Everything You Need to Know About Twin Commander Fuel Leaks

a Twin Commander flying over clouds

All Twin Commander Aircraft have fuel cells. Depending on your model, there may be a combination of fuel cells and a wet wing area. If your twin Commander is an older model, it can have up to 22 fuel cells. The fuel cells typically perform well however, over time they can develop leaks.

As a way to extend the life of your fuel tanks, it is a good practice to keep a sufficient amount of fuel in the aircraft to keep all the areas wet with fuel. Keeping the aircraft moving and hangared also helps prolong the life of the fuel storage system.The fuel will keep the interior of the storage system moist and healthy.

If you do spot a fuel leak with your Twin Commander, however, there are a few things you should keep in mind.

Understanding the Fuel System of a Twin Commander

The fuel system enables fuel to be loaded, stored, managed, and delivered to the engine of an aircraft. Systems will vary from aircraft to aircraft based on the size and complexity of the aircraft they are installed in. As a general rule across all aircraft makes and models, a fuel system consists of a gravity-fed fuel tank with a fuel line connecting it to the engine. When considering a larger, multi-engine or cargo aircraft, the fuel system likely consists of multiple fuel tanks that are either located in the wing, fuselage, or both.

As the size and complexity of an aircraft increases, so does the complexity of the fuel system. That complexity typically includes more system automation, and a tangle of tanks, pumps, and switches. However, in the case of most Twin Commanders, the fuel systems are kept surprisingly simple.

A network of rubber bladder fuel cells in each wing gravity feed to a common tank either in the  fuselage or a common tank in each wing. Boost pumps in the fuel system send fuel to the engines. From there, engine-driven fuel pumps provide the proper fuel supply to the engine fuel controls to deliver fuel to the engines.

Depending on your model of aircraft, there may be a single fuel gauge or one for each wing. On the Twin Commander aircraft, there is no need to worry about switching fuel tanks or moving fuel around as the fuel system is kept simple—making fuel management simple and seamless for the operator. As always, referring to the Aircraft Operating Manual for proper operation of the fuel system is the pilot’s responsibility.

How to Check for Fuel Leaks

With a single fuel gauge, fuel maintenance is a breeze. In the case of Twin Commander 690, 690A, and 690B, for example, there are also fuel filters, sump drains, fuel vents, and shutoff valves in place to keep fuel from flowing the wrong way.

In the case of identifying fuel leaks, however, there are a handful of things you can keep an eye out for:

  • If fuel is dripping from any overboard lines on the underside of the engine, it could be coming from the fuel pump, fuel control unit, or a component on the engine.
  • If you’ve parked your aircraft and spot a leak area in the hangar, on the underside of the wings, that’s the most obvious sign that you could have an issue with either a fuel cell or the wet wing area.

If you notice or even suspect a fuel leak, it’s important to consult your MRO so they can get to work identifying the cause of the leak and making a plan to either repair or replace the failing part.

How an MRO Should Address a Fuel Leak

Typically, if you bring your Twin Commander in because of a fuel leak, your MRO should park your aircraft in a designated hangar area. The aircraft can be topped off of fuel and allowed to sit and be monitored to check for leaks. If any areas are found, the MRO will mark the general area of the leak, and the aircraft will be defueled, opened, vented, and allowed to dry.

The leak could be coming from a fuel cell or the wet wing area. If a fuel cell appears to be leaking, the cell will be removed and capped off, pressurized and checked with a soap solution. If an area is found, the fuel cell can be sent for repair or it may have to be replaced with a new tank.

If the wet wing is determined to be leaking, the maintenance team will inspect the internal confines of the wing and remove any sealant and replace fasteners if necessary. They’ll then prep the area for a reseal, and allow the new sealant to dry. From there, they’ll pressurize the fuel tank and they’ll spray the area in question with a soap solution. Fuel leaks are often identified in this phase because of the bubbles that the soap solution will create around any leaking areas. If bubbles appear, the process is repeated and fasteners are replaced with oversized fasteners as required.

If no bubbles appear, the wing will be refueled and the maintenance team will again monitor the areas for leaks.

Once no leaks appear, the aircraft will be returned to service.

Understanding Fuel Leak Classification

When an MRO is investigating a leak, there are certain fuel leak classifications that they’ll keep in mind. The size of the area that a fuel leak moistens within 30 minutes is what an MRO will use as a classification standard.

They will clean and dry the area entirely and dust the leak area with talcum powder. The talcum powder provides a better visual background to determine areas in question. If a leak is present the fuel wets it, making the size of the leak area easier to see.

At the end of the 30 minutes, all leak areas (if there are multiple) are broken into four classification areas:

  • Slow Seep
  • Seep
  • Heavy Seep
  • Running Leak

A slow seep is a leak that isn’t more than ¾-inch in diameter. A seep is a leak that covers an area from ¾-inch to 1 ½-inches in diameter. A heavy seep is a leak that wets an area from 1 ½-inch to 3-inches in diameter. Finally, a running leak is when the fuel runs, flows, or drips even at the end of the 30 minute waiting period. 

Contact Winner Aviation for Your Twin Commander Fuel Leak Repair

If you notice a fuel leak, or even suspect one, contact our maintenance team. Winner Aviation is one of 14 Twin Commander Factory Authorized Service Centers in the country. We’ll take care of your Twin Commander and get your aircraft back in service with minimal downtime.

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