Towards the end of 2008, the FAA issued an Airworthiness Concern Sheet related to float failures inside carburetors of reciprocating aircraft engines. The objective of the document was to raise awareness of the issue, in addition to requesting that comments or experiences be forwarded to the Agency for review. Such a gathering of data could, should a pattern emerge from the results, either lead to the issuance of an airworthiness directive, or to no action being suggested at all. In any case, it’s a matter of which to be aware. What follows is a summary of the FAA’s concerns with a brief overview of the float’s purpose inside your carburetor.
Fuel flows into the carburetor through the needle valve located in the carburetor’s float chamber. The fuel’s level in the chamber is controlled by a float which opens or closes the needle valve as the float rises or falls. When fuel in the chamber rises to a predetermined level, it closes off fuel flow through the needle valve. No additional fuel can then enter the carburetor until fuel is drawn into the engine. When fuel in the chamber drops owing to its intake into the engine, the float drops and opens the valve, allowing more fuel into the carburetor until the float rises sufficiently to close off the valve again.
The level of the fuel in the float chamber governs the level of fuel in the nozzle of the carburetor’s venturi. One of the problems of the carburetor is that the float can deteriorate and develop a hole. Should this happen, fuel can leak into it, thereby increasing its weight. If its weight increases, it drops. If it drops, it opens the needle valve allowing more fuel into the carburetor. As a result, the level of fuel in the chamber rises and fuel overflows from the nozzle, thereby flooding the carburetor.
The buoyancy of a carburetor float comes from the hollow cavity formed by its thin outer shell (usually made of brass or plastic). If this thin shell becomes cracked, springs a small leak, or in any way becomes porous, the result will be a breach that will allow fuel to enter the hollow floatation cavity, reducing the float’s buoyancy. Reduced float buoyancy will lead to improper metering of fuel into the engine, and is often the source of fuel leaking from the carburetor. It is these conditions that can lead to engine power loss, or engine fires.
Over the past two decades, service bulletins have been issued expressing concerns regarding carburetor float issues and their causal relationship to poor idle cut-off and carburetor leaks after engine shutdown. Many of these conditions are the direct result of fuel leaking into a deteriorated carburetor float. Accident investigations that have lead to float issues being identified as their cause have been reported by many aviation authorities throughout the world.
Newer carburetor floats may be made from closed cell epoxies that have greater resistance to wear and tear. The buoyancy of these newer floats is provided by hundreds of closed cells as opposed to a few larger hollow cavities inside the float. Should one cell of these newer floats become compromised for whatever reason, the buoyancy provided by the hundreds of other cells will continue to allow the float to operate as designed.
Why the float fails in the first place can be the result of any one of many factors: a weld joint may separate or, if the float’s pivot becomes excessively worn, the float’s shell may get chafed from rubbing on the carburetor bowl. Also, fuel evaporating from within the carburetor can leave residues and sediments that, over time, can degrade the surface of the float. Eventually, a hole pokes through the float. That hole, tiny though it may be, can bring down your airplane prematurely.