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.
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 Federal
Aviation Administration take appropriate action to mandate
compliance with Service Bulletins DME 700-34-10, 17, 23, 34, and
35.
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.