The Whistle Slot

We often tend to believe—what we know, everyone knows. While participating in a flight instructor refresher recently, a young lady from Maine provided a reminder that this is often not the case.

This lady and her husband fly in Maine throughout the year. During the winter, they and their aircraft are frequently exposed to extremely cold temperatures During the past winter, they had an unfortunate experience. The end of the engine breather tube froze over, a pressure build up occurred in the crankcase, and the crankshaft nose seal ruptured. The oil leak that resulted covered the aircraft with oil from nose to tail. Fortunately, a safe landing was made before all oil was lost.

As she related her story, another flight instructor quickly indicated that he had also experienced the same problem several years earlier. The safe landings in both cases are good news. The bad news is the expense incurred to repair the engine.

An incident like this is preventable, and for that reason it is important that we repeat ourselves from time to time. We should not assume that everyone knows about the "whistle slot" or other methods of insuring adequate crankcase venting.

First, the cause of this incident. Moisture is expelled from the engine crankcase through the breather tube which often extends through the bottom of the engine cowling into the air stream. Under very cold conditions, this moisture may freeze and continue a buildup of ice until the tube is completely blocked.

It is normal practice for the airframe manufacturer to provide some means of preventing freeze-up of the crankcase breather tube. The breather tube may be insulated, it may be designed so the end is located in a hot area, it may be equipped with an electric heater, or it may incorporate a hole, notch or slot which is often called a "whistle slot." The operator of any aircraft should know which method is used for preventing freezing of the breather tube, and should insure that the configuration is maintained as specified by the airframe manufacturer.

Because of its simplicity, the "whistle slot" is often used. Although the end of the tube may extend into the air stream, a notch or hole in the tube is located in a warm area near the engine where freezing is extremely unlikely. When a breather tube with whistle slot is changed, the new tube must be of the same design. Replacing a slotted tube with a non-slotted tube could result in an incident like the one described by the lady from Maine.

The Flyer may have carried information on this subject in the past, but the reminder from someone who had an unfortunate incident prompted this story. Preventing possible freezing of the crankcase breather tube by use of a whistle slot or other means is an important little detail which all of our readers should be aware of. Many may benefit from the knowledge.