HOW an Impossible Failure CRASHED this Boeing 767!

a boeing 767 is climbing out of the dense jungles of northern thailand the pilots are settling into their seats for the long flight ahead when suddenly a warning related to the aircraft's trust reversal system appears this is the start of a terrifying chain of events that the aircraft manufacturer didn't even think was possible stay tuned on the 26th of may 1991 a crew from lauda air was getting ready for a two-leg flight bringing them from kaitake airport in hong kong towards bangkok in thailand and then finally towards a home base in vienna austria this was part of a three times weekly service that laodaya was doing between bangkok and vienna and the aircraft that was being flown was a boeing 767 300 that had been purchased new to allow the air only about two and a half years earlier in october 1989. this aircraft was a long-range version of the boeing 767 and it was equipped with two pratt whitney 4060 high bypass turbofan engines these engines were state of the art they were very very efficient but the way that they were constructed is going to have a major impact on what's about to happen so we're going to have to become a little bit technical now a high bypass turbofan engine is basically an engine where the majority of the thrust that is pushing the aircraft forward is produced by the big fan in front of the engine rather than the air that passes through the core and the burn chambers in the case of the pratt and whitney 4060 engines the bypass ratio was about five to one meaning that there was five times as much volume of air that passed the bypass duct of the engine than what passed to the core of the engine this high bypass ratio meant a lot better fuel economy and also much lower noise on these new engines because it turns out that using the fan to move a larger volume of air at a slower speed is much more efficient than trying to accelerate a smaller volume of air into higher speed this is by the way why all modern engines seems to be getting bigger and bigger but that's a slightly different story another consequence of this shift in design was that the engine thrust reversers could now be constructed differently on older type of jet engines the reverse truss was constructed by two buckets being extended and then deployed into the core airflow basically redirecting the whole core airflow into a forward angle but with these new high bypass engines it was found that it was much more efficient to use two translating sleeves that would move back and they would then extend blocker doors into the bypass duct that would redirect the bypass flow forward through some fixed cascade veins and that would create the reverse trust however another consequence of this was that the reverse truss was now being created in a slightly different place while in the old engines the reverse truss was being created at the very back of the engines now the reverse truss was being created much further ahead and that is going to have an impact in what's about to happen but why is reverse trust so important well as the aircraft kept becoming bigger and heavier and started to be able to fly during all sorts of different types of weather it became very apparent for the industry that the aircraft needed more than one way to decelerate of the landing or during a rejected takeoff the thrust reversers created a different type of braking than what the wheel brakes would and it turned out that especially during operations on wet or slippery runways the reversers were extremely important and even if they operated the aircraft on dry runways the reversers helped to take some energy away from the brakes which meant that the temperature of the brake discs could be lowered substantially now the reverses on commercial aircraft were only certified to operate on the ground and because of that the aircraft manufacturers had built in several redundant systems to make sure that they couldn't be deployed once the aircraft was airborne on the boeing 767 this was accomplished by a safety mechanism that connected the weight and wheel switches of the aircraft those are switches that can feel if the aircraft is on the ground or if it's airborne was connected to two electrically activated hydraulic valves that both independently could make sure that the truss traverses remained closed whilst the aircraft was airborne the first valve was called a hydraulic isolation valve the hiv and it received signals from the weight on wheel switches and when the weight the wheel switch is indicated that the aircraft was airborne the hydraulic isolation valve just cut access from hydraulic fluid into the trust reversal mechanism and this was true any time that the aircraft was airborne except when a system called the resto circuit was active but we'll get to that later on if for whatever reason the hydraulic isolation valve would malfunction during flight and allow hydraulic fluid into the thrust reversers that would send a warning to the pilot called reverser isolation notifying them that this was going on remember that now following the hydraulic isolation valve was the second weld called the directional control valve the dcv the directional control valve then directed hydraulic fluids to either deploy or close the trust traverses and the directional control valve was defaulted into the closed position so together these two valves were built in a way that should ensure redundancy to keep the truss reversers closed whenever the aircraft was airborne on top of this the engines were constructed to automatically reduce the trust on the engine back to idle in case it felt that any of the trust reversal sleeves was starting to move but of course no system is completely foolproof and that had been proven by a couple of uncommanded trust reversal deployment in flight on some other aircraft types including the dc-8 the dc-10 the boeing 707 and the boeing 747 and because that had happened the faa had mandated boeing to before certification prove either through flight tests or implied modeling that the aircraft would be able to be handled properly in case a trust reversal would deploy in flight those tests and models were built based on a couple of assumptions namely that this trust reversal deployment would happen to an aircraft which was at a relatively low both altitude and air speed and the reason it was made like that was because that was seen as the most critical scenario because if the air speed is low it means that the effect of the flight control surfaces would also be lower and that would be detrimental when it came to handling the effects of the asymmetric trusts that the trust reversal deployment would cause and also the loss of lift on the wing that had happened the test and models also assumed that the engine would be back at idle thrust before the trust reverser deployment event would happen and that was because of the way that the engines had been designed that the trust reversal sleeves shouldn't be able to start moving unless the engines were first back at idle with all of these assumptions met the model showed that the wing that was associated with the deployed truss traverser would only lose about 10 percent of its lift due to the air that was now being forced forward by the transversal disrupting the airflow at the leading edge of the wing this would cause a moderate roll and together with the yaw that was induced by the asymmetric trust this would only lead to moderate problems with controlling the aircraft something that was well within the capabilities of a trained pilot on the 26th of may the day started just like any other day for the crew the pilots met up in the crew room in hong kong and started going through the flight plan the no temps the weather and all of the other briefing material once they were done with that they went they talked to the cabin crew and then together they went out to the aircraft when they came into the aircraft the captain went through the technical log and the technical status of the aircraft was perfectly fine it was perfectly airborne now if the captain would have looked back through the technical history of the aircraft he would have noticed that there had been some work carried out on this aircraft by the technical department specifically there have been several instances of fault messages being sent from the left hand trust traverser these were minor fault messages but the maintenance department in lauda had been troubleshooting this on and off for the last six months or so they had followed the fault isolation manual that boeing had provided to them and that had led them to replace some of the components inside of the trusteversa that would work for a little while but after a few flight similar fault messages would start reappearing again now because these faults were indicated as minor and not critical the dispatch deviation procedure guide actually allowed the aircraft to be dispatched and to fly with these fault codes up to a maximum of 500 hours before it had to be rectified and every time that they did something that rectified the fault these 500 hours would be reset so this was why the aircraft had been flying with these reoccurring fault messages for the last six months the technical engineering department was not happy with this though so they had started to inspect all of the electrical wire bundles that was going out to the left-hand trusster bus to see if maybe there was something in there that was causing these fault messages to appear but since this was not a mandatory inspection and not part of the normal maintenance schedule they did this on and off and meanwhile the aircraft kept flying so even if the captain would have gone back through the tech logan scene that this maintenance work had been carried out and was going on it is unlikely that he would have reacted to it because according to the tech log it was fully airworthy with no limitations whatsoever the captain of the flight was a 48 year old american citizen with about 11 750 hours of total flight experience he was assisted by a 41 year old first officer with just over 6 500 hours and together with them were eight cabin crew members on this flight on the first flight from hong kong over towards bangkok the first officer was pilot flying they taxed it out in kaitak took off completely normally and proceeded to fly over to bangkok without any indication of any type of problems they landed on time according to schedule taxed it into their stand where they proceeded to do a normal turnaround uploading more fuel passengers and cargo getting ready for their flight back to watch vienna but before we get into that flight which is going to turn out to be the accident flight i just want to share this exclusive offer to you from my sponsor this video is sponsored by nordvpn come a little bit closer closer i'm going to tell you a secret i can be anywhere i want within seconds canada us sweden japan you just name it a platform 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flight of the day the captain was going to be pilot flying the pre-flight setup the walk around and all the general preparation was completely normal and at time 1602 that's 23 or 2 local time the aircraft received its pushback clearance and pushed back from the gate to start the taxi out for departure runway 2-1 left in bangkok onboard the aircraft was 213 passengers eight cabin crew members and the two pilots and at time 23-15 lauda airflight cirrosito iv started rolling down runway two one left for takeoff the takeoff was completely normal they started climbing initially to seven thousand feet and the turn towards the northwest the weather in the area was quite good it was a little bit cloudy temperature of 26 degrees with some light winds and some forecasts of thunderstorms in the general area but nothing indicated that any thunderstorms were close to the flight path of flight 004 after takeoff the first officer contacted the bangkok departures atc frequency the flight was then cleared to continue to climb to 11 000 feet and turn towards a waypoint called limbo this was read back by the first officer who was then told to contact the next departure controller the new controller told the flight to continue climb to flight level 310 that's 31 000 feet and continue on the flight plan route the flaps were attracted as per normal procedures and once the flaps were up the first officer completed the after takeoff checklist and he then started to complete the flight plan to fill in the actual departure time to calculate the times over each waypoints and the fuel they could expect at each waypoint and then once he was done with this he passed on those figures onto the ground controller the operations controller in bangkok for loud air so that they could follow the progress of the flight about four minutes and 45 seconds into the flight the first officer suddenly exclaims that keeps coming on referring to a left reverser isolation warning the way he says it indicates that this morning must have been coming on and going out a couple of times before he actually says something about it now do you remember what the reverser isolation warning actually means exactly this means that the hydraulic isolation valve that keeps the hydraulic fluid away from the truss reverser mechanism is now open in flight the captain acknowledges this and it tells the first officer to please take out the quick reference handbook non-normal checklist for this specific failure the first officer does so he finds the checklist but when he starts reading the checklist it doesn't really have any action items it gives a little bit of information about the fall and then it says that additional failures might cause in-flight trust reversal deployment it also says that the pilots can expect normal operation of the transversal of the landing this likely reassures the pilots because any time that a curate checklist doesn't have any action items in it and it doesn't tell the pilots to land at the closest suitable airport it basically means that the failure is not that critical and the fact that the checklist just casually refers to any additional failures might cause an in-flight deployment of the trust traversal that just says that because remember there's still the directional control valve that is supposed to keep the truss traverses closed and the in-flight testings that have been done prior to the certification of the aircraft have shown that even if the truss reverses does deploy it should be okay easily handled by the pilots now there is an additional checklist for an actual in-flight deployment of the trust reversal and that checklist basically just tells the pilots to shut the associated engine down to minimize the effect of the reverser the pilots now keep discussing this indication as they're climbing away further and the first officer suggests that maybe they should contact the handling frequency in bangkok to tell the engineering department that they have this problem and see if they have any guidance for them the captain said that that's a great idea contact them but tell them that it's likely moisture or something because the indication keeps coming on and off all the time now i understand why the captain thought this because a lot of warnings in an aircraft is triggered by microswitches these microswitches are monitoring the position of certain components and sometimes moisture or contamination can get into these microswitches and make them give faulty warnings actual faults tends to be more permanent in nature so when you have a small warning like this going on and off blinking like that given the amount of information that the captain knows at this point i would probably have thought the same in any case the aircraft continues to climb up towards 31 000 feet that have been cleared and a few seconds later the first officer marks to the captain that it looks like he needs a little bit of left trim what the first officer likely means is that he can see that the steering wheel is slightly displaced towards the left this can happen when there is some unexpected drag on one side of the aircraft in this case on the right hand side that causes a little bit of a yaw and the autopilot responds to this by putting aileron in against that side to make sure that the aircraft keeps flying straight ahead but obviously flying with ailerons like that is not the most economical thing to do so what we do is that we have a little rudder on the back side of the main rodder that you can displace in order to counteract that yaw that will then center the steering wheel and the aircraft will continue to fly straight ahead but in a more clean configuration now this remark is unlikely to have anything to do with what's about to happen next about four and a half minutes after the first officer made a comment about the trim this is 15 minutes and one second into the flight the first officer suddenly exclaims oh the trust reverse is deployed at this point the aircraft is at an altitude of twenty four thousand seven hundred feet climbing with a speed of mach point seven eight and with the engines producing full climb thrusts the next thing that can be heard on the voice recorder is sound similar to the aircraft shroudering and a metallic snap the aircraft now enters into a rapid roll to the left accompanied by a violent pitch down and within seconds several warnings can be heard on the cockpit voice recorder including the altitude alert and several master caution warnings a second metallic snap can be heard followed by the captain exclaiming jesus christ followed 12 seconds later by the captain saying here wait a moment and then five seconds after that damn it evidence indicates that it took the pilots approximately 10 seconds to shut the left engine down but at that point it was already too late as this is all happening you can hear on the cockpit voice recorder how the background noise is steadily increasing indicating a higher and higher indicated air speed this is then later accompanied by a continuous over speed warning but why is this happening the aircraft is supposed to be controllable even if one of the thrust reversers are deployed well it turns out that when the modeling was done it was based both on a much lower airspeed about 200 knots and this aircraft is actually making mach 0.78 and it also assumed that the engine would have been spooled back to idle before the reverse thrust was activated in this case whatever failure that caused the truss reverser sleeves to start moving backward did so while the engines were still producing climb thrust and remember how we were talking about these new reverses they produced the thrust a little bit further forward on the engines well this meant that as these trust reversal sleeves were now moving backwards they produced an enormous plume of reverse air that came out just in front of the leading edges of the wings likely reducing the lift available on that wing with as much as 25 not the 10 that the models had shown this sudden and violent onset of reverse thrust on the left-hand side initiated at 28 degrees per second rolled towards the left together with a violent jaw due to the asymmetric truss situation the pilots would have had to immediately initiate full right rudder and full right aileron to try to counteract this together with reducing the trust on the operating right engine to minimize the trust asymmetry but here the pilots would have faced their next hurdle which is that as this descent initiated they were already at a speed of mach 0.78 and as the speed now continued to increase as we could hear from the cockpit voice recorder and the overspeed warning it meant that the pressures and the stresses on the aircraft would have built up enormously and that would meant that the pilots would have had to be very very careful in order to not over stress the aircraft structure as they tried to both maneuver out of the situation and pull the aircraft out of the dive simultaneously as the aircraft is now accelerating and getting closer to the speed of sound the effect of the flight controls will start to become less effective due to elasticity and aerodynamic mac effects and if you put that together with the fact that they're still suffering from asymmetric trust from the now deployed trust reversal it together means that this situation became completely uncontrollable within seconds after the truss reverser deployed what now follows is a terrifying example of what happens to an aircraft when it exceeds its maximum both speed and maneuvering loads we don't know exactly how high the speed became in the end but we do know from the eec memory on the left hand side that it recorded mach 0.99 as its highest value and that was likely the highest value that it could record in reality it is a possibility that this aircraft exceeded mach 1 during its descent down towards the ground the first part that separated the aircraft were sections of the rudder and the left elevator probably due to buffeting overload this was then followed by a large section of the right horizontal stabilizer that was ripped off the aircraft as the pilot tried to pull it out of the dive the fact that most of the right stabilizer was now gone and not on the left hand side started creating asymmetrical forces on the back of the aircraft basically acting like a giant hand turning a corkscrew this damaged both the skin and the connecting structure in the tail leading to the separation of what was left of the horizontal stabilizer and the tail fin without the stabilizing downforce from the stabilizer the aircraft now turned into an almost vertical dive increasing the speed even further this led to the stresses on the wings just becoming too great which meant that the wings now broke off and as that happened that ignited the fuel inside of the fuel tanks leading to an enormous fireball that could be seen by eyewitnesses on the ground on the cockpit voice recorder the complete breakup of the aircraft could be heard as a series of banks and metallic snaps about 26 seconds after the initial indication of the trust reversal deployment this was calculated to have happened at approximately four thousand feet and about three seconds later the aircraft impacted the ground there were no survivors of this accident making this the first deadly accident of a boeing 767 and the worst aviation disaster in thailand to this day unfortunately the post-impact fire also completely destroyed the flight data recorder making it impossible to find out for sure what parts of the system caused the in-flight trust reversal deployment the crash site was also looted before the investigation team could get there and one of the things that the looters took with them was the hydraulic directional control valve from the left hand engine which was a crucial piece of evidence the valve was eventually returned to the investigation team about nine months after the accident but there were indications that it had been tampered with when it was returned so now the investigation team had to try to figure out what had happened and they didn't have the flight data recorder in order to help them so they started looking back to any kind of similar fault that happened on the boeing 767 fleet and they quickly found that there were some wiring issues leading up to the trust reversal system that could potentially leave the hydraulic isolation valve to open up in flight causing the kind of indications that the pilots had reported during this flight now if that fault existed and it was combined with another short circuit within the electrical system that could potentially lead to the directional control valve activating a truss reverser deployment in flight the investigation team also found that if the hydraulic isolation valve was open for whatever reason and there was some contaminations or blockage inside of the directional control valve that blockage could also lead to an activation of the trust reversal as soon as the investigation revealed this boeing issued a technical service bulletin that mandated several upgrades to the boeing 767s that were fitted with pratt and whitney 4 000 model engines the first thing that was mandated was that all trust reversals was deactivated until the upgrades could be implemented and the upgrades included things like a dedicated restore circuit much better and individual electrical wiring and a few other things as well that after the upgrade made a truss reverser deployment in flight virtually impossible but it was still the issue with why this aircraft couldn't be controlled it had been certified the pilot should have been able to handle it lauda air spent a lot of resources on this and nikki lauda himself was deeply involved in investigation work among other things they tried to replicate the flight in a flight simulator and when they did that it showed that this event should have been controllable but it turned out that the simulators just did not have the correct performance parameters to accurately simulate the handling characteristics in this incident and when boeing heard about this they initiated wind tunnel testing with a model very similar to boeing 767 in low-speed wind tunnels and then they extrapolated to see what would happen in higher speed scenarios and when they got the data from that they put that into one of their own simulators and they had the boeing chief technical pilot actually try to fly it and he very quickly realized that unless the pilots did everything right within four to a maximum of six seconds after an in-flight deployment with this kind of situation it would have been impossible for the pilots to recover so boeing basically conceded that an untrained flight crew that was given this type of situation would not be expected to be able to handle it this accident really opened the eyes of the industry into the possibilities and the risks associated with an in-flight trust reversal deployment event it spurred a lot of innovation into system design improvements and redundancies to keep these things from happening the investigation recommended revisions in how the faa certified aircraft with the trust traverses to make sure that they couldn't deploy in flight and also improvements on how the cockpit voice recorder and the flight data recorder could be further insulated and kept away from damage in case of a post accident fire now if you want to learn more about the black boxes and whether or not we will eventually be streaming that data up into the cloud rather than having it stuck inside of the black boxes then check out this video that i just did on my new channel mentor now which i hope you have subscribed to if you want to continue to binge watch incidents on accident and check out this playlist up here and if you want to support the work that i do then consider becoming part of my brilliant patreon crew or get yourself some merch have an absolutely fantastic day and i'll see you next time bye

Channel: Mentour Pilot

Views: 2,972,411

Keywords: captain, mentour pilot, mentor pilot, crash investigation, full epsiodes, aviation, pilot, crash, air crash investigation, final report, boeing, airbus, piper, cessna, disaster, fatal crash, air incident, air disasters 2022, storytelling, stories, crime, non-fiction, lauda, nicki lauda, 767, dive, reverser, hull loss, hong kong, austria, vienna., lauda air flight 004, lauda air flight 004 crash animation, plane crash, True crime, True crime storytelling, Kai tak

Id: WsYU7tjOvm0

Length: 28min 30sec (1710 seconds)

Published: Sun Jul 03 2022