Recommendation R19980241

Recommendation issued to: Civil Aviation Safety Authority

Recommendation details
Output No: R19980241
Date issued: 30 March 1999
Safety action status:



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.


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".


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.

Output text

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.

The Bureau simultaneously issues the following recommendations:


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.


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.


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 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".

Initial response
Date issued: 14 April 2000
Response from: Civil Aviation Safety Authority
Action status: Closed - Accepted
Response text:

BASI Recommendation R980241: CASA does not agree that fuel filter drains are essential for fuel system integrity checks and that adequate provision already exists in the design standards for allowing the detection of water contamination of the aircraft's fuel supply.

I have attached the Technical Specialists review of the BASI report for your information. Please do not hesitate to contact me if you have any further queries on this matter.

Last update 01 April 2011