October 21, 1999

Matt Taylor
Director of Maintenance
Middle Tennessee State University
Flight Operations Maintenance
1940 Memorial Boulevard
Murfreesboro, TN 37130

Robert E. Scovill
P.O. Box 347
Smyrna, TN 37167

Dear Bob,

With regard to your continued trouble with your 1981 Cessna 172P, N52749; I wanted to submit the following. As you are aware, we recently had the opportunity to evaluate the integral fuel tanks on one of our Cessna 172RG's. This aircraft, SN: 172RG0003, had some fuel tank rivets and screws that began to "stain" or "seep" a little fuel. In order for us to remedy this problem we had to drain the fuel, remove access panels and reseal the interior of the tanks. While performing these functions we were able to conduct tests and evaluate the integral tanks.

This aircraft is equipped with integral fuel tanks, as mentioned. The fuel tank part number differs from the fuel tank part number for a 172P. However, all of the structural components have the same part numbers. From this, I concluded that the only difference between the tanks is the application. Therefore, it is conceivable to do a comparative analysis between the two systems.

We first sumped the fuel tanks, drained about one fourth of the fuel, removed the access panels and visually inspected for water and/or other contamination. Then we drained the fuel through the wing drains and monitored for water presence. We found no signs of water or other contaminants in the fuel tanks. We refilled the right tank to about half capacity and introduced a known quantity (20 oz.) of water with red dye. The water was introduced through the inspection panels and the filler neck in small quantities to simulate condensation and/or contaminated refueling. The red dye made the water very distinguishable and prominent. We sumped the tank various times during the water introduction and did not remove any amount of water from the wing sump. After introducing 20 oz. of dyed water into the integral wing fuel tank, through normal preflight sumping no water was removed through the wing fuel sump. It was not until we shook the wing vigorously that we were able to remove 6 oz. of water through the wing sump. We were unable to remove the remaining 14 oz. of water through the wing sump. We drained the remaining fuel, used a chamois cloth to remove the remaining water and let the tank air dry overnight.

After the remaining fuel and water was removed, we inspected the interior of the fuel tank. The tank was found to be in original condition with no evidence of any previous repairs. The fuel tank sealant appeared to be properly installed and not posing a threat to impede flow of liquids through or around any rib, stringer, gusset, doubler or baffle. The structural make up and integrity of the tank appeared to be the same as that of your 172P.

While studying the behavior of water in fuel during this test, I noticed that water takes on characteristics we normally associate with jelled substances. The pressure exerted by the fuel forced the water to adhere to the surface of the tank and not allow it to move freely like it moves when only in the presence of air. The water droplet's external pressure causes the water particles to resist separation. As the small droplets of water slowly move about and join, they form larger and larger droplets. As these droplets grow so does the pressure of the fuel on the water droplet. With growing external pressure, the water particles resistance to separation grows which prevents it from easily traveling from one section of the fuel tank to another. When a droplet is larger than the hole it comes in contact with, it resists traveling through the hole. The relatively small gaps and holes in and around the ribs, stringers, baffles, etc. do not allow the water to move freely from the different sections of the tank to the fuel strainer. There is very little dihedral or angle of incidence in the wings of these aircraft which does not allow gravity to assist in the movement of the water.

It is very conceivable that over an extended period of time these aircraft could accumulate an appreciable amount of water (due to many factors; such as, condensation, contaminated fuel source, leaking fuel cap seals, the possibilities are numerous). This water may not be evident until it receives help from steep turns or other maneuvers which let gravity assist the water in moving from the different sections and accumulating around the fuel pick up screens. By this time it is too late to do anything about it.

These aircraft suffer from a poorly designed fuel tank and contaminant monitoring system. We are currently evaluating these findings and establishing a proposed inspection program that will eliminate the hazard of contaminated fuel reaching the engine.

Thank you for your assistance,

Matt Taylor