|Understanding Your Air Box and Filter|
|Written by Jacqueline Shipe|
|Thursday, 24 February 2011 10:15|
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Carburetor Air Boxes
There is probably no other section of the induction system that needs as much regular maintenance as the air filter and carburetor box. The aluminum structure that houses the air filter usually attaches to the cowling, the engine mount, or directly to the carburetor. They are subjected to a lot of vibration, because they are connected in some fashion to the cowling and the engine. The carburetor heat flapper valve or the alternate air door also operates at least once every flight, often multiple times, and the moving parts wear with use.
Spots Prone to Wear
The flapper valve in the carburetor air boxes generally consists of a flat piece of aluminum with sealing material extending out around the edges. The valve is attached to a steel shaft. Older-model air boxes had the flapper valve attached to the top of the shaft. This caused the valve to be sucked up against the outlet hole that feeds air to the engine if the valve ever loosened and came off the shaft. This would close off the air source to the engine. Newer models come with the valve attached to the underside of the shaft to prevent this.
The shaft is held in place with two bearing assemblies, which are connected to each side of the box. An arm on the shaft itself provides a connection for the carburetor heat cable. These items not only wear from continuous vibration during flight, they also wear each time the carb heat is pulled on and pushed off. As the sealing material erodes, it can leave large gaps where air can seep in around the sides, making the carburetor heat less effective when it is on and allowing some hot air in even when the cable is pushed off. This hot air is unfiltered, plus it can cause the engine to have a richer-than-normal mixture, because the hotter air is thinner. Both of these situations are bad for you and your aircraft.
The box itself is also subject to wear. The flanges where the scat hoses are connected vibrate, which causes them to occasionally become loose or chaff holes in the metal. Also, several areas on the box are subject to cracks. Care needs to be taken when patching the box to be sure that no fasteners are extending through to where the flapper valve seals. Fasteners or pieces of metal that become loose over time can get sucked into the engine, so this area is in need of regular inspections. Alternate air mechanisms are not used as often as the carb heat is, but they still wear.
The boxes can be very costly to replace completely, depending on the model. The parts to rebuild the complete mechanism are expensive, as well. Fortunately, some companies make several replacement parts for the boxes that are much more reasonably priced than the factory parts.
Air Filter Replacement Requirements
The air filter element needs to be replaced periodically. The sponge-type filters in the Brackett assemblies are required to be replaced every 12 months or 100 hours. They may need replacement more often in dusty conditions. The front surface can be brushed off if a small amount of dust is on it, but the element can’t be submersed in anything to be cleaned. Any paper induction air filter has a required replacement time of 500 hours, per airworthiness directive (AD) 84-26-02. Inspection of the element is required on every 100-hour and annual inspection.
Fabric Air Filters
There are new air filters that are made of neither paper nor sponge material but, instead, are made of fabric. These are called Challenger filters, and they have to be treated periodically with a special spray from K&N. Challenger Aviation states that these filters allow nearly 50 percent more airflow than paper filters and 60 percent more airflow than the sponge-type filters. This gives a slight increase in horsepower (two to four percent). These filters have to be cleaned with a special cleaning agent and must also be recharged every 100 hours or annually (or more often when in dusty conditions). They can be serviced up to 25 times over 2,500 hours before replacement is required.
ADs for Brackett Air Filter Assemblies
The Brackett air filter assemblies are the most widely used and are, for the most part, pretty reliable, although there are a couple of ADs on some of them. AD 81-15-03 calls for the replacement of the assemblies with an aluminum retaining screen and inspection for a gasket retainer. AD 96-09-06 requires installation of a gasket-retaining strip on certain assemblies to allow operation up to 500 hours and then calls for a complete replacement of the assemblies with a new type that is manufactured with the gasket-retaining lip from the manufacturer. AD 2002-26-03 calls for inspection for a retaining screen on the downstream side and installation of one if it’s not there. This one applies to the Cessna 206 and 210 models.
Backfiring in the Induction System
The backfiring that can occur in an induction system while the airplane is being started can severely damage the element. There are times when there can actually be a fire in the carb air box that is undetected, because it is extinguished once the airplane starts. A sponge-type filter that is exposed to this will look eroded on the surface. Paper filters look blackened on the inside. Filter elements should be replaced if a fire has occurred.
Breathe EasyDespite the fact that maintenance on the carburetor air box and air filter is imperative, the work involved is fairly simple. The most time-consuming part is often the periodic examining of the system. A little maintenance on the induction system can help you and your engine breathe easier.