Recommendation

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.