Why did SilkAir Flight 185 crash? (1997, Musi River, Indonesia)
Hi everyone! Given the end of my A Level exams, I have finished my write-up on something I have been interested in for some time. Follow my blog, as I will continue to cover such events in the future.
On to the tale today...
The aircraft involved: 9V-TRF (Source: Mike Cornwall - https://www.flickr.com/photos/hilifta/19342313372/, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=149149473)
1: An Tragic Day
Musi River Delta, Indonesia
19 December 1997
It is an ordinary day for many people who live on the banks of the Musi River. Located on the Indonesian island of Sumatra, the river connects the Barisan Mountains that form the backbone of Sumatra to the Bangka Straits that extends to the South China Sea. Large ships and smaller ones regularly navigate this river to reach the city of Palembang that has large port facilities that export petroleum, rubber and palm oil. Many villages line the banks of the Musi River, including the leisurely village of Sukamaju. Many villagers go about their daily routines by tending to their families or their crops.
Here’s a map for better visualization:
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| Location of Palembang within Indonesia |
However at about 16:13 local time, this peace was shattered by a loud sound which distracted the villagers from their work. They then saw a plane plunge from the sky and smash into the river just a few hundred meters from their village. In the words of Amran, a villager:
I was working and then the plane came from that side [the opposite bank of the Musi River]. It made a sound like "Rahhhh...", and then it made a U-turn to that side and crashed into the river with a big boom, and a water splash.
They rush to the crash site with their boats, but find nothing but shattered debris and personal effects. Soon, rescuers and investigators arrive at the scene and come to an inevitable conclusion: there are no survivors.
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| Location of crash site near Musi River (Source: KNKT) |
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| Location of crash site vis-a-vis Sumatra (Source: KNKT) |
2: The Plane
Jakarta, Indonesia
19 December 1997
At 15:37 local time, SilkAir Flight 185 departs on time from Jakarta, on a scheduled flight bound for Singapore. It is a clear and sunny day. The Jakarta to Singapore flight connects two major financial centres in Southeast Asia and is widely sought after by travellers. SilkAir, a Singaporean regional airline, obviously did not wish to miss out on this important route. This time, there were 97 passengers and 7 crew on board.
The flight was operated by a Boeing 737-300. It was new, being only 8 months old at this point, and later inspections of maintenance records of the plane revealed no defects or anomalies. By all accounts, there should have been no technical defects with the plane.
At the controls are 41-year-old Captain Tsu Way Ming, and 23 year-old First Officer (FO) Duncan Ward. Captain Tsu was quite experienced with over 7000 flight hours, with more than 3000 hours on the type. He was described as "well accepted" and "a leader among the Singaporean pilot community in SilkAir". FO Ward, while younger, already had over 2500 flight hours, with over 2300 hours on the type. He was described as "a well-balanced and well-adjusted person, keen on his job, and planning to advance his flying career".
The weather remained clear and sunny along most points along the route, except some small islands near Singapore, which experienced isolated thunderstorms. A perfect day for flying. SilkAir 185 climbed to its cruising altitude of 35,000 feet as instructed by Jakarta Air Traffic Control (ATiC). At 16:10 local time, Jakarta ATC contacted SilkAir 185 to ask them to continue on to waypoint PARDI, which was the reporting point to contact Singapore ATC. The crew acknowledged the call. For the next 2 minutes, all appeared normal. Then, a data return on Jakarta ATC's radar screens detected a sudden descent to 400 feet below their assigned altitude of 35,000 feet. After that, all hell broke loose as the altitude of the plane plunged rapidly. Just 30 seconds at the first indications of a sudden descent, the plane was already at 19,500 feet.
Then, the plane vanished from radar.
Rescuers arrived at the crash site, only to discover nothing except shattered pieces of wreckage.
3: The Investigation Begins
Singapore mourned the first fatal crash of a Singaporean airline. It was a tragic end to an overwhelming year for many, as 1997 was the year of the Asian Financial Crisis that caused economic woes for many. The news of the crash did not help.
As families mourned the loss of their loved ones, investigative efforts were well underway. The task fell onto the hands of the National Transportation Safety Committee (KNKT) of Indonesia, assisted by the US National Transportation Safety Board (NTSB), along with Singapore's Ministry of Transport (MOT). Their first task was to recover the wreckage of the fallen plane, and with it, the Cockpit Voice Recorder (CVR) and the Flight Data Recorder (FDR) of the plane. The problem was that the wreckage of the plane was in small pieces embedded into the thick silt of the Musi River. Recovering the wreckage on the riverbed would be a difficult task.
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| Source: KNKT |
The first breakthrough by the investigators was the discovery of wreckage outside of the Musi River. Pieces of the empennage of the plane were found in the Sumatran forest, the furthest of which was 4 kilometres from the crash site. This indicated that the plane had broken up in mid air. Analysis of the wreckage showed that they had failed at high altitude due to flutter owing to extreme forces on the plane during the dive that exceeded the design specifications of the empennage.
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| What makes up an empennage |
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| Source: KNKT |
4: Bizarre Discoveries
Examining the pieces of wreckage, investigators noted something interesting about the jackscrew on the plane. The jackscrew is a threaded rod connected to the horizontal stabiliser that rotates when pilots press a trim switch on their control column. It moves the stabiliser up or down, changing the pitch of the plane (angle of nose relative to the horizontal).
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| Diagram of yaw, pitch, and roll |
The jackscrew is stiffened by a ball nut that prevents it from swinging dangerously to an unsafe position. The screw was in the 2.5 units position. This was interesting as it suggests that the plane was configured to push the nose of the aircraft down into a dive at the time of the crash. Examination of the jackscrew suggested that it was not defective, given that its ball-nut was quite tightly connected to the jackscrew.
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| Diagram of a jackscrew assembly in a Boeing 737-300 |
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| Recovered jackscrew (Source: KNKT) |
Given the mystery already generated by the existing evidence, investigators were naturally eager to find out the data recorded in the Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR). As their name suggests, the FDR records flight data (in this case, over 296 parameters, such as altitude, and airspeed), and the CVR records the conversations in the cockpit.
Investigators were left aghast when they discovered that the CVR stopped operating 6 minutes before the crash. Analysis of the CVR recording revealed that the last few moments on the CVR was the sound of Captain Tsu getting out of his seat to head to the lavatory. Looking at the FDR data, investigators were very much astonished to discover that it had also stopped recording before the crash, but this time, it was 1 minute before the crash.
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| Note the gap between end of plot, which denotes when SilkAir 185 crashed, and the end of data from the flight. (Source: KNKT) |
All the existing circumstantial evidence appeared to point towards a disturbing cause of the crash: the pilots themselves...
5: The Pilots
Both Captain Tsu and FO Ward were competent pilots. That much could not be disputed. There was no question of any bad blood between them as they had flown 19 flights together over the preceding months.
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| Captain Tsu |
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| FO Ward |
As for Captain Tsu, it appears that he was also competent. A former air force pilot, he joined SilkAir and became a leader among the Singaporean pilot community there. He became a line inspector pilot. However, there were some blemishes to his record that were of concern.
On 3 March 1997, Captain Tsu was the commander of SilkAir Flight 274, which left Singapore for Manado. As the aircraft approached Manado, it was positioned too high and was moving too fast to be able to land at the airport. In such circumstances, Captain Tsu should have proceeded on a "go-around" before attempting to land. His co-pilot on this flight stated that instead of taking this course of action, Captain Tsu took the aircraft on "S turns" in an attempt to bring it down to a much lower altitude in order to land, involving quick rolls to the left and right. Unable to achieve his purpose, Captain Tsu then took the aircraft on a "go-around" before it landed at Manado airport. The co-pilot said that Captain Tsu had promised to report the "go-around" to SilkAir but did not do so. It is worth noting that no passenger filed a complaint about the flight in question.
On 24 June 1997, Captain Tsu and the same co-pilot discussed the Manado incident while they were in the cockpit of a SilkAir aircraft that was about to take off on a flight from Singapore to Jakarta. Captain Tsu told the co-pilot that he was upset by what he had heard from other crew members about the Manado incident. As a result of something which the co-pilot said, Captain Tsu pulled out the CVRs circuit breaker in order to preserve the recording of the conversation. As the co-pilot made it clear that he was not prepared to fly without the CVR in operation, Captain Tsu, who wanted to have the CVR tape downloaded, requested for and was given clearance to move the aircraft back to the gate. Subsequently, he changed his mind and reset the CVR. After that, the aircraft took off and proceeded to Jakarta without any further incident. The flight safety department at SilkAir took this incident seriously and removed him from his line safety pilot position.
On 20 November 1997, Captain Tsu was the commander of Flight MI 916, which was scheduled to fly from Singapore to Kunming. On take-off, one of the aircraft's engines did not achieve the target thrust. Shortly after taking off, Captain Tsu had second thoughts about proceeding to Kunming. After discussions with an engineering officer, a joint decision was made for the aircraft to return to Singapore. The aircraft, a Boeing 737, which had just taken off, did not have a fuel dumping capability. As such, it exceeded the maximum landing weight when it returned to Changi Airport. Such an overweight landing should have been recorded in the Technical Log and Incident Report Form. However, Captain Tsu did not do so. For this infringement, both he and his co-pilot received a "be more mindful" letter from SilkAir.
Looking into his financial history, investigators found that the captain had been involved in securities trading since 1990. His net worth decreased from 1994 to 1996 and increased “marginally” in 1997. Captain Tsu's trading activities were stopped on two occasions due to the non-settlement of his securities-trading debt. His trading activities were stopped from April 9, 1997, through Aug. 15, 1997, and again on Dec. 9, 1997. On the morning of 19 December 1997, Captain Tsu promised to make a payment when he returned from his flight. Captain Tsu had several loans and debts at the time of the accident. Captain Tsu and his immediate family’s monthly income was calculated to be less (about 6 percent) than their monthly expenditure at the time of the accident. He had several insurance policies that provided benefits in the event of his death, most of which were acquired many years before the accident.
A Singaporean police investigation was launched into Captain Tsu. Interestingly, the investigation found that there was no motive for him to crash the plane (although this was heavily debated).
To investigators, the pilot suicide theory was becoming increasingly likely. But did it fit in with the evidence? To check, investigators used the plots of the radar returns obtained from Jakarta ATC to test out the descent profiles involving all possible scenarios that could have caused the crash, like mechanical failures or deliberate pilot input. Of all the possible scenarios, only one fit the profile: the manual use of ailerons and/or rudder along with manipulation of the elevators in the horizontal stabiliser. The others did not result in as rapid a descent and were recoverable. This appears to indicate that the crash of SilkAir Flight 185 was the result of a deliberate pilot input.
6: Disputes Begin
After the majority of the investigative efforts were completed, the NTSB and KNKT sat down to discuss their conclusions. At the meetings, it was agreed by investigators that the cause of the crash was likely pilot suicide, probably by Captain Tsu. The KNKT was tasked with the writing of the report for publication. Interestingly, the Chairman of the KNKT, Professor Oetarjo Diran, rejected the report. As an investigator later put it:
We submit[ted] it to the chairman, but the chairman didn't like what we submitted.
In an interview, Professor Diran said:
As an engineer, I would like to have a one-to-one connection: cause and effect. I didn't see the cause, and ... I see only an effect, an aircraft went down. I didn't know what the cause is, and I cannot prove that the cause of the accident...wasn't suicide. It could also be mechanical failure.
The US NTSB was unhappy with this. In their view, they ,along with the KNKT, had painstakingly developed the whole theory of pilot suicide, only for it to be rejected. In the appendix to the final report for the crash, the KNKT appended 57 pages of arguments and rebuttals between the KNKT and NTSB. The KNKT and the NTSB argued back and forth about the significance of, among other things, Captain Tsu's work history, the cause of CVR stoppage, and ultimately, the cause of the crash.
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| Comments and dissent of NTSB to the KNKT (Source: KNKT) |
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| Conclusion of KNKT report (Source: KNKT) |
Ultimately, the KNKT published their report, which was over 200 pages long. They ultimately concluded:
The NTSC has to conclude that the technical investigation has yielded no evidence to explain the cause of the accident.
7: Lawsuits
The story of the crash does not end with the dispute between investigators. Families of the victims were divided into camps; some of whom supported the suicide theory, and others who supported another interesting theory, namely that the rudder PCU had malfunctioned and caused the crash.
I return to the point about the rudder PCU that I made before. NTSB investigators concluded that the rudder was free of corrosion that could have caused it to malfunction. However, when lawyers from some of the victims' families sent it to a laboratory for examination, it was found that it contained metal shavings that were imperfections from the manufacturing process.
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| Rudder pedals are visible at where the foot should be if you are seated in the pilot seat. (Source: Flicker) |
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| Source: Federal Aviation Administration (FAA) of the US |
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| Source: FAA |
At this juncture, it would be useful to explain the PCU's functions. It is made up of a primary slide, secondary slide, and a slide housing. When a pilot steps on a rudder pedal, it moves one or both of the slides, and the hydraulic fluid passes through under extreme pressure by moving through the holes on the slides, moving the rudder. To put into perspective how precise the process must be, the slides only move a distance equivalent to the width of a dime. However, under certain conditions, the secondary slide could jam against the housing of the slide, causing the hydraulic fluid to pass through the wrong holes, leading the rudder to move in the opposite direction to that commanded by the pilot, or even in the absence of any pilot input.
In 1991 and 1994, a rudder hardover happened on two different flights: United Flight 585 and USAir Flight 427, killing 157 people in total. In both cases, metal shavings were found in the PCU, further narrowing the distance between the secondary slide and the housing, ensuring that both jammed together. The cause of the United Flight 585 crash was initially found by the NTSB as “undetermined” before it was reexamined after the crash of USAir Flight 427. The families believed that a rudder hardover also happened on Flight 185.
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| United 585 wreckage (Source: Cockpit Voice Recorder Database) |
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| USAir 427 wreckage (Source: Take to the Sky: The Air Disaster Podcast) |
Furthermore, analysis of flight path trajectories of all 3 of the flights were consistent: a roll to the side, followed by a nosedive. In addition, Boeing had also instructed pilots to push the nose down to increase the airflow over the wings to ensure that the pilots could overcome the rudder hardover (although to me, it seems rather unlikely that a pilot would push the plane's nose down continuously to the point of entering a nosedive).
As for why the CVR and FDR stopped recording, they had a different theory. They argued that based on an examination of the FDR data, the FDR failed to record 4000 seconds of data across 60 parameters in the last 25 hours before the crash (or about 12 flights worth of data), for periods of as short as 4 seconds and as long as 11 minutes.
Armed with these arguments, some families of the victims sued Boeing and Parker-Hannifin, the manufacturer of the valve, in the Los Angeles Superior Court for compensation for the deaths of their loved ones. After a trial before a jury, Boeing and Parker-Hannifin lost out on all issues of liability and were ordered to pay US$43.6 million to the families. Eventually, the parties settled before an appeal was scheduled to be heard, even as Parker-Hannifin rejected liability.
Meanwhile, some families sued SilkAir in Singapore under the Warsaw Convention for "willful misconduct" of the flight crew, asserting that the pilots either committed suicide or otherwise wilfully misconducted themselves. After a trial in a courtroom that was packed with spectators, Singapore High Court judge Tan Lee Meng dismissed the lawsuit. He rejected the claims that Captain Tsu's work history was significant in pushing Captain Tsu to crash the plane, stating that instances of disciplinary action were not too serious. He also found it unlikely that Captain Tsu would have committed suicide just 2 days before his father's birthday party that he had thoroughly planned and cause his family to lose a son, father and breadwinner. He rejected the projections from the corrected radar data used by the NTSB in their simulations as unreliable as the plane had exceeded the proper flight envelope, making it debatable if the trajectory depicted was accurate. He also found that the CVR and FDR were not proven to have been disconnected deliberately. He found the NTSB vehement, and referred to the United Flight 585 crash as evidence that they may be wrong. The judge found that it was not proven that the pilots had crashed the plane deliberately or out of recklessness. It was also held that it was not proven that the pilots did not attempt to recover the plane or were reckless in failing to recover. Hence, the lawsuit was dismissed, a decision affirmed on appeal.
8: Thoughts
Personally, I am more partial towards the deliberate act theory. The jackscrew was convincing to me, as I still find it difficult to imagine why a pilot would continually cause his plane to plunge from cruising altitude if there was an emergency. The rudder theory was interesting, but ultimately, it cannot satisfactorily explain how both the FDR and CVR somehow failed at different timings.
Ultimately, I still feel that the evidence was rather circumstantial as I still can't seem to fit the last piece of evidence of how exactly the pilots crashed the plane given that the FDR stopped functioning before the crash. I do agree though that this is what most likely happened amongst all the theories.
9: The Aftermath
In the midst of all the controversies surrounding the cause of the crash, many of the bereaved were left confused and lost for answers. As the mother of a victim said in an interview as to what she thought was the cause of the crash:
So many professors came down. Already told you, "We don't know what happened." All the experts already say, "Cannot answer." How do I answer?
For many, the loss of their loved ones left a gaping hole in their lives. A family member refused to change her phone number for years, hoping that her son would return home. Another, who lost both her parents in the crash, lamented that whatever the cause of the crash, it would not change anything for her.
Regardless of whatever the cause of the crash was, it cannot be denied that those on the flight had hopes and dreams that were inexplicably shattered and forever lost that fateful day, leaving those who cherished them grieving and shattered forever in the aftermath. Perhaps it is best to lament, and spare a thought for those who perished on what should have been a short flight to Singapore 27 years ago…
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| Photo of SilkAir Flight 185 Memorial |
In memory of those who perished on SilkAir 185.
Sources:
Mayday / Air Crash Investigation: Pushed to the Limit / Pilot Under Pressure - a 2012 documentary about the crash
SilkAir 185: Pilot Suicide? - a 2006 documentary about the crash
Final report by the KNKT - published on 14 December 2000
Official Report Provides No Conclusions About Cause of SilkAir B-737’s Fatal Plunge from 35,000 Feet - Flight Safety Foundation, March 2001
Families lose SilkAir lawsuit - Today, 25 October 2001
25 years after SilkAir crash: Why mum who lost son on flight never changed her mobile number - The Straits Times, 18 December 2022
SilkAir crash anniversary: Her parents were planning to spend Christmas in Singapore - The Straits Times, 18 December 2022
Beryl Claire Clarke (as personal representative of the estate of the Late Eugene Francis Clarke) v Silkair (Singapore) Private Limited and other actions [2001] SGHC 326 - judgement of the Singapore High Court on 24 October 2001
Clarke Beryl Claire (as personal representative of the estate of Eugene Francis Clarke) and others v SilkAir (Singapore) Pte Ltd [2002] SGCA 26 - judgement of the Singapore Court of Appeal on 15 May 2002
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