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Safety Advisory Notice issued to: Civil Aviation Safety Authority

Recommendation details
Output No: SAN20000278
Date issued: 27 February 2001
Safety action status: Closed


Recently two reports of DME (Distance Measuring Equipment) unit internal failures have occurred, which have led to fumes and smoke in the cabin on Boeing 767 aircraft. None of these failed units had recommended modifications completed to improve reliability as outlined in the manufacturer's service bulletins. The operator had not complied with these service bulletins as they are not mandatory, choosing instead to incorporate the modifications detailed in the service bulletins on their fleet units during the next required repair of the units.


Occurrence 200000055

The Boeing 767 was cruising at flight level 370 passing overhead Jakarta when the crew observed that the left DME (Distance Measuring Equipment) circuit breaker had popped. After the circuit breaker was reset, the crew noticed a strong electrical burning smell and both DME circuit breakers popped. The electrical smell persisted. The crew declared an emergency and diverted the aircraft to Jakarta, where a normal approach and landing was conducted.

The operator's technicians examined the aircraft and removed both control panels in the area of the DME interrogator units. All wiring was examined for an overheat condition. The problem was isolated to the DME interrogator units. The units were sent to the DME manufacturer for a complete teardown investigation. Investigation revealed that the A5 modulator had overloaded the 86 volt DC power supply and overheated the power transformer, by placing excessive demand on the secondary windings. The A5 modulator and two resistors within the circuit card also displayed evidence of severe heat damage. One of the two resistors was believed to have produced the fumes detected by the crew. The primary failure component was identified to be a transistor within the A5 modulator, which drives the modulation output transistor.

Related Occurrence

Occurrence 200003857

The Boeing 767 had just reached cruise altitude at flight level 330 approximately 40 minutes out of Singapore enroute to Perth, when the flight crew noted smoke and electrical fumes on the flight deck. The source of the smoke and fumes could not be readily identified. The pilot in command elected to have the flight crew don oxygen masks, and diverted to Jakarta.

The operator's engineering personnel examined the aircraft and found the right DME (Distance Measuring Equipment) circuit breaker open. Technicians isolated the problem to the right DME interrogator unit. The malfunctioning DME unit was disabled in accordance with the MEL (Minimum Equipment List) guidelines to allow the aircraft to continue to Perth. Following arrival in Perth, the unit was replaced.

Examination of the unit by the manufacturer revealed that the DME unit's A5 modulator had overheated. This failure mode was similar to two other units, which had overheated on a different aircraft in January 2000 (see Occurrence 200000055). The failure mode of those units was such that the A5 modulator had overloaded the positive 86 volt DC power supply and overheated the power transformer. Compliance with service bulletins recommending product improvements to this unit were not mandatory, and the recommended modifications had not been incorporated into this unit, or the previous two units that had sustained failures.

Service Bulletin background

The DME manufacturer issued Service Bulletin DME-700-34-18 in April 1991. The service bulletin addressed modifications to a power supply transformer, an increase in the power rating of a resistor, and installation of a fuse. The bulletin was related to a product reliability improvement and implementation was not considered mandatory.

In January 1992, the DME manufacturer released Service Bulletin DME-700-34-23, outlining a replacement transistor for a non-procurable transistor Q7 on the PA modulator card. The bulletin also outlined replacement of several resistors, transistors, inductors, and also resistors on the PA modulator card to improved operation under high stress conditions caused by low input power. This modification was designed to increase reliability of the modulator card and was related to a product reliability improvement. Incorporation of this modification was not considered mandatory.

The DME manufacturer issued Service Bulletin DME-700-34-34 in March 1998, outlining a replacement transistor for a non-procurable transistor Q8 on the PA modulator card. This modification was released to allow continued productivity of the unit. Again, this modification was not considered mandatory.

The DME manufacturer issued Service Bulletin DME-700-34-35 in December 2000, outlining the addition of one fuse to the Power Supply Circuit Card A2, and wiring changes. Incorporation of this modification was not mandatory. In addition, this service bulletin recommended incorporation of Service Bulletins DME-700-34-10, 17, 23, and 34 prior to or in conjunction with Service Bulletin DME-700-34-35. It also stated a requirement that Service Bulletin DME-700-34-18 be installed prior to incorporating Service Bulletin DME-700-34-35.

Other related occurrences

A search of the Federal Aviation Administration Incident Database revealed three documented overheated DME units resulting in smoke and/ or fumes in the cockpit. Part number information of the failed units was not on record. The DME manufacturer has investigated seven units, out of 8,509 units worldwide, that have failed due to a secondary failure of the transformer in the power supply. Analysis of these units revealed that a variety of component failures have caused the transformer to overheat. Three of the seven failures were due to tantalum capacitors breakdown.


Aircraft electrical DC (Direct Current) and AC (Alternating Current) circuits are protected by circuit breakers or fuses. The designers normally place the critical circuit breakers and fuses in the cockpit area or where they are easily accessible. Circuit breakers are usually of the thermal type, that is, excessive current is determined based on increases in heat. The circuit breaker protection for the DME on the incident aircraft type is rated at 2 amperes. The left DME unit receives power from the AC Left Transfer Bus. The right DME unit receives power from the Right AC Bus. The DME units referenced are installed in the cockpit area and are not vented to outside atmosphere.

The right DME circuit breaker in the September 2000 occurrence tripped following overheat of the unit modulator, but not before causing the smoke and fumes noted. During the January 2000 occurrence, the left DME circuit breaker tripped and was reset. Following resetting of this circuit breaker, both left and right DME circuit breakers tripped accompanied by the reported smoke and fumes. It is believed that following the tripping of the left circuit breaker, the unit modulator overheated. When the crew reset the left circuit breaker, the right unit failed internally, creating the appearance of a simultaneous failure. The operator's non-normal checklist for smoke or electrical fumes or electrical fire directs the flight crew to remove power from the affected equipment if the source can be determined. In this case, the circuit breaker was reset prior to the flight crew noting electrical fumes.

Although the circuit breakers acted as designed to prevent damage to the circuitry, the introduction of smoke and fumes into the cockpit is a serious safety concern. Because of the pressurisation of the aircraft and the cycling limitations of fresh air into the cabin, smoke and fumes produced from overheating of the modulator may take several minutes to dissipate. During this time, the technical crew may discount any other indications of smoke and fumes in the cockpit or cabin, resulting in decreased reaction time to an actual in-flight fire. Additionally, the technical crew might be predisposed to complacency concerning smoke and fumes in the cockpit, which would also reduce reaction times in the event of an actual in-flight fire.


The DME unit reliability is significantly increased through compliance with modifications recommended in the applicable service bulletins. Incorporation of these modifications eliminates a failure mode of the unit, which includes overheating of the modulator and resulting smoke and fumes in the cabin. This failure mode impacts flight safety because of the false indication of an in-flight fire. The recurrence of this type of DME failure may lead to pilot complacency and reduced reaction times to actual emergencies. The referenced service bulletins are not being incorporated because of the non-mandatory requirement of the bulletins.

Output text

The Australian Transport Safety Bureau suggests the Civil Aviation Safety Authority take appropriate action to mandate compliance with Service Bulletins DME 700-34-10, 17, 23, 34, and 35.

Initial response
Date issued: 23 March 2001
Response from: Civil Aviation Safety Authority
Response text:

I have reviewed Occurrence Report 200003857, forwarded under your BO/200003857 dated 2 February 2001. The occurrence involved B767 VH-OGC, from Singapore on 6 September 2000, and occurred due to an internal failure of a DME which resulted in diversion of the aircraft to Djakarta.

A copy of the related ATSB Safety Advisory Notice, SAN20000278 was requested, and forwarded to us on 9th March 2000. The SAN provides detail of the event and a related event, and corrective action developed by the equipment manufacturer. This response can be considered to also provide a response to SAN20000278.

The faulty equipment is subject to TSO-C66b, requiring that all materials must be self-extinguishing. Also, the aircraft MMEL allows continued operation while one DME system is unserviceable.

Rockwell Collins produced Service Bulletin (SB) DME-700-34-35 dated 18 Dec 2000 to prevent recurrences. The SB notes that other SBs must be installed prior to or in conjunction with SB DME-34-35. Rockwell Collins highly recommend that the modification be accomplished at the next shop visit. Boeing used All Operator letter L30-01-003/AOL/CAB dated 10 January 2001 to advise MD-11 operators of this SB. The views of the FAA and their response to SAN20000279 are not known at this time.

Qantas is proceeding with incorporation of SB DME 700-34-35 in all affected equipment. The motivation for incorporation of the SB is partly commercial, to reduce the possibility of future diversions of large aircraft.

Ansett reviewed the SB, and decided on the basis of information in the SB that incorporation of the SB was not justified.

SAN20000278 proposes that a potential unsafe situation exists because the smoke and fumes that result from the equipment failure may mask other failures and predispose technical crew to complacency concerning smoke and fumes in the cockpit. The events resulted in a perceptible smoke or burning smell in the aircraft cockpit. However, the strong aversion of all aircrew to all forms of smoke or fumes in aircraft is such that this incident, which in itself was not detrimental to aircraft safety, would not deter aircrew from taking appropriate action if other smoke or fumes are detected.

CASA therefore proposes to disseminate SAN20000278 after the embargo on public distribution lapses, and recommend that operators incorporate SB DME 700-34-35. Should the FAA, with greater knowledge of the aircraft, subsequently require mandatory action, CASA would consider taking further action.

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Last update 05 April 2012