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National Transportation Safety Board
Washington, D.C. 20594

OCT 11 1994

 

Messrs. Normal L. Horton & Jerry L. Wells
Aviation Safety and Management Consultants
4582 Old Stage Road
Central Point, Oregon 97502

 

Gentlemen:

 

The Safety Board shares your concern regarding potential water-in-fuel problems in Piper Apache and Aztec PA-23 series airplanes, as outlined in your letter of August 22, 1994. As you know, the Board has submitted several safety recommendations to the Federal Aviation Administration (FAA) concerning such problems. Therefore, to put this matter in proper perspective, I believe it is important to recall, chronologically, some of the contents of these recommendations and the sequence of events that have transpired since they were issued. Moreover, it is equally important to recognize that the National Transportation Safety Board is an investigative rather than a regulatory agency and is empowered only to recommend, not mandate, accident prevention remedial measures.

 

On October 16, 1985, the Safety Board issued Safety Recommendations A-85-77 and -78 concerning Piper Apache models PA-23-150, PA-23-160, and PA-23-235. The Board recommended that the FAA issue an airworthiness directive (AD) requiring the installation of individual fuel line drain valves in these airplanes and that the FAA require the Piper Aircraft Corporation to publish service and operating information fully describing preflight fuel sampling procedures applicable to these airplanes, and in the case of airplanes fitted with optional auxiliary fuel cells, specific procedures that take into consideration the difference in drainage time required for lines for the auxiliary fuel cells. The following excerpts from its safety recommendation letter outline some of the Board's rationale for these remedial measures.

 

In the standard Apache, a 36-gallon nylon and neoprene main fuel cell is located outboard of each engine, and auxiliary cells, such as were installed in N2160P, may be installed optionally in the outboard sections of the wings. Neither the main nor the auxiliary cells (if fitted) in Apache airplanes are provided an individual fuel line drain valve. Fuel sampling or sumping to detect and remove water during preflight must be accomplished through the main fuel strainers located in the inboard sides of the main wheel wells. In order to sample fuel from each cell, the fuel selector valve in the cockpit must be positioned alternately to each of the main and auxiliary fuel cells while the fuel strainer is being drained. In the case of the more remotely located auxiliary fuel cells, allowance must be made for additional drainage time and fuel flow to ensure that any residual or free water in these cells is removed. The Apache Pilot's Operating Handbook, however, states only that the fuel strainers should be drained regularly; there is no explicit procedure outlined to ensure adequate drainage and sampling of individual fuel cells which takes into account the differences in the drainage time of the auxiliary cells.

 

 

 

The Piper Aztec, the successor design to the Apache, incorporates individual drain valves for each fuel line on the airplane which are located near the main fuel strainers This system obviates the need for the pilot to reposition the fuel selector valve in the cockpit to sample each cell.

On December 30, 1987, the Safety Board issued several additional safety recommendations to the FAA regarding potential water-in-fuel problems in Piper Apache and Aztec PA-23 series airplanes and included reference to your open letter of June 11, 1987, concerning this matter. The Safety Board explained that:

 

The Piper PA-23 Apache/Aztec fuel systems were the subject of an open letter of June 11, 1987, addressed to both the National Transportation Safety Board (NTSB) and the Federal Aviation Administration (FAA) from Messrs. Norman L. Horton and Jerry L. Wells of Aviation Safety and Management Consultants (ASMC), Central Point, Oregon. Messrs. Horton and Wells emphasized the potential for water entrapment in the PA-23 bladder-type fuel cells due to wrinkles or folds as outlined above and further stated that water might also be entrapped because of the relative position of the fuel cell outlet port. Based on the results of tests they conducted, they contend that even if the fuel bladders are not wrinkled, some water may still not be drainable but may remain entrapped in the aft inboard portion of the fuel cell with the airplane in its normal ground attitude. Part 3, subpart 3.444, "Fuel Tank Sump" of the Civil Air Regulations (CAR 3) governing certification of Piper Apache airplanes states:

 

(c)

If a separate sediment bowl is provided in lieu of a tank sump, fuel tank outlet shall be so located that, when the airplane is in normal ground attitude, water will drain from all portions of tank to the sediment bowl.

In view of the number of accidents involving engine stoppage in Apache airplanes due to water or undetermined reasons, the potential for entrapping water in the fuel cells of these airplanes is particularly disturbing. Moreover, a review of several accident reports, including those wherein water in the fuel was specifically cited as a casual element, clearly disclosed attempts by the pilots to remove water from the Apache fuel system during normal preflight procedures.

Piper model PA-23-250 Aztec airplanes, the successor design to the Apache, uses similar bladder-type fuel cells. However, unlike the Apache, the Aztec incorporates individual drain valves for each fuel line on the airplane that are located near the main fuel strainers. The Safety Board believes that the additional drain valves may contribute significantly toward elimination of (drainable) water from the fuel. Engine stoppage has been attributed distinctly to water in only one accident involving an Aztec since 1975. Both persons aboard that airplane were killed. However, 17 other accidents recorded since 1975 involved engine stoppage in the Aztec for undetermined reasons. Thus, the Safety Board is concerned that there is a similar potential for water entrapment in Aztec fuel cells. As a result, the Safety Board believes that the FAA, in conjunction with the Piper Aircraft Corporation, should conduct tests to determine whether or not significant amounts of water can be entrapped in the fuel cells installed in both the Piper Apache and Aztec airplanes.

The fuel filler compartments on both Apache and Aztec airplanes are recessed within the tops of the wings and incorporate hinged access covers with seals. If the seals are defective, precipitation or wash water may enter the fuel filler compartment and leak into the fuel cell. Both airplane fuel systems also incorporate thermos-type fuel cell caps which often loosen, harden, or crack (especially on the older Apache airplanes) and are ineffective in sealing the fuel cells.

One May 24, 1971, the Piper Aircraft Corporation issued Service Bulletin No. 340, "Fuel Cell Vent/Drain Lines Inspection," applicable to both Apache and Aztec airplanes, to prevent leakage of water into the fuel filler compartments and fuel cells, and to assure that the vent/drain hole inside the fuel filler compartment is not blocked or cracked. In view of the previous water in fuel/engine stoppage accidents involving Apache and Aztec airplanes and the potential for the influx of water into the fuel cells provided by the recessed fuel filler compartment design, the Safety Board believes that the FAA should mandate compliance with this service bulletin.

 

As a result, the Safety Board recommended that the FAA:

 

Conduct tests to determine whether or not water can be entrapped in the fuel cells installed in Piper model PA-23 Apache/Aztec airplanes because of wrinkles or folds in the bladders and/or because of the orientation of the fuel bladder cell with the airplane in a normal ground attitude. (A-87-127)

 

 

 

If the Piper Apache/Aztec fuel cell tests disclose that unsafe quantities of water can be entrapped, issue an airworthiness directive containing appropriate remedial action that must be taken to purge the fuel cells of such water or to prevent the water from reaching the engine. The remedial action may include use of special fuel cell suction/siphoning devices, installation of fuel cell sump drains, larger capacity fuel strainers, and/or directed, special preflight action that a pilot must take to change the airplane's normal ground attitude and to optimize drainage of water through the existing fuel strainers. (A-87-128)

 

Issue an airworthiness directive applicable to Piper model PA-23 Apache/Aztec airplanes, requiring compliance with Piper Service Bulletin No. 340 concerning inspection of fuel cell vent and drain lines at every 100-hour or annual inspection, whichever occurs first. (A-87-130)

The FAA, in response to Safety Recommendation A-87-130, issued AD 88-21-07 (effective November 6, 1988) mandating compliance with Piper Service Bulletin (SB) No. 340. On November 7, 1990, the FAA issued AD 90-23-18 (effective December 10, 1990) in response to Safety Recommendations A-85-77 and -78 and A-87-127 and -128. The actions outlined in the AD provided for preflight drainage of Apache fuel systems in accordance with the detailed system operating instructions contained in Part 1 of Piper SB No. 827A dated November 4, 1988, and the installation of dual-fuel drain kits in accordance with the instructions in Part II of the SB. The AD also required the fuel tank wedge kits be installed in certain Apache and Aztec airplanes with unbaffled fuel tanks in accordance with the instructions in Part I of Piper SB No. 932, dated January 12, 1990, and that enlarged fuel bowls be installed in Aztec airplanes with baffled fuel tanks in accordance with the instructions in Part II of the SB.

 

The wedge kits were intended to eliminate any potential for significant amounts of water to be trapped in the fuel cells by ensuring that any water that may have migrated to the aft inboard portion of the cells was drainable with the airplane in its normal ground attitude. The enlarged fuel bowls would have prevented engine stoppage due to water in the fuel by providing an increased capacity to store any water that might flow to and overload the smaller (3-ounce) fuel bowls during flight. However, the Aircraft Owners and Pilots Association (AOPA) petitioned the FAA to withdraw AD-90-23-18 because, in its judgment, the water-in-fuel problems were caused by poor maintenance (poorly maintained fuel filler caps and doors) rather than inadequate design. As a result, the FAA suspended the AD on December 13, 1990. On April 24, 1991, the FAA issued an advance notice of proposed rulemaking (ANPRM) seeking comments from interested persons regarding the best action (if any) to be taken to correct potential water-in-fuel problems affecting the aforementioned airplanes. Subsequently, the FAA issued AD 92-13-04, effective August 21, 1992, and applicable to Piper model PA-23-150 and PA-23-160 airplanes. The AD retained the preflight fuel system drainage provision previously contained in AD 90-23-18 (for airplanes not modified by the dual-fuel drain kits), but no Apache or Aztec fuel system design modifications., i.e., fuel tank wedge kits, enlarged fuel bowls, were deemed necessary.

 

The Safety Board's investigation of the accident at Manteo, North Carolina, on July 26, 1991, involving a Piper Aztec model PA-23-250, N54746 (NTSB Identification: ATL91FA142 [SumpThis]), disclosed a loss of engine power on takeoff for an undetermined reason. Although water in the fuel might have been involved in this accident, an intense postcrash fire that consumed the cockpit, fuselage, and wings of the airplane, and damaged the engines, precluded any conclusive examination of the aircraft's fuel system for discrete evidence of water or other potential contributing factors.

 

The Safety Board, with assistance from parties to the investigation, makes every effort to evaluate all evidence derived from an examination of aircraft wreckage. Such evidence, however, must be positive and unambiguous. Moreover, while the Board does not normally reexamine wreckage after its investigation is complete and the wreckage has been released to its registered owner(s), it recognizes that subsequent accident-related activities by parties or other persons may occasionally produce potentially significant supplemental information. Therefore, the Safety Board's investigation of an accident is never officially closed, since relevant information or evidence pertinent to the accident may be submitted to the Board by anyone at any time. For example, under 49 CFR 845.41, "Petitions for Reconsideration or Modification," petitions requesting the Board to reexamine and/or amend its findings and determinations of probable cause may be filed by parties to an investigation or hearing other persons having a direct interest in the accident investigation. Such petitions need to be submitted in the manner and format described under this part, and will be entertained only if based on the discovery of new evidence or on a showing that the Board's findings are erroneous.

While the Board's examination of the wreckage of N547646 could not substantiate a positive finding of water in the fuel, such a condition was not ruled out. Moreover, because several years have elapsed since the accident, it is now even more unlikely that sufficient evidence could be identified in the wreckage to support this possibility. Therefore, the Safety Board believes that loss of engine power for an undetermined reason is the most appropriate conclusion. Nonetheless, the Safety Board remains concerned about potential water-in-fuel problems in Piper Apache and Aztec PA-23 series airplanes, and will continue to give special consideration to this possibility during its investigations of accidents involving loss of engine power in these aircraft.

The Safety Board is most appreciative of your continuing interest and efforts concerning this and other matters relating to general aviation safety.

 

 

Sincerely,

Jim Hall
Acting Chairman

 

 

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