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Purpose of trip:
Examine Mr. Scovill's airplane with respect to our prior review of the Cessna type design data thus provided, listen to Mr. Scovill's explanation of the events and the issues at hand, gather any other knowledge helpful to our investigation. |
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Report:
We arrived late the afternoon of Feb. 23 and met Mr. Scovill and Mr. Allen at one of the Middle Tennessee State University's (MTSU) flight hangars in which his airplane resided. This is not the hangar in which Mr. Scovill keeps his airplane normally. He has a private hangar on the same field. We listened to Mr. Scovill's explanation of what he has found, what he has tested, and what has been done on the other MTSU airplane (172RG) at the field. We found his Model 172P to be in exceptionally clean condition. |
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We did not conduct a review of Mr. Scovill's logbooks for, as we understand, this has been done previously by representatives of the Nashville Flight Standards District Office (FSDO). We did discuss the Cessna representative's participation in Mr. Scovill's situation thus far. He has had one visitor (supposedly and engineer) whose qualifications and goals of the visit Mr. Scovill questioned. The Cessna representative, did not do a through examination of his airplane, but reviewed his logbooks. |
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Per his accounts Mr. Scovill has had no luck with the Cessna factory service/customer support. He at one time was interested in upgrading to another airplane, but could not get the Cessna personnel to understand nor take any action on his behalf, per Mr. Scovill's accounts. Mr. Scovill has discussed his situation with as many aviation related personnel as he can. He has had a representative of Embry Riddle Aeronautical University (ERAU) be present during one of the ground tests in which water has been added to Mr. Scovill's airplane wing fuel tanks. This person is Mr. Bob Cunningham. Mr. Cunningham appears on videotape during one of the tests. The writer received and viewed this videotape during our visit. |
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Mr. Scovill repeatedly explained his history on this situation to us. His belief is that he had no problems over the many years (prior to 1996) because he had not accumulated an amount of water in his wing tanks, if when displaced and delivered to the fuel tank outlets (finger strainer areas) during his normal flying would cause an interruption in power, or complete loss of power. He believes this water has come from condensation, as he does not have another source of water ingress into the tanks. We explained our theory, that the frequency of his problems seemed to increase after a certain period of time, possibly indicating a point in time in which he picked up a load of fuel with a greater than normal amount of water entrained in it. This may be an explanation of increased frequency, but as Mr. Scovill repeatedly stated, this does not account of the fact that who cares how the water gets into the tanks, if the water is in the tank, it cannot be sumped out (drained in the normal ground attitude) from the single standard configuration wing tank drain, nor the optional Service Bulletin (SB) installed 4 additional drains. This is Mr. Scovill's primary concern. If there is water in the tanks, from whatever source, the pilot cannot be assured of sumping (draining the water) from the single location on their airplanes prior to flight. It is his assertion that if an individual follows the Cessna Pilots Operating Handbook (POH) and operates the airplane to the book, he will not be assured of removing the water from his wing fuel tank system with the current configuration of his airplane and the standard single drain, nor with the 4 SB added drains. |
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We conducted an external evaluation of Mr. Scovill's airplane which was in the mid part of the hangar, not situated on a level floor from RH to LH wing. The airplane was in an area of the floor where fluid would want to flow to a floor drain, located at the approximate LH wing strut area. The following Table I defines course measurements of the attitude which the airplane was initially reviewed. |
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TABLE I |
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Item |
Measured where |
Value |
Item |
Measured where |
Value |
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RH wing tip to floor, forward |
At the leading edge skin break and outboard closure rib |
76 1/8" |
LH wing tip to floor, forward |
At the leading edge skin break and outboard closure rib |
73 3/16" |
|
RH wing tip to floor, aft |
At the aileron leading edge skin break and outboard closure rib |
75 5/16"" |
LH wing tip to floor, aft |
At the aileron leading edge skin break and outboard closure rib |
72 1/4" |
|
RH wing root forward |
At the leading edge skin break and first outboard wet rib |
68 3/8" |
LH wing root forward |
At the leading edge skin break and first outboard wet rib |
68 5/8" |
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RH wing root aft |
At the flap leading edge break and first outboard wet rib |
66 5/8" |
LH wing root aft |
At the flap leading edge break and first outboard wet rib |
66 1/2" |
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RH height to strut |
Outboard corner of strut |
67 9/16" |
LH height to strut |
Outboard corner of strut |
66 1/4" |
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RH wing dihedral |
Inboard closure rib to outboard closure rib on the forward fastener line |
1 1/2" over 41 1/8" |
Not recorded |
Not recorded |
Not recorded |
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RH axle |
To center of hex head |
7 1/2 " |
LH axle |
To center of hex head |
7 3/8" |
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Nose axle |
To center of hex head |
6" |
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Item |
Measured at |
Value |
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Nose axle |
To center of hex head |
6" |
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Nose strut extension |
Strut face |
2 1/2" |
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Height to Prop shaft |
From hangar floor |
46 1/4" |
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All measurements were made using a standard Stanley tape measure, which was restrained at the hangar floor and held manually at the specified wing surface points. Although crude, this gave us a starting point to understand any flow that may be occurring in Mr. Scovill's wing tanks. For our evaluation of RH wing the wing had a little more rise to it than it would normally have had on a fore and aft true level surface. |
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We then conducted an examination of Mr. Scovill's airplane, which was provided to us with the upper wing access panels having been previously opened, finding them now held in place with two loose screws each. The interior of the wing tanks did not reveal any areas of damage and/or repair. The interior wing skins, stringers, and ribs were of an untreated appearance, no paint present. The fuel tank sealer found in the wing appeared to be that of an original installation and not in excess per the criteria outlined in the Cessna 172 Service Manual. There were no areas of wipe off or stick marks, some times typical of areas where sealer is removed and replaced, possibly due to a maintenance action inside the wing. We found the stringers bonded to the lower wing surface. There existed approximately 10 gallons of fuel in each wing, along with an indeterminate amount of red-dyed water, left over from prior evaluations of Mr. Scovill's airplane. We initially reviewed the RH wing tank interior structure. We did not note any repairs of areas where maintenance action had been taken. Using a .218, .175, and .115 drill bit along with .032 safety wire, the writer proceeded to evaluate all sheet metal joints in the integral fuel tanks. This was accomplished by probing a specific area with the largest drill bit down to the safety wire to see what could pass through any gaps present in the particular interface. The forward and aft side of each stringer was evaluated. The following Table II and Table III summarizes these results: |
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We noted that the approximately .50" diameter holes were present at eight points along the ribs at WS 51.05 and 39.63. These holes are up above the bend radius of the lower flange of each of these ribs. This puts the lowest opening through the rib at these holes at approximately .115 (as measured with a drill bit) from the lower rib flange upper surface. Additionally, it appeared that all "gussets" (stringer to rib clip) appear to be of the type with a slot at the lower corner, which interface with the rib face. The only deviation from this was the two square corner gussets, which were used at the flap box forward inboard corners. |
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In addition to the items noted in Tables I and II the following figures indicate a variation in the manufacture of the tanks on Mr. Scovill's airplane, which we found. Figure A depicts the RH integral wing tank and Figure B depicts the LH integral wing tank. |
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Two tests were run on the RH wing of Mr. Scovill's airplane. The first involved introducing red-dyed water directly at the aft fuel pick-up location until the pick-up finger strainer was just covered. This yielded an "ashtray" (sheet metal dam at each fuel tank outlet) that was not quite half full of water. Access panels off; fuel cap open, the selector in the RH position. The fuel gascolator pull chain was opened for approximately 4 seconds (3.93) per Pilots Operating Handbook (POH) procedures. This yielded less than 4 ounces of fuel and water mix. This was approximated at 3.5 ounces total. The goal of this test was to determine what quantity of fluid was drained in the 4 second POH procedure. |
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The second test involved draining the gascolator with the pull chain until water free fuel came out the gascolator drain. At this time red-dyed water was added directly at the aft fuel finger strainer location. Access panels off; fuel cap open, the selector in the RH position. The gascolator pull chain was opened until red-dyed water appeared at the gascolator drain outlet. This took approximately 13 seconds, and measured out to be approximately 6.5 ounces of fuel. The goal of this test was to determine the amount of fluid contained in the fuel line from the aft fuel pick-up to the gascolator outlet. |
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For record purposes, we have created the following table which presents a summary of fuel system tests, which have been conducted by Mr. Scovill on his airplane, and a test conducted on an MTSU Model 172 RG. This airplane happened to have a need for maintenance to reseal the access covers, and knowing of Mr. Scovill's problems MTSU conducted their own test, on their airplane. FAA personnel from the Nashville FSDO witnessed all tests. Tables IV through VIII present the results of these tests. |
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05/19/99
Cessna Model 172P; S/N: 17274599
Normal ground attitude, hangar floor, approx. 15 gallons of fuel LH tank, all 5 drains sumped prior to test |
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Event |
Total time elapsed |
Orig. drain |
Opt. drain A |
Opt. drain B |
Opt. drain C |
Opt. drain D |
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0.00 |
No water |
No water |
No water |
No water |
No water |
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8.75 ounces of water added to wing via access panels and fuel filler port |
80 min. |
.053 oz. of water after 3 samples |
No water |
No water |
No water |
No water |
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5.25 ounces of water added via access panels and fuel filler port |
120 min. |
1.14 oz. of water after 3 samples |
No water |
No water |
No water |
No water |
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No action |
194 min. |
.973 oz. of water after 4 samples |
No water |
No water |
No water |
No water |
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Rocked wing, up and down approx. 4" for 30 seconds |
210 min. |
4.1 oz. of water after 5 samples |
No water |
No water |
No water |
No water |
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End of test |
215 min. |
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Total water remaining in wing |
6 ounces |