The Mooney Ovation Ultra, like many high-performance aircraft, utilizes “wet wing” fuel tanks, where the wing structure itself is sealed to form the fuel containment area. Unlike aircraft with removable rubber or flexible bladders that can be easily pulled out and repaired, Mooney’s wet wing tanks rely on sealant applied to the internal ribs, spars, and skin joints. This design, while efficient for space and weight, makes fuel leaks a common and often challenging issue for Ovation Ultra owners in California, Arizona, and Nevada.
At The Aero Center, we have extensive experience with Mooney’s unique fuel tank systems. Our 24/7 maintenance center offers specialized diagnostics and repair services for fuel leaks, minimizing your aircraft’s downtime. We operate with the principle of Social Proof, with a strong track record of successfully resolving complex fuel leak issues that other facilities might shy away from.
UNDERSTANDING MOONEY OVATION ULTRA FUEL LEAKS
Mooney fuel tanks are notoriously prone to leaks as they age, with the average expected life for original sealant being around 20 years¹. However, some leaks can occur much earlier. The main failure mode is the deterioration of the sealant caused by aging, vibrations, and stress from landings (the landing gear is attached to the spar in the tank area, and even normal landings can flex the structure enough to loosen old sealant)¹.1
Common Locations for Leaks:
- Rivets: Leaks often begin as a “weep” around rivets, especially those along the wing walk area or where internal stiffeners attach to the skin. These areas experience significant stress and flexing².
- Seams/Joints: The sealant at the internal joints between wing skins, ribs, and spars can degrade and develop pathways for fuel to escape.
- Access Panels/Sender Units: Gaskets and sealant around fuel quantity sender units and other access panels can fail over time.
- Fuel Selector Valve/Lines: While not part of the wet wing itself, leaks can occur at the fuel selector valve or in lines leading to or from the tanks, which can mimic a tank leak.
Identifying a Leak:
- Visible Stains: Fuel stains on the underside of the wing, particularly near rivets or access panels, are the most common sign.
- Fuel Smell in Cabin: If fuel leaks into the leading edge of the wing, it can sometimes migrate into the cabin, indicating an urgent need for inspection.
- Fuel Loss During Ground Operation: A noticeable drop in fuel level while the aircraft is parked, especially when tanks are full.
- Only Leaks When Full: A leak that only appears when the tank is filled above a certain level does not necessarily mean the leak is above that level. The weight of the fuel can force leaks lower or expose weaknesses under pressure¹.
FUEL BLADDER LEAK PATCHING (WET WING TANK)
For Mooney Ovation Ultra (and most M20 series aircraft), “patching” refers to applying new sealant to a localized area of the wet wing tank, as there isn’t a removable bladder to physically patch outside the wing. This is often an attempt to stop small, isolated “weeps” or minor leaks without resorting to a full tank reseal.
Advice for Patching (for certified A&P Mechanics ONLY):
- Locate the Exact Source: This is the most challenging and crucial step. Fuel can track along internal structures before appearing on the exterior.
- Drain the Tank: Completely drain the affected fuel tank.
- Ventilation: Ensure excellent ventilation to remove fuel fumes.
- Inspection: Use a bright light and a mirror to meticulously inspect the internal surfaces of the tank through access panels. Look for streaks, discolored sealant, or areas where sealant has peeled.
- Pressure Test/Vacuum Test: A mechanic may apply a very light pressure (never over-pressurize a fuel tank!) or vacuum to the tank while applying a leak detection solution to the exterior suspected area or observing internal signs.
- Preparation is Key:
- Cleaning: The area to be patched must be meticulously cleaned. This involves removing all traces of fuel, old sealant, dirt, and corrosion. Specialized aerospace-grade cleaning solvents are used. Any contamination will prevent the new sealant from bonding properly.
- Surface Roughening: Lightly abrade the surface to provide a mechanical key for the new sealant.
- Approved Sealant:
- Use only aviation-grade, fuel-resistant, two-part polysulfide sealant (e.g., PPG Aerospace P/S 890, Flamemaster CS 3204) that is approved for use in aircraft fuel tanks.2
- Ensure the sealant is within its shelf life and mix ratio is strictly followed.
- Application:
- Apply the sealant precisely to the leak location. For rivets, a small “dollop” or bead around the rivet head can sometimes be effective, as mentioned in Mooney owner forums².
- For seams or small cracks, a thin, even layer that extends beyond the damaged area is applied.
- The sealant must be worked into the joint or rivet head to ensure good adhesion and fill any voids.
- Curing:
- Allow the sealant to cure completely according to the manufacturer’s instructions. This usually takes 24 to 72 hours at room temperature, but warmer temperatures can accelerate curing. Do not refuel the tank before full cure.
- Post-Patch Test:
- After curing, the tank should be pressure tested (very carefully and to manufacturer specifications, usually very low pressure) or filled with fuel and closely monitored for any signs of continued leakage.
Limitations of Patching:
- Temporary Fix: Patching is often considered a temporary solution, especially if the underlying cause is widespread sealant degradation due to age. It may stop that specific leak, but other leaks may soon appear as surrounding sealant continues to fail¹.
- Difficulty Locating: As noted, locating the exact source of a leak can be extremely difficult, and patches applied in the wrong place are ineffective.
- Interior Access: Most patching must be done from inside the tank through small access panels, requiring specialized tools and a mechanic with experience in tight spaces.
WHEN TO CONSIDER A FULL RESEAL OR BLADDER CONVERSION
If multiple leaks appear, or if a patch fails, a full tank reseal is often the next step.
- Full Reseal (“Strip and Reseal”): This involves completely stripping out all the old sealant from the tank interior (a messy, labor-intensive process often using chemical strippers), meticulously cleaning the metal to bare surface, and then applying fresh, new sealant throughout the entire tank. This is a significant undertaking, often taking several weeks and costing upwards of $10,000-$15,000 USD (as of mid-2020s)¹. However, a properly done reseal can last for decades.
- Bladder Conversion: For some older Mooney M20 models (up to J-model), replacing the wet wing with vulcanized neoprene bladders is an option. While this used to come with a penalty in reduced fuel capacity and higher empty weight, new systems from companies like O&N Aircraft Modification can retain original fuel capacity with a modest weight penalty¹. This is also a significant investment but can offer peace of mind if recurring wet wing leaks are a persistent problem. For Ovation Ultra (M20U), which is a later model, direct bladder conversions may be less common or available, but a mechanic familiar with STCs could advise.
For Mooney Ovation Ultra owners in California, Arizona, and Nevada, the appearance of fuel leaks should be addressed promptly by a qualified aviation mechanic. While localized patching might offer a temporary solution for minor seeps, a comprehensive evaluation is crucial to determine if a full reseal or, if applicable, a bladder conversion is the more cost-effective and safer long-term solution.
The Aero Center has the expertise and facilities to accurately diagnose and expertly repair fuel bladder leaks in your Mooney Ovation Ultra, offering both localized patching (where appropriate) and guidance on more extensive resealing options. Our 24/7 service ensures rapid response to keep your Mooney safely in the air.
The Aero Center is located at William J. Fox Airfield KWJF | Lancaster, CA. Contact us at 209.885.6950 for questions or appointments.
Footnotes
- Aviation Consumer. “Fuel Tank Reseal: Thankless, Expensive.” Aviation Consumer, September 27, 2024.
- Mooneyspace.com. “Ovation fuel tank sealing – Modern Mooney Discussion.” Mooneyspace.com Forums, April 9, 2022.
- The Mooney Flyer. “Patch or Reseal?” The Mooney Flyer, March 3, 2018.