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SR 111 Investigation Report

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1.18.8  Chronological Sequence of Events General ACARS Odour Detected in the Cockpit Diverting to Halifax Multiple Systems Failures Autopilot Disconnect Altitude Alerts Declaration of Emergency Cabin Crew Use of Flashlights Loss of Lower Yaw Damper A Loss of Flight Control Computer 1 Parameters Loss of Left Emergency AC Bus Loss of Altitude, Airspeed, and Total Air Temperature Loss of Display Electronic Unit Change of Slat Parameter "Retract to Transit" Change of TCAS Parameter "TR Advisory to Standby" Change of Captain's Pitot Heat "On to Off" Loss of VHF-1 Reception CVR P1 Channel Intermittent Loss of Audio Channels on CVR Loss of First Officer's Display Units Loss of Upper Yaw Damper Flight Data Recorder Stoppage Loss of Cockpit Voice Recorder Possible VHF Transmission from SR 111 Mode C Returned Loss of Transponder Engine 2 Shutdown

Back to the top  General

As part of the investigation, relevant information from the FDR, CVR, ATC tapes, FADEC NVM, recorded radar data, and ACARS was compiled in a timeline. (See Appendix D – Timeline.) Some of the information included in this section is also depicted on a map of the flight path (see Appendix A – Flight Profile: Selected Events.)

Back to the top  ACARS  Initial Anomalies

Monitoring systems at ARINC and INMARSAT indicate that these two systems were functioning normally during the flight of SR 111. The INMARSAT logs indicate that the satellite telephone service on SR 111 was not used during the flight. The occurrence flight remained within the normal range for VHF coverage. When VHF coverage is available, it is the primary path for data exchange.

Audit logs from the service providers SITA, ARINC, and INMARSAT show that both the ACARS and SATCOM systems of SR 111 initialized and functioned as expected while the aircraft was at the gate at JFK airport in New York. The systems logged onto their networks at 2318:55 and 2330:18 respectively. The SATCOM logged on as a Class 3 mode for voice and data transmission, which indicates that the ACARS management unit (MU) and the satellite data unit were operational at this time.

The ACARS is designed such that if it is not used for 10 minutes, the ACARS MU will send a tracker message to let the service provider know that the aircraft ACARS receiver is still within the coverage area. An ARINC message was sent by SR 111 at 0021:18. Tracker messages would have been expected from the aircraft at about 0031:18 and 0041:18. There is no record that either of these tracker messages were received by a ground station, although the internal ACARS MU system and message counters were updated, as shown by subsequent messages recorded by the service providers. This indicates that messages were logged by the ACARS as being sent during that time frame. It is possible that the system could have logged onto one of two other networks that had some overlap coverage in the area. This can happen if one system becomes saturated. Records for verification of this information were unavailable by the time this aspect of the investigation was conducted and the data were requested by the investigation team.[106]  Thirteen-Minute Gap in VHF Communications

About 15 minutes after take-off, a 13-minute gap in radio communications occurred between SR 111 and Boston ARTCC. The last communication from SR 111 prior to the gap occurred at 0033:12 when the captain acknowledged a radio frequency assignment change from Boston ARTCC (124.52 MHz to 128.75 MHz). Nine seconds later (0033:21), the FDR recorded a VHF 1 microphone keying event that would be consistent with the pilots attempting to contact Boston ARTCC. No transmission from SR 111 was heard on frequency 128.75 MHz or on any other recorded ATS frequency.

The SR 111 FDR recorded 11 microphone keying events by SR 111 during the 13-minute gap: 9 on VHF 1 and 2 on VHF 2. During this time, Boston ARTCC attempted to contact SR 111 four times on the assigned frequency of 128.75 MHz, three times on the previous frequency of 124.52 MHz, and at least once on the aviation emergency frequency of 121.5 MHz. None of the 11 keying events from the aircraft coincided with the times of the transmissions from Boston ARTCC, indicating that the SR 111 crew was not likely receiving the ATS radio calls.

At 0046:27, SR 111 called Boston ARTCC using VHF 1 on 134.95 MHz, a frequency that had not been assigned to the flight. This transmission was recorded on the ATS tape; however, the Boston ARTCC controller did not comprehend the call that was made on an unassigned frequency and did not immediately respond to this first SR 111 call.

The FDR indicates that at 0047:02, SR 111 attempted another brief call on VHF 1 on an unknown frequency. At 0047:03, INMARSAT logs show a downlink from SR 111 indicating that VHF 3 data communications were lost. This downlink would be consistent with VHF 3 being switched by the pilots from data to voice mode. At 0047:15, SR 111 again called Boston ARTCC using VHF 1 on 134.95 MHz. Communications with SR 111 was restored when Boston ARTCC heard and acknowledged this transmission, and instructed SR 111 to switch to the appropriate frequency for the area control sector they were in (133.45 MHz).

Two-way communications were then restored, and the controller established that the SR 111 crew could hear ATS clearly. There is no record of either the pilots or the controllers at Boston ARTCC making any further comments about the gap in communications. There were no reports of communications difficulties between ATS and any other aircraft in the area. No technical anomalies were recorded on the FDR during the 13-minute gap, and no plausible technical failures were determined during the investigation. It should be noted that FDRs record only a small percentage of the total electrical and systems activity that occurs on an aircraft. Radio communication gaps periodically occur when pilots inadvertently select an incorrect radio frequency when reassigned a new frequency. It is unknown whether this occurred in this instance; however, no other explanation was found.  Additional ACARS Information

About 32 minutes of recorded information was retrieved from the CVR, starting at 0053:17 while SR 111 was cruising at FL330. INMARSAT records indicate that at 0053:51, there was a downlink from SR 111 confirming that VHF 3 communications had been lost for more than seven minutes. The time of this message correlates with the downlink message at 0047:03, indicating the loss of VHF 3 communications, and is consistent with the pilots having switched VHF 3 from data mode to voice mode.

At 0104:14, the ACARS MU sent a downlink message changing coverage from INMARSAT back to ARINC. This would be consistent with the pilots switching VHF 3 from voice mode back to data mode.

After 0104:14, the ACARS functioned as expected. The pilots successfully requested weather information via ACARS at 0113:13 and at 0114:37. The latter request was completed at 0115:18. There was no further crew-initiated communication using ACARS. The last message from ACARS was recorded at 0125:08, when a tracker message for flight following was sent and acknowledged by the system. At 0126:01, the ACARS MU failed as a result of the fire event.

Back to the top  Odour Detected in the Cockpit

The first indication of an abnormal situation was at 0110:38, when the first officer referenced an unusual odour in the cockpit. There were no alerts, warnings, or indications recorded on the FDR to identify any technical problem with the aircraft. No mention was made by the pilots at this time, or during the previous 17 minutes, about any technical problem.

At 0110:57, the captain said "look,"[107] indicating something was visible in the cockpit; it is probable that it was a small amount of smoke that he observed, based on the comment he made later at 0112:24: "It's definitely smoke which came out."

Having been given permission to stand up at 0111:06, the first officer transferred flying control of the aircraft to the captain at 0111:14, indicating that he was getting up. Fifteen seconds later, at 0111:29, the first officer indicated that there was nothing more "up there." This indicates that the visible smoke ceased within 30 seconds of first being noticed.

At 0112:06, the captain summoned to the cockpit a flight attendant working in the first-class cabin. A few seconds later, she opened the cockpit door and entered the cockpit. In response to a query from the captain, the flight attendant indicated that she could smell the odour in the cockpit, but had not noticed any odour in the cabin where she was working. No references were made to visible smoke at this time.

At 0112:24, based on the comment by the captain, it appears that wherever the smoke may have been originally spotted, the amount was likely small, momentary in nature, and no longer visible. Twice within a period of 18 seconds, the CVR recorded sounds of an electrically driven cockpit seat moving, each time for a period of two seconds. It is unknown whether it was the first officer's or the captain's seat that was moving. The captain commented "Air conditioning, is it?" The first officer answered "Yes." The manner in which the captain framed his inquiry suggests that he was confirming with the first officer the course of action they would undertake, such as selecting the System Display Air Page or the Air Conditioning Smoke Checklist. The first officer's confirmation indicates agreement, suggesting that both pilots agreed that they were dealing with an air conditioning anomaly. The captain indicated that something should be closed; most likely he was requesting that the flight attendant close the cockpit door, as within two seconds, sounds consistent with the cockpit door closing were recorded.

At 0112:52, the FDR recorded that the Air Page was selected on the system display. This selection could have been made anytime within the previous 63 seconds and would not have been immediately recorded by the FDR because of the 64-second sample rate interval for recording this FDR parameter. Selection of the Air Page is an action that the pilots would be expected to take to troubleshoot a suspected air conditioning smoke/fumes anomaly.

At 0112:54, the seat belt lights were activated in reaction to light turbulence being experienced.

At 0113:14, a discernable amount of smoke again became visible to the pilots. They considered potential diversion airports and the need to bring the navigation charts forward from the ship's library. Weather conditions were considered in the assessment of various destinations. The ACARS recorded request was for the following airports: LLSG (Geneva, Switzerland), KJFK (New York, New York), KBOS (Boston, Massachusetts), and CVQM (unknown; it is probable that the pilots meant to input CYQM, which is Moncton, New Brunswick, an airport 90 nm northwest of Halifax).

At 0113:53, the captain commented "That's not doing well at all up there." At 0114:05, the captain attempted to call Moncton ACC, but the radio transmission was blocked by a simultaneous transmission from another aircraft. The frequency had been, and continued to be, busy with calls from other aircraft. These other transmissions would have been heard by the SR 111 pilots.

At 0114:15, the captain radioed Moncton Centre and declared "Pan, Pan, Pan," requesting an immediate return to a convenient place. The captain's tentative airport selection for the diversion was Boston, an airport with which he was familiar. The flight was cleared to proceed to Boston and to maintain FL310. A right turn was initiated toward Boston. At the time of the "Pan Pan Pan" call, the aircraft was at FL330, 66 nm from the threshold of Runway 06 at the Halifax International Airport.

At 0114:48, the captain's oxygen mask was removed from its stowage box, and the sound of oxygen flowing from the mask was evident. The M/C indicated to a flight attendant that he had been advised that there had been some smoke observed in the cockpit, and that the captain did not want the cockpit door to be opened.

Back to the top  Diverting to Halifax

At 0115:06, the controller asked the pilots whether they would rather go to Halifax. Having identified Halifax as the closest airport, it was chosen. Halifax was a Swissair-designated intermediate alternate airport, and therefore was approved for MD-11 operations. At 0115:29, the first officer was reassigned the flying duties and instructed to descend immediately. Seven seconds later, the aircraft began descending initially at about 2 000 fpm. The airspeed was at or close to the selected airspeed value of 292 knots, which provided a ground speed of just over 8 nm per minute. The captain continued with radio communication duties. At 0115:36, the captain advised the controller that they would prefer Halifax. At 0115:41, SR 111 was cleared by the controller to proceed directly to Halifax and to descend to FL290. At this time, the aircraft was at FL328, about 56 nm from the threshold of Runway 06.

At 0115:56 and 0116:03, respectively, the captain and first officer donned their oxygen masks. Donning of smoke masks was not included on the Swissair smoke checklists, as it was considered to be a memory item and was a procedure that was practised in the flight simulator by all Swissair flight crews. In their simulator training, flight crews were instructed to don oxygen masks whenever smoke is present. It is not known how much smoke was being seen, if any, but it is likely that at least a smell would have been evident.

Between 0116:08 and 0116:27, the Halifax weather information was passed to SR 111 by the crew of an overflying aircraft. The controller cleared SR 111 to continue descent to 10 000 feet. Moncton ACC was coordinating the arrival of SR 111 with the Halifax tower via a land line. The Moncton controller asked SR 111 for the amount of fuel and the number of passengers on board so that he could pass the information to the Halifax Aircraft Firefighting Services through Halifax tower personnel. SR 111 told the controller to "stand by" for that information.

At 0117:19, the aircraft passed through FL297 and the speed brakes were fully extended. The rate of descent increased to 4 000 fpm, and then reduced to about 3 500 fpm by 0119:28. At 0117:20, the instrument approach plates for the Halifax Airport were not readily available to the pilots to provide information about the runway, minimum safe altitudes, and published approach details. A cabin call chime sounded a few seconds later. The captain then briefed the M/C that there was smoke in the cockpit, that the cabin crew was to prepare for landing in Halifax in about 20 minutes to half an hour, and that he was about to start a checklist. The tone of the captain's voice did not indicate that the situation was sufficiently critical to warrant an emergency; however, he indicated that the passengers were to be briefed that the flight was landing immediately.

With the autopilot engaged, the desired airspeed can be selected by either pilot using a rotary speed-set dial. Based on the FDR sample rate intervals, it is known that the selected airspeed was changed from 292 KIAS to 310 KIAS, during the interval between 0117:16 and 0118:20. Within this time period, at 0117:38, the captain indicated to the first officer that he should not descend too fast, likely referring to the airspeed that was being selected[108] rather than the aircraft's rate of descent. It is possible that some higher speed had been momentarily selected and then adjusted to 310 KIAS. At about 0119:24, the selected airspeed was further increased to 320 KIAS. The aircraft's maximum operating airspeed (barber pole speed) was 365 KIAS; the aircraft remained below that airspeed.

At 0118:17, SR 111 was directed to change to Moncton Centre frequency 119.2 MHz. The first officer, who continued as the pilot flying, was also assigned the radio duties. SR 111 was cleared to descend to 3 000 feet, but the first officer advised Moncton Centre of a preference to descend to an intermediate altitude of about 8 000 feet while the cabin was being prepared for landing.

At 0119:12, the controller asked the SR 111 pilots whether they would like radar vectors to Runway 06 at Halifax. The first officer asked for the latest wind information. The controller did not relay the wind information, but repeated that Runway 06 was the active runway and asked whether he should start the radar vectors. SR 111 accepted radar vectors for Runway 06 and the controller instructed SR 111 to turn left to a heading of 030.

The crew bag containing the approach charts for Halifax was stored in the ship's library beneath the right observer's station, an area that is not within reach of the pilots while they are in their seats. The captain had been attempting to contact a flight attendant directly for some time. At 0119:27, a flight attendant entered the cockpit and moved the crew bag containing the approach chart information to within the captain's reach.

At 0119:37, the controller informed SR 111 that the instrument approach to Runway 06 was a back-course approach. He provided the localizer frequency, and advised the pilots that they were 30 miles from the threshold of Runway 06. The aircraft was descending through FL210, and the first officer informed the controller that more than 30 miles would be required. SR 111 was instructed to turn to a heading of 360 degrees, to lose altitude.

At 0120:14, an announcement was made by the M/C to the passengers, informing them that the aircraft would be landing in Halifax in 20 to 25 minutes. The pilots agreed that a quick descent was warranted in case the smoke thickened. The first officer asked the captain whether he agreed with conducting a backbeam approach to Runway 06, indicating that it would be the quickest approach and would result in landing into wind. The first officer also mentioned fuel dumping and asked the captain about his preference for where and when to dump fuel. The captain seemed to concur; however, his verbal response to these inquiries was interrupted by a physical activity involving stretching, consistent with retrieving something that was out of normal reach, perhaps a checklist or an approach chart.

At 0121:20, the controller requested the number of persons and the amount of fuel on board. The first officer responded that there was 230 tonnes of fuel on board; this was actually the current gross weight of the aircraft, not the weight of the fuel alone. He did not relay the number of persons on board. He queried the controller about whether fuel dumping could be done in that area during descent. The controller responded by asking whether SR 111 was able to turn back to the south, or whether they wanted to stay closer to the airport. When conferring about this with the captain, the first officer stated that the controller would prefer that fuel dumping be done to the south, and asked the captain whether they should do that or whether they should go and land. Given their understanding of the current situation, the pilots decided that turning to the south for fuel dumping would be appropriate. The first officer informed the controller that a left or right turn toward the south was acceptable. The controller instructed SR 111 to turn left to a heading of 200 degrees, requested that the pilots indicate when they were ready to dump the fuel, and advised them that it would be about 10 miles before they were off the coast. He advised SR 111 that they were still within about 25 miles from the airport. The first officer informed the controller that they would stay at 10 000 feet, and the controller cleared SR 111 to maintain that altitude. At 0122:21, the speed brakes were retracted as the aircraft descended through 12 550 feet. The rate of descent reduced to 1 000 fpm, then subsequently increased to 2 000 fpm until the aircraft levelled off between 10 150 and 10 300 feet.

At 0122:33, the first officer asked the captain whether he was in the emergency checklist for air conditioning smoke. The captain indicated that he was. At 0122:37, the FDR recorded that the selected indicated airspeed (IAS) had been changed from 320 to 249 KIAS. This is consistent with applicable regulatory requirements, which stipulate that airspeed be reduced to a maximum of 250 KIAS when aircraft are at 10 000 feet or below. At 0122:41, the airspeed began to decrease from 320 KIAS.

At 0122:48, the captain provided some FMS (STI1-119) advice as the first officer was "inserting" Halifax airport into the FMS to be able to display airport information, such as runway length and instrument approach information. At 0123:00, as the airspeed was decreasing through 306 KIAS, the first officer asked the captain for his agreement to reduce the speed only slightly. The captain indicated that he was proceeding with the checklist, and that the first officer could fly the aircraft as he thought best.

At 0123:22, the airspeed stabilized at 300 KIAS, never reaching the previously selected 250 knots. (It is likely that the selected IAS was increased to 300 KIAS prior to 0123:22, as the selected airspeed FDR sample at 0123:41 was 300 knots.) At 0123:30, the controller instructed SR 111 to turn to a heading of 180 degrees and advised that they would be off the coast in about 15 miles. The first officer confirmed they were maintaining 10 000 feet. At about 0123:51, possibly in consideration of the information that the coast was still 15 miles ahead, the selected airspeed was further increased to 320 knots. This would be consistent with a desire to start the fuel dumping as soon as possible.

At 0123:45, the captain referred to the CABIN BUS switch and asked for confirmation, which the first officer provided. Selecting this switch to the OFF position is the first item on the Swissair Smoke/Fumes of Unknown Origin Checklist (see Appendix C – Swissair Smoke/Fumes of Unknown Origin Checklist). With the CABIN BUS switch in the OFF position, the recirculation fans are turned off, and the airflow above the forward ceiling area would have changed from a predominant flow aft toward the fans, to a predominant airflow forward toward the cockpit.

At 0123:53, the controller informed SR 111 that the aircraft would remain within 35 to 40 miles of the airport in case they had to land quickly. The first officer indicated that this was fine, and asked the controller to inform them when fuel dumping could start.

Up until this time, there were no failures recorded on the FDR, and there were no indications of any systems anomalies reported by the pilots. As well, no smoke had been reported in the cabin area.

Back to the top  Multiple Systems Failures

Starting at 0124:09, and for the next 92 seconds, the FDR recorded a number of technical failure events that were associated with the failure of aircraft systems, as discussed in the report sections that follow. Both flight recorders and the VHF radios (communications with ATS) stopped functioning at about 0125:41. Near the end of this 92-second period, a reference was made within the cockpit to something burning. It is assessed that the location to which this reference was made was the overhead ceiling area of the cockpit.

Back to the top  Autopilot Disconnect

At 0124:09, the FDR recorded a disconnect of Autopilot 2. It would be normal for the captain to have his PFD displayed on DU 1, and the first officer to have his PFD set on DU 6. The changes to the PFD displays triggered by the autopilot disconnect would be as follows: the Autopilot 2 status indication text, (displayed as "AP 2 "), which normally appears in a cyan colour, would change to a red flashing "AP OFF," and the lateral and vertical windows would become flashing red boxes. There would also be an aural warning tone; this aural warning tone was heard, beginning at 0124:09 and continuing until the CVR ceased to record.

At 0124:18, the captain noted, and the first officer confirmed, that the autopilot had disconnected. At 0124:25, the first officer informed Moncton ACC that they had to fly manually, and asked for a protected block of altitudes between 11 000 and 9 000 feet. The controller assigned the altitude block between 5 000 and 12 000 feet.

Although the pilots did not verbalize any attempt to cancel the aural tone, this would be an expected reaction in accordance with their training. It is not known whether the pilots attempted to engage Autopilot 1; however, the aural tone did not stop, and Autopilot 1 did not engage. If the crew attempted to engage Autopilot 1 and it was unavailable, the circuit for Autopilot 1, and the circuit required to cancel the aural tone must have already been compromised by the fire. The electrical power circuit for AFS 2 is powered from the 28 V DC Bus 3 through CB E-07 located on the avionics CB panel. One of the wires coming from this CB goes to the control wheel autopilot disconnect switches. The loss of power through this wire (e.g., as a result of being compromised by fire damage) would disconnect the autopilot, and prevent the autopilot aural tone from being reset.

If Autopilot 1 could not be engaged, this indicates that its armed status was inhibited. A failure of Autopilot 1 would, after two minutes, produce annunciator indications that likely would have been noticed by the pilots. The pilots did not mention any annunciator indications associated with Autopilot 1 in that time frame. Therefore, if Autopilot 1 failed it would have done so within the two minutes before Autopilot 2 disconnected and the "AP 2 OFF" alert appeared in the PFD. The electrical power circuit for the AFS 1 is powered from the 28 V DC Bus 1 through CB C-07 located on the avionics CB panel.

Back to the top  Altitude Alerts

The CVR recorded an altitude alert tone at 0124:38.4 and again at 0124:41.6. The aircraft altitude selector was set to 10 000 feet at this time.

As the aircraft approached 10 000 feet from above, it started to climb again. Information from the FDR indicates that the first officer had changed his altimeter setting to 29.80 in. Hg, whereas the captain had left his altimeter on the standard setting of 29.92 in. Hg, which is the setting required when aircraft are flying above 18 000 feet. The altitude alert system has a 150-foot alert threshold. The two different barometric settings would cause an altitude difference of about 100 feet between the two altimeters. Each FCC would then generate a separate altitude alert tone as their respective thresholds were exceeded, as recorded on the CVR.

Back to the top  Declaration of Emergency

Starting at 0124:35, and lasting intermittently until 0125:27, a land line conversation took place between Moncton ACC and the Halifax FSS, during which Moncton ACC advised Halifax FSS of the anticipated fuel dumping. The provision of this information, which is in accordance with standard practice, was meant to ensure that other aircraft were informed of the fuel dumping location so that they could stay clear of the area.

At 0124:42, the captain called Moncton ACC and declared an emergency. The first officer, in an overlapping radio transmission, acknowledged that SR 111 was cleared between 12 000 and 5 000 feet, and advised that they were declaring an emergency at time zero-one-two-four (0124). This "emergency" declaration from SR 111 was coincident with the land-line information exchange taking place between the Moncton ACC low-level supervisor and the Halifax FSS.

At 0124:53, the captain called Moncton ACC and indicated that they were starting to dump fuel and had to land immediately. At 0124:57, the controller replied that he would contact them in just a couple of miles, to which the first officer replied "Roger," at 0125:01. At 0125:02, the first officer restated that they were declaring an emergency; the controller acknowledged at 0125:05.

Back to the top  Cabin Crew Use of Flashlights

At 0124:46, the cabin crew indicated that they had lost electrical power in the passenger cabin and that they were using flashlights to continue to prepare the cabin for landing. This is consistent with the CABIN BUS switch in the cockpit having been selected to the OFF position using Swissair's procedures, which had been referenced by the pilots at about 0123:45.

Back to the top  Loss of Lower Yaw Damper A

At 0124:54, the FDR recorded the failure of lower yaw damper A. The failure was likely the result of a loss of power to the circuit and may not have been obvious to the crew. A message "YAW DMP LWR A OFF" would have been displayed on the EIS status page; however, the information would only be available to the crew if that status page had been selected. It is unknown whether this page was selected.

The lower yaw damper control circuit A is electrically powered from the 28 V DC Bus 1 through CB C-11 located in the avionics CB panel. Lower yaw damper B, which is also powered by the 28 V DC Bus 1 through CB C-12 in the avionics CB panel, did not fail at the same time. This indicates that the 28 V DC Bus 1 was still powered at this time. It is likely that either CB C-11 tripped or that a wire in its circuitry shorted or opened, possibly as a result of heat damage.

Back to the top  Loss of Flight Control Computer 1 Parameters

At 0124:57, Channel A of FCC-1 lost primary power, and within 15 seconds (at 0125:12), all of the data being reported to the FDR by FCC-1 stopped. The loss of FCC-1 Channel A would drop off some non-flight critical information displayed on the captain's PFD on DU 1, but would not affect the first officer's PFD on DU 6. The first officer was the pilot flying. Under these conditions, the master caution light would illuminate, and faults and various messages would appear on the EAD (typically DU 3), a cue light would illuminate on the SDCP, and warning lights would illuminate on the AFS panel.

The FCC-1 Channel A is electrically powered from the 28 V DC Bus 1 through CB C-17 on the avionics CB panel.

Back to the top  Loss of Left Emergency AC Bus

Based on the loss of ADC-1, DEU 1 and the captain's pitot heat, all of which are electrically powered from the left emergency AC bus, it was concluded that this bus was lost at 0125:06. The arced condition of the left emergency AC bus feed wire corroborates this explanation. This arcing event would have tripped the left emergency AC RCCB B1-136 causing the left emergency AC and DC buses to switch to their emergency power source, the battery and static inverter. Subsequently, several system anomalies and failure events occurred, which were recorded on the FDR. A brief explanation of the resulting system losses follows.

Back to the top  Loss of Altitude, Airspeed, and Total Air Temperature

Between 0125:06 and 0125:07, the pressure altitude, computed airspeed, and total air temperature parameters, as recorded in the FDR, became static. After about seven seconds, a no-data-update sequence began, indicating that the data updates to the FDR from ADC-1 were lost.

The aircraft's transponder Mode C, which provides aircraft altitude information to ATC radar, stopped transmitting at 0125:06. This correlates with the loss of functionality of ADC-1. The ATC transponder Mode A (which provides aircraft identification) was still available, indicating that the transponder had not failed.

ADC-1 is electrically powered from the left emergency AC bus through CB F-04 located in the overhead CB panel.

Back to the top  Loss of Display Electronic Unit 1

At 0125:06, the data from DEU 1 was lost; a switchover to DEU 3 occurred after 0125:14. Normally, data recorded on the FDR is received through DEU 1. If DEU 1 data is unavailable, the data will freeze for eight seconds, and the source will automatically switch to DEU 3.

Electrical power for DEU 1 is supplied by the left emergency AC bus through CB F-03 on the overhead CB panel. The loss of the left emergency AC bus would result in the captain's DU 1 and DU 3 going blank; the subsequent loss of DEU 1 would cause a red X to appear across DU 2.

Back to the top  Change of Slat Parameter "Retract to Transit"

Between 0125:06 and 0125:14, the slats proximity sensor electronic unit B sensors changed from "target near" to "target far." This was a result of electrical power being lost to the B sensors.

The B sensors are powered from the 28 V DC Bus 1 through CB E-09 on the avionics CB panel. The loss of power to this CB would not have any effect on the cockpit displays.

Back to the top  Change of TCAS Parameter "TR Advisory to Standby"

Swissair's practice is to use transponder ATC-1 on odd number flights and ATC-2 on even number flights. If ATC-1 loses altitude data from ADC-1, the traffic alert and collision-avoidance system (TCAS) goes into a Standby mode (as was recorded on the FDR) as long as the ATC-2 still has altitude information. The TCAS senses ATC-2 transponder input but does not use that information unless ATC-2 is selected on the ATC control panel. "TCAS STBY" would be displayed on the PFD and NDs, and an "ATC XPDR1 FAIL" message would be displayed on the EAD even though ATC-1 is still transmitting Mode A information. If no air data were available from ATC-2, the TCAS would go into the "OTHER" mode and "TCAS FAIL" would be displayed on the PFD and the NDs. The TCAS "OTHER" mode is a recorded parameter but did not appear on the SR 111 FDR.

This combined information signifies that ADC-2 was still functional at this time.

Back to the top  Change of Captain's Pitot Heat "On to Off"

Between 0125:06 and 0125:14, the power for the captain's pitot heat was lost. The heater is electrically powered by the left emergency AC bus through CB F-05 located on the overhead CB panel.

The loss of the captain's pitot heat would activate the master caution light, and various cues and alerts would appear on the SDCP, the EAD, and the SD.

Back to the top  Loss of VHF-1 Reception

At 0125:05.6, an ATC transmission of less than one second was received through VHF 1 and recorded on the CVR. The next ATC transmission, starting at 0125:16, was not recorded by the CVR. The start of the next ATC transmission, starting at about 0125:40, was received and recorded for about 0.2 seconds.

For the CVR to not record an ATC transmission, the VHF signal would have to be lost. The VHF signal can be lost either through a loss of power to the VHF-1 transceiver, or through the transceiver going below a minimum threshold voltage. As the audio signal came back briefly before the end of the recording, it is concluded that the transceiver had gone below the minimum threshold voltage. The cut-out voltage, where the transceiver would stop operating, was found to be about 13 volts.

The VHF-1 transceiver is electrically powered by the left emergency DC bus through CB D-08 on the overhead CB panel.

Back to the top  CVR P1 Channel Intermittent

At 0125:06, the 28 V DC power supply to AMU-1 began to fluctuate around 12 V DC, but did not cut out. This assessment is supported by the fact that starting at this time, there was intermittent, attenuated, and distorted recording by the CVR captain's audio (P1) channel of inputs associated with AMU-1.

Back to the top  Loss of Audio Channels on CVR

From 0125:06 to 0125:34, the recording of the P1 channel was intermittent. The first officer's audio channel, the cabin interphone, and the CVR CAM all continued to record normally until 0125:41.4, when the CVR recording stopped.

At 0125:16, Moncton ACC transmitted a clearance to SR 111 to dump fuel on their present track, and to advise when the dump was complete. This transmission was not recorded on the SR 111 CVR. Therefore, it is unlikely that it was heard in the cockpit. A second clearance from Moncton ACC to dump fuel was transmitted at 0125:40. A 0.2 second fragment at the start of this second transmission was recorded 1.4 seconds before the CVR stopped recording.

Back to the top  Loss of First Officer's Display Units

At 0125:16, the first officer advised the captain that he was just flying, and not doing anything else. At 0125:20, the captain referred to something that was burning already, and the first officer made a reference to landing. At 0125:33, the first officer indicated that his side was all dark, and also made reference to standby instruments and speed. It is likely that the first officer's three DUs (DU 4, DU 5, and DU 6) had gone blank. For all three displays to go blank, each unit has to lose its own power source. All three units are powered from the right emergency AC bus, which was still powered, through three CBs located on the overhead CB panel at positions F-29, F-30, and F-31. Therefore, either the individual CBs tripped or the integrity of the wiring was compromised.

Back to the top  Loss of Upper Yaw Damper A

At 0125:34, the FDR recorded the failure of upper yaw damper A. The failure was likely the result of a loss of electrical power to the circuit. This failure may not be obvious to the crew; however, a message "YAW DMP UPR A OFF" would be displayed on the EIS status page, if that page had been selected. It is unknown whether this page was selected.

The upper yaw damper control circuit A is electrically powered from the 28 V DC Bus 3 through CB E-11 located in the avionics CB panel.

Upper yaw damper control circuit B, which is also powered by the 28 V DC Bus 3 through CB E-12 on the avionics CB panel, did not fail at the same time. This indicates that the 28 V DC Bus 3 was still powered, and that either CB E-11 tripped or a wire in its circuitry experienced an arcing event, possibly as a result of heat damage.

Back to the top  Flight Data Recorder Stoppage

Between 0125:39.8 and 0125:40.2, the DFDAU started a warm reboot sequence resulting from a power interruption. The DFDAU re-synchronized and provided additional valid data for 1.3 seconds before the FDR stopped recording.

The DFDAU and the FDR are powered from the 115 V AC Bus 3 through CB D-31 on the avionics CB panel. The warm reboot was the result of either a temporary short or temporary open circuit.

Back to the top  Loss of Cockpit Voice Recorder

At 0125:41.4, the CVR stopped recording. This is attributed to a loss of electrical power. The CVR is powered from the right emergency AC bus (phase C) through CB F-20 located on the overhead CB panel.

Back to the top  Possible VHF Transmission from SR 111

At 0125:46, Moncton ACC recorded an unintelligible fragment of audio that could have been from SR 111.

Back to the top  Mode C Returned

At 0125:50, transponder Mode C data was regained by ATC until 0126:04.1. Mode C data had been lost with the loss of ADC-1 at 0125:06. If the captain switched to DEU-Auxiliary, DU 2 would be reconfigured to a PFD with the air data parameters showing red Xs. To regain air data, he would have had to switch from ADC-1 to ADC-2. This would have restored air data resulting in ATC regaining Mode C.

Back to the top  Loss of Transponder

The radar recorded the last transponder Mode A and C returns at 0126:04.1. This failure is attributed to a loss of power to ATC-1 transponder after this time. The ATC-1 transponder is powered from the 115 V AC (phase A) Bus 1 through CB B-21 located on the avionics CB panel.

Back to the top  Engine 2 Shutdown

Damage to Engine 2 was consistent with an engine that was not producing power at the time of impact. The fault data that was recorded in the NVM of the FADEC indicated that the engine was shut down by use of the FUEL switch at about 1 800 feet;[109] the airspeed was about 227 knots TAS. In general, during flight operations, the pilots could be expected to shut down an engine if they determined that a malfunction existed that would either cause severe engine damage or collateral aircraft damage, or be a hazard to continued aircraft handling. However, prior to the stoppage of the CVR and the FDR, there was no recorded indication of any mechanical malfunction of the engines, and no indication of any crew intention to shut down Engine 2.

Three FADEC fault entries revealed that Engine 2 had a loss of TRA. Loss of TRA inputs would cause the Engine 2 to revert to a fixed thrust mode and to maintain power at the last validated EPR value (thrust setting); it was determined that this thrust setting was flight idle. Flight crews would not normally be prompted to shut down an operating engine if the only discrepancy was that the thrust setting was fixed at flight idle, unless the planned operation of the aircraft, such as an imminent landing, required such an action.

The power wire for the fire detector loop A for Engine 2 shows clear indication of electrical arcing. This damage to the power wire would cause the loss of power to the fire detector loop A circuit. This would result in the FIRE DET 2 FAULT display, a Level 1 (amber) alert on the EAD, and would not result in a false fire alarm. The wire shorting or opening would not cause any overhead lights to illuminate. The crew never mentioned any alerts being displayed and the arcing event was considered to have occurred later in the sequence after the fire was already established.

If the ground wire for the light circuit to Engine 2 fire handle and fuel condition switch was grounded because of fire damage, then both lights would illuminate. The ground wire for this circuit was routed through an area of known fire damage where arcing events had occurred. This specific wire was not recovered; therefore, its condition could not be confirmed. It is possible that fire damage to this wire triggered illumination of the Engine 2 fire handle lights or FUEL switch light, which could prompt the pilots to shut down Engine 2. The Swissair Engine Fire checklist directs the flight crew to reduce the throttle to idle, then turn off the FUEL switch. This latter switch movement would have registered the faults that were recorded on the FADEC.

This was the last recorded information prior to the time of impact at 0131:18.

[106]    By the time investigators became aware of the aircraft communications addressing and reporting system anomalies and requested the data from the service providers, some of the service providers had already deleted the recorded data.

[107]    This and other references to speech in this section are translations from Swiss–German.

[108]    A commanded airspeed was being dialed in with the IAS/MACH rotary SELECT knob.

[109]    The tolerance for the FADEC altitude data is ±470 feet.

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Updated: 2003-03-27

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Important Notices