays.
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Another common place for fuel leaks is at the entry door (under the wing walk tape) through loose rivets caused by the flight crew stepping on them.
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Another common leak is through damaged fuel nuts (due to using screws of the wrong length) or damaged access panels.
However, the most common repair is a poor repair of the tank seal (incorrect procedure or sealant) and its subsequent degradation.
A leak in the upper wing skin may cause water to enter the fuel tank.
ALTERNATIVE:
Installation of vulcanized neoprene bladders. The system from Griggs O&N Aircraft Modification maintains a capacity of 64 gallons with a weight increase of 30 pounds.
RECOMMENDATIONS:
Mooney SB M20-230 - Inspection of Fuel Tank Sealant Application
When purchasing an older Mooney aircraft (30 years or more) with wet wings, consider the potential cost of resealing or bladder installation.
Resealing is recommended if you plan to own the aircraft long-term.
RESEALANT:
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Chemical removal of old sealant using PolyGone.
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Mechanical removal of old sealant (scraping).
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Application of new sealant in a three-part process with a curing time for each step.