On the 4th of August 2018, an antique Junkers Ju 52 took off for a sightseeing trip across the Swiss Alps. The weather was perfect and the vintage WWII-era transport plane seemed to be in good working order. But as the flight traversed a precipitous mountain pass, it suddenly rolled over and spiraled into the ground in full view of numerous witnesses, slamming nose first into a rocky basin high above the tree line. None of the 20 passengers and crew survived. With both pilots dead and no flight recorders to explain what happened, Swiss investigators turned to an unorthodox source of information: videos and photographs taken by passengers in the minutes leading up to the crash. What they found was a flight path that led inexorably to disaster as the pilots flew into a box canyon at too low an airspeed, leading to a stall from which they could not recover. But this was hardly a one-off: pilots at Ju-Air had been flying extremely dangerous flight paths for years, and this wasn’t even the only serious issue at the airline. The scale of the problem was so great that authorities banned Ju-Air from flying passengers and grounded its planes — yet another unfortunate setback for the small and dwindling cohort of vintage aircraft still flying today.
What would you give to fly through the Alps at low altitude in a classic airplane from the second world war? Your first thought might be “that sounds unsafe,” but maybe only because you already know how the story ends. For 17 unsuspecting tourists, however, it sounded like a wonderful time. And indeed, for many other aviation enthusiasts, travelers, and photographers before them, it was. Out of thousands of military planes produced during WWII — known collectively as “warbirds” — a tiny percentage are still flying today, operating short commercial and private flights aimed at the sort of people who think it might be cool to fly on a B-17 or a Ju 52 simply for its own sake. In Switzerland, one of these airlines was Ju-Air, a small company based at an airbase in Dübendorf which operated a fleet of three antique Junkers Ju 52 propeller planes that originally belonged to the Swiss Air Force.
The Junkers Ju-52 originally entered service in Germany in 1931 as a civilian aircraft. The design featured three radial piston engines — two on the wings, and one in the nose — and was capable of carrying 17 passengers, which was fairly standard for the time. Mass production of the Ju 52 as a military transport plane began after the rise of the Nazi Party, and continued for several years after Nazi Germany’s collapse, with the last one built in 1952. Among the over 4,800 Ju 52s produced during this 21-year period was HB-HOT, which was built in Germany in 1939 and immediately sold to the Swiss Air Force. The plane remained in the service of the Air Force until 1985, when it was retired and permanently loaned to a group called the “Association of the Friends of the Swiss Air Force,” known by its German acronym VFL, whose mission was to preserve old military aircraft for future generations. VFL sold tickets for sightseeing flights on its three Ju 52s under the brand-name Ju-Air, and had carried passengers with an apparently perfect safety record ever since. The Ju 52s, nicknamed “Auntie Jus” or “Iron Annies,” were a common sight on summer weekends over the Alps.
In 2017, Ju-Air began advertising a special two-day scenic trip to the Ticino region of southern Switzerland using HB-HOT. The plan was to take off from Dübendorf on August 3rd, 2018, then fly to the town of Locarno in Ticino, where the passengers would receive a guided tour of various cultural attractions, followed by an overnight stay in a hotel. On August 4th, they would get back on board HB-HOT and fly back to Dübendorf. The trip package cost $1,136 per person — not a small amount by any means, but how else could you get to fly on a Ju 52? By spring of 2018, all 17 seats had been filled, and the plan seemed to be proceeding smoothly. But just one week before the trip, both of the scheduled pilots bailed, forcing Ju-Air to scramble to find someone else to cover their shifts. A 63-year-old Ju-Air pilot, named in official reports only as “Pilot A,” suggested that he and his good friend, 62-year-old “Pilot B,” could fill in, on a couple of minor conditions. The airline accepted without hesitation.
Pilot A and Pilot B were both former Air Force pilots with over 20,000 hours of flying experience, who had taken up flying for Ju-Air in retirement. Pilot A had only about 300 hours on the Ju 52, while Pilot B had a little over 900. They had both been flying for Ju-Air for several years, but the airline typically only flew on weekends, so they accumulated hours at a much slower rate than regular line pilots.
The outbound flight from Dübendorf to Locarno on August 3rd went as planned. As the plane flew low over the central Alps, the passengers snapped photos and recorded videos of the scenery; after landing, they disembarked and boarded a bus for their tour of the region. The flight attendant, who was also the group’s tour guide, went with them, while the pilots boarded a small private plane and flew back home for the night. The following day, they returned to Locarno and met the passengers at the airport for the flight back to Dübendorf.
After loading the passengers and their bags, the pilots calculated the plane’s weight and center of gravity, which they found to be within limits. In reality the center of gravity was farther aft than permitted, but Ju-Air pilots had been systematically miscalculating these figures — not because they couldn’t do math, but because the empty weight and center of gravity values provided to them in the operations manual were incorrect. Nevertheless, the exceedance was small, and the plane had no trouble getting into the air.
As HB-HOT climbed sedately through the precipitous valleys of the Alps, passengers once again pulled out their phones and digital cameras to document the scenery. Indeed, the pilots always made sure the passengers got the best possible view: they even flew through a special “countryside quiet zone,” meant to reduce noise pollution and protect wildlife, at less than 300 meters above the ground. What could possibly go wrong?
Around 45 minutes into the flight, HB-HOT began to approach the last major east-west ridge before reaching the Swiss lowlands to the north. The mood on board remained upbeat. The flight attendant texted her friend to report that they were about to pass over the friend’s home town of Ruschein, and the pilots made a sharp turn to overfly the village, per her request. They then turned toward the mountains to the north, flying past the Berghaus Nagens lodge at low altitude while making comments on the scenery, which were piped to the passengers’ head phones.
Cruising at about 9,000 feet and in a slight descent, HB-HOT approached a high-altitude valley between two peaks: the 10,167-foot Piz Segnas on the right, and the scraggly 9,347-foot Tshingelhörner on the left. Between them lay a narrow gap called the Segnas Pass, which topped out at about 8,760 feet (2,625m). The main attraction in the valley was a rock formation called the Martinsloch, a massive window in a descending ridge of the Tschingelhörner just west of the pass. In order to give the passengers the best possible view of the Martinsloch, the pilots slowed down, descended until they were level with the window, and charted a course directly down the middle of the valley.
This course, while giving the passengers a fairly good photo op, violated every principle of safe mountain flying. The most basic rule to follow while flying in mountains is to always leave yourself an escape route if something goes wrong, such as an engine failure, turbulence, or other surprise event. In practice, this often means flying close to one side of the valley rather than down the middle, so as to leave room to turn around and go back out the way you came. It also means staying high enough to be able to recover control in the event of an in-flight upset. The pilots of HB-HOT observed neither of these rules. The optimal route through the valley below the Segnas Pass would have been to fly down the far right side of the valley at a height of at least 11,000 feet, but they chose to fly down the middle of the valley at barely 9,000 feet, only a couple hundred meters above the ground.
To make matters worse, the pilots of HB-HOT were maintaining a speed of about 100 knots (185 km/h), which was too slow for the conditions. When flying close to mountains, pilots are likely to encounter turbulence, which can include not only crosswinds and rotors but also significant updrafts and downdrafts. The danger of this type of turbulence is amplified at low speeds due to basic principles of aerodynamics. The amount of lift generated by the wings of an airplane is a function of its airspeed and its angle of attack — the angle of the wings relative to the airstream. At lower airspeeds, a higher angle of attack must be used to achieve the same amount of lift; but if the angle of attack gets too high, the airflow will separate from the wing surfaces, causing a catastrophic loss of lift leading to a stall. The particular airspeed at which this critical angle of attack is reached is known as the stall speed. 100 knots was about 44% above the Ju 52’s stall speed at that altitude, and although this might seem like a lot, it really isn’t. The problem is that a powerful updraft can considerably increase the angle of attack without warning, causing the plane to stall at a much higher airspeed than normal; consequently, it’s important to maintain higher speeds when flying near mountains where downdrafts and updrafts are common. For the Ju 52, the bare minimum recommended speed for the conditions under which HB-HOT was flying would have been 106 knots, or 197km/h. Any lower than this and normal mountain turbulence could cause the airplane to stall.
As HB-HOT entered the valley of the Segnas Pass, it flew through an area of downdrafts which caused the plane to enter a shallow descent. The pilots reacted instinctively by pulling the nose up to try to stay above the level of the pass. They probably didn’t think too much about this: at that moment, they were distracted not only by their efforts to thread the needle between the two mountains, but also by an attempt to synchronize the thrust from the three engines, which had gotten out of sync during the course of the flight. But behind the scenes, their energy state was worsening. By pitching up to counter the decreased angle of attack (and consequent descent) caused by the downdraft, they set themselves up for a problem if that downdraft suddenly ceased, which it soon did. As the plane approached the upper part of the valley, the downdraft gave way to an updraft with a strength of about 11km/h (6 knots). Whereas the downdraft reduced the plane’s angle of attack, the updraft had the opposite effect, causing the angle of attack increase substantially. But the plane was still pitched upward at around 10 degrees in order to counter the reduced angle of attack caused by the downdraft. Thus, the updraft combined with the already high pitch angle of the plane, and the angle of attack increased to more than 20 degrees, exceeding the critical point, without any change in the airplane’s orientation in space. At that moment, HB-HOT was beginning its left turn to cross the pass. When the Ju 52 exceeds its critical angle of attack while in a turn, the inside wing tends to stall first; as a result, the left wing began to lose lift, and the plane banked 30 degrees to the left, more than the pilots had intended.
Noticing the excessive left bank, Pilot A, who was at the controls, attempted to steer back to the right using the ailerons, but this had no effect because the left wing had stalled. He quickly realized what was occurring and turned his control column back to the left, steering into the turn. Although it seems counterintuitive, this was exactly the right move. The Ju 52’s ailerons turn the airplane by raising the angle of attack of the outside wing, increasing its lift and causing it to rise, while doing the opposite to the inside wing. But if the outside wing has stalled, moving the aileron to increase its angle of attack will cause it to lose lift instead, making the stall worse. Therefore, trying to turn right while the left wing is stalled is the worst thing a pilot can do in this situation; instead, Ju 52 pilots are trained to steer into the stalled wing, using the aileron to decrease the wing’s angle of attack and delaying or preventing the stall. It is therefore possible to avoid a loss of control in this situation by allowing the plane to enter an extremely sharp 180-degree turn around the stalled wing, while pushing the nose down to reduce the overall angle of attack and increase airspeed.
The problem was that the flight path the pilots had chosen didn’t leave them enough room to perform this maneuver. To recover, they needed to pitch down and turn hard to the left, but they were only a few dozen meters above the height of the pass and would strike it if they pitched down, and if they tried to do a 180-degree turn to the left, they would crash into the side of the Tschingelhörner. For fear of striking terrain, the pilots were unable to make the large inputs needed to keep the plane under control, and the stall quickly escalated beyond all hope of recovery. The left wing lost all ability to generate lift; the plane rolled 186 degrees to the left and entered a near-vertical dive. Within seconds, it was all over: pitched down almost 90 degrees, HB-HOT slammed nose first into a barren, rocky field just below the pass, instantly killing all 20 passengers and crew on board.
Numerous hikers and climbers in the valley and on the pass witnessed HB-HOT’s final seconds with a mixture of shock and disbelief. One hiker, reacting to the onset of the loss of control, whipped out their phone and managed to capture video of the last six tenths of a second of the flight before it struck the ground (shown above). Those who were closer to the scene rushed to the crash site to search for survivors, but upon arriving they discovered that the cabin had been crushed like an accordion into the ground, leaving little hope that anyone would be found alive. Indeed, within minutes it was obvious that no one had survived the crash. Rescuers soon arrived by helicopter, but there was nothing they could do but radio back the tragic news.
The crash of HB-HOT was the first fatal commercial airplane accident in Switzerland in 17 years. How was it that this crash could occur in a country that had maintained a perfect safety record for such a long period? The Swiss Transportation Safety Investigation Board (known as the STSB) was tasked with finding out. Their job would not be easy: as a vintage airplane, HB-HOT was exempt from rules requiring that commercial aircraft carry a flight data recorder and a cockpit voice recorder, meaning that details about the performance of the airplane and the actions of the crew would have to be gleaned from other sources. Fortunately for the investigators, it turned out that this was Switzerland’s first big accident of the digital age: not only did dozens of people on the ground photograph or film the airplane during its final flight, some of the electronic devices belonging to the passengers had survived the crash as well. In addition to a vast trove of photographic evidence submitted by the public, investigators were able to recover data from 10 of the 44 mobile phones and digital cameras found in the wreckage, an effort which yielded no less than 23 videos taken from on board HB-HOT. These included a video taken inside the cockpit during the first phase of the flight, along with footage from a go-pro camera which recorded the final two minutes right up until the moment of impact. By dispassionately analyzing the gut-wrenching videos, investigators were able to reconstruct most of the plane’s flight path, along with its airspeed, pitch angle, engine power settings, and control movements throughout the critical final phase leading up to the loss of control.
Upon combining the video evidence with a detailed computer simulation of the air movements in the valley at the time of the crash, investigators were able to determine what happened with surprising accuracy. First, the plane entered the valley at too low an airspeed. Then a downdraft coming over the Tshingelhörner pushed the plane down, causing the pilots to pull the nose up to try to stay above the pass. Their efforts to keep the plane above the pass and synchronize the engines likely distracted them from their deteriorating energy state. When the downdraft transitioned to an updraft while the plane was in a left turn, the increase in angle of attack caused the left wing to stall. Although the pilots attempted to begin the appropriate recovery maneuver, terrain constraints prevented them from executing the procedure properly, and they lost control of the airplane.
The root cause of the crash was not the pilots’ failure to keep the plane under control per se, but their decision to fly into the valley in the first place. The question was why a pair of experienced pilots who had been flying in the Alps for years would decide to fly a route which offered them no possibility of escape if something went wrong. The answer would blow the case wide open.
It turned out that this was far from the first time that Ju-Air pilots had flown recklessly while crossing the Alps — in fact, it was far from even the hundredth time. An analysis of radar data from 216 Ju-Air flights between April and August 2018 showed that over 36% of them were conducted in a manner that multiple specialists independently judged to be medium- to high-risk, including 13 which scored 10 out of 10 on a risk scale devised for the purpose. High risk maneuvers on these flights included operating for extended periods in box canyons with no escape route; continuous flight over terrain at heights well below 300 meters; lateral clearance of cliff faces by less than 30 meters; and several extremely low crossings of alpine passes, including one which was performed at just 5 meters above the ground. Sixteen of Ju-Air’s 27 pilots were found to have been at the controls during at least one of these high-risk flights. Both Pilot A and Pilot B were among the worst offenders — when placed together, they flew high-risk flight paths more than twice as often as the average crew.
Needless to say, flying so close to terrain is not merely unsafe, it’s also against regulations. The Ju-52 is required to stay at least 2,000 feet above any terrain within 9.3km of the route during cruise flight, because if an engine fails while above 8,250 feet (2,500m), it is unable to maintain altitude. For HB-HOT to have flown the route of the accident flight while observing this rule, it would have had to maintain a cruise altitude of over 12,500 feet — but this was also against regulations for a totally different reason. Because the Ju 52 is not pressurized, Ju-Air did not allow its pilots to fly above 10,000 feet, which is about as high as the average person can comfortably go without acclimatizing first. Therefore, HB-HOT could not have flown its planned route without violating one or both of these rules. It was clear which set of rules Ju-Air pilots adhered to: in fact, minimum terrain clearance distances weren’t even mentioned in the Ju-Air operations manual.
Looking into the history of both pilots, they found numerous additional violations. Pilot A routinely violated minimum altitudes during training flights as well as during regular operations, but his instructors nevertheless graded his performance as “standard” to “high standard” and described his choice of flight path as “appropriate” and “sensible.” Pilot B performed similarly dangerous maneuvers in training but was graded “high standard,” while his choice of flight path was described as “anticipatory” and “considerate.” Investigators also found that in 2013, Pilot B flew the exact same route over the Segnas Pass at a height of 30 meters above the terrain and with a flight path that left no avenue of escape, just like on the accident flight.
It was apparent that a culture of disregard for terrain clearance rules had taken hold among Ju-Air pilots and instructors alike. But where did this disregard come from? The STSB identified two main factors. One thing investigators discovered was that the rate of violations was much higher among pilots who had joined Ju-Air after serving in the Air Force than it was among those who had careers in civil aviation. The Swiss Air Force operated under a different set of regulations and tolerated a considerably higher level of risk in its day-to-day operations, and this had apparently bled over into ex-Air Force pilots who were now flying passengers for Ju-Air. Tellingly, both pilots on the accident flight had previously flown with the Air Force.
The second major factor behind this culture of high-risk flying was the fact that the airline operated vintage airplanes. While one might expect pilots of vintage airplanes to be extra careful due to the age of their aircraft, the opposite is true equally as often. Ju-Air had received a number of exemptions from regulations that could not easily be applied to the antique Ju-52 (such as the requirement to carry black boxes), and in the absence of careful management, this had evolved into a certain implicit sentiment that regulations in general did not apply, or ought not to apply, to Ju-Air. There was a sense that the rules were meant for the “big boys,” and that a small association flying WWII transport planes on weekends in visual flight conditions could do pretty much whatever it wanted as long as it stayed out of the way.
Immediately following the accident, investigators also found a large number of deficiencies in areas not directly related to the crash. HB-HOT suffered from extensive corrosion of wing and fuselage components, and a spar in the left wing was found to be in an advanced stage of fatigue. According to documents from the plane’s last overhaul, the aircraft was not capable of meeting its design specifications with regard to performance. Numerous major repairs and overhauls had no accompanying documentation at all. Empty weight and center of gravity figures for the planes were incorrect and pilots had been chronically miscalculating these parameters as a result (including on the accident flight, where an out-of-limits aft center of gravity might have exacerbated the increase in angle of attack which led to the crash). Because the manufacturer disappeared decades ago and no one held the type certificate for the Ju 52, Ju-Air had been fabricating spare parts on its own through subcontractors, not all of which were qualified for the work, and none of the commissioned spare parts had received approval from the Federal Office of Civil Aviation (the Swiss FAA) since 2002.
Despite these mechanical problems and the reckless flying by its pilots, Ju-Air had never faced real scrutiny from the FOCA during the 33 years leading up to the crash. Part of the problem was that Ju-Air had somehow fallen into the wrong inspection category. Following Switzerland’s adoption of European Union air safety regulations in 2006, Swiss regulators reviewed the type certificates of all airlines in Switzerland to determine whether they met standard airline rules or needed a special status, but Ju-Air somehow ended up under “standard” rather than “special,” despite the fact that its fleet of Ju 52s could not possibly meet the standards that European regulations required of a normal airline with a normal fleet. The “special” category even had a subcategory specifically for historic aircraft, but Ju-Air was never placed in it. Therefore Ju-Air became the responsibility of FOCA inspectors who were more used to A320s and 737s and had no idea what to make of the airline’s old 1930s warbirds. Although Ju-Air had successfully requested 15 regulatory exemptions from the FOCA, individuals within the agency did not agree on a single reason as to why these exemptions had been granted to an airline which wasn’t in the “historical” subcategory. Across numerous inspections of Ju-Air planes and facilities, the FOCA did little more than confirm that the proper procedures existed, with no regard for whether they were being followed. When FOCA inspectors rode along with Ju-Air pilots, they did so without passengers and they didn’t go anywhere near the mountains, so the reckless low-altitude flying was not detected. At one point a FOCA inspector did fly on a scheduled Ju-Air flight in the Alps, but despite the fact that this flight flew dangerously close to terrain, the inspector reported no violations. The FOCA had also received numerous complaints from the public about Ju-Air planes conducting low-altitude flyovers of quiet zones, wildlife refuges, villages, and even restricted military airspace, but besides the agency politely telling the airline to knock it off, no concrete action was taken.
Ju-Air’s internal checks and balances somehow managed to be even less effective than the FOCA. Although it had a safety management system as required by regulations, the system seemed to exist in order to comply with the law rather than to actually improve safety. Despite all the reports of airspace and terrain clearance violations, the safety management department only ever assessed each case individually (and usually took no action). The purpose of a safety management system is to identify unsafe trends that manifest through repeated incidents, but it seemed that no such analysis of these incidents was ever performed. Furthermore, the airline’s internal compliance watchdog often reported that items were in compliance with regulations when they manifestly were not, and sometimes simply reported that procedures for compliance existed without verifying whether they were being followed.
The result of all these failures by Ju-Air and by the FOCA was that nobody who mattered ever learned about or took any action to correct numerous shortcomings, from the chronic problems with center of gravity calculations to the dangerously low flying to the poor structural condition of the Ju 52s. It seemed like nobody really took Ju-Air that seriously, because of the type of planes it flew and the manner in which it flew them. Lost in the fog of regulatory exemptions and informal good-ol’-boys culture was the fact that the lives of real people were being put at risk.
In its final report, the STSB issued several safety recommendations intended to make historic aircraft regulations more compatible with the real use cases of the airplanes, and to improve the effectiveness of safety management systems and FOCA inspections. Also as a result of the findings of the investigation, the FOCA withdrew the certificates of airworthiness for both of Ju-Air’s remaining planes shortly after the accident, then later allowed them to resume flying on a limited basis —but all passengers would have to be members of a special Ju-Air club for at least 30 days, and would receive education about the risks associated with flying on vintage airplanes before ever boarding a Ju-52. This only lasted until March 2019, when the FOCA revoked the airline’s operating certificate and imposed a fine. Simultaneous with this decision, the FOCA announced an end to commercial flights using vintage aircraft in Switzerland. From that point on, such flights could not be conducted for profit or carry paying passengers. This is similar to the system that exists in the United States, where vintage aircraft cannot directly carry paying passengers, but members of the public can still get the chance to fly on the airplanes by donating to non-profit organizations dedicated to the preservation of vintage aircraft as “living history” exhibits. However, even this arrangement is under renewed scrutiny following the October 2019 crash of the WWII-era B-17 bomber “Nine-O-Nine” in Connecticut, in which 7 of the 13 passengers and crew were killed.
The question of how to balance safety and history only grows more salient with each passing year. Should we still let people fly on B-17s and Ju 52s? For aviation enthusiasts, the answer is a resounding “yes,” and so far regulators tentatively agree, with some fairly restrictive conditions. For regulators around the world, the eventual (and most likely inevitable) decision to ground these airplanes for good will be a difficult one, as many of those making the decisions are themselves lifelong aviation enthusiasts who don’t want to have to face the day when flying aboard a WWII bomber is no longer possible. And for the rest of us, the question will always be polarizing. For many people, the thought of getting on such an old plane is terrifying, and some may incredulously ask why they haven’t been grounded already. But for others, until the day the last warbird lands for the last time, the legendary glory days of military aviation will never cease to beckon.
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