Analysis of Flight 103 Cockpit Voice Recorder
ANALYSIS OF RECORDED DATA
appendix describes and analyses the different types of recorded data which were
examined during the investigation of the accident to Boeing 747 registration
N739PA at Lockerbie on 21 December 1988. The recorded data consists of that from
the Cockpit Voice Recorder (CVR), the Digital Flight Data Recorder (DFDR), Air
Traffic Control (ATC) radio telephony (RTF), ATC radar, and British Geological
Survey seismic records. The time correlation of the records is also discussed.
Digital flight data recorder
flight data recorder installation conformed to ARINC 573B standard with a
Lockheed Model 209 DFDR receiving data from a Teledyne Controls Flight Data
Acquisition Unit (FDAU). The system recorded 22 analogue parameters and 27
discrete (event) parameters. The flight recorder control panel was located in
the flight deck overhead panel. The FDAU was in the main equipment centre at the
front end of the forward hold and the flight recorder was mounted in the aft
DFDR strip and examination
inspection of the DFDR showed that there was considerable disruption to the
control electronics circuits. The crash protection was removed and the plastic
recording tape was found detached from its various guide rollers and tangled in
the tape spools. There was no tension in the negator springs. This indicated
that the tape had probably moved since electrical power was removed from the
recorder. The position of the tape in relation to the record/replay heads was
marked with a piece of splicing tape in order to quantify the movement. To
ensure that no additional damage was caused to the tape it was necessary to cut
the negator springs to separate the upper and lower tape reels.
crinkling and stretching of the tape and the damage to the control electronics
meant that the tape had to be replayed outside the recorder. AAIB experience has
shown that the most efficient method of replaying stretched Lockheed recorder
tapes is to re-spool the tape into a known serviceable recorder, in this case a
25 hour duration of the DFDR was satisfactorily replayed. Data relating to the
accident flight was recorded on track 2. The only significant defect in the
recording system was that normal acceleration was inoperative. There was one
area on the tape, 2 minutes from the end, where data synchronisation was lost
for 1 second.
and reduction of the data from the accident flight showed that no abnormal
behaviour of the data sensors had been recorded. The recorded data simply
stopped. Figure C-1 is a graphical representation of the main flight parameters.
order to ensure that all recorded data from the accident flight had been decoded
and to examine the quality of the data at the end of the recording, a section of
tape, including both the most recently recorded data and the oldest data (data
from 25 hours past), was replayed through an ultra-violet (UV) strip recorder.
The data was also digitised and the resulting samples used to reconstruct the
tape signal on a VDU.
methods of signal representation were used to determine the manner by which the
recorder stopped. There was no gap between the most recently recorded data and
the 25 hour old data. This showed that the recorder stopped while there was an
incoming data stream from the FDAU. The recorder, therefore, stopped because its
electrical supply was disconnected. The tape signal was examined for any
transients or noise signals that would have indicated the presence of electrical
disturbances prior to the recorder stopping. None was found and this indicated
that there had been a quick clean break of the electrical supply.
last seconds of data were decoded independently using both the UV record and the
digitised signal. Only 17 bits of data were not recoverable (less that 23
milliseconds) and it was not possible to establish with any certainty if this
data was from the accident flight or if it was old data from a previous
working group of the European Organisation for Civil Aviation Electronics (EUROCAE)
was, during the period of the investigation, formulating new standards (Minimum
Operational Performance Requirement for Flight Data Recorder Systems, Ref:-
ED55) for future generation flight recorders which would have permitted delays
between parameter input and recording (buffering) of up to ? second. These
standards are intended to form the basis of new CAA specifications for flight
recorders and may be adopted worldwide.
analysis of the final data recorded on the DFDR was possible because the system
did not buffer the incoming data. Some existing recorders use a process whereby
data is stored temporarily in a memory device (buffer) before recording. The
data within this buffer is lost when power is removed from the recorder and in
currently designed recorders this may mean that up to 1.2 seconds of final data
contained within the buffer is lost. Due to the necessary processing of the
signals prior to input to the recorder, additional delays of up to 300
milliseconds may be introduced. If the accident had occurred when tha aircraft
was over the sea, it is very probable that the relatively few small items of
structure, luggage and clothing showing positive evidence of the detonation of
an explosive device would not have been recovered. However, as flight recorders
are fitted with underwater location beacons, there is a high probability that
they would have been located and recovered. In such an event the final
milliseconds of data contained on the DFDR could be vital to the successful
determination of the cause of an accident whether due to an explosive device or
other catastrophic failure. Whilst it may not be possible to reduce some of the
delays external to the recorder, it is possible to reduce any data loss due to
buffering of data within the data acquisition unit.
is, therefore, recommended that manufacturers of existing recorders which use
buffering techniques give consideration to making the buffers non-volatile, and
hence recoverable after power loss. Although the recommendation on this aspect,
made to the EUROCAE working group during the investigation, was incorporated
into ED55, it is also recommended that Airworthiness Authorities re-consider the
concept of allowing buffered data to be stored in a volatile memory.
Cockpit voice recorder (CVR)
aircraft was equipped with a 30 minute duration 4 track Fairchild Model A100 CVR,
and a Fairchild model A152 cockpit area microphone (CAM). The CVR control panel
containing the CAM was located in the overhead panel on the flight deck and the
recorder itself was mounted in the aft equipment centre.
channel allocation was as follows:-
1Flight Engineer’s RTF. Channel 2Co-Pilot’s RTF. Channel 3Pilot’s RTF.Channel
4Cockpit Area Microphone. 3.1 CVR strip
gain access to the recording tape it was necessary to cut away the the outer
case and saw through part of the crash protected enclosure. No damage to the
tape transport or the recording tape was found. The endless loop of tape was cut
and the tape transferred to the replay equipment. The electronic modules in the
CVR were crushed and there was evidence of long term overheating of the dropper
resistors on the power supply module. The CAM had been crushed breaking internal
wiring and damaging components on the printed circuit board.
erase facility within the CVR was not functioning satisfactorily and low level
communications from earlier recordings was audible on the RTF channels. The CAM
channel was particularly noisy, this was probably due to the combination of the
inherently noisy cockpit of the B747-100 in the climb and distortion from the
incomplete erasure of the previous recordings. On two occasions the crew had
difficulty understanding ATC, possibly indicating high cockpit noise levels.
There was a low frequency sound present at irregular intervals on the CAM track
but the source of this sound could not be identified as of either acoustic or
electrical in origin.
CVR tape was listened to for its full duration and there was no indication of
anything abnormal with the aircraft, or unusual in crew behaviour. The tape
record ended with a sudden loud sound on the CAM channel followed almost
immediately by the cessation of recording. The sound occurred whilst the crew
were copying their transatlantic clearance from Shanwick ATC.
Analysis of the CVR record
The stopping of the recorder
determine the mechanism that stopped the recorder a bench test rig was
constructed utilizing an A100 CVR and an A152 CAM. Figures C-2 to C-5 show the
effect of shorting, earthing or disconnecting the CAM signal wires. Figure C-8
shows the CAM channel signal response to the event which occurred on Flight
PA103. From this it can be seen that there are no characteristic transients
similar to those caused by shorting or earthing the CAM signal wires. Neither
does the signal stop cleanly and quickly as shown in Figure C-5, indicating that
the CAM signal wires were not interrupted. The UV trace shows the recorded
signal decaying in a manner similar to that shown in Figure C-6, which
demonstrates the effect of disconnecting electrical power from the recorder. The
tests were repeated on other CVRs with similar results and it is therefore
concluded that Flight PA103’s CVR stopped because its electrical power was
C-9A to C-9D show the recorded signals for the Air India B747 (AI 182) accident
in the North Atlantic on 23 June 1985. These show that there is a large
transient on the CAM track indicating earthing or shorting of the CAM signal
wires and that recorder power-down is more prolonged, indicating attempts to
restore the electrical power supply either by bus switching or healing of the
fault. The Flight PA103 CVR shows no attempts at power restoration with the
break being clean and final.
order to respond to events that result in the almost immediate loss of the
aircraft’s electrical power supply it was therefore recommended during the
investigation that the regulatory authorities consider requiring CVR systems to
contain a short duration (i.e. no greater than 1 minute) back-up power supply.
Information concerning the event
C-8 is an expanded UV trace of the final milliseconds of the CVR record. Three
tracks have been used, the flight engineer’s RTF channel which contained similar
information to the P2’s channel has been replaced with a timing signal.
Individual sections of interest are identified by number. On the bottom trace,
the P1 RTF track, section 1 is part of the Shanwick transatlantic clearance.
During this section the loud sound on the CAM channel is evident.
of the DFDR event recordings shows that the Shanwick oceanic clearance was being
received on VHF2, the aerial for which is on the underside of the fuselage close
to the seat of the explosion. Section 2 identifies a transient, on the P1
channel, typical of an end of ATC transmission transient for this CVR. The start
and finish of most of the recorded ATC transmissions were analysed and they
produce a similar signature to the three shown in Figure C-10. The signature on
the P1 channel more closely resembles the end of transmission signature and it
is open to conjecture that this transient was caused by the explosion damaging
the aerial feeder and/or its supporting structure.
3 shows what is considered to be a high speed power supply transient which is
evident on all the RTF channels and is probably on the CAM channel, but cannot
be identified because of the automatic gain control (AGC), limiting the audio
event. This transient is considered to coincide with the loss of electrical
power to the CVR. Section 5 identifies the period to the end of recording and
this agrees well with tests carried out by AAIB and independently by Fairchild
as part of the AI 182 investigation. The typical time from removal of the
electrical supply until end of recording is 110 milliseconds.
the period identified as section 4 it is considered that the disturbances on the
RTF channels are electrical transients probably channelled through the
communications equipment. Section 6 identifies the 170 millisecond period from
the point when the sound was first heard on the CAM until the recording stopped.
CAM unit is of the old type which has a frequency response of 350 to 3500 Hz.
The useable duration of the signal is probably confined to the first 60
milliseconds of the final 170 milliseconds and even during this period the AGC
is limiting the signal. In the remaining time the sound is being distorted
because power to the recorder has been disconnected. The ambient cockpit noise
may have been high enough to have caused the AGC to have been active prior to
the event and in this event the full volume of the sound would not be audible.
Distortion from the incomplete erasure of the last recording may form part of
the recorded signal.
is not clear if the recorded sound is the result of the explosion or is from the
break-up of the aircraft structure. The short period between the beginning of
the event and the loss of electrical power suggests that the latter is more
likely to be the case.
some of the frequencies present on the recording were not present in the
original sound, but are the result of the rise in total harmonic distortion
caused by the increased amplitude of the incoming signal. Outputs from a
frequency analysis of the recorded signal for the same frequency of input to the
CVR, but at two input amplitudes, are shown in Figures C-11 and C-12. These
illustrate the effects on harmonic distortion as the signal level is increased.
Finally the recorded signal does not lend itself to analysis by a digital
spectrum analyser as it is, in a large measure, aperiodic and most digital
signal analysis algorithms are unable to deal with a short duration signal of
this type, however, it is hoped that techniques being developed in Canada will
enable more information to be deduced from the end of the recording.
the aftermath of the Air India Boeing 747 accident (AI 182) in the North
Atlantic on 23 June 1985 the Royal Armaments Research and Development
Establishment (RARDE) were asked informally by AAIB to examine means of
differentiating, by recording violent cabin pressure pulses, between the
detonation of an explosive device within the cabin (positive pulse) and a
catastrophic structural failure (negative pulse). Following the Lockerbie
disaster it was considered that this work should be raised to a formal research
project. Therefore, in February 1989, it was recommended that the Department of
Transport fund a study to devise methods of recording violent positive and
negative pressure pulses, preferably utilising the aircraft’s flight recorder
results from these trials indicates that if a suitable sensor can be developed
its output will need to be recorded in real time and therefore it may require
wiring into the CVR installation. This will further strengthen the requirement
for battery back up of the CVR electrical power supply.
Flight recorder electrical system
CVR/DFDR electrical wiring.
flight recorders were located in the left rear fuselage just forward of the rear
pressure bulkhead. Audio information to the CVR ran along the left hand side of
the aircraft, at stringer 11. Electrical power to the CVR followed a similar
route on the right hand side of the aircraft crossing to the left side above the
rear passenger toilets. DFDR electrical power and signal information followed
the same route as the CVR audio information.
Flight recorder power supply
DFDR, CVR and the transponders were all powered from the essential alternating
current (AC) bus. This bus was capable of being powered by any generator,
however, in normal operation the selector switch on the flight engineers panel
is selected to "normal" connecting the essential bus to number 4
generator. When the cockpit of Flight PA103 was examined the selector switch was
found in the normal position.
Aircraft alternating current power supplies
electrical power to the aircraft was provided by 4 engine driven generators, see
Figure C-13. Each generator was driven at constant speed through a constant
speed drive (CSD) and connected to a separate bus-bar through a generator
control breaker (GCB). The 4 generators were connected to a parallel bus-bar
(sync bus) by individual bus tie breakers (BTBs). Control and monitoring of the
AC electrical system was achieved through the flight engineer’s instrument
panel. In normal operation the generators operated in parallel, i.e with the
of the CVR CAM channel signal indicated that approximately 60 milliseconds after
the sound on the CAM channel an electrical transient was recorded on all 4
channels and that approximately 110 milliseconds later the CVR had ceased
recording. Within the accuracy of the available timing information it is
believed that the incoming VHF was lost at the same time, indicating an AC power
AC electrical system was protected from faults in individual systems or
equipment by fuses or circuit breakers. Faults in the generators or in the
distribution bus-bars and feeders were dealt with automatically by opening of
the GCBs and opening or closing of the BTBs. In the event of fault conditions
causing the disconnection of all 4 generators electrical power for essential
services, including VHF radio, was provided by a battery located in the cockpit.
short time interval of 55 milliseconds after which the AC supply to the flight
recorders was lost limits the basis on which a fault path analysis of the AC
electrical system can be undertaken. On the available information only a
differential (feeder) fault could have isolated the bus-bar this quickly, with
the generator field control relay taking 20 milliseconds to trip. However, in
normal operation, the generators would have been operating in parallel and the
essential AC bus-bar would have been supplied via the number 4 BTB from the sync
bus. If the fault conditions had continued, a further 40 to 100 milliseconds
would have elapsed before the BTB opened. If the BTB was open prior to the fault
it would have attempted to close and restore the supply to the essential bus.
Any automatic switching causes electrical transients to appear on the CVR and
data losses on the FDR. Both the CVR and the FDR indicate that a clean break of
the AC supply occurred with no electrical transients associated with BTBs open
or closing in an attempt to restore power. In the absence of any additional
information only two possibilities are apparent:
That all 4 generators were simultaneously affected causing a total loss of AC
electrical power. The feeders for the left and right side generators run on
opposite sides of the aircraft under the passenger cabin floor. The only
situation envisaged that could cause simultaneous loss of all 4 generators is
the disruption of the passenger cabin floor across its entire width.
That disruption of the main equipment centre, housing the control units for the
AC electrical system, caused the loss of all AC power. However, again it would
have to affect both the left and right sides of the aircraft as the control
equipment is located at left and right extremes of the main equipment centre.
nature of the event may also produce effects that are not understood. It is also
to be noted that a sudden loss of electrical power to the flight recorders has
been reported in other B747 accidents, e.g. Air India, AI 182.
British Geological Survey has a number of seismic monitoring stations in
Southern Scotland. Stations close to Lockerbie recorded a seismic event caused
by the wing section crashing on Lockerbie. The seismic monitors are time
correlated with the British Telecom Rugby standard. Using this and calculating
the time for the various waves to reach the recording stations it was possible
for the British Geological Survey to conclude that the event occurred at
19.03:36.5 hrs ? 1 second.
were made to correlate various smaller seismic events with other wreckage
impacts. However, this was not conclusive because the nearest recording station
was above ground and due to the high winds at the time of the accident had
considerable noise on the trace. In addition, little of the other wreckage had
the mass or impact velocity to stimulate the sensors.
sources of each time encoded recording were asked to provide details of their
time standard and any known errors in the timings on their recordings. Although
the resolution of the recorded time sources is high it was not possible to
attach an accuracy of better than ?1 second due to possible errors in
synchronising the recorded time with the associated standard. The following time
sources were available and used in determining the significant events in the
communications were recorded along with a time signal. The time source for the
ATC tape was the British Telecom "Tim" signal. Any error in setting
the time when individual tapes are mounted was logged.
Recorded rada data
time signal derived from the British Telecom "Rugby" standard was
included on radar recordings. The Rugby and Tim times were assumed to be of
equal accuracy for timing purposes.
The DFDR had UTC recorded.
source of this time was the flight engineer’s clock. This clock was set manually
and therefore this time was subject to a significant fixed error as well any
inaccuracy in the clock.
The CVR had no time signal.
the CVR was correlated with the ATC time through the RTF and with the DFDR, by
correlating the press to talk events on the FDR with the press to talk signature
on the CVR.
recordings included a timing signal derived from the British Telecom Rugby
Analysis and correlation of times
Scottish and Shanwick ATC tapes were matched with each other and with the CVR
tape. The CVR recording speed was adjusted by peaking its recorded 400 Hz AC
power source frequency. This correlation served as a double check on any fixed
errors on the ATC recordings and to fix events on the CVR to UTC. The timing of
the sound on the CAM channel of the CVR was made simpler because Shanwick was
transmitting when it occurred. From this it was possible to determine that the
sound on the CVR occurred at 19.02:50 hrs ?1 second.
the CVR now tied to the Tim standard it was possible to match the RTF keying on
the CVR with the RTF keying events on the FDR. These events on the FDR were
sampled and recorded once per second, it was therefore possible for a 1 second
delay to be present on the FDR. This potential error was reduced by obtaining
the best fit between a number of RTF keyings and a time correlation between the
FDR and CVR of ?? second was achieved. From this it was determined, within
this accuracy, that electrical power was removed from the CVR and FDR at the
the recorded radar data it was possible to determine that the last recorded SSR
return was at 19.02:46.9 hrs and that by the next rotation of the radar head a
number of primary returns, some left and right of track, were evident. Time
intervals between successive rotations of the radar head became more difficult
to use as the head painted more primary returns.
point at which aircraft wreckage impacted Lockerbie was determined using the
time recorded by seismic activity detectors. A seismic event measuring 1.6 on
the Richter scale was detected and, with appropriate time corrections for times
of the waves to reach the sensors, it was established that this occurred at
19.03:36.5 hrs ?1 second. A further check was made by triangulation techniques
from the information recorded by the various sensors.
Recorded radar information
radar information on the aircraft was available from from 4 radar sites. Initial
analysis consisted of viewing the recorded information as it was shown to the
controller on the radar screen, from this it was clear that the flight had
progressed in a normal manner until Secondary Surveillance Radar (SSR) was lost.
There was a single primary return received by both Great Dun Fell and Claxby
radars approximately 16 seconds before SSR returns were lost. The Lowther Hill
and St. Annes radars did not see this return. The Great Dun Fell radar recording
was watched for 1 hour both before and after this single return for any signs of
other spurious returns, but none was seen. The return was only present for one
paint and no explanation can be offered for its presence.
Limitations of recorded radar data
evaluating the recorded radar data it is important to highlight limitations in
radar performance that must be taken into account when interpreting primary
radar data. The radar system used for both primary and secondary radar utilised
a rotating radar transmitter/receiver (Head). This means that a return was only
visible whilst the radar head was pointing at the target, commonly called
painting or illuminating the target. In the case of this accident the rotational
speeds of the radar heads varied from approximately 10 seconds for the Lowther
Hill Radar to 8 Seconds for the Great Dun Fell Radar.
it was possible to obtain accurate positional information within a resolution of
0.09? of bearing and ? 1/16 nautical mile range for an aircraft from SSR,
incorporating mode C height encoding, primary radar provided only slant range
and bearing and therefore positional information with respect to the ground was
structural break-up of an aircraft releases many items which were excellent
radar reflectors eg. aluminium cladding, luggage containers, sections of skin
and aircraft structure. These and other debris with reflective properties
produce "clutter" on the radar by confusing the radar electronics in a
manner similar to chaff ejected by military aircraft to avoid radar detection.
when the target is not masked by clutter repetitive detection of individual
targets may not be possible because detection is a function of the target
effective area which, for wreckage with its irregular shape, is not constant but
fluctuates wildly. These factors make it impossible to follow individual returns
through successive sweeps of the radar head.
Analysis of the radar data
detailed analysis of the radar information concentrated on the break-up of the
aircraft. The Royal Signals and Radar Establishment (RSRE) corrected the radar
returns for fixed errors and converted the SSR returns to latitude and longitude
so that an accurate time and position for the aircraft could be determined. This
information was correlated with the CVR and ATC times to establish a time and
position for the aircraft at the initial disintegration.
the purposes of this analysis the data from Great Dun Fell Radar has been
presented. Figures C-14 to C-23 show a mosaic picture of the radar data i.e.
each figure contains the information on the preceding figure together with more
recently recorded information. Figure C-14 shows the radar returns from an
aircraft tracking 321?(Grid) with a calculated ground speed of 434 kts. Reading
along track (towards the top left of Figure C-14) there are 6 SSR returns with
the sixth and final SSR return shown decoded: squawk code 0357 (identifying the
aircraft as N739PA); mode C indicating FL310; and the time in seconds (68566.9
seconds from 00:00, i.e. 19.02:46.9 hrs).
the next radar return there is no SSR data, only 4 primary returns. One return
is along track close to the expected position of the aircraft if it had
continued at its previous speed and heading. There are 2 returns to the left of
track and 1 to the right of track. Remembering the point made earlier about
clutter, it is unlikely that each of these returns are real targets. It can,
however, be concluded that the aircraft is no longer a single return and,
considering the approximately 1 nautical mile spread of returns across track,
that items have been ejected at high speed probably to both right and left of
the aircraft. Figure C-15 shows the situation after the next head rotation.
There is still a return along track but it has either slowed down or the slant
range has decreased due to a loss of altitude.
rotation of the radar head thereafter shows the number of returns increasing
with those first identified across track in Figure C-14 having slowed down very
quickly and followed a track along the prevailing wind line. Figure C-20 shows
clearly that there has been a further break-up of the aircraft and subsequent
plots show a rapidly increasing number of returns, some following the wind
direction and forming a wreckage trail parallel to and north of the original
break-up debris. Additionally it is possible that there was some break-up
between these points with a short trail being formed between the north and south
trails. From the absence of any returns travelling along track it can be
concluded that the main wreckage was travelling almost vertically downwards for
much of the time.
geographical position of the final secondary return at 19.02:46.9 hrs was
calculated by RSRE to be OS Grid Reference 15257772, annotated Point A in
Appendix B, Figure B-4, with an accuracy considered to be better than ?300
metres This return was received 3.1?1 seconds before the loud sound was
recorded on the CVR at 19.02:50 hrs. By projecting from this position along the
track of 321?(Grid) for 3.1?1 seconds at the groundspeed of 434 kts, the
position of the aircraft was calculated to be OS Grid Reference 14827826,
annotated Point B in Appendix B, Figure B-4, within an accuracy of ?525 metres.
Based on the evidence of recorded data only, Point B therefore represents the
geographical position of the aircraft at the moment the loud sound was recorded
on the CVR.
almost instant destruction of Flight PA103 resulted in no direct evidence on the
cause of the accident being preserved on the DFDR. The CVR CAM track contained a
loud sound 170 milliseconds before recording ceased. Sixty milliseconds of this
sound were while power was applied to the recorder; after this period the
amplitude decreased. It cannot be determine whether the decrease was because of
reducing recorder drive or if the sound itself decreased in amplitude. Analysis
of both flight recorders shows that they stopped because the electrical supply
was removed and that there were valid signals available to both recorders at
most important contribution to the investigation that the flight recorders could
make was to pinpoint the time and position of the event. As the timescale
involved was so small in relation to the resolution and accuracy of many of the
recorded time sources it was necessary to analyse collectively all the available
recordings. From the analysis of the CVR, DFDR, ATC tapes, radar data and the
seismic records it was concluded that the loud sound on the CVR occurred at
19.02:50 hrs ?1 second and wreckage from the aircraft crashed on Lockerbie at
19.03:36.5 hrs ?1 second, giving a time interval of 46.5 ?2 seconds between
these two events. When the loud sound was recorded on the CVR, the geographical
position of the aircraft, based on the evidence of recorded data, was calculated
to be within 525 metres of OS Grid Reference 14827826.
Eight seconds after the sound on the CVR the Great Dun Fell radar showed 4 primary radar returns. The returns indicated a spread of wreckage in the order of 1 nautical mile across track. On successive returns of the radar, two parallel wreckage trails are seen to develop with the second trail, to the north, becoming evident 30 to 40 seconds after the first.