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Central Region
601 E. 12th Street
Kansas City, MO 64106

SEP 23 1987

 

Messrs. Norman L. Horton and Jerry L. Wells
Aviation Safety and Management Consultants
4582 Old Stage Road
Central Point, OR 97502

 

Gentlemen:

 

This is in response to our previous letter of July 29, 1987, in which we stated we would respond back to you after review of the original type certification data for the Piper PA-23 Series airplanes and comparison of the tests conducted by your company. The following conclusions have been made:

 

Your open letter of June 11, 1987, states that it is your opinion that the PA-23 does not meet the requirements of Civil Air Regulation (CAR) 3.553, CAR 3.444(c), Federal Aviation Regulation (FAR) 23.971(b) or FAR 23.999 and that this non-compliance is the cause of some PA-23 accidents. The certification basis for the design of the Piper PA-23 is CAR Part 3 and compliance questions should be limited to only those requirements. References to the requirements of Part 23 of the FAR are not germane to our discussions of the PA-23. FAA's review of Piper's drawings and design layouts, plus examination of a conforming wing tank show that the PA-23 fuel system is in literal compliance with CAR 3.444(c) and CAR 3.553 in that a very small amount of fuel (or water, if present) could be retained in the aft inboard corner of the tank after normal preflight draining.

 

The bottom of the PA-23 tank design is flat, except for a recess 5.5" x 4.0", approximately 0.75 inch deep at the tank outlet. The design intent was that the bottom inboard edge of the tank would be level with the airplane in the normal ground attitude. Our investigation supports your finding that the bottom of the tank is not level, but low at the aft end. The exact amount of inclination determined by our study is 1 degree allowing a small amount of liquid (1 to 3 fluid ounces) to be trapped in the fuel tank. Your demonstration that more than 3 ounces of water can be trapped in a non-conforming fuel tank that has a ridge around the recess and/or wrinkles in the cell bottom, should not be used as evidence of non-compliance. Ridges and wrinkles in the fuel tank you described are not part of the type design. However, this condition is typical of fuel bladders in other designs and the evaluation of these designs has not shown any significant problems. Our evaluation of this system suggests that any trapped contaminants would not flow into the main fuel supply line during or shortly after take-off. Contaminants aft of the recess at the start of take-off would then tend to be held in the aft position of the tank during take-off by acceleration forces, then would stay there when the aircraft was rotated into the take-off and climb attitude. If released later in the flight, contaminants would be trapped in the gascolator, which has a capacity of 3.3 fluid ounces.

 

The fact that five of the eight reported PA-23/23-160 contaminated fuel accidents/incidents occurred on take-off or initial climb-out, suggests that a large amount of water which should have been drained during preflight was not. This is the same conclusion reached in our earlier investigation which led to the publication of expanded fuel draining preflight inspection procedures. This conclusion is further supported by the PA-23-250 accident/incident data, with a similar fuel system that at that time had more detailed preflight fuel draining instructions. The PA-23-250 accident/incident record lists only three fuel related accidents with one on climb-out none on take-off. This is from a population of approximately 5,000, contrasted with approximately 2,000 PA-23/23-160.

Even though fuel contamination in PA-23 fuel tanks has not been established as an unsafe condition, the FAA will continue to monitor service experience.

In view of the potential economic impact of your suggested action, and considering the positive results of expanded preflight inspection procedures, we cannot concur with your recommended action, although we understand your concern and appreciate the desire that even these old aircraft should have a perfect safety record.

As stated earlier, the FAA and Piper are committed to pursuing reasonable ways to improve the PA-23/23-160 fuel system. We appreciate your suggestions and investigations. Should you have any more questions or need to discuss these findings further, please feel free to contact Mr. Robert R. Goodall of my staff at (404) 991-3810.

 

Sincerely,

Paul K. Bohr
Director, Central Region

 

 

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