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Output Number
R19980245
Approval Date
Published Date Time
Recommendation type
Recommendation
Mode
Aviation
Date released
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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.

The Bureau of Air Safety Investigation recommends that Boeing
Rotorcraft Light Helicopter Division review McDonnell Douglas
Helicopter Systems mandatory Service Information Notice No: HN-237
with a view to restoring the removed filter drain valves and
resolving the suspected air problem by other means.



The Bureau simultaneously issues the following
recommendations:



"R980241



The Bureau of Air Safety Investigation recommends that the Civil
Aviation Safety Authority review the design standard for rotor
craft in the normal category to ensure that fuel filter drains are
a requirement, particularly for gas turbine helicopters, and that
they be retrospectively fitted to all affected helicopters.





R980242



The Bureau of Air Safety Investigation recommends that the Federal
Aviation Administration review the design standard of FAR part
27.997 (b) to ensure that fuel filter drains are a requirement,
particularly for gas turbine helicopters, and that they be
retrospectively fitted to all affected helicopters.



R980244



The Bureau of Air Safety Investigation recommends that Boeing
Helicopter Systems review the fuel filter warning light colour and
the appropriateness of the Flight Manual instructions that allow a
flight to be completed after such a warning.



R980246



The Bureau of Air Safety Investigation recommends that the Civil
Aviation Safety Authority amend CAO section 20.2 paragraph 5.1A to
accord with the note in section 20.10 paragraph 1A.1 to ensure that
appropriate inspections for water in the fuel are conducted prior
to the commencement of operations".

Organisation Response
Date Received
Organisation
R19980245
Response Text

MD Helicopters, Inc. (MDHI) and The Boeing Company appreciates
the opportunity to review the safety recommendations contained in
the reference letter. Enclosed is MDHI's response to the BASI's
recommendations.



A copy of the enclosure was reviewed by Boeing and the FAA Los
Angeles Aircraft Certification Office (LAACO) and found acceptable.
In addition, MDHI is aware that a similar response has been
submitted to the Australian BASI by FAA Headquarters in Washington
D.C.



If there are any questions on the above, please let us know.



Subject: 369D Engine Fuel Filter



Reference: BASI - Australia 369D Occurrence #9800067



As a result of a power loss incident on a 369D helicopter, the
Australian BASI has issued five recommendations for design changes
to the 369D fuel system. Three of these recommendations are for the
FAA and two are for MD Helicopters, Inc.



The purpose of this memo is to review the facts and analysis
presented by BASI for this incident and provide an engineering
response to the recommendations.



FACTUAL INFORMATION



The Factual Information section of the recommendation does not
contain some important information that would assist in identifying
the cause of this incident:



1 . Was the operator performing the fuel tank sump fuel drain and
inspection required by the Pilot's Flight Manual prior to the first
flight of every day? If so, was any contamination ever found?

2. Was a vacuum leak check per Boeing Publication CSP-11MI-2
Servicing and Maintenance Instructions, Section 28-00-00 performed
on the engine fuel system after the incident?

3. Was the engine fuel filter differential pressure switch
inspected after the incident for proper operation per Boeing
Publication CSP-11MI-2 Servicing and Maintenance Instructions,
Section 28-00-00?

4. Was the engine fuel filter bypass valve inspected after the
incident for proper operation per Rolls-Royce Allison Publication
1OW2 250-C20 Series Operation and Maintenance, Section
73-10-01?

5. Was there any inspection of the fuel bladder installation of
this helicopter to confirm and quantify the speculation that there
were ripples in the bladder that could trap water?

6. Was the engine fuel control inlet screen inspected for
contaminates and/or bypass condition?

7. Was the engine fuel nozzle screen inspected for
contamination?



The Factual Information section should also include the fact that
the 369D helicopter fuel tank has an 8 ounce drainable fuel sump,
required by CAR 6.423(b), that the engine fuel system can not draw
from in any ground or flight attitude.



ANALYSIS



The first conclusion reached in the Analysis section of the
recommendation is that the lack of a drain valve at the engine fuel
filter bowl significantly reduces the ability of the pilot to
detect contamination of the fuel system in either daily or
post-fueling inspections. There is no argument in the analysis that
the fuel system contamination came from anywhere but the fuel tank.
The tank sump is sized to contain more than all of the entrained
water that could migrate out of the fuel from a single refueling.
Any contamination in the fuel tank will migrate to the sump, either
while on the ground because of the sloping bottom of the tank or
during flight operations at different helicopter pitch attitudes
regardless of "ripples" in the bottom of the fuel bladder. If not,
then the contaminates are effectively trapped in the tank and can
not enter the engine fuel system. If the fuel contamination was
present in the fuel tank, then the daily preflight inspection
required by the Pilot's Flight Manual is sufficient to detect fuel
system contamination.



In this specific case, the analysis states that the helicopter was
operated for 15 hours after the contamination supposedly occurred.
That would presumably require at least 5 refuelings and 5 sump
drains. It stretches credibility to assert that no contamination
would be found in the fuel tank sump during this time but that
contamination would be accumulating in the engine fuel filter. It
seems more likely that fuel tank sump samples were not taken. If
the required fuel tank sump drain samples were not taken during
this time, then why would the operator take samples from the engine
fuel filter bowl if a drain were available?



Also note that Rolls-Royce Allison 250-C20 Series Commercial
Service Letter 1193, dated September 15, 1997, recommends use of
the optional airframe fuel filter when operating 250-C20B engine
powered aircraft from isolated locations where the fuel is stored
in drums or similar extended fuel storage containers. The letter
states: "The accessory filter furnishes an additional measure of
safety whenever the fuel transportation and storage is less than
optimal."



The second conclusion drawn by the analysis is that the engine fuel
filter impending bypass amber caution light should be changed to a
red warning light and that the Pilot's Flight Manual instructions
following the indication should be revised from "turn start pump
on, monitor instruments and continue flight' to land immediately".
The reason given for this is that the pilot didn't see the amber
caution light prior to engine failure due to fuel contamination.
The logic apparently is that the amber light was lit, but was
ignored by the pilot. The pilot's statement is reported to be that
the amber light was not lit.



The engine fuel filter and engine fuel filter impending bypass
indicating system are designed to allow continued flight without
engine failure after cockpit indication of impending bypass. The
impending bypass indication occurs when the filter differential
pressure is .78,98 psid. The filter bypasses when the filter
differential pressure exceeds 2 to 2.5 psid. Depending on the
concentration of contaminates in the fuel, it may take a few hours
to dozens of hours or more of operation after the engine fuel
filter differential pressure reaches the impending bypass setting
for the filter to bypass. Under no circumstances would a properly
operating system fail to indicate an impending bypass prior to
engine failure. This system is equivalent to having a "popout" type
impending filter bypass indicator on the engine filter bowl. A
popout indicator can only be inspected between flights; therefore,
it must be acceptable to continue the flight after the indication
occurs.



Changing the impending bypass light to red and revising the Pilot's
Flight Manual to require immediate landing after an impending
bypass indication is a serious step and should be considered
carefully. Impending bypass indications during flight will likely
occasionally happen within the fleet, depending on the quality of
each operator's fuel supply. Consistently operating with moderately
or heavily contaminated fuel will likely prevent the filter from
lasting the 300 hour replacement lifetime. The impending bypass
setting is intentionally set low enough relative to the filter
bypass differential pressure that continued flight after the
indication will not result in engine failure. Forcing the pilot to
land immediately upon impending filter bypass indication may result
in hazardous landings away from prepared landing facilities.



In this specific case, it is possible that either the engine fuel
filter impending bypass indication system was inoperative or that
the engine fuel filter bypass valve was either leaking or opening
early. In either case, there would be no indication to the pilot of
an impending filter bypass. Changing the colour of an unlit light
won't make any difference.



The third conclusion made is that CAO 20.2 paragraph 5. 1 A should
be clarified to ensure that daily preflight fuel system inspection
requirements to apply to helicopters that are being hot -refueled.
MDRI has no recommended procedures for hot-refueling of the 369D
helicopter; however, the MDHI position has always been that a daily
preflight fuel tank sump contamination inspection is
required.



R980245



"The Bureau of Air Safety Investigation recommends that Boeing
Rotorcraft Light Helicopter Division review McDonnell Douglas
Helicopter Systems mandatory Service Information Notice No: HN-237
with a view to restoring the removed filter drain valves and
resolving the suspected air problem by other means".



MDHI's position is that the 369D helicopter currently has a fuel
filter drain. The drain consists of a plug, which may be removed
for maintenance draining. A drain valve is not required. MDHI does
not believe that there is a need for daily preflight draining of
the fuel filter bowl.