SUBJECT - NORMAL CATEGORY ROTORCRAFT FUEL FILTER
SAFETY DEFICIENCY
The ability to detect fuel contamination between the fuel tank and
the engine on Boeing Rotorcraft Light Helicopter Division Model 369
and 500N has been significantly reduced because of the approved
removal of the fuel filter drain valve.
Note: This aircraft was previously McDonnell Douglas Helicopter
Company (MDHC) 369 and 500N.
FACTUAL INFORMATION
Occurrence 9800067
A Boeing 369D helicopter was engaged in a sling operation to lift a
powerline to the top of a transmission tower. The pilot was
hovering the helicopter about 60 ft above the 30 ft tower, and had
just passed the powerline to the linesman when the engine
experienced a total loss of power. The pilot immediately banked the
helicopter to the right to avoid personnel on and below the tower,
and attempted a landing in light scrub. The helicopter came to rest
on its right side, incurring substantial damage, and slightly
injuring the pilot.
A maintenance investigation found that the engine fuel system was
contaminated with water, discoloured fuel, and particle debris.
After the engine fuel system was cleaned, the engine was
successfully ground-run. The helicopter had recently spent 45 hours
engaged in firefighting operations in the 60 hours since the last
maintenance inspection. During the firefighting operations,
refuelling was routinely undertaken from an assortment of drum and
mobile bowser fuel sources. Hot refuelling also accounted for a
significant proportion of those refuelling operations. It is
probable that the fuel system was contaminated during that period;
however, the investigation was not able to conclusively prove when
or how water and other contaminants entered the fuel system.
The pilot reported that he was not forewarned of a fuel filter
problem and did not recall seeing the amber fuel filter
differential pressure warning light during the accident sequence.
The first warning light that he recalled was the red engine-out
light at the time the engine failed. He explained that functional
tests had been carried out on the fuel filter differential pressure
warning system during the pre-flight sequence. The system had been
functioning normally.
Fuel system design and maintenance requirements
The Boeing 369D fuel tanks are lined with a bladder. As bladders
rarely sit smoothly and flat on the tank floor, some water may be
trapped between the ripples in the bladder and consequently may not
be drained from the fuel tank drain valve. In addition, the fuel
system is fitted with a fuel filter differential pressure warning
system to alert pilots of filter contamination and an impending
bypass of the filter. The flight manual provides the following
instructions regarding the fuel filter indicator:
"Amber fuel filter indicator illuminated indicates clogged filter;
turn start pump on, monitor instruments and continue flight; the
lighted indicator indicates that the pressure through the filter is
0.8 psi differential or more";
"Warning, after the fuel filter indicator has lighted, and
following the completion of the flight in progress, additional
flight is prohibited until the fuel filter has been
serviced".
The fuel system filter is readily accessible for drain purposes, as
the engine cowls, behind which the fuel filter is located, are
easily unlatched. However, the aircraft maintenance manual provides
the following warning:
"Air in the fuel system will cause a power reduction or flameout.
Do a fuel system vacuum leak check and system air bleed after
opening the fuel system to atmosphere and prior to releasing
helicopter for flight".
This maintenance must be carried out any time that the fuel system
filter is removed either for inspection or replacement. In most
cases, pilots are not appropriately qualified to perform this
maintenance.
The manufacturer does not call for scheduled inspection of the
filter, only its replacement every 300 hours. In addition, the fuel
filter housing is not transparent. Therefore, unless the fuel
filter differential pressure warning system alerts the pilot to a
developing problem, contamination of the filter may only be
detected by dismantling the fuel filter for a visual
inspection.
In this occurrence, a certificate of airworthiness was issued for
the aircraft on 26 September 1997 at 2,751 hours aircraft total
time in service. The fuel filter was not inspected or changed at
the subsequent 100-hourly inspection on 23 December 1997, prior to
the accident on 7 January 1998. The fuel filter had been in service
for 160 hours.
Design and subsequent modification of the Boeing 369D fuel
filter
The helicopter manufacturer was issued a Federal Aviation
Administration (FAA) type certificate against Civil Aviation
Regulation (CAR) 6. CAR 6.427 states that:
"a strainer incorporating a sediment trap and drain shall be
provided in the fuel system between the fuel tanks and the engine
and shall be installed in an accessible position. The screen shall
be easily removable for cleaning".
The helicopter fuel system in this occurrence had been subsequently
modified in accordance with a mandatory MDHC Service Information
Notice No: HN-237, dated 26 September 1994, which approved the
removal of the engine fuel filter drain valve. The FAA approved
this modification in accordance with the later design requirement
of Federal Aviation Regulation (FAR) 27.997(b). This regulation
states that:
"there must be a fuel strainer or filter between the fuel tank
outlet and the inlet of the first fuel system component which is
susceptible to fuel contamination, including but not limited to the
fuel metering device or an engine positive displacement pump,
whichever is nearer the fuel tank outlet. This fuel strainer or
filter must have a sediment trap and drain except that it need not
have a drain if the strainer or filter is easily removable for
drain purposes".
The manufacturer elected to remove the filter drain valve to
prevent engine flameouts suspected to be the result of air entering
via the firewall and filter drain valves, pooling in the filter and
forming an air slug. This possibility was apparently not proven in
laboratory simulation. Another manufacturer overcame the possible
air slug scenario by establishing the tolerance level of the engine
to air in the fuel, and installing a calibrated air bleed in the
filter to remove the air safely.
Fuel system inspection requirements
Civil Aviation Order (CAO) section 20.2 refers to safety
precautions before flight. Paragraph 5.1 (b) recommends that all
fuel system filters and collector boxes be checked for water
contamination at frequent intervals. The intent of the order is to
check for the presence of water before the start of each day's
flying and after each refuelling. However, CAO 20.2 paragraph 5.1 A
states that "paragraph 5.1 does not apply to helicopters that are
being hot refuelled in accordance with section 20.10".
CAO section 20.10 refers specifically to requirements for hot
refuelling in helicopters. The note in paragraph 1A.1 states that
"operators and pilots should note the provisions of paragraph 5.1
of section 20.2 of the CAO's relating to the inspections and tests
for the presence of water in an aircraft's fuel system before the
start of each day's flying are applicable to helicopters to which
this section applies".
ANALYSIS
Quality control of fuel entering the fuel system is a valuable
defence against the consequences of contaminated fuel. However, the
ability of the pilot to detect contamination of the fuel system
during routine inspections is an equally important safety defence.
These two safety defences should not be considered mutually
exclusive. Analysis of this occurrence revealed a design deficiency
in Boeing 369D helicopters manufactured without, or modified to
remove, the fuel filter drain valve. The removal of this drain
significantly reduces the ability of the pilot to detect
contamination of the fuel system in either daily or post-fuelling
inspections.
Fuel system design and maintenance requirements
Water and other contaminants could have accumulated in the filter
for a total of 160 hours prior to the accident. Whilst it could not
be conclusively proven, it is most likely that the water
contaminated the fuel system during the refuelling operations that
took place during fire-fighting activities. Ash particles found in
the fuel filter during the post-accident inspection are consistent
with fuel contamination at that time. Further, water in the fuel
that may not have drained from the tank sump drain valve, possibly
because of retention in ripples in the tank bladder, may have
continued to accumulate in the filter during the 15 hours
subsequent to the firefighting refuelling operations.
In view of this occurrence, and the CAO 20.2 recommendation for
safety precautions before flight that checks for the presence of
water in the fuel filter be conducted at frequent intervals, the
Bureau considers that the replacement schedule of 300 hours for
this fuel filter does not meet with the intent of the
recommendation. Changing the fuel filter in accordance with this
maintenance schedule, and with no provision for a daily filter
contamination inspection, does not provide an adequate safety
defence for fuel system integrity.
The Bureau also considers that allowing the removal of the filter
drain valve on this aircraft on the basis of easy removal of the
filter for drain purposes, is flawed logic. Removal of the filter
for drain purposes is a good feature; however such maintenance
action would only normally be carried out if there were a known
contamination problem. In addition, in order to prevent subsequent
engine flameout, the fuel system must be bled and tested for air
leaks prior to releasing the helicopter for flight. Pilots are not
normally approved to perform this maintenance. None of this may be
accomplished easily for any gas turbine engine installation,
especially if the helicopter is operating in a remote region and in
hostile refuelling conditions. Provision of a fuel filter drain
would enable a pilot to readily conduct a check for contaminants,
without requiring further maintenance to return the aircraft to
service.
Fuel filter warning system
The pilot did not see the fuel filter differential pressure amber
warning light. However, had the pilot been alerted to an impending
problem by the warning light, the flight manual stated that the
flight may continue. In this occurrence, there was very little
warning, if any, before the engine flamed out. This accident
demonstrated that the fuel filter warning light could not be relied
upon to provide adequate warning of the possible consequences of
filter contamination and an impending bypass. A red warning light
and a requirement to land immediately may be more appropriate in
order to alert pilots to take immediate precautionary action
against an uncertain outcome.
Fuel system inspection requirements
Pilots reading CAO 20.2 paragraph 5.1A may wrongly assume that the
paragraph 5.1 requirements for fuel system inspection before the
start of each day's flying do not apply to helicopters that are
being hot-refuelled. The note in CAO 20.10 paragraph 1A.1 however,
states that it is applicable. This ambiguity in what is a critical
safety check should be removed.
