One Disaster Caused Another! United Airlines Flight 608 & 624

0:00 [Music] fire every Pilot's worst nightmare and for good reasons as you will soon see but what if the very mechanism created to control that threat turns out to be even more dangerous stay tuned these days everyone involved in aviation is standing on the shoulders of over A Century of Aeronautical Knowledge but I rarely go back to those early days when that knowledge was first emerging which is why in this video we're stepping into a time machine and going back almost 80 years to tell the story of two accidents from the 1940s who truly changed

0:45 aviation in the second half of the 1940s as the world emerged from the devastation of the second world war a fierce competition emerged as aircraft manufacturers sought to build bigger better and faster airliners in the United States lock had been working on a large pressurized 4 engine passenger airliner since way back in 1937 and their design became the beautiful constellation which by the way I have covered in quite some length over at my other channel Mentor now anyway the constellation entered service in 1943 as the first mass-produced pressurized airliner and that came as a real wakeup call for Douglas aircraft which hadn't even started designing a suitable competitor at that

1:31 point they did have the 4 engine 44 passenger DC4 But Not only was it substantially smaller than the constellation it also wasn't pressurized meaning that it was slower less efficient and less comfortable than the constellation was but the US military had purchased hundreds of the DC4 military variant the c-54 so in 1944 they approached Douglas to ask for a lengthened pressurized version of this already popular military transport and what Douglas came up with was a modified DC4 with new more powerful engines a pressurized fuselage and a slightly increased passenger capacity but by the time the first prototype flew in 1946 the war had

2:16 already ended so Douglas rebranded the model as the dc6 and started marketing it as a passenger airliner instead powered by four pratton Whitney r2800 piston engines and with room for between 48 to 68 passengers the original dc6 was well received by Airlines and passengers when it entered service in 1947 but unfortunately that honeymoon would be very short because it turns out that the dc6 had been delivered with a truly shocking design flaw on October the 24th 1947 the crew of a nearly brand new United Airlines dc6 reported for duty at Los Angeles International Airport in California and they had been scheduled for a Non-Stop CrossCountry

3:03 flight over to Chicago and it was a mostly beautiful day along the Route the flight crew consisted of A 42-year-old Captain with around 15,000 hours of Total flying time and a 25-year-old first officer with 3,000 hours but because the dc6 was so new at this time the two men combined had less than 200 hours on the type on the ramp at Lex the three stewardesses soon welcomed 47 passengers in through the aircraft air stairs taking the total number of people on board up to 52 once the boarding was complete then after completing the pre-flight checks taxi and runup The Lax Tower granted takeoff clearance to flight 608 and with a deafening chorus

3:48 from its four massive piston engines The dc6 Accelerated down the runway and eventually became airborn at time 10:23 they proceeded normally and soon left the immediate Los Angeles area and climbing to their cruising altitude of 19,000 ft the flight exited control airspace and continued Northeast under their own navigation the flight crew then checked in with United Airlines dispatchers at certain predetermined reporting points allowing the company to track the flight's progress across Southern California past Las Vegas and into Southwestern Utah and it was over a remote area of Utah that things started to go very wrong but before we get to that I have to explain some of the systems on board the dc6 the fuel system being the most

4:35 important for this story the DC 6's fuel system had basically been inherited from the DC4 and it consisted of eight fuel tanks in total one main tank and one alternate tank for each of the four engines in the cockpit several levers allowed the pilots to configure which fuel tank was supplying which engines something that on Modern aircraft is heavily automated but ful manual on the dc6 each of the four engines had a corresponding fuel source selector which could be set to main alternate or off now this was pretty straightforward if it was set to main it would draw fuel from its main tank and if it's set to alternate it would then draw from its alternate tank but there were also two

5:20 cross feed levers one for the right wing and one for the left wing which allowed the pilots to feed multiple engines from the same fuel tank normally the cross feed levers were in the off position but if the pilot wanted to run both of the left engin of a common source they could move the left cross feed lever to the on engines one and two position or do the same with the right engines moving the right cross feed lever to on engines 3 and four now let's say for the purposes of an example that the pilots wanted to run engines three and four off of the number three main tank in order to do that the pilot would set the right cross withed lever to on engines three and four set the number three fuel source selected to Main and then set the number four fuel source selected to off this would then

6:08 cause the number four engine to receive fuel from the number three main tank instead of either of its own tanks every fuel tank also contained a boost pump whose purpose was just to push fuel out of that tank and towards the engine in order to get the fuel flow started now as far as my team and I have been able to gather the most common fuel management procedure on the dc6 was to run each engine of its own main fuel tank during the climb and then switch each engine over to its alternate tank at the start of the cruise and then to switch back to the main tanks once the alternate tanks were empty so this then begs the question when would the pilot ever use the cross feed system well as it turns out the inboard and outboard tanks weren't the same size

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7:41 it's also a powerful shield for your online privacy whenever I am using public Wi-Fi at hotels airports or cafes nordvpn keeps my connection secure and their threat protection tool blocks malware fishing attempts and even those annoying trackers trying to follow my every click right now Nord has a fantastic offer for you my viewers sign up for their 2-year plan using the link here below which is nordvpn.com pilot and you'll get four extra month for free plus with their 30-day money back guarantee there is actually no risk in trying them out thank you n now back to the story on the version of the dc6 used on flight 608 the outboard alternate tanks were actually bigger than the inboard alternate tanks and what this

8:28 meant was that when all engines were run from their alternate tanks during Cruise then assuming that all of the tanks were full to start with the number two and number three alternate tanks would run out of fuel before the number one the number four would So that obviously wasn't great now the final report on this accident is really light on the details about the fuel system configuration and procedures this is actually one of the main differences to Modern report so I'm going to have to extrapolate a little bit here from a Manual of an airline that actually is still operating the dc6 today at that Airline at least the procedure for this dc6 version is to run all engines of their main tanks until 450 lbs have been used and then switch over to running all engines of the

9:15 inboard alternate tanks using the cross feeds after an unspecified amount of time Pilots then switched the running engine onto each of its alternate tanks with a cross feed closed presumably until the smaller in board tanks run low on fuel and at that point the procedure calls for the crew to run all engines of the outboard alternate tanks using the cross feeds again until those tanks run low as well at which point they would then switch back to the main tanks now if all of that sounds very very complicated it really is and this procedure requires careful monitoring of the fuel levels to make sure that the smaller tanks aren't allowed to run completely empty which could cause an

10:00 engine flameout now the reason that the fuel tanks had different quantities had to do with their positioning within the wings and it's a quite common feature on 4 engine airliners managing the resulting challenges was normally the duty of the flight engineer but the dc6 was one of the first airliners that had been constructed with a two pilot cockpit instead of three this issue with complicated crossfeeding can be resolved through the automatic transfer of fuel from the larger tanks into the smaller ones but Douglas hadn't designed the dc6 with that capability either or had they according to the accident report it actually turns out that fuel transfer between tanks was regularly taking place on the dc6 fleet as well as on its

10:47 related predecessor DC4 Douglas stated that it had never intended for fuel to be transferred between tanks on either of these aircraft and neither Douglas nor any airlines had ever published a procedure for doing so but they hadn't specifically forbid it either and doing fuel transfers on those aircraft wasn't only possible it was actually incredibly easy for example to transfer fuel from the larer number four alternate tank to the smaller number three alternate tank all the pilots had to do was to set the right cross feed lever to on engines three and four set the fuel source selector for both engines three and four to alternate and turn on the boo pump in the alternate tank number four the only

11:32 difference between this procedure and the procedure for running both engines of alternate tank number four was that the number three fuel source selector was set to alternate instead of off this setting kept a fuel line open between the number three alternate tank and the number three engine but with the number three alternate boost pump off no fuel would actually flow from the tank to the engine instead with number four alternate boost pump turned on fuel would now flow from the the number four alternate tank not only to engin three and four but also back down the open fuel feed line from engine number three into the number three alternate tank this procedure was common knowledge among DC4 pilots and when the dc6 sented service the procedure had been passed on to that aircraft as well now according

12:19 to a first officer who regularly flew with the captain of flight 608 that Captain regularly used a fuel schedule involving a fuel transfer which he described in the following way after running each engine of its main tank during climb the captain would normally switch all engines to run off their respective alternate tanks the smaller number two and three alternate tanks would then run low and fuel faster so at some point before the quantity in those tanks fell below 500b he would transfer fuel over from the larger number one and number four alternate tanks to equalize the levels and then switch back to running each engine of its alternate tank until they were all empty now the final report didn't really

13:04 explain why he would do this but we think that this was likely a workload reducing procedure which when you hear about an experienced Captain modifying procedures is usually the case as I mentioned earlier the official fuel scheduled required close monitoring and the timing of the configuration changes was pretty inflexible but by contrast equalizing the fuel levels could be done at any time whenever it was convenient because the exact fuel level remaining at the start of the procedure didn't matter very much but what most dc6 Crews didn't know was that transferring fuel in this way actually came with a huge hidden risk connected to the fuel tank ventilation system you see all fuel tanks have to

13:50 have pressure relief vents to allow the pressure inside of the tank to equalize with the outside pressure as the aircraft climbs and on the 6 these relief vents had a dual purpose which was to make sure that the pressure inside of the tanks didn't get too low either you see the dc6 had suction based fuel feed systems which could cause the tank to collapse in on itself if the fuel level was too low unless pressure was added to it so in order to solve that Douglas had decided to locate the tank vents in places where aerodynamic pressure in Flight would force air into the tank if the internal pressure dropped too low these vents were therefore placed near the edge of the wing and because the inboard alternate tanks were mostly

14:35 inside of the fuselage itself the vents for those tanks were on the underside near the wing route Leading Edge now as the dc6 was originally designed there was no valve regulating air flow from inside to outside the tank only the other way around but this pretty quickly resulted in reports of fuel slushing out through the tank vents whenever the pilots made sharp turns while the aircraft was taxiing with full tanks as a result of that Douglas added a valve on the tank vents that would only open once the pressure differential between the inside and outside of the tank exceeded 2.5 lb and because they made this modification Douglas was obviously aware that fuel could sometimes escape from the tank vents which was a potential concern given that the entire

15:23 tank ventilation system was certified under the assumption that this could never happen certification regulations adopted by the civil aviation Authority the predecessor to the FAA require that quote Vents and drainage shall not terminate at points where the discharge of fuel from the vent Outlet would constitute a fire hazard but Douglas hadn't actually tested whether fuel escaping from the tank vent would create a fire hazard which they and the CAA claimed was because the possibility of fuel escaping from the vents had not been envisioned H anyway the ca certified the location of the tank vents as safe based on this assumption which was to be perfectly

16:09 honest here kind of ridiculous to start with and unsurprisingly also turned out to be completely wrong in reality it was actually very simple to cause fuel to overflow from any of the tank vents while transferring fuel between the different tanks but because fuel transfer wasn't an intended feature of the the e6's design even though it was really easy to do as I've just shown you there wasn't any kind of automatic shut off to prevent more fuel from being pumped into a tank that was already full and if the Boost pump kept pumping in more fuel after that well then the fuel would just start to escape through that relief vent in fact the only assurance against this happening was that the pilot would remember to turn the Boost

16:55 pump off before the target fuel tank was full investigator later found that if the pilot of flight 608 had started transferring Fuel and then forgot about it the excess fuel would soon start to pour out through the tank vent of an astonishing rate of 47 l or 12.5 gallons per minute and this was obviously a huge problem by itself but it gets even worse a few leak this size from most of the tank vents would just fall back relatively harmlessly into the slipstream and disappear out into the there but do you remember that inboard alternate tank vent close to the wing route that I mentioned before it turns out that when those went leaked the fuel would flow after along

17:40 the exterior of the fuse Lush instead and then directly into the combustion air intake for the cabin heating system you see air conditioning on the dc6 was accomplished using a gasoline powered heater and it required a constant flow of air so Douglas had included a scoop on the out outside of the fuselage near the heater's location in the tail in order to divert some air from the outside Airstream into that combustion chamber and it just so happened that this intake scoop was almost directly in line with the pressure relief vent from alternate tank number three so if fuel were to escape from this vent and flow into the cabin heater intake scoop that fuel would then go directly into the he's combustion

18:25 chamber making the fuel air mix to Rich and therefore causing the heater to backfire violently this backfiring could then Set Fire to the walls of the inlet chamber and could even cause the combustion chamber to completely explode now this was obviously an extremely serious flaw that didn't comply with the caa's regulation regarding tank vents to terminate somewhere that wasn't going to constitute a fire hazard but because Douglas had finagled its way out of demonstrating the vent's compliance this issue wasn't discovered or fixed back on United flight 608 the pilots obviously knew nothing about any of this back in those days there were no flight recorders yet so we don't know

19:12 exactly what the pilots were doing as the flight proceeded over Utah but we can take an educated guess most probably the captain followed his own usual improvised fuel schedule and after running the engines from the alternate tanks for a while he then decided to eat equalize the fuel levels as I mentioned earlier he normally made sure to do this before the fuel quantity in the inboard alternate tanks dropped below 500 lb and according to my team's calculations if he started the fuel transfer earlier than that with at least 950 lb still remaining in the inboard alternate tanks well then there would have still been enough fuel left in the outboard alternate tanks to completely overflow the inboard tanks if the transfer was

19:57 forgotten now again we obviously don't know exactly what happened here but the crew probably became distracted with something after starting the fuel transfer process and they therefore forgot to close the cross feed between engines three and four and turn off the number four alternate boost pump after the fuel levels had been equalized and as a result the Boost pump just kept pushing more fuel from the number four alternate tank into the number three until that tank overflowed and fuel started gushing out from the vent that fuel would have then gone straight into the cabin heater in Tech scoop which soon started a fire at the time this happened the aircraft was somewhere over a remote area of Southwestern Utah with an estimated arrival time over

20:43 Bryce Canyon National Park at 1222 but at 12:21 the flight crew reported to a dispatch that they had a fire in the baggage compartment which they were unable to extinguish the pilot reported that they were going to make an emergency landing at Bryce Canyon airport which was a remote field serving the national park and a few moments later they also added that the tail is going out meaning that they were having issues with pitch control now most probably the crew thought that they had a baggage fire because the baggage compartment was the first place the fire spread to that was equipped with a smoke alarm however in reality the fire at this point was actually spreading throughout the tail section and forward towards the center Wing section along the on the side of

21:29 the fuselage this meant that debris soon started falling from the plane while it was still 26 miles away from Bryce Canyon airport and when they were about 20 M out Witnesses on the ground could see smoke pouring from the plane by 15 Mi the fire was also visible on the outside of the aircraft and around that time the fire had spread far enough forward to reach the DC 6's Deployable emergency landing flare which we used to Signal an emergency without the use of a radio those barium nitrate flares were stored in a compartment in the lower part of the right wing route and when they ignited they massively accelerated The Blaze and created a glow that must have been visible for

22:15 miles and as a result the fire also started to weaken the airplane structure in this area and a number of large items soon fell down from the plane including the trailing Edge right-wing flap the main cabin entrance Door the alcohol Supply tank for the alcohol injected engines and an ice box from the galley at this point the fire had also likely spread inside of the cabin with such intensity that investigators believed that all of the passengers probably perish while the airplane was still in the air but the pilots were somewhat shielded inside of the cockpit and at 12:27 they transmitted uh we may make it approaching a strip unfortunately that was the very last anyone ever heard from flight

23:02 608 at time 12:29 within sight of Bryce Canyon airport the fire finally destroyed the control cables connecting the Pilot's controls to the elevator in the tail which caused a complete loss of pitch control the airplane then entered a dive from a low altitude and crashed into a plateau finally killing everyone that was still left alive on board now as tragic as this accident was it sadly still wasn't the end of this story investigators from the Civil Aeronautics board were only just beginning their inquiry into the crash of United flight 608 when on November 11th Less Than 3 weeks later an American Airlines dc6 made an emergency landing in Gallup New Mexico also with a fire on

23:48 board and in that case although the fire destroyed the aircraft all of the passengers and crew managed to evacuate without any loss of life but examination of the wreckages soon showed that both the crash of Flight 608 and the gallop fire had been caused by Fuel overflowing from the number three alternate tank V into the cabin heating system this prompted Douglas to ground every dc6 In America which at the time also included Harry Truman's presidential plane the findings also caused Douglas the CA and the cab to hash out a massive modification program and complete recertification a of the dc6 in order to improve the safety of its design now we don't really know all

24:34 of the exact details of the modifications in this program because they were not fully described in the report but we do know that the inboard alternate fuel tank vents were moved guards were placed over the fuel boost pump switches check valves were added to prevent transfer of fuel between tanks smoke detectors were added in the air conditioning compartment and fireproof shrouds were play bed around the emergency landing flares available documents also describe unspecified but extensive modifications to the electrical system air condition system and finally the fire protection system but what nobody knew was that this modification program had now introduced a new design flaw one that would soon claim even more lives and

25:21 this new issue involved that revamped fire extinguishing system precise information about how the system was modified just isn't available but it does look like Douglas increased the number of fire extinguishes in the under floor compartment possibly by a factor of two now this change meant that Douglas had to redemonstrated the system's compliance with a CA certification requirement ensuring that passengers and crew wouldn't be exposed to hazardous quantities of extinguishing agent the dc6 had previously met this requirement when it was originally certified in early 1947 but with the beefed up Banks of extinguishers the ca now required new flight tests to prove that the original protections still held up which kind of makes sense the dc6 is

26:09 fire extinguishers were all using carbon dioxide which Smothers the fire by displacing the oxygen but obviously that would also smother human beings if the concentration would be too high and since no compartment on the dc6 was completely airtight activating the extinguishers below the floor would cause some of the carbon dioxide to also enter the passenger cabin and cockpit where it could incapacitate the occupants and this was obviously a especially significant concern given that Douglas had now just doubled the amount of CO2 that could be released in fact during an early test flight when the new system was just installed Douglas test pilots activated the system and nearly got themselves killed they

26:56 started the test while they were in a descent and large quantities of CO2 then entered the cabin and because CO2 is heavier than air it then flowed down into the cockpit due to the nose down pitch of the aircraft the concentration became so great that it partially incapacitated the flight crew as well as representatives of the CAA and the pilots Union reps who were also present on board now fortunately in that incident they eventually managed to recover but after that incident the ca made Douglas install a cabin pressure relief valve that would allow the pilots to increase the rate of air flow out of the cabin and prevent the CO2 from reaching hazardous concentrations but in 1948 no one was completely sure

27:42 what a safe level of carbon dioxide was so Douglas hired a medical expert to review the latest research and make a recommendation to them this expert concluded that CO2 concentrations of 5% was an appropriate upper limit for a limited time but he also warned that the tests that Douglas had conducted were insufficient to prove that this limit wouldn't be exceeded on top of that Douglas hadn't tested the new relief valve in all possible descent configurations and the expert was worried that if a slower speed descent was done using the landing gear and the flaps the cabin air wouldn't be drawn out as quickly as it would in a highspeed descent potentially allowing a harmful CO2 buildup even with

28:28 the relief valve open the expert ended his report by urging Douglas to correct this issue to avoid embarrassment at a later date and to share these findings with the CIA but according to later court records Douglas decided to bury this report and the company's Chief pilot even wrote do not share with the CIA on the front cover as a result the ca approved the new Fire extinguishing system and the relief valve without any further testing but the lack of testing wasn't the only issue with Douglas's new solution the problem was that opening the relief valve required a separate action by the pilot and it had to be accomplished before activating the extinguishers making the procedure overly complicated

29:14 in a very high stress situation the cab would later point this out in the report explaining that humans were extra prone to mistakes when they were under stress and panic and this meant that even as early as back in 1948 the industry understood that activating emergency systems should be as simple as possible probably representing one of the first real shows of understanding of human factors in accident investigations anyway what this all meant was that in this case the only assurance that the system would work correctly was that the pilots would exactly follow the procedures in the manual and to make things even worse United Airlines version of the manual included no information about why opening the relief valve first was so

30:03 important so you can probably see where this is going and actually some people at the time did too in April of 1948 shortly after the modified dc6 had return to service the airline pilots Association wrote a letter urging the CIA to require fullface smoke prooof oxygen masks in part due to the risk of incapacitation by fire extinguishing agents back in the Pilots oxygen masks were of the rebreather type which mixes oxygen with the surrounding air instead of supplying pure oxygen much like passenger oxygen masks do today but this meant that these masks were only really useful in a depressurization event and didn't protect against smoke or fumes the union clearly saw this as

30:49 insufficient and the ca agreed to study the possibility of implementing smoke proof oxygen masks like the kind that we have today but that study was still ongoing when events escalated further on May the 13th 1948 a tww constellation made an emergency landing in Missouri after the pilots were partially incapacitated by CO2 from their fire extinguishers during the investigation into that incident a test flight nearly met the same fate which prompted the Air transport Association and Industry Group to send out an urgent telegram on June the 10th to several Airlines descri describing the problem on the constellation and warning that they had reason to believe that a similar incident could happen on the dc6 as well

31:36 but the ATA hadn't consulted Douglas before issuing that warning and Douglas responded quite angrily on June the 15th with a telegram of its own which reminded that the dc6 had passed all smoke and fume tests with entirely satisfactory results and Douglas therefore demanded that the ATA must withdraw it warning which the ATA also did only 2 days later on June the 17th 1948 uh United Airlines dc6 operating as flight 624 departed Chicago for New York's LaGuardia Airport with 39 passengers and four crew on board in command were a 35-year-old captain and a 26-year-old first officer who had a

32:22 combined experience of more than 10,000 hours but much like the crew of flight 608 they were very new to the dc6 in fact the captain had only 30 hours on the type and it's virtually certain that he didn't know the full reasoning behind the newly added emergency cabin pressure relief valve anyway after climbing to its Cruise altitude of 17,000 ft the flight proceeded normally and the captain reported that time 11:55 that the plane was in good working order but that time 12:31 somewhere over Eastern p pen Sylvania flight 624 suddenly transmitted a frantic and garble distress call in which the pilot said that they had received a fire alarm in the baggage compartment that they

33:07 were activating the extinguishers and that they were now in an emergency descent the tone of voice in the transmission was very agitated and it was clear that the pilots were under great stress as I can definitely understand unfortunately In the Heat of the Moment they likely therefore forgot to open the cabin relief valve before activating the fire extinguishers and as a result carbon dioxide pulled into the cockpit during the emergency descent causing the partial or total incapacitation of the pilots Witnesses observed the plane flying erratically for around 10 minutes over flying Sunbury airport at 4,000 ft without attempting an emergency landing before descending towards a mountainous area the aircraft came within 500 to

33:54 1,000 ft of the ground near Shamokin before enter a shallow left turn towards even higher terrain where it was seen flying only 200 ft above the ground near the town of Mount carel the aircraft then turned abruptly right and climbed sharply but at that time it was too late it soon clipped a couple of high voltage power lines and then crashed into a forested mountain side completely destroying the aircraft and killing all 43 passengers and crew now after the accident the cab determined that the crew must have been incapacitated since they flew for 10 minutes without making any further Radio Calls overflying several suitable emergency landing sites and maneuvering towards High terrain that should have

34:40 been clearly visible to them normal functions of any aircraft systems were found except for the baggage fire detector which had unfortunately issued a false alarm something that was far too common back then in its final report the cab criticized Douglas aircraft for designing an emergency system that was too complex to be reliable in a real emergency and for downplaying the risk of CO2 incapacitation on the dc6 they also criticized Douglas for failing to demonstrate that its safeguards were effective in all the scent configurations after testing later proved that the medical expert was indeed right about the risk of CO2 contamination even with the relief valve open in a high drag descent the fire

35:26 extinguishing system was then redesigned so that the cabin relief valve and the extinguishers could be activated using a single switch and Douglas increased the number of vents to ensure that CO2 was properly evacuated in every possible configuration on top of that the CAA also mandated fullface fume proof oxygen masks for all dc6 Pilots which has since become the standard throughout the industry now Douglas ultimately ended up losing a court case filed by the victim's families and was forced to pay a record penalty after the defense lawyers had been caught trying to erase the words do not discuss with CAA from the cover of the medical report before submitting it as evidence and this wouldn't be the last

36:13 time that the Douglas company or later McDonald Douglas would find themselves in hot water because of a partially covered up design flaw as you can actually see in these videos that I recommend you see next now if you want to enjoy my next video before anyone else then please join my patreon crew and we can then discuss any questions that you might have in our next Zoom hangout my name is Peter hornfeld you're watching Mentor pilot have an absolutely fantastic day and I'll see you next time bye-bye