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Detection Unknown: Is it an Extreme Preflight Procedure, or is it a Bizarre Fuel-Drain Operation? Testing Cessna's Rock & Roll Procedures

A SumpThis Fact Sheet

The following excerpt (taken from the May 1983 issue of Aviation Safety, Volume 3, Number 5 [at that time published by Belvoir Publications, Inc.]) discusses the rock & roll preflight procedure that is used during a preflight check on some general aviation aircraft manufactured by Cessna Aircraft Company.

"Cessna ... 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 [emphasis added] (it contained warnings about not harming the wings or tail while performing it)"

Today, this extreme preflight procedure is often referred to as a rock & roll procedure, but it has also been referred to as a shake and rock procedure, and a chicken dance. Of course, as pilots who use the procedure are aware, its purpose is to aid in the detection of fuel contamination.

Regardless of the type of aircraft and regardless of the type of fuel system involved, a preflight check must include a procedure that uses aircraft components to insure positive detection of fuel contamination and positive detection of prevention of fuel contamination. The aircraft components used in the procedure for "positive detection" must be reliable to insure that a pilot can determine before leaving the ground if fuel contamination is present or not. Of special interest to this issue is fuel contaminated by water.

Preflight procedures must include a means for positive detection of hazardous quantities of water. When the positive detection procedure reveals that no fuel contamination is present, it must mean absolutely "positive detection of prevention of fuel contamination has indeed occurred." If this latter condition is unknown, then a pilot should not rely on the procedure, which is normally part of the fuel sampling process. This fact is true for any unknown. For example, no qualified, safety conscious pilot would takeoff without knowing how much fuel is in his or her aircraft's tank. In the same way that a pilot must have a positive detection of quantity of fuel, he or she must have positive detection of fuel contamination and positive detection of prevention of fuel contamination.

Historically, pilots, the NTSB and universities have tested the extreme preflight procedure published by Cessna in 1982. All of the tests indicate that the procedure is unreliable.

1. An aerospace engineer and part-time flight instructor, Rodney Gross, appears to be the first person to test the rocking procedure on a Cessna 182 in 1978 (Gross's test was performed before the procedure was published by Cessna). His results indicate that the procedure is unreliable.

2. An Atlanta NTSB investigator tested the rocking procedure in accordance with Cessna's letter on a Cessna 182 in 1982. The test results indicate that the procedure is unreliable.

3. At the request of the NTSB, the University of Illinois Institute of Aviation in February 1983 tested the rocking procedure on a Cessna 182. The test results indicate that the procedure is unreliable. After this test, the NTSB said, "Clearly, Cessna's advice to owners will not solve the problem..."

4. In April 1999, Robert E. Scovill JR tested the rocking procedure on a Cessna 172. His test results indicate that the procedure is unreliable.

5. In October 1999, Middle Tennessee State University Flight Operations Maintenance tested the rocking procedure on a Cessna 172. The test results indicate that the procedure is unreliable.

In his book, Cessna Wings for the World--The Single-Engine Development Story, William D. Thompson (Ref: 1) discusses problems with the bladder tank and Cessna's approach to find solutions to the problems. He refers to a fuel-draining operation that he calls bizarre. It appears that this bizarre fuel-draining operation is what Aviation Safety calls an extreme preflight procedure. Here's an excerpt from Thompson's book.

"The C-182 has remained a most durable airplane throughout the years, but, ironically, the two design features borrowed from military aircraft component manufacturers have been troublesome as the airplanes aged. These are the flush-mounted fuel caps and the rubber fuel bladders. Engine stoppages have occurred due to water accumulation in the fuel bladders that could not be easily drained in standard pre-flight procedures. Deteriorating rubber O-rings in these flush fuel caps could admit a surprising amount of rain water in a parked airplane's fuel tanks. Part of this problem was the relatively flat surface which allowed water to stand on the fuel cap assembly, and, then, gradually seep into the tanks.


Complete pre-flight drainage of this collected water was hampered by wrinkles in the bottom of the fuel bladders that became more pronounced as they aged. Thus, pulling the gascolator knob or pushing the quick-drain vale in the wing sumps could leave some water in the bladders. Later, while flying in rough air or in normal maneuvers the entrapped water would find its way into the fuel feed lines.

These problems had never faced us in dry Kansas, and we had to explore the solutions by piping water into the prototype's fuel tank in flight. To our surprise, the engine could tolerate very large quantities of water (in the pints) in the fuel before it would run roughly and eventually quit. Turning to the opposite uncontaminated fuel cell would immediately restore engine power from a windmilling propeller condition.

A series of Cessna Service Letters and FAA Airworthiness Directives was issued to Cessna owners to require the installation of redesigned fuel caps and the smoothing or replacement of worn-out fuel bladders. It was suspected that owners who had never encountered the problem simply ignored these directives, and subsequent owners would be surprised with contaminated fuel incidents in later years. One of the instructions suggested a bizarre fuel-draining operation with the tail depressed to within 5-inches of the ground and the wings being rocked 10-inches up and down at the tip at least 12-times. Continued incidents of water-contamination engine stoppages (despite the above precautions) plus the high cost of fuel bladder replacement prompted the author's good friend, William Barton, in Oakland, Oregon to develop completely redesigned semi-rigid plastic fuel cells and a raised (and water-proof) fuel cap assembly as retrofit kits."

SumpThis has presented the above facts in the interest of promoting safety in general aviation. It is not presented for argument sake. Clearly, the rock & roll procedure is unreliable. Various sources have communicated with SumpThis, but no source has provided facts in the form of test results or in the form of in service data that proves the rock & roll procedure is reliable for "positive detection."

Rather than expend energy and resources debating over an unreliable procedure, SumpThis encourages the general aviation community to work together to find alternative procedures that will provide a means for positive detection. Collaboration on this topic by pilots, aircraft owners, aircraft manufacturers and government agencies can lead to ideas that may solve the problem.

In the mean time, pilots should communicate to each other not to rely on the rock & roll procedure for positive detection until it is proven to be reliable. The FAA, NTSB, Cessna and aviation organizations can assist by disseminating information about this matter.

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Ref: 1, Page 91; Copyright William D. Thompson; Second Printing 1992, Printed by Maverick Publications, Inc.; P.O. Box 5007; Bend Oregon 97708; ISBN 0-89288-221-2.

About the author, Thompson, William D. (Jan. 26, 1920-March 17, 2001)

"Mr. Thompson earned a degree in aeronautical engineering in 1947 from Purdue University. For the next 28 years, he worked for Cessna Aircraft Company as an engineering test pilot and later as the manager of Flight Test and Aerodynamics. He was a Fellow in the Society of Experimental Test Pilots, and at Cessna, he was a long-time member of the SAE Cockpit Standardization Committee and the sole representative of the General Aviation Industry on NASA's Aerodynamics/Aeronautics Committees."

 

 

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