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.
SAFETY DEFICIENCY
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.
FACTUAL INFORMATION
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.
ANALYSIS
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.
CONCLUSION
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.
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.
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.