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20 October 1999



Federal Aviation Administration
Aircraft Certification office
1801 Airport Road
Wichita, Kansas 67209


Mr. Carlos Blacklock


Response to FAA Safety Recommendations 99.283 and 99.284: Cessna 172P Integral Fuel Tanks


Dear Mr. Blacklock:

The FAA notified Cessna of FAA Safety Recommendations 99.283 and 99.284 by letter dated 23 September 1999. These recommendations surrounded several incidents whereby a Model 172P experienced engine stumbling or stoppage, with the cause extrapolated to the wing design of the aircraft. According to the safety recommendations, the integral wings of the aircraft trapped water which was released into the fuel system in a short enough period of time and with sufficient quantities to cause engine interruptions. Furthermore, tests were described whereby colored water was poured into a tank, then drained until no more was recoverable. A certain amount of water remained in the tank.

In 1980, Cessna began production of the Model 172P with integral fuel tanks, which carried over into the 172Q. With Service Bulletin SEB92-25, Cessna required installation of four additional drains per tank, each placed in a corner to facilitate more complete removal of any contaminates from the fuel tank. This design was incorporated into the 172R and 172S at restart of production operations in 1996. During the process of certification basis negotiations between the FAA and Cessna in 1995, FAA Service Difficulty Reports were reviewed, as were Cessna's records. At that time, the integral tanks were regarded by all parties as having substantially reduced the number of fuel contamination cases reported for those aircraft so equipped since their inclusion into the standard design of the Model 172, and the added drains were only considered to improve on a successful design. The additional drains were provided to assure that the tanks would be drained of contaminants, regardless of the resting attitude of the aircraft.

Cessna considers it unlikely that the aircraft engine problems described were actually caused by water ingestion unless the aircraft in question has a serious water contamination source, such as contaminated storage tanks from which it is filled or poorly sealing fuel caps with precipitation, etc. It may also be possible that, due to some kind of work done to the aircraft in question, the aircraft experiencing problems may no longer meet type design for the model. A simple check of the gascolator after an incident would have shown whether engine stoppage could have been due to water, although no mention was made of such a check in the letter.

The description was given whereby the owner of the 172P in question poured colored water into one tank of his aircraft, then was able to drain off all but 13 ounces of fluid. Cessna considers that, given the amount of surface area inside the wing, 13 ounces is not an unexpected amount of undrainable water, due mainly to surface adhesion within the tank. In the early 1980s, Cessna performed extensive tests to determine the behavior of water in fuel systems. The tests included water ingestion into the engines themselves, as well as the behavior of water in the tanks. During these tests, Cessna added colored water to the tanks of test articles with both integral and bladder tanks, and drained them in a similar fashion to that described in the letter. It was found that, after all readily drainable water has been removed, the remaining water collects on the tank surfaces largely in the form of droplets and small amounts of collected water. These will continue to adhere to the tank walls and floor for some time, and will individually work their way toward the tank outlet. Upon exiting the tank, the water will collect in the fuel strainer where it will be removable prior to the next flight. Cessna's experience is that, for aircraft, meeting type design, 1) an engine, and particularly a carbureted engine, requires a substantial amount of water virtually unmixed with fuel before it will quit due to water ingestion, 2) the strainer will readily trap undrainable water that exits the tank during engine operations, and 3) that once the tank has been purged of drainable water, only small amounts of water will escape the tanks during operation, certainly much less than that required for engine stoppage. Cessna's test aircraft had small amounts of colored water appearing during preflight checks for several months following the test described, but never experienced engine problems due to ingestion of water left over from the tests.

In addition during an FAA audit in the fall of 1997 Cessna's approval of the 172R integral tanks was reviewed. Cessna was able to show the FAA's satisfaction that the fuel tanks met all regulatory requirements, and that they perform as intended when in conformance with type design.

Safety recommendation 99.283 asked for a review of the water/contamination egress capability of the Cessna integral fuel tanks which include all similar designs to the 172P model. Several such reviews have taken place since certification of the original integral tanks on the Model R172K, both of the tanks themselves, and of the field experience of planes using the design. In each case, the tanks have proven to meet all requirements, and to operate in a predictable and safe manner when recommended procedures are followed for water contamination removal, and all components of the fuel system are in good shape and in accordance with type design.

Safety recommendation 99.284 suggested a review of the effectiveness of Service Kit SK182-100. The addition of four drains per tank ensures that, even in unusual parking attitudes, both tanks will have at least one drain located such that water running to any corner can be removed prior to flight. If the customer's drains were properly installed per Service Bulletin SEB92-25, the additional drains will only add to the customer's ability to drain water from his tanks. The fact that none of the drains removed any water prior to an incident only serves to strengthen Cessna's position that the problem is not likely to be water contamination.

Regarding the FAA's request for copies of pertinent data and correspondence showing compliance to certain CARs, attached you will find selected pages from Cessna's Master Compliance Checklist for the 172P, a copy of the 172P Wing Structures report which shows that the 172P integral tank was originally installed and substantiated on the 1979 Model R172K, a copy of Cessna's notification to the FAA, Letter no. 80L6/8-38 dated 13 June 1978, showing that the 1979 R172K would be the first to receive the integral tanks, and copies of selected pages from Cessna Report DMR 172K-0 Addendum 2 , which covers the requested items. Please note that Cessna had already begun using the FAA's Form 8110-5 by that time, so that substantiation is shown to the requirements, although shown in the attachment as complying with Part 23 counterpart paragraphs. Car 3.411 later became FAR 23.901, 3.442 became 23.967, 3.44 became 23.971, and 3.553 became 23.999.

Should the FAA have any further questions or comments, please do not hesitate to contact the undersigned at your convenience.

Very Truly Yours,

Mike McClary
Single Engine Executive Engineer



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