Safety Advisory Notice SAN20000279

Safety Advisory Notice issued to: Federal Aviation Administration

Recommendation details
Output No: SAN20000279
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 Federal Aviation Administration take appropriate action to mandate compliance with Service Bulletins DME 700-34-10, 17, 23, 34, and 35.

Initial response
Date issued: 11 September 2001
Response from: Federal Aviation Administration (FAA)
Response text:

The Office of Accident Investigation is in receipt of your recommendation regarding "B-767 DME Units."

Your recommendation has been forwarded to the appropriate office for response, which is normally 90 days. Your recommendation has been identified, as 01.130, and inquiries should reference this number.

You will be kept informed as to the progress and final resolution of your submission. If you have any questions, or need additional information regarding this safety recommendation, please notify Mr. [name supplied], AAI-210, at [number supplied].

The Office of Accident Investigation convened a Safety Recommendation Review Board to review the enclosed response to FAA Safety Recommendation 01.130. As a result, the Review Board classified this recommendation as "Closed-Not Adopted"

If you have any questions, please contact Mr. [name supplied], AAI-210, at [number supplied].

This is in response to the July 18, 2001, FAA Safety Recommendation 01.130 regarding reports of DME unit failures in Boeing 767 aircraft. The Wichita ACO was requested to determine if the type of failures identified in the safety recommendation affect other airplanes that are equipped with the avionics equipment manufactured under TSOA.

We requested information from Rockwell Collins about their investigation of the DME-700 failures. A copy of their response is attached to this Memo as reference. They investigated the failure of seven DME-700 units out of over 8500 units in service. The seven DME-700 units failed due to a secondary failure of the transformer in the power supply. Of the seven transformer failures, four were caused by a shorted A5A 1 modulator and three were caused by shorted capacitors. Further investigation showed that the capacitors were solid tantalum, and therefore not subject to aging related failures. All failures appear to be random.

A Rockwell Collins listing of eligible aircraft the DME-700 may be installed on is indicated below: The DME-700 is installed in, but not limited to the following aircraft: Airbus A-310/319/320/321/330/340 and Airbus A300600 MD80 and MD 11 Boeing 747-400, 737-300/400/500 classic Boeing 757 and 767 Fokker 100 and 70.

Although compliance with Rockwell Collins Service Bulletins DME 700-34-10, -17, -23, -34, and -35 is encouraged, there is not sufficient reason to make these service bulletins mandatory as part of an Airworthiness Directive. We do not consider these seven random failures a systemic failure of the approximately 8500 units in service. We recommend Safety Recommendation 01.130 be closed.

The following information is provided in response to your facsimile, Ref. (A), and the Safety Advisory Notice, Ref. (B), based on information documented in Refs. (C) through (G).

Rockwell Collins has thoroughly investigated the seven DME-700 units (out of over 8,500 units in service) that failed due to a secondary failure of the transformer in the power supply. The evidence indicates that these are independent events. Even in the one case where two failures occurred during the same flight, it is important to note that the failures were not simultaneous and in fact the second failure occurred some time after the first, and after crew reset circuit breakers that had tripped.

An analysis of the seven failures revealed that a variety of component failures have caused the transformer to overheat. There was no evidence of flames, and the "self-extinguishing" materials used in the DME-700 were effective to preclude propagation. Three of the seven failures were due to the failure of tantalum capacitors. However, it should be noted that solid tantalum capacitors do not age and therefore aging is not a factor in these failures. Rockwell Collins Service Bulletin 35, dated 18 Dec '00, which is available free of charge, will protect the transformer from all of these and other component failures.

This activity has been coordinated with our own Product Integrity Committee, reviewed with the FAA, and concurred by other customers that this is not considered a safety issue. In addition, the text of the referenced SIL and SB were carefully coordinated with Airbus and Boeing. They agreed that "it is highly recommended that this modification be accomplished at the next shop visit", while also agreeing that SB 35 should not be a mandatory modification, nor one that requires a part number change.

The overall MTBF of this product is currently in excess of 130,000 flight hours and the MTBUR is in excess of 80,000 flight hours across the industry. Additional safety margins include aircraft circuit breakers (which operated as expected) and flight crew training for smoke events.

In summary, it is Rockwell Collins' position that the DME-700 has demonstrated performance levels that do not warrant Airworthiness Directive (AD) action.

Last update 05 April 2012