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Cessna's Bladder Tank from the Years 1978 to 1986
Test results that reveal the indicated design flaw in Cessna's Integral Wing Tank resemble results from tests performed on Cessna's Bladder Tank.
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The FAA's Office of System Safety maintains aviation safety data that includes NTSB safety recommendations made to the FAA. Correspondence between the NTSB and the FAA over the years 1983 through 1986 that discusses water contamination issues with Cessna Bladder Tank equipped aircraft is available in the following report.
Report Number: A-83-6
The above report concludes with NTSB's letter to the FAA dated 3/25/86 stating that NTSB's "Safety Recommendation A-83-6 [to the FAA] has been classified as 'Closed--Unacceptable Action.'" The NTSB substantiated their classification by stating that, "While AD 84-10-01 may help to reduce the entrapment of water within the fuel tanks, it does not provide for the positive detection and/or elimination of water from the fuel."
The AD 84-10-01 was amended with amendment 39-5940, which became effective on July 5, 1988, about two years following NTSB's final comments given above. You may read the AD 84-10-01 with its amendments here.
The above history of the bladder tank problem reveals how the NTSB and the FAA addressed undetectable, water contamination in the context of an indicated system design flaw. Clearly, the NTSB made strong recommendations for an engineering fix that enables positive detection of water contamination. AD 84-10-01 with its amendments does not appear to provide a solution that enables positive detection during a practical and correct preflight check.
In 1983, the NTSB and the FAA were not alone in addressing the indicated water contamination problems in Cessna's bladder tanks. In that year, Eugene R. Odou, M.D. tragically lost his daughter due to an accident related to this problem. With his permission, the following letter written by him is provided to reveal his efforts to alert others of the dangers. His message is as important today as it was in 1983.
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ODOU MEDICAL GROUP
401 WEST BEVERLY BOULVARD
MONTEBELLO, CALIFORNIA 90540-3693
September 6, 1983
Dear Cessna 182 P Owner:
On April 13, 1983, my daughter was killed in an accident caused by undrainable water in the fuel system of our Cessna 182 P.
I am notifying all 2,605 Cessna 182 P owners at my own expense because I want to prevent further accidents.
Two weeks after her death, Aviation Safety, Volume 3 #5 (1111 East Putnam Ave., Riverside, CT. 06878) carried the enclosed article about this serious problem.
The August 1, 1983 FAA Airworthiness Directive was not sent to the 182 P owners. This Ad concerns fuel water problems. The data from NTSB indicates an accident rate of 9.6/1000 AC for the Cessna 182 P, which is the highest of all aircraft listed!
Besides faulty gas caps and wrinkles in the fuel cells, I have learned that some of the quick drains are non-standard. They are 1½" instead of 5/8" which would compound the problem of undrainable water!
If you find any of these major defects in your 182 P fuel system, notify FAA, NTSB or me.
Sincerely,
Eugene R. Odou, M.D.
ERO: ag |
With his letter, Dr. Odou attached the May 1983 issue of Aviation Safety, Volume 3, Number 5 (at that time published by Belvoir Publications, Inc., 1111 East Putnam Ave., Riverside, CT. 06878). The following excerpts taken from this publication reveals why the NTSB's safety recommendations to the FAA in report A-83-6 stress urgent concern.
Going back to the year 1978, the Aviation Safety publication documents Rodney Gross's experiences with water contamination in a Cessna 182K. The article mentions that Gross, in 1978, was an aerospace engineer and part-time flight instructor. On February 18, 1978, Gross and his son went out to fly the airplane…. He conducted a careful preflight--including sumping the tanks…
On climbout, the engine quit dead without warning, and Gross crashed surviving with serious injuries. Post-crash investigation revealed water contamination, but Gross conducted a careful preflight, so he did his own post-crash investigation to determine why he did not detect water contamination.
… He obtained use of a Cessna 182, [added] an extra drain valve at [a]… low point area, levelled [sic] the plane and put the selector on the right tank, which was half-full of fuel. He now introduced quantities (in increments totaling about 76 ounces, or more than half a gallon) of water into the right tank, then tried to get the same amount of water out.
It was readily apparent that typical preflighting methods would not get the water out… Gross drained roughly a gallon and a half of fuel out of the strainer drain, but got not a trace of water.
Taking a sample from the newly added low-point drain did now show a trace of water (half an ounce in a pint's worth of fuel)…
Now going to the wing drain, Gross collected lots of the water on the first try (47 ounces). But then the fuel ran clear for a further half a gallon on sampling.
Gross had now accomplished rather more draining than any typical pilot does on a preflight, and yet there was still more than a pint of water in the tank.
He now tried rocking the plane vigorously, to encourage the water to flow to the drain… The water didn't. The first sample after rocking contained about three ounces of water in a pint of fuel, but then three more pints of fuel were drained--with rocking in between--and no water came out. Gross quit for the night, with 15 ounces of water remaining in the tank.
The next day, nearly two quarts of clear fuel flowed from the wing drain without evidence of that water.
Gross siphoned off the top of the tank's fuel (but not the water), until there was about five gallons remaining in the tank. He now used the low-point drain to take out the five gallons. None of the water came out with it.
Finally, by jacking up the plane's right main wheel an inch and vigorously rocking the wings, Gross persuaded the residual fuel and the 15 ounces of water out of the tank.
From Gross's efforts, a series of events occurred over the years 1978 to 1982, which eventually lead Aviation Safety to inform the NTSB of Gross's experiments.
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NTSB officials contacted Cessna and asked the company to analyze the problem and suggest a solution.
Amid this context, Cessna now issued a rather unusual service letter to owners of the entire single-engine fleet (bladder-equipped or not). Owner Advisory SE82-36A, issued July 30, 1982, reminded owners to do proper preflight checks for water and other contaminants in the fuel. This was hardly newsworthy.
Tucked into the letter, however, was the advice that the pilot should "gently move the wings and/or lower the tail to the ground (on nose gear aircraft) to move the contaminants to the sampling points and assure that they are drained from the fuel system."
This is rather an extreme preflight procedure (it contained warnings about not harming the wings or tail while performing it)…
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The Atlanta NTSB investigators decided to use Cessna's new procedure in a test of their own.
They put 32 ounces of water in a nearly full wing tank of a Cessna 182P, allowed it to settle, and checked the wing drain--no water evident.
They now rocked the wings and held the tail to the ground, in accordance with the Cessna letter. Draining several quarts of fuel produced only five ounces of the water. They now held the tail down and jacked the left wing up, and in this manner got out another 10-12 ounces of water. But nothing would make the remaining 15-17 ounces of water come out of the tank. Eventually, they had to drain the entire tank and swab out the cell to get the remaining water out.
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NTSB apparently considered the investigators' test a little informal, so it arranged for a further test, conducted by the University of Illinois Institute of Aviation in February of this year [1983]. The academics improved on the test by coloring the water red (the fuel was blue 100LL) and by measuring very precisely, in milliliters. When all was said and done (including holding the tail down and jacking up the wing on a Cessna 182Q), some 500-ml (about 17 ounces) of water remained in its tank…
Clearly, Cessna's advice to owners will not solve the problem, NTSB said….
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NTSB called on the FAA to issue an AD… applying to bladder-equipped Cessnas from the Model 180 through the 207…
NTSB also suggested that a "fuel system design change is warranted…"
The above history reveals an apparent, common problem inherent in both the bladder tank and the integral wing tank designs. Neither design provides a means for positive detection of water contamination. At best, these tanks offer a pilot a random chance of detecting fuel contaminated with water using FAA-Cessna procedures during a preflight check. Yet if a pilot has an incident or an accident resulting from this lack of positive detection, they are statistically very likely to have weather, pilot error or reasons unknown stated as the probable cause.
Similar tests have been performed on the bladder tank and the integral wing tank. The tests have similar results. But this discovery is not surprising when one considers that both tank designs rely on low point sump drains for fuel sampling. Additionally, the interior of these tanks from a functional point of view is not very different when one considers that a wrinkle in a bladder tank will equivalently function like a rib in an integral wing tank. For example, the integral wing tank is designed with ribs that can dam water in a way similar to how wrinkles in a bladder tank can dam water. Consider the following excerpt from Bill Allen's June 2, 1999 report about a test performed on a Cessna integral wing tank.
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Cessna's Integral Wing Tank Ribs appear to have the same damming effect as wrinkles can have in Cessna's rubber bladder tank.
In the June 2, 1999 report that discusses the results of a test performed on aircraft number 17274599, Bill Allen, Facilities Manager for MTSU Aerospace, makes the following observation.
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…You can see in the following [figure 2] how this could collectively trap a considerable amount of water. |
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The FAA agreed that the rubber wrinkles in the bladder tank had to be fixed. They should agree that the ribs in the integral wing tank must be fixed.
Additionally, the FAA and the NTSB both know that shaking the wings and rocking the aircraft's tail is a futile effort that does not reliably cause water to run to the sump area where it can be drained. Both agencies should work to have these dangerously misleading procedures removed from AD's and POH's.
Interested readers who wish to make a more detailed comparison between the bladder tank tests and the integral wing tank tests should review the following documents, which are also available from links on the document page.
Detection Unknown
April 19, 1999 Letter to the FAA
June 2, 1999 Report of inspection results for aircraft number 17274599.
October 21, 1999 Report of inspection results for aircraft number 172RG0003
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