June 2, 1999

Nashville Flight Standards District Office
#2 International Plaza Drive
Suite 700
Nashville, TN 37217

Dear Sirs,

I am writing this letter to inform you of what appears to be a potential problem with at least one Cessna 172 aircraft. The aircraft in question belongs to Mr. Robert E. Scovill and is a 172P bearing the serial number 17274599 with a total time of approximately 1060 hours. The aircraft was purchased new in 1981 by Mr. Scovill and has been hangared ever since except when parked away from home. I know that Mr. Scovill has already sent you a complete history of the problems with his aircraft so I will only cover the history briefly.

During a time span of approximately the last 24 months, the aircraft in question has had one precautionary landing due to a rough running engine. In addition, the aircraft has had three engine failures one of which resulted in him having to perform an emergency landing in a field while avoiding utility lines. According to Mr. Scovill, the representatives from the Nashville FSDO that came to his landing site at first believed the cause to be carburetor ice. To my knowledge, a cause has not been determined absolutely for these problems but the biggest probability is water in the fuel. Mr. George Erdel, draining a sizeable quantity of water from the aircraft after the off-airport landing, evidenced this. This came after Mr. Scovill attempted to taxi to a suitable place to take off within the field and due to the roughness of the field, water was apparently dislodged from places unknown. This caused the engine to begin to run roughly and prompted the further investigation. The question would be how did the water get in there? Why was it not drained before flight? The easy answer would be, Bob did not sump the tanks thoroughly due to a less than adequate preflight, but this would be an incorrect assumption. Mr. Scovill indicated that he had performed a proper preflight before he took off. This would mean that there was a hazardous quantity of water in the fuel tanks when he took off from Fayetteville that he could not drain from any of the ten sumps. Mr. Scovill indicated that he had phoned Cessna about the problem and they advised him to have the carburetor checked which he did.

The maintenance records for this aircraft indicate that it has had proper maintenance since it was delivered. The aircraft has normally been kept out of the weather in a private hangar and has never had any type of repairs to the fuel tank area that would make it be in any condition other than how it was delivered. The only exception to this is the fact that Mr. Scovill recently had Cessna Service Kit SK182-100 installed to try to help alleviate the water problem that he believed he had. Given that the aircraft is 19 years old, it still has the aesthetic and mechanical appearance of new. The aircraft and owner combination could not be much better for this case study and experiment.

This brings me to the point of this paper. On May 10, 1999 I was approached about the feasibility of bringing Mr. Scovill's airplane to our hangar to check out the fuel tank area with our borescope and to check for any anomalies. I agreed and the airplane was brought in on the morning of May 19, 1999. In addition to Bob Scovill and myself, Paul Jones and Craig Roberts were also present as was a friend of Bob's, Glenn Tarpley. We checked the attitude of the aircraft and determined that it was as level as you could expect any aircraft on a hangar floor to be and the ambient temperature was 72 degrees Fahrenheit.

We started the experiment and decided to take off the fuel tank access covers on top of the left wing. We looked inside of the fuel tank area and noted some water inside the wing but not a considerable amount. We then decided to introduce water mixed with red food coloring to different parts of the wing and to then note the results. We put water into the areas shown in the table and on the diagram below and listed what the results were as well:

 

NOTE: ALL SUMPING WAS DONE WITH A STANDARD FUEL SAMPLER CUP OF 1.75-OUNCE CAPACITY SIMILAR IN APPEARANCE TO SPORTY'S PILOT SHOP PN 6912A.

EVENT #

TIME

LOCATION

PROCEDURE

RESULTS

1

10:30

All 5 sumps

Preflight sump

No Water

2

 

Area 2, Fig. 1

Introduced 1.75 Oz. Water

 

3

 

Area 3, Fig. 1

Introduced 1.75 Oz. Water

 

4

10:40

Sump 1

Sumped 1.75 Oz. Fuel

No Water

5

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

6

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

7

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

8

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

9

10:55

Sump 1

Sumped 1.75 Oz. Fuel

No Water

10

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

11

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

12

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

13

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

14

 

 

No action this event

 

15

11:25

Sump 4

Introduced 1.75 Oz. Water

 

16

11:35

Sump 1

Sumped 1.75 Oz. Fuel

No Water

17

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

18

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

19

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

20

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

21

11:40

Area 21, Fig. 1

Introduced 1.75 Oz. Water

 

22

11:42

Sump 1

Sumped 1.75 Oz. Fuel

No Water

23

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

24

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

25

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

26

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

27

11:46

Area 27, Fig. 1

Introduced 1.75 Oz. Water

 

28

11:50

Sump 1

Sumped 1.75 Oz. Fuel

.053oz. water after 3 samples

29

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

30

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

31

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

32

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

33

12:04

Sump 2

Introduced 1.75 Oz. Water

 

34

12:10

Sump 1

Sumped 1.75 Oz. Fuel

No Water

35

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

36

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

37

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

38

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

39

12:17

Area 39, Fig. 1

Introduced 1.75 Oz. Water

 

40

12:22

Sump 1

Sumped 1.75 Oz. Fuel

No Water

41

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

42

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

43

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

44

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

45

12:25

Area 39, Fig. 1

Introduced 1.75 Oz. Water

 

46

12:36

Sump 1

Sumped 1.75 Oz. Fuel

1.14oz. water after 3 samples

47

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

48

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

49

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

50

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

51

13:44

Sump 1

Sumped 1.75 Oz. Fuel

.973oz. water after 4 samples

52

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

53

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

54

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

55

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

56

14:04

 

Rocked wings approx. 4 in.

for 30 seconds

57

14:05

Sump 1

Sumped 1.75 Oz. Fuel

4.1oz. water after 5 samples

58

 

Sump 2

Sumped 1.75 Oz. Fuel

No Water

59

 

Sump 3

Sumped 1.75 Oz. Fuel

No Water

60

 

Sump 4

Sumped 1.75 Oz. Fuel

No Water

61

 

Sump 5

Sumped 1.75 Oz. Fuel

No Water

 

 

 

To summarize this experiment, we introduced 14 ounces of water into the left wing and sumped a total of approximately 6 ounces from the five wing sumps (it is also interesting to note that we found no water at any of the retrofit sumps). The water left in the wing is 8 ounces with the gascolator volume being around 5 ounces. If we assume the other wing is somehow perfect, that leaves 3 ounces to go to the carburetor, which it apparently has done in the past.

In light of what our experiment showed, Mr. Scovill began to wonder if his aircraft met the requirements of FAR Section 23.971 which basically states that the fuel tank and sump configuration must drain a hazardous quantity of water from the wing with the aircraft sitting level. Paul Jones and I determined that the aircraft in question was certified under CAR 3 not FAR 23 and advised Mr. Scovill of this. He took a copy of the CAR 3 and found a section that pertained to this area and contained similar verbiage. To my knowledge, he has forwarded this to you.

  After we finished our testing, we drained the fuel from the wing and observed the remaining water in various parts of the wing. It seemed to us that the water flow to the original sump was being blocked by the design of the flow-through holes in the ribs of the left wing. It appears as if the holes are punched into the ribs near the bottom wing skin rather than a notch being cut out of the rib. This provides a "dam" to water flow at no less than 8 places in the left wing. The height of this hole above the surface of the bottom wing skin appears to be about an eighth inch at it's highest. You can see in the following figures how this could collectively trap a considerable amount of water.

 

The following picture shows the same Fuel Bay depiction with a suggested method of transferring water from the back of the ribs along the trailing edge of the wing to the original fuel sump area.

 

 

This cut-out method has been employed on many aircraft fuel tanks and allows water to flow with a minimum of resistance.

  It is apparent that there are several Cessna models and years of manufacture that employ this design of wing fuel bay. There is no easy way to tell if this affects this aircraft only or if it is or could be a problem for many other aircraft. It is definitely a problem with this aircraft at this time and further investigation by FAA engineers or Cessna is warranted. I made a video tape of what we saw with the borescope inside the fuel tank area including water puddled inside up against the ribs away from the retrofit sump assemblies. I will supply a copy to anyone who needs it. Please feel free to contact me at 615-893-9260 if you have any questions. Thank you.

  

Bill Allen
Facilities Manager, MTSU Aerospace
M.Ed., A+P, IA, Private Pilot ASEL

 

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