Narrative:

Before departing on an instrument flight plan I conducted a thorough preflight; which included among other things checking the fuel tanks; including the tip tanks; for fuel (which I had filled); and that there was no noticeable clogging of the fuel vents. There was no noticeable blockage in any of the vents that could be seen with the naked eye; and the ground run up; including a ground run-up to verify proper fuel flow from the tips confirmed that the system was behaving normally. Upon departing I burned from the left tank first; and later switched to the tip tanks which flowed normally. I continued to burn from the tips until the fuel in them was virtually exhausted (the tips are really only to be used in straight and level flight); and then returned to the mains. All systems behaved completely normally to this point. While enroute; I requested a lower altitude and was granted a descent from my trip-long cruise altitude of 6;000 feet down to 4;000 feet. At approximately 5;000 feet during the gradual shallow (and normal) decent; I heard a thud noise; looked to the right; and observed a substantial disfiguration of the tip tank; which had imploded. I did not know what the problem was; what the cause was; and also noticed that the right side of the airplane was leaking fuel. I was immediately concerned that the plane could be coming apart mid-air. After a brief immediate explanation of the problem; I asked the controller for an immediate diversion to the nearest airport. He provided the vector and asked if I would like to declare an emergency. Uncertain of the precise situation from the cockpit; I explained to him that we were losing fuel and the plane could be coming apart on us (I had one passenger with me); and that I did want to declare an emergency. The controller stayed with me to the ground; and the landing was successful. After inspection by an aviation mechanic; it was determined that the fuel vent for the tip tank had become clogged. While not visible on the ground; this clogged vent was the cause in creating a vacuum within the tip tank that was at such a low pressure; the descent toward the ground into higher pressure air created enough of a pressure differential to cause the tank to implode. According to my conversation the tank manufacturer; this is not the first time that this has happened. While the tank has its own vent and shares the vent with the main tank and fuel system; the tank by itself has only a single vent. I believe that an additional fuel cap vent (or any secondary vent at all on the tank itself) would have prevented this problem. Perhaps a device or specially-designed (like a fuel sump) that can be pushed through a fuel vent to verify that it is clear; could also be used as a standard preflight tool on the ground to verify the vent is completely clear to prevent this and other similar fuel vent related incidents. It is very difficult or impossible to see with the naked eye if there is a blocked fuel tank vent line; when the lines are long; or as the case is commonly; the lines are very thin. Because of this; a technique for verifying that fuel vent lines are clear should be established as part of training and part of every pilot's preflight routine. Blowing on the fuel vent with an open gas cap is one method; but this method is not always practicable. Some method should be established for every fuel vent line in every aircraft whether there are redundant ventilation systems or not. Additionally; I must wonder about the structural integrity of a tank system that is incapable of handling the kinds of pressure that it was subjected to with the clogged vent. While we can expect pressure to cause a fuel bladder to change form and possibly lead to a reduction in available fuel flow or fuel pressure; it is rather disconcerting to experience the failure of part of the airframe as a results of these differential pressures it should be noted that the fuel tanks are an aftermarket fuel tank. Given the number of these tanks that are out in the field; mandating that a fuel cap with a vent to be used; or some other form of at-the-tank backup ventilation would be a good idea. That being said; the tanks may simply not be built with the necessary structural integrity; should these kinds of blockages be common.

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Original NASA ASRS Text

Title: A Beechcraft Bonanza V35B fuel tip tank collapsed during a descent through 5;000 feet from the cruise altitude of 6;000 feet. The tank vent was apparently plugged allowing a vacuum to develop that collapsed the tank.

Narrative: Before departing on an instrument flight plan I conducted a thorough preflight; which included among other things checking the fuel tanks; including the tip tanks; for fuel (which I had filled); and that there was no noticeable clogging of the fuel vents. There was no noticeable blockage in any of the vents that could be seen with the naked eye; and the ground run up; including a ground run-up to verify proper fuel flow from the tips confirmed that the system was behaving normally. Upon departing I burned from the left tank first; and later switched to the tip tanks which flowed normally. I continued to burn from the tips until the fuel in them was virtually exhausted (the tips are really only to be used in straight and level flight); and then returned to the mains. All systems behaved completely normally to this point. While enroute; I requested a lower altitude and was granted a descent from my trip-long cruise altitude of 6;000 feet down to 4;000 feet. At approximately 5;000 feet during the gradual shallow (and normal) decent; I heard a thud noise; looked to the right; and observed a substantial disfiguration of the tip tank; which had imploded. I did not know what the problem was; what the cause was; and also noticed that the right side of the airplane was leaking fuel. I was immediately concerned that the plane could be coming apart mid-air. After a brief immediate explanation of the problem; I asked the Controller for an immediate diversion to the nearest airport. He provided the vector and asked if I would like to declare an emergency. Uncertain of the precise situation from the cockpit; I explained to him that we were losing fuel and the plane could be coming apart on us (I had one passenger with me); and that I did want to declare an emergency. The Controller stayed with me to the ground; and the landing was successful. After inspection by an aviation mechanic; it was determined that the fuel vent for the tip tank had become clogged. While not visible on the ground; this clogged vent was the cause in creating a vacuum within the tip tank that was at such a low pressure; the descent toward the ground into higher pressure air created enough of a pressure differential to cause the tank to implode. According to my conversation the tank manufacturer; this is not the first time that this has happened. While the tank has its own vent and shares the vent with the main tank and fuel system; the tank by itself has only a single vent. I believe that an additional fuel cap vent (or any secondary vent at all on the tank itself) would have prevented this problem. Perhaps a device or specially-designed (like a fuel sump) that can be pushed through a fuel vent to verify that it is clear; could also be used as a standard preflight tool on the ground to verify the vent is completely clear to prevent this and other similar fuel vent related incidents. It is very difficult or impossible to see with the naked eye if there is a blocked fuel tank vent line; when the lines are long; or as the case is commonly; the lines are very thin. Because of this; a technique for verifying that fuel vent lines are clear should be established as part of training and part of every pilot's preflight routine. Blowing on the fuel vent with an open gas cap is one method; but this method is not always practicable. Some method should be established for every fuel vent line in every aircraft whether there are redundant ventilation systems or not. Additionally; I must wonder about the structural integrity of a tank system that is incapable of handling the kinds of pressure that it was subjected to with the clogged vent. While we can expect pressure to cause a fuel bladder to change form and possibly lead to a reduction in available fuel flow or fuel pressure; it is rather disconcerting to experience the failure of part of the airframe as a results of these differential pressures It should be noted that the fuel tanks are an aftermarket fuel tank. Given the number of these tanks that are out in the field; mandating that a fuel cap with a vent to be used; or some other form of at-the-tank backup ventilation would be a good idea. That being said; the tanks may simply not be built with the necessary structural integrity; should these kinds of blockages be common.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.