Ep 27 de Havilland Comet
Engineering News – Surfside Condominium Building Collapse in Miami Florida on June 24, 2021. (1:35)
In this week's episode, I am joined by Brian, a former pilot, to talk about the de Havilland Comet (6:30). While the Comet had some wins (14:45), it also suffered significant losses. The first fatal crash occurred after take-off from Karachi, Pakistan on March 3, 1953 (19:45), followed by another crash on May 2, 1953, 6 minutes after take-off from Calcutta, India (20:35). Then in 1954, Yoke Peter crashed on Jan 10th (25:55) and Yoke Yoke crashed on April 8th (29:30), both about 20 minutes after takeoff from Rome.
Engineering News - Surfside Condo Collapse
de Havilland Comet
Nicole: Hi and welcome to Failurology, a podcast about engineering failures. I'm your host Nicole and I'm from Calgary Alberta. Joining me today is Brian, he's a former commercial pilot with over 10 years of experience in geomatics engineering and project management. He holds a diploma in aviation from Mount Royal College in Calgary, now mount royal university as well as a bachelor of science in Geomatics Engineering from the University of Calgary. Welcome, Brian.
Brian: It's my pleasure to be a part of Failurology today.
Nicole: Brian, what's geomatics engineering? It's not something I've heard a lot about.
Brian: Geomatics Engineering is the engineering discipline that deals with the acquisition, modelling, analysis and management positions above on and below the surface. So Google maps and GPS navigation are built on and from geomatics engineering principles.
Nicole: Fascinating. If you've been listening to Failurology, which I hope you have, today's episode is going to sound a little bit different. I'm trying some new things so let me know how you like it.
Brian: This week's episode of Failurology. is brought to you by the Sit Down Stand Up Paddleboard Company.
Nicole: Whether you like to sit down or stand up,
Brian: the Sit Down Stand Up Paddleboard Company has something for you.
Nicole: Don't miss our paddle sale, it's quite the oar-deal.
This week's engineering news is about Champlain towers in Miami Florida. At around 1:00 AM eastern time on June 24 this year the 12 storeys beachfront condominium Surfside Florida, built in the 1980s partially collapsed. Looking at the initial photos it looks like only some balconies collapsed, but I found a video that was captured by a nearby surveillance camera and the section that collapsed was much larger. 55 out of 136 total units were destroyed. And I imagine the remainder of the building has been evacuated and will likely be deemed condemned. As of this recording, at least 5 people were killed, 11 more were injured and 99 people remain missing. While it is possible that the missing people have been presumed dead, one of the people rescued from the Sampoong department store collapse we covered in episode 14 was discovered after 16 days. So I personally have not given up hope yet.
Nicole: Same here. 16 days is crazy. That woman drank rainwater that had dropped down through the structure to stay alive, which is insane I can't imagine.
Brian, I mean just the boredom too right. 16 days of just being trapped. No cellphone or no tv.
Nicole: And you can't move so all of your muscles are probably super-stiff, I mean that's the least of your worries though right. It's way too early to tell what the cause of this failure is, but it's also extremely on-brand for the show and I just had to, I mean I've been doing so much research and I just had to talk about it. And the lovely people of Reddit have definitely helped me. So I'm just gonna share some of the information that I found. I think almost all of this came from the subreddit r/engineering so thank you to you out there if you're on r/engineering. So there was a structural field survey from 2018 that talked about some issues with waterproofing below the pool deck, entrance drive, and planter wall. The report basically says that waterproofing is beyond its useful life and must be completely removed and replaced. One of the significant problems with that is the waterproofing membrane is underneath a concrete topping so it's very very disruptive to access. It's causing major structural damage to the concrete slab below the waterproofing layer. The report also notes that the waterproofing is laid on a flat surface so the water sits on the waterproofing until it evaporates. This was noted as a major error in the original design within that report. The report also notes that there's abundant cracking and spalling of varying degrees observed on the concrete columns, beams and walls within the parking structure and lower levels of the building. And in some cases, the rebar was exposed to and deteriorating. There were some concrete repairs that had taken place but they don't seem to have worked and they were causing additional cracking. I also came across some emails that were released by the city. The emails appear to be written from a member of the building's that collapsed board and they're related to the adjacent property; which states that that property is quote digging too close to our property and we have concerns regarding the structure of our building end quote. Now, this building is located, apparently located in another jurisdiction which is really interesting and unfortunate. And so the city wasn't super, didn't appear to be super helpful. At least from the emails that are released. I mean I didn't read them all there's a lot of pages to go through, but I did kind of skim them. And then there are a couple more things so, from a few of the surveillance footage which you may have seen, the people that have reviewed are suggesting that the failure occurred near the base of the structure almost like the legs were kicked out from under it. But also important to note is that Florida is very unstable ground, there have been numerous sinkholes, there are entire communities basically that are stinking. So the foundation could be playing a factor or it could simply be that the structure was under-designed, the rebar was corroded, or the concrete and steel they used was substandard. These are all just speculation, it's far too early to tell but there's a lot of people watching. This failure has been noted as potentially the worst structural failure of an occupied building in U. S. history and I mean based on the ones I researched I would have to agree with that. So we'll definitely continue to follow this story. It'll probably be a little while before we know exactly what happened and depending on whether or not this goes to court and those documents are made public we may never know, I hope that's not the case, but I'm sure the people of Reddit will keep us informed because they always do. So if you wanna read more on the collapse we're gonna put a couple of links on the website page for this episode you can head out head-on over to failurology.ca if you wanna check those out.
Brian: Now on to this week's engineering failure, the de Havilland Comet. Which was the world's first commercial jet airliner. Brian, I don't know about you but I kept singing the Steve Miller Band song when I was researching this.
Brian: I did not sing the Steve Miller Band song when I was researching this. In fact, I didn't even think of it until you brought it up right now.
Nicole: yeah well I couldn't get it out of my head, I don't know why. Big old jet airliner.
Brian: And now it's gonna be stuck in my head.
Nicole: Sorry about that. So the Comet was developed and manufactured by the de Havilland aircraft company at Hatfield Aerodrome in Hertfordshire U. K. which at the time was a leading UK aircraft designer and manufacturer. At the end of World War 2, the U. K. wanted a trans-Atlantic mail plane with a pressurized cabin that could carry 1000 kilograms, travel at a speed of 640 kilometres per hour, nonstop across the ocean, at an elevation of 12,000 meters. This was a risky design, nearly all of the design elements were untried at the time and there was also a huge financial commitment that de Havilland took to undertake this project. There were no jets like this at the time, this was literally the world's first jet airliner, which is crazy to think about because now they're everywhere and we fly in them all the time. So some of the hallmark designs were a pressurized cabin, large square windows and four ghost turbojet engines hidden in the wing. Brian, I wanted to ask you, the ghost turbojet engines, are they called ghosts because they're hidden in the wings? Because you don't see that anymore now you see the engines hung below the wings and so I'm just curious, I wasn't able to confirm if that's what the ghost part of the name meant.
Brian: yeah I couldn't find anything either way that was the reason that they were named ghost turbine engines or if the ghost was just the name that they had for the design, but either way, it just referred to now as a ghost engine. It's hidden from sight, it's not immediately visible, it's essentially inside the wing.
Nicole: Why did we use those then and not use them now? They seem like a pretty cool streamlined design.
Brian: Yes, so it definitely helped a lot with streamlining of the airplane because there wasn't a lot of drag with the engine hanging below the wing. But it made maintenance on these engines incredibly difficult. There were a lot of panels that they had to take apart and it was just a difficult area to access. One of the advantages of having the engines inside of the wing is that there was less of a risk picking up foreign object debris or debris that's on the runway or taxiway surface the airplanes are landing or taking off and ingesting it into the engine. So there was a design consideration that went into this.
Nicole: Yeah I think you mentioned as we were talking about this failure before and something I hadn't considered, you know today all, I mean all the runways I've ever been on have been paved. But that might not have been the case in the fifties.
Brian: Yes certainly some of the airports that the Comet may have operated off of could have been a gravel runway. Most of them were likely paved surfaces, but at the same time, they were just trying to minimize as much as they could any chance of ingesting foreign object debris. So by putting the engine inside of the wing, it did give them an extra couple feet of clearance above the ground from what we have now.
Nicole: Interesting. So the comets flight crew consisted of two pilots, a flight engineer and a navigator. Brian, I'm so glad you're here because planes are, it's like learning a whole new language and it's not my forte I will say, so thank you for being here and answering all my questions because my next question is what is a flight engineer? Is that like what we would think of traditionally as professional engineer P. Eng or are they more like a train engineer?
Brian: I'm gonna say they're more similar to the train engineer if we're trying to equate them to an engineering discipline of today. So the flight engineer would be responsible for systems operations and engine operations in the airplane, so you'd be dealing with the hydraulic system, fuel system and managing engine loads, and the electrical system. So it was somebody who had a really really good, or he or she, had a really good understanding of all of the systems that went into the airplane. Oftentimes these would be people that have a flying background or an aviation background so they would also be commercial pilots or have flown other aircraft, so they had an ability to understand what was going on with the aircraft.
Nicole: Interesting. That's all automated now right? we don't have flight engineers anymore?
Brian: Yes so in modern aviation now a lot of that has been, like you said, automated out. That role doesn't exist. There are some airplanes that are still operating, usually not in North America, that have a flight engineer position. But through systems automation and just advances in technology, that position has essentially ceased to exist.
Nicole: So the original Comet was the same length as a Boeing 737 but not quite as wide. It also carried fewer people because it was much less spacious you know this was the forties and fifties and I think partially they weren't sure how many people really fit on a plane and also we haven't had decades of time and energy spent into condensing everyone into the smallest possible space that they purchase to fly.
Brian: Yeah this is still over the golden era of air travel. Kind of the luxury of travelling on an airliner, this wasn't something that you just did because you had to go to a business meeting. These were the elites of the world. This is how the elites would travel. So it was very important, yeah I think, it's great people dressed up to go on flights in suits and fancy dresses. Yeah unfortunately that doesn't seem to happen a lot right now, I think the last flight I was on last year, before COVID, I believe I was wearing sweatpants. So not to the same level of people back in the early fifties.
Nicole: They also had a galley for hot cold food and drinks, a bar, and separate men and women's washrooms; which is also crazy, so many amenities.
Brian: Yeah it was very similar to cruise ships. A lot of early air travel was based on cruise ships design.
Nicole: Yeah okay that makes a lot of sense. So a few more things about the Comet, it flew at a faster speed and it climbed faster than the traditional piston-engine aircraft at the time, it was also relatively quite uncomfortable for the time. I in my non-aviation brain kinda liken this to the difference between flying on a Dash 8 versus a 737. So the Dash 8 are the, what I call the 2 by 2 rows, the smaller planes that the puddle jumpers as we say, you know those plans are very loud. I usually have my headphones turned all the way up to be able to hear what I'm listening to. And then you know you compare that to the 737s which you know have 3 seats on either side of the aisle, that's how I tell the difference how many seats are there, how wide is this plane. It was just the 737s quite a bit quieter, it's got a lot more room, it's quite a bit nicer. But you know I kind of like the Dash 8s, they're kind of cozy.
Brian: Dash 8s are great airplanes. I'm a big fan of Dash 8s and their Canadian built, so that's a point right there for them.
Nicole: Oh even better. I think too my brain knows that when I'm on a Dash 8 I'm not going very far so I don't have to sit on the plane for very long. I'm really built for 90-minute flights max, anything more than that's just too long.
Brian: Hours in a Dash 8 is not fun; I've done that a number of times. And it's not the most pleasant flying experience.
Nicole: Yeah no thank you. Some other uncommon design elements at the time were swept wing leading edge, integral wing fuel tanks, and 4 wheels bogie main undercarriage; which I think means that the landing gear was four-wheeled instead of two.
Brian: That's correct yeah it would have four wheels.
Nicole: The fuel tanks in the wings, that seems weird to me. Is that normal? Do we still do that?
Brian: Yeah we definitely still store fuel in the wing tanks and lots of aircraft actually store fuel in the tail. There are lots of spots where fuel will get stored in the aircraft and during flight, so in the flight engineer position or in the automation now we can actually move fuel around between the wings and the belly tanks and the tail. So we can move it around for the weight and balance proposed if we need to. So the integral fuel tanks within the wings, yeah that's still a common thing that we do now in aircraft design.
Nicole: Before we get into the bad stuff, do you want to tell us about some of the successes of the Comet? Make us, you know build us up, before we tear ourselves down.
Brian: Yes so like Nicole mentioned the de Havilland Comet was the first modern jet engine, commercial passenger airliner. Prior to that, there were a lot of two-engine and four-engine piston-powered aircraft. They weren't pressurized, they flew fairly slowly, they were noisy, they couldn't climb above the weather. So that was one of the big things that the Comet was designed for. To fly above the weather, to fly faster and to be able to climb a lot quicker about the weather. So the Comet was typically about 50 percent faster than piston-engine aircraft of the same era. British Overseas Airways Corporation, the launch customer for the Comet, planned a nine-stop flight from London to Tokyo that would take 36 hours and some of the competitor aircraft would complete the flight in 46 or 47 or 48 hours. So even though it was 9 stops to get there, you still saved 10 hours on your trip. And just looking at that too, it seems ridiculous that 36 hours was seen as an incredibly quick flight to get from London to Tokyo. Now it's probably gonna be an 11-hour flight. I haven't done this flight. Nicole, have you done this London to Tokyo flight before?
Nicole: No I haven't but you can fly from Calgary to Tokyo. I think it's 14 hours one way obviously but you obviously go the other way, you go over the Pacific.
Brian: But either way it's like London to Tokyo or Canada to Tokyo, it's gonna be much shorter than 36 hours.
Nicole: It's also weird to think about the fact that we have all these nonstop long-distance flights right. I mean the planes can fly over 10 hours nonstop. It's crazy to think you would have to stop 9 times.
Brian: That's essentially every 4 hours they were stopping for fuel, to get to their destination. yeah, and I think this is part of travel back then you just stopped a whole bunch of times on the way to your destination. It was part of the experience, I guess similar to a train trip now where you stop every few hours in the city to load people and unload people.
Nicole: So was that the biggest limiting factor between how many stops was the length of travel before it ran out of fuel?
Brian: Yeah fuel is definitely a driver of how long you fly between each point.
Nicole: And how, so is it that they found better ways to store the fuel, the fuel improved, or the engine efficiency improved or is it a combination?
Brian: It's gonna be, I'm gonna say 99 percent engine efficiency improvement. Just the amount of horsepower that modern turbojet engines and turbofan engines, especially high bypass turbofan engines that are on transatlantic aircraft now so the Boeing 777 or the Airbus A350. Those just have incredible efficiencies and we're just able to store more fuel on airplanes now just because we have so much more room on airplanes than the size of the Comet. And so in its first year of service, the Comet carried 30,000 passengers including some members of the royal family. so for the first year, it was phenomenally successful. It was the creme de la creme of air travel.
Nicole: Even with those successes and the challenges of designing a whole new kind of plane the Comet did have some serious problems. I mean we wouldn't be here if it did. There were overall 13 fatal crashes, 426 fatalities and 26 hull loss accidents. Brian, what is the hull loss accident?
Brian: A hull loss accident happens when the insurance company deems that it costs more money to fix the airplane than what it's worth; so it's the same thing as writing off your car in an accident. Yeah and in total there were only, I believe just over 100 airframes of the Comet produced. So they had a fairly substantial number, percentage-wise, of airplanes that were lost.
Nicole: Yeah I read so I read there are 114 of the Comets built, including the prototypes. Do you know how many 737s there are? I'm curious just comparison-wise.
Brian: There are over 10,000 737s that have been built since 737 went into production. So substantially more 737s than de Havilland Comets out there.
Nicole: 114 verses over 10,000, big difference. There were 3 catastrophic in-flight breakups of the comet in the first 12 months of travel. investigations took place and it did take a while for all this all to flush itself out but in the end, they determined that two of the failures were caused by metal fatigue in the frame and the other was caused by overstressing the frame due to severe weather. Following these investigations they determined that the design flaws were related to improper riveting, dangerous concentration of stress around those square windows. In the future prototypes, the windows were revised to oval and the structure was significantly reinforced. and we're gonna get into all of that as we talk about the failures. so Brian , do you want to talk about the first accident that occurred?
Brian: Yes the first accident occurred with the Comet, actually occurred with an airframe that was supposed to go to Canadian Pacific airlines. and it wound up crashing in Karachi Pakistan and the investigation into that blamed pilot error. they failed to become airborne while attempting a night takeoff, they'd over rotated the aircraft, the angle of attack was too high, they suffered a loss of lift and ultimately the aircraft went into a dry drainage canal and it collided with an embankment. Unfortunately, all 5 crew and 6 passengers on board were killed. so in response to that, Canadian Pacific cancelled its remaining order for a second Comet and never operated the type in commercial air service in Canada.
Nicole: Do you know when that Karachi failure occurred?
Brian: That was in 1953, so March 3 of 1953 is when the first accident of the Comet occurred
Nicole: Okay then just about 2 months later on May 2nd, 1953, another crash occurred. so this is the weather one. This one occurred after the Comet took off from the airport in Calcutta, India on route to Delhi as part of a Singapore to London flight. so it's one of those stops that it had made. this one occurred 6 minutes after takeoff, the airport lost radio communication with the plane. The plane, unfortunately, flew through a severe thundersquall and did not survive killing all 43 people on board. The witnesses claim that the plane was wingless and on fire when it crashed, which is...
Brian: Never a good thing to happen to an airplane. If the airplane doesn't have wings, that's not a good spot to be.
Nicole: No, and I mean imagine, you can't unsee that. The fact that the plane had lost wings led the investigators to suspect structural failure. The England ministry of civil aviation undertook an investigation in agreement with the Republic of India. They located the wreckage 24 miles from the airport in a nullah which is a dry riverbed or ravine. There were no scratches on the soft ground where the aircraft had fallen so that suggested that it came pretty much straight down, it didn't you know do any sliding across the ground to make any scuff marks. There was evidence of fire and smoke damage as well as impact damage but there were also some parts of the plane that showed no evidence of fire. and there was also no evidence that the crew had undertaken any emergency procedure. so you know I don't know what the procedures were like back then, but I have to think that if the plain, you know if the fire was the first part of the failure, they would have taken some type of procedure to address the fire or at least to protect themselves from the fire before the plane, I guess spontaneously combusted. But based on the remaining pieces of the wreckage there was no evidence of that. so the summary of findings that I read in one of the reports that I found, which is really interesting to read reports from the 1950s, they're written a little bit different than the one's today, but they're pretty detailed and direct and to the point which I like. so they said that their certificate of airworthiness and the certificate of maintenance were valid at the time of the crash, the captain had considerable experience with the route, the weight and center of gravity which deals with the weight positioning relative to the plane, those are both found to be acceptable, the captain was given all of the relevant meteorological data including the risk of the thundersquall before the plane took off, that said the plane still encountered the thundersquall while it was climbing to cruising altitude, which they believe because structural failure in the air, resulting in the fire. examination of the wreckage didn't reveal any sign of sabotage, lightning damage, faulty workmanship, defective material, or power plant failure. and they believe that the probable cause was that the plane encountered extreme negative G forces during take-off and the severe turbulence-induced downloading, and loss of the wings. After examining the cockpit, the pilot may have inadvertently over-stressed the plane while trying to pull out of a steep dive. The investigation doesn't appear to have considered metal fatigue as a contributing cause, it sounds like they understand that there was a structural failure but they kind of blame it on the pilot and what he did to the plane because of the weather.
Brian: Yeah and I think that's probably a reasonable conclusion for them to reach. This is the first real significant incident that occurs with this aircraft. They're flying into thunder squalls, there's a whole bunch of thunderstorms there. There are wings that aren't attached to the airplane so certainly, I think that the direction was correct that you know there's an overstressing in the airframe, likely from, possibly pulling out a steep dive or turbulence-related issues from the thunderstorm. so I do think that they settled on probably the correct cause or at least most likely cause based on the evidence that they had here.
Nicole: At the time yeah. In hindsight, you know, 70 years later looking back it's more probable that the structure was to blame and it was a structural failure. but yeah, at the time, based on the little evidence they had this was only really the second crash that had occurred. but even that said there were still some changes that went into place so out of this came stricter speed limits during turbulence, and the planes were equipped with weather radar, and a "Q feel" system was introduced to keep the stick forces proportional to control loads. Brian, can you explain a little more what the "Q feel" system is?
Brian: Yes so the de Havilland Comet and aircraft at that time, they had wires and pulleys and cranks that would translate those control forces to the control column or the yolk all over to the control surfaces. So with the "Q feel" system, the pilots were able to get feedback on how much pressure they were applying to those controls surfaces, they could feel it through the control column so they would know if they were pulling back too hard out of a dive, or if they were rolling the plane too hard. So it would just give them some positive feedback into the control column. Very similar to video game controllers now, where there's some feedback that you know that the control vibrates a little bit. It was kind of the same concept in the control column for the "Q feel" system.
Nicole: I think that would allow the pilots to fly it but a lot more intuitively. I've never flown a plane, I'm not qualified, but you know you get onto the airplane and you look in the cockpit and there are just buttons and dials and knobs and things. Wow, that's a lot of stuff there. so you know I think the feedback and allowing you to be a little more intuitive would probably be helpful.
Nicole: So do you want to tell us about the next crash that occurred?
Brian: Yes, we're onto the third crash now. so the third crash occurred January the 10th of 1954 about 6 or 7 months after the second crash. So 30 minutes after take off, there's another Comet travelling from Singapore to London and it broke up again in mid-air and crashed into the Mediterranean Sea. and unfortunately killed all thirty-five people on board. up until that point, the aircraft had operated just under 1300 flights before the incident. so a fairly low number of flights in terms of modern commercial aviation aircraft.
Nicole: So what is a typical flight cycle number for modern aircraft? Just an average.
Brian: Easily into, multiple thousands is not unrealistic. There are certainly aircraft out there that are into 10-20 thousand cycles. and again just through maintenance, everything gets replaced, checked, certified, so the aircraft can operate for a long time. so this 1300 or 1290 flights or cycles, is still a fairly low number. The witnesses that were present said that the airplane fell into the sea again on fire. but they were only partial radio transmissions that could be recovered from ATC, which would lead us to believe or which would suggest that whatever had happened it happened very suddenly, and obviously had very catastrophic consequences. So out of this, the Abell Committee was formed to investigate this further, this is the third incident that's happened to this airplane. The third fatal accident that's happened in less than a year. so the Abell Committee was established to figure out what was going wrong with these airplanes. On the previous couple legs that this airplane had flown, there were problems in the fuel valve on the port wing, the engine hydraulic flow warning and the automatic temperature control selector. so there are workarounds for all these, and the investigations team didn't really feel like these were contributing factors to the crash. They looked at the weather, it was also not a contributing factor. There was some minor turbulence, but it was nothing more than what would be expected or what the plane would normally encounter on these flights. From the recovery, the probable failure was the fuselage, which was above the engines, or what the wings were connected to. Autopsies that were performed on the victims of this incident revealed post mortem burns which suggested the fire or the burns to their body had occurred after the victim had passed away; which would suggest that their death occurred in the air at some point before the fire started in the wreckage. Out of this, the Abell Committee identified a number of potential causes of why this aircraft would be failing. It identified the flutter of the control surfaces, there was excessive vibration, structural failure due to high loads or metal fatigue of the wing structure, failure of power-play control, explosive decompression of the cabin, they thought this might be caused by failure of the window panels, engine installation which could have led to the fire. they had a number of causes but they couldn't pin this down to one specific cause. so initially concluded that fire was the most likely cause of this failure. They suggested a number of changes to the engine to protect against fire and failure of the turbine blades, and the wings from damage due to the fire, reinforced underneath the wing. The possibility of fatigue in the wing structure due to gusts was more likely than the fatigue of the pressure cabin. so there weren't really any changes that were made to the fuselage portion of the windows of the aircraft. Not too long after that, we have a fourth incident that happened.
Nicole: This is where it gets good, real good. okay. So on April 8th, 1954, while travelling from Rome to Cairo, another Comet crashed into the Mediterranean Sea and killed all 21 passengers on board. so the flight that Brian was just telling us about that one was nicknamed Yoke Peter; I find these names really amusing. that was Yoke Peter, this one is Yoke Yoke, which I definitely giggled a little bit.
Brian: Yoke Yoke Yoke Yoke.
Nicole: exactly. so Yoke Yoke had made 900 previous flights. so Yoke Peter had made 1300, this one had only made 900 when it crashed; which is very few. and the investigation determined that the accident occurred at roughly the same height and after approximately the same lapse of time after departure as the Yoke Peter flight.
Brian: So these sound like very similar, two very similar crashes. They both crashed into the Mediterranean, they both seem to have occurred at the same height and the same time after departure, so what happens with this, since they have two very similar incidents?
Nicole: Well actually this is really interesting, for example, you know obviously not an aviation expert, said that before, but if I was investigating this failure, now I basically have twice as much data on the same failure. And I mean as unfortunate as it was, I think it really gives them a better understanding and ability to piece together what happened because they can take parts and pieces from each accident and put them together to see one picture. and that's kind of what they did. They kind of looked at both of these. For example, the research that I found on these two failures was in the same report because they happened two months apart and they were very very similar and they were from the same location. going from Rome to Cairo. it seems like we almost kind of looked at it as one flight.
Brian: yes so this is gonna be like a 737 Max from a few years ago. there were a number of incidents in a very short time frame that were all very similar and ultimately did have the same cause of the accident
Nicole: So because these failures happened so close together and they were the third and fourth failure of the Comet within the first year of service the fleet was grounded, their certificate of airworthiness was revoked, basically the plane was not allowed to fly, and the production line was suspended. Then the Cohen Committee was formed, and they considered metal fatigue as the most likely cause of these accidents. finally someone was looking at the metal fatigue, which again hindsight's 2020, but going back and reading about this and I'm reading about the first three failures, I just keep thinking "why is no one looking at the metal fatigue, why is no one looking at this part?" Typically I mean before this they had been testing the planes with air, but air is compressible and whenever the fuselage is tested and there's a failure, it's catastrophic and it becomes really hard to continue testing. so they decided this time to test with water. They basically built a big tank from, I've got some pictures on the website, they look like a really big seacan that they stick the cabin inside of and then the wings stick out the side; which it looks kind of weird in the pictures. but by doing this they could test it over and over again, and when a piece of the cabin failed or a piece of the plane failed, it was really easily repairable, at least structurally. so that they could fix it and test it again. which was good because that would allow them to find multiple points of failure. to do this, they place the whole cabin in the tank they fill both the tank in the cabin with water simultaneously, and then they pressurize the inside of the cabin using a pump. and they can run this sequence in cycles to the same or different pressures to simulate flight cycles, take-off and landing pressures, and turbulence. so they have a lot more control when they test with water. The plane if they actually ran these tests on was another plane called the Yoke Uncle. Yoke Uncle had been flying. it had done just over 1200 actual flights. and then after 1800 simulated flights, about just over 3,000 total flights cycles between actual and simulated, the hull burst open during one of the tests. They drained the water and revealed that the failure had occurred at one of the cabin windows. so remember that the windows are square, 90-degree angles, not rounded ovally windows like we see today. and due to the algae in the water, they were able to see that the crack had endured several pressurizations before it spread catastrophically. So what that tells me is that that failure did not occur on that one single test, that crack had been growing and growing with every test that they did and that test is just the one that finally blew.
Brian: So in this case Nicole, having the hull burst open was something that was a good thing in this case right?
Nicole: Yeah, because it told them what was wrong. but not good in the air; good in the tank, not in the air. one other thing that I thought was interesting, so the tank tests were able to simulate real-life conditions to an extent, but they don't consider it a one to one life cycle for simulated to service. so the estimated that the tank failure at about 3000 flights was equivalent to closer to 2500 in practice. but you know today's flights, I would assume the average plane flies at least 4 flights a day, so that's less than 2 years of flying which is not very many. and also between Yoke Peter which failed at 1300 flights and Yoke Yoke which failed at 900 flights, that number wasn't consistent. it's almost like they didn't know where it was going to fail, just that it could feel at any moment. it was like Shroedinger's fuselage explosion.
Brian: which is a real thing, that we just made up.
Nicole: It is but also terrifying. so the Cohen Committee determined that the cause of the failure of Yoke Peter was the bursting of the pressure cabin. and once they found all the wreckage and closely examined it, they confirmed that. They even built, this is really interesting, they even built a scale model out of wood and they flew at appropriate speeds and had it made to break at the point where the failure was suspected based on the tank test. and then they watched how those wooden plane fragments landed and they determined that the way the wooden plane landed was In line with how Yoke Peter's wreckage was found. so they kind of almost reverse-engineered the crash and showed that their test matched real-life conditions and they matched the wreckage, which was really interesting. I mean this is 1954. today you could run computer simulations and finite element analysis and you could see how the entire plane would fail and at which point, and you could even see damage. So when I run those tests before, different colours will show different stresses, and so you may have a bunch of red spots which are the higher stress spots and then you'll have some green spots which aren't as stressed, at least in the program I was using. this wasn't available in 1954, they literally had a tank of water and a pump.
Brian: and slide rules, slide rules were big back in 1950
Nicole: Yeah and wood, they had made a wooden plane. so they had stuff, but not compared to today. I think it's pretty cool that they were able to figure this out. one thing they also determined. So no one had attempted to measure the stress on the skin material which is basically the outside of the plane; no one attempted to measure where it might be higher than expected. so they had calculated what the stresses would be but no one actually checked that in real-life conditions. And part of that reasoning was that it was quite a laborious process. so they essentially had to install a ton of strain gauges on the skin and then run it in a test. but based on everything that had happened and transpired, they decided now's a great time to do this; it was worth it at this point.
Brian: So Nicole with the square windows, all this stress, so would the square window kind of concentrate all this fatigue stress and fatigue cracking right at the corner of the window? or was it all over the window?
Nicole: They found that it was at the corner. After this gauge test, the strain gauge test, yeah they found significantly higher stresses; I think 5 or 6 times what they had seen elsewhere on what they're calling the skin of the plane, concentrated at the corners of the windows. which honestly doesn't surprise me
Brian: I feel like rectangular and square doors are probably a little bit easier to fabricate than an oval-shaped door or a circular-shaped door. and a crack in your door, there's no wings that could fall off so it's probably less important we have oval-shaped access doors.
Nicole: But today all the plane windows are oval and all the doors are oval; so thanks de Havilland. They also found that the punch rivets that they used which was how they attached the windows, those openings were imperfect, the hole wasn't completely round, and that also added to some of the fatigue cracking that they saw. They even ran a finite element analysis in 2012 and they also confirmed the extra stresses at the rivets and near the corners of the windows. Based on this analysis they expected that the stress was 5 times at the corners of the windows then any areas that were further away from windows. one last thing, the windows were supposed to be glued and riveted but they were riveted only; so I don't know how much they didn't lack of glue really contributed. I don't know if that would have prevented the cracking from spreading. I can't really comment on that, but I just thought that was interesting, so I thought I would share. but out of this, they proposed much thicker gauge materials in the pressure cabin and they redesigned the windows and cut-outs to lower the general stress, so that's I think how we ended up with the oval windows that we have today. which is interesting to think about how that all kind of came to be. because today I would just assume that they just started with oval windows, but clearly they did not.
Brian: you have to start somewhere, and the earlier airplanes that they based a little bit of the Comet's design on, those airplanes had rectangular or square windows, because those airplanes weren't pressurized, it wasn't that important to have rounded windows in the aircraft, or oval-shaped windows. So it was fine just to have square or rectangular windows. So as a result of this, everyone around the world, all the different aircraft manufacturers, were watching and trying not to repeat the Comet's mistakes. Having that many airframes crash in that short of a period is something that no aircraft manufacturer wants. So, as a result of the Comet findings and the fatigue cracking, Boeing Aircraft Corporation, and McDonald Aircraft Corporation and Douglas Aircraft Corporation, they all changed any preliminary designs that they had to make use of windows that had rounded corners in them. de Havilland did try to recover a little bit, they made some additional prototypes and additional models of the Comet that did have rounded windows or the corners were at least rounded, but it was never an aircraft that recovered and it didn't see the same level of sales or commercial success that de Havilland had envisioned. There are a few that did remain in service until 1981, a few flying with the military until 1997, and a heavily modified version of Comet 4 operated until 2011.
Nicole: I've said this on the show many times, I'm gonna say it again; failure is an integral part of engineering. again no crystal ball, but I believe that whoever decided to take on the first jet airline design likely would have been where de Havilland is today. maybe they wouldn't have encountered this type of failure, but you're in entirely new engineering territory, you're designing systems and things that have not ever been done before. there is no way you're going to get that right the first time. and it's unfortunate that they didn't survive, but you know we have them to thank because had they not gone through that process, we wouldn't be where we are today with air travel. failure is an integral part of engineering, it needs to happen so that we can learn so that we can grow, so we can make things better. it's really unfortunate what happens, of course, we don't anybody get hurt, that's really crappy. I hate, that is the part of this podcast that I like the least, is when people get hurt. But also things need to break so we can figure out why and fix them, so they don't break next time. I think it's important. so there you have it, the de Havilland Comet. even though they were not successful long term, they are the reasons we have jet airlines today. I hope that this story doesn't prevent you from flying as we go back to whatever our new normal's gonna look like. in an average year, there are an estimated 39 million flights, although probably not in 2020. In 2019 there were 20 fatal airline accidents, resulting in 283 fatalities; which equates to 1 in 2 million flights. statistically driving a car is more of a risk than flying. Based on the number of accidents 2019 was the seventh safest year ever and the third safest based on the number of fatalities. the safest year on record was 2017 based on 10 accidents and 44 deaths. yes sometimes bad things happen but statistically, it's unlikely. so Brian before we end, do anything you want to add?
Brian: I don't have anything to add, I just want to thank you so much for having me on Failurology. This is the first podcast that I've ever been a part of. So thank you again for allowing me to be on your podcast.
Nicole: Yes, thanks for joining me. I liked the discussion part, I think that's a little bit more fun, at least for me then you know just kind of talking into the mic by myself. Also, you added a lot, airplanes are hard. I don't there's a lot of things I don't know. It's like learning a whole new language. I did Air France flight 447 in episode 9, and I went into it thinking it was gonna be easy. I mean it's pitot tubes, they got filled with ice; how complicated can this be? but there's so much. I had to use basically a jargon cheat sheet to read the report because I didn't understand all of the things that they were saying, there were so many things to learn. just to understand what had happened.
Brian: Aviation loves abbreviations and acronyms. It's one of the founding things of aviation.
Nicole: It wasn't in words that I could understand easily so it was tricky. So thank you for being here and explaining things to me.
Brian: it was my pleasure
Nicole: For photo sources and transcripts from this week's episode, head to failurology.ca. and if you're enjoying what you're hearing please rate, review, and subscribe to the show so more people can find it. If you want to chat with us, my Twitter handle is @failurology, you can email us at email@example.com or you can connect with us on LinkedIn; check out the show notes for links to all of these. and thanks everyone for listening. Brian, do you think you'll join me for another episode
Brian: I think I'll join you for another episode. This was a lot of fun.
Nicole: Maybe we'll take a little break from planes though.
Brians: That's fine, we can do some buildings or bridges or cars or other exciting things.
Nicole: yeah I really want to do, there's two floating bridges in Washington that both sank, which is really unfortunate obviously, but I mean floating bridges are kind of weird right they're not the norm. but more on that next time. Bye everyone, talk soon.