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Recommendation issued to: Civil Aviation Safety Authority

Recommendation details
Output No: R19980258
Date issued: 21 December 1999
Safety action status:
Background:

SUBJECT - CAO FUEL TANK DRAIN REQUIREMENTS FOR LIGHT AIRCRAFT


SAFETY DEFICIENCY

Some older general aviation aircraft used for passenger-carrying flights do not have fuel drains that meet current or past regulatory requirements.


FACTUAL INFORMATION

As a result of research leading from occurrence 199503537, this recommendation has been developed.

The aircraft was cruising at 1,000 ft above ground level when the engine lost power. Although the engine regained power, the pilot turned the aircraft back towards Jandakot and advised the aerodrome controller that he was experiencing an engine problem. Shortly after, the engine stopped completely, and the pilot elected to land in a farm paddock. The aircraft sustained substantial damage during the landing attempt.

It was the aircraft's first flight following a periodic servicing. A substantial quantity of water was found in the fuel system. A visual inspection of the main fuel tank indicated that the drain sump was not located at the lowest point of the tank when the aircraft was on the ground. Undrainable water was able to accumulate in the tank aft of the sump while the aircraft was positioned in the ground attitude.


Fuel tank drain regulations

A general airworthiness directive (DCA/GENERAL/19 FUEL SYSTEM -REQUIREMENTS)
was issued in 1951 following an accident involving a Dragon Rapide. Water remained in the aircraft's fuel tanks despite prior careful draining through the standard drain cocks. The directive required that a modification to fuel systems be made, if necessary, to ensure that:

"2.1 All fuel tanks, including auxiliary tanks, are capable of being completely drained when the aircraft is in the normal ground attitude. In this context "normal ground attitude" shall include the possibility of the aircraft standing on a surface which is not completely level."

"2.3 It is not possible for water which may accumulate in small quantities in the bottoms of tanks to be conveyed to an engine in any normal attitude of the aircraft.

GENERAL NOTE. The foregoing requirements will be most easily complied with where fuel tanks are fitted with effective drainable sumps with the fuel take-off points located above the sump level. Where a conventional tank sump arrangement is impracticable a separate sediment bowl may be accepted provided it can be clearly demonstrated that the capacity of the sediment bowl is adequate for the purpose and that water will automatically drain from all portions of the tank to the sediment bowl when the aircraft is in the normal ground attitude.

This directive requires modifications to be made to any aircraft that does not meet these requirements, to ensure that it does meet these requirements".

The directive was cancelled without replacement in 1973 because its requirements had been incorporated into the British and the American design requirements, which were the basis for the Australian design requirements. The justification for cancellation without replacement stated that many aircraft, including DH-82 and DC-3 aircraft had been modified to meet the requirement.

The American design requirement, Federal Aviation Regulation (FAR) 23.971, stated:

"(a) Each fuel tank must have a drainable sump with an effective capacity, in the normal ground and flight attitudes...

(b) Each fuel tank must allow drainage of any hazardous quantity of water from any part of the tank to its sump with the airplane in the normal ground attitude.

(c) Each reciprocating engine fuel system must have a sediment bowl or chamber that is accessible for drainage...and each fuel tank outlet is located so that, in the normal flight attitude, water will drain from all parts of the tank except the sump to the sediment bowl or chamber."

The British design requirements were the same.

Civil Aviation Order 101.22 para. 3.2(1) stated the requirements for aircraft design for aircraft of less that 5,700 kg imported into Australia since 1 May 1981. It required that aircraft meet the requirements of FAR 23 or the equivalent British Civil Aviation Regulation. Australian manufactured aircraft of less than 5,700 kg and amateur-built aircraft had to meet similar requirements.

Replacement fuel tanks for aircraft manufactured before 1951 are required to meet the airworthiness specifications valid at the time of manufacture. The justification for cancellation without amendment of DCA/GENERAL/19 stated that there had been a need to modify the fuel systems of aircraft manufactured before 1951 to meet this requirement. Since the cancellation of DCA/GENERAL/19 there has been no requirement to put fuel drains in the lowest point in the system when in the ground attitude in replacement fuel systems for these aircraft, and to do so without a Civil Aviation Regulation 35 approved modification or an equivalent Supplemental Type Certificate would be a regulatory breach.

At present, the main supplier of new fuel tanks for Tiger Moths is based overseas, and does not incorporate a fuel drain that is at the lowest point of the fuel tank when the aircraft is in its ground position.


Scope of the problem

Evidence from maintenance engineers who are experienced in maintaining this type of aircraft indicates that not all DH82 Tiger Moths meet the requirements of either DCA/GENERAL/19 or the Civil Aviation Orders in force since 1973. There are approximately 200 DH82 Tiger Moths on the Australian register. Many of these aircraft are used for passenger-carrying activities. There have been occasions during regular maintenance in which significant quantities of water have been found in the sediment bowl located between the fuel tank and the engine.

ANALYSIS

The reason given for the cancellation without replacement of DCA/GENERAL/19 was that even though legislation in force at the time of the cancellation was believed to adequately address the requirement for fitment of fuel drains in aircraft fuel tanks, it did not address the requirement for fuel drains in replacement or repaired fuel tanks for aircraft constructed before the date of issue of DCA/GENERAL/19.

No occurrence has been found in which the correct design and use of fuel tank drains in accordance with certification requirements since 1951 has been a factor; therefore, no inadequacy has been demonstrated in the regulations presently in force.

It is possible that an aircraft manufactured before 1951, and originally modified in accordance with DCA/GENERAL/19, can now have its fuel tanks repaired or replaced with components designed to the regulations in force before 1951. As a consequence, it may not have an effective fuel drain system installed.

It is recognised that aircraft affected by this safety deficiency are of a considerable age and that the commercial implications of any required change must be considered. However, the number of fare-paying passengers carried in such aircraft is sufficient to warrant further consideration of this safety deficiency.

Output text

The Australian Transport Safety Bureau (formerly the Bureau of Air Safety Investigation) recommends that the Civil Aviation Safety Authority review the regulatory requirement for the provision of adequate fuel drains in aircraft manufactured before 1973 that do not have a certification requirement for a fuel drain at the lowest point in the fuel tank when the aircraft is in its ground attitude.

Initial response
Date issued: 09 June 2000
Response from: Civil Aviation Safety Authority
Action status: Closed - Partially Accepted
Response text:

Water drains are a significant safety requirement, which we had cause to review during the recent fuel crisis and subsequent water washing of contaminated aircraft. That crisis showed that a few aircraft did not drain well, and required an unexpected effort in order to drain all water from the aircraft. I note that the issue has not resulted in any incidents to date, and that the design standard for certification of new aircraft, FAR 23. 971, is satisfactory.

CASA therefore proposes to issue an Airworthiness Advisory Circular on the importance of water drains in aircraft. Given your concern regarding DH82 aircraft in particular, we will send a copy to all CoR holders of these aircraft.

 
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Last update 01 April 2011