Ju-Air JU52 at Piz Segnas on Aug 4th 2018, lost height in spiral descending trajectory

Last Update: January 28, 2021 / 14:43:16 GMT/Zulu time

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Incident Facts

Date of incident
Aug 4, 2018

Classification
Crash

Aircraft Registration
HB-HOT

Aircraft Type
JUNKERS Ju-52/3m

ICAO Type Designator
JU52

Switzerland's SUST released their final report concluding the probable causes of the crash were:

Direct cause

The accident is attributable to the fact that after losing control of the aircraft there was insufficient space to regain control, thus the aircraft collided with the terrain.

The investigation identified the following direct causal factors of the accident:

- The flight crew piloted the aircraft in a very high-risk manner by navigating it into a narrow valley at low altitude and with no possibility of an alternative flight path.

- The flight crew chose a dangerously low airspeed as regard to the flight path. Both factors meant that the turbulence which was to be expected in such circumstances was able to lead not only to a short-term stall with loss of control but also to an unrectifiable situation.

Directly contributory factors

The investigation identified the following factors as directly contributing to the accident:

- The flight crew was accustomed to not complying with recognised rules for safe flight operations and taking high risks.

- The aircraft involved in the accident was operated with a centre of gravity position that was beyond the rear limit. This situation facilitated the loss of control.

Systemic cause

The investigation identified the following systemic cause of the accident:

- The requirements for operating the aircraft in commercial air transport operations with regard to the legal basis applicable at the time of the accident were not met.

Systemically contributory factors

The investigation identified the following factors as systemically contributing to the accident:

- Due to the air operator’s inadequate working equipment, it was not possible to calculate the accurate mass and centre of gravity of its Ju 52 aircraft.

- In particular, the air operator’s flight crews who were trained as Air Force pilots seemed to be accustomed to systematically failing to comply with generally recognised aviation rules and to taking high risks when flying Ju 52 aircraft.

- The air operator failed to identify or prevent both the deficits and risks which occurred during operations and the frequent violation of rules by its flight crews.

- Numerous incidents, including several serious incidents, were not reported to the competent bodies and authorities. This meant that they were unable to take measures to improve safety.

- The supervisory authority failed to some extent to identify the numerous operational shortcomings and risks or to take effective, corrective action.

Other risks

The investigation identified the following factors to risk, which had no or no demonstrable effect on the occurrence of the accident, but which should nevertheless be eliminated in order to improve aviation safety:

- The aircraft was in poor technical condition.

- The aircraft was no longer able to achieve the originally demonstrated flight performance.

- The maintenance of the air operator’s aircraft was not organised in a manner that was conducive to the objective.

- The training of flight crews with regard to the specific requirements for flight operations and crew resource management was inadequate.

- The flight crews had not been familiarised with all critical situations regarding the behaviour of the aircraft in the event of a stall.

- The supervisory authority failed to identify numerous technical shortcomings or to take corrective action.

- The expertise of the individuals employed by the air operator, maintenance companies and the supervisory authority was in parts insufficient.

The SUST sumarized the sequence of events:

At 16:14 on 4 August 2018, the historic Junkers Ju 52/3m g4e commercial aircraft, registered as HB-HOT and operated by Ju-Air, took off from Locarno Aerodrome (LSZL) for a commercial VFR flight to Dübendorf (LSMD). On this flight, pilot A was sitting in the left-hand seat in the cockpit and piloting the aircraft as the commander, while pilot B was assisting him as the co-pilot sitting on the right.

Following take-off from concrete runway 26R westwards and a 180-degree turn over Lake Maggiore, the flight led into the Blenio valley via Bellinzona and Biasca.

HB-HOT steadily gained altitude in the process. North of Olivone, the aircraft turned into the valley of the Lago di Luzzone reservoir and thus into the Adula/Greina/Medels/Vals countryside preservation quiet zone. This zone was crossed at between 120 and 300 m above ground and at times with a minimal lateral separation from the terrain.

At 16:45, as the aircraft was flying over Alp Nadels, the ISP sent a text message to a friend in Ruschein (municipality of Ilanz) to say that the Ju 52 was approaching the area. The flight subsequently continued eastwards into the Surselva region at approximately 2,500 m AMSL. At 16:51, the aircraft crossed the Vorderrhein valley in the region of Ilanz on a north-easterly heading and initially made a relatively tight left turn, taking it over Ruschein. The flight path then led generally northwards past the Crap Sogn Gion mountain and towards the basin south-west of Piz Segnas. At first, the aircraft approached this basin on the left-hand, western side of the valley.

HB-HOT was climbing at this time, and reached an altitude of 2,833 m AMSL in the Nagens region.

The aircraft made a slight right turn when flying past the Berghaus Nagens lodge (see figure 2). During this phase, at 16:55, one of the pilots informed the passengers of the scenery over the speakers in the cabin and through the passengers’ personal headphones.

To start with, the aircraft was flying at a ground speed of 165 km/h during this phase. By point F2, the ground speed had decreased to 135 km/h, and roughly remained so until shortly before point F3.

Towards point F3, the aircraft’s altitude dropped slightly and the ground speed briefly increased by around 65 km/h to approximately 200 km/h. During this time, its pitch attitude3 was 5 to 7 degrees. Towards the end of this phase, just before point F4, the flight path angle4 changed from -3 degrees to approximately -1 de-gree and the speed of each of the three engines decreased steadily by approximately 20 revolutions per minute (rpm). At point F4, the aircraft was at an altitude of 2,742 m AMSL.

At 16:56:02, shortly after point F4, the speed of each of the three engines in-creased by approximately 40 rpm. At 16:56:09, HB-HOT entered the basin south-west of Piz Segnas at an altitude of 2,755 m AMSL (point F5, see also figure 14) and was therefore approximately 130 m above the elevation of the Segnes pass. The flight crew then navigated the aircraft on a north-north-easterly heading almost in the centre of the valley. HB-HOT climbed slightly during this phase and its flight path angle was approximately 2 degrees; its pitch angle remained at 5 to 7 de-grees. At 16:56:17, the aircraft reached an altitude of 2,767 m AMSL at point F7 and was therefore approximately 140 m above the elevation of the Segnes pass.

HB-HOT flew past the Tschingelhörner mountain peaks and began to reduce in altitude, dropping more than 15 m in approximately 6 seconds. During this phase, the power of the engines was rapidly reduced by 30 to 50 rpm, which meant that the engines were increasingly running at a similar speed5. During this process, the pitch angle increased and the flight path angle continuously became more nega-tive.

When the aircraft was approximately abeam the Martinsloch and at an altitude of approximately 2,766 m AMSL (point F8), the flight crew initiated a right turn during their descent and then made a left turn (point F109, see figure 5). The ground speed was approximately 170 km/h and the difference between the aircraft’s pitch and flight path angles increased to approximately 15 degrees during the right turn. When transitioning into the left turn (between points F9 and F10), the pitch angle was approximately 11 degrees and the flight path angle was around -10 degrees. At this time, the aircraft was flying at approximately 125 m above the elevation of the Segnes pass (see figure 3).

During roughly the next 4 seconds, the aircraft descended by 25 m and the already negative flight path angle became even more negative,During roughly the next 4 seconds, the aircraft descended by 25 m and the already negative flight path angle became even more negative.

After point F13, the roll to the left increased steadily and did not decrease even when a significant aileron deflection to the right was made. The ailerons were then brought into a neutral position and temporarily deflected into a position for a left turn.

At the same time, the pitch attitude began to decrease and the flight path ran in-creasingly steeper downwards whilst the left bank attitude constantly increased (see figure 6).

During this last flight phase, the aircraft experienced low-frequency vibrations. Ul-timately, when the aircraft was 108 m above ground (point F16) its longitudinal axis was pointing downwards by 68 degrees from horizontal. By this time, the elevator had deflected upwards by approximately 13 degrees and the rudder was pointing 2 degrees to the right.

The speeds of the three engines had increased slightly compared to the beginning of the downward spiral trajectory and were between 1,720 and 1,750 rpm shortly before impact.

The roll to the left accelerated significantly during this phase. Shortly after 16:57, the aircraft hit the ground in a vertical flight attitude with an almost vertical flight path and at a speed of approximately 200 km/h.

Reconstructions revealed that, at the time of the accident, HB-HOT’s centre of gravity was at 2.071 m behind the wing’s leading edge. In the images and video footage available that had been captured from inside HB-HOT, there was no evidence of anyone moving within the aircraft or not sitting in their seat between the period when the aeroplane entered the basin south-west of Piz Segnas and up to the beginning of its downward spiral trajectory.

The captain (63, ATPL, 20,714 hours total, 297 hours on type), named Pilot A, was pilot flying, the first officer (62, ATPL, 19,751 hours total, 945 hours on type) named Pilot B was pilot monitoring.

The SUST annotated:

In the last two months prior to the accident flight, pilot A carried out a total of 33 flights on the accident type; 28 of these were with pilot B, who carried out the accident flight with him.

In the months and years prior to the accident flight, various safety-critical flights had been documented on which pilot A had been part of the crew, flying below a safe altitude9 or taking high risks. Between April 2018 and including the day of the accident, at least six flights have been logged which involved flight paths with a risk score of 8 to 10 (see section A1.18.4); on four of these flights, he was working with pilot B. On 6 July 2018, pilot A acting as commander flew, together with pilot B acting as co-pilot, over Munich in the Junkers Ju 52/3m g4e aircraft, registered as HB-HOT, at an altitude considerably below the minimum required level.

During his last line check on 7 April 2018, pilot A flew significantly below the safety altitudes as specified in the Aeronautical Information Publication (AIP) VFR guide. Furthermore, he disregarded essential principles for safe mountain flying. These principles have been published since 1981 and, at the time of the accident, were listed under RAC 6-310 in the AIP VFR guide. The Ju-Air training captain entrusted to carry out pilot A’s line check was, among other things, also a TRI11 and TRE12 for Ju 52 aeroplanes. This training captain rated the per-formance of pilot A as ‘standard’ to ‘high standard’. The choice of flight path was described as “appropriate” and “sensible”.

In the last two months prior to the accident flight, pilot B carried out a total of 41 flights on the accident type; 28 of these were with pilot A, who carried out the accident flight with him.

In the months and years prior to the accident flight, various safety-critical flights had been documented on which pilot B had been part of the crew, flying below a safe altitude or taking high risks. Between April 2018 and including the day of the accident, at least eight flights have been logged which involved flight paths with a risk score of 8 to 10 (see section A1.18.4); on four of these flights, he was working with pilot A.

During his last line check on 12 May 2018, pilot B flew significantly below the safety altitudes as specified in the Aeronautical Information Publication (AIP) VFR guide. Furthermore, he disregarded essential principles for safe mountain flying. These principles have been published since 1981 and, at the time of the accident, were listed under RAC 6-3 in the AIP VFR guide. The Ju-Air training captain who was entrusted to carry out pilot B’s line check and also worked as a ground instructor for the air operator, rated the flight as ‘high standard’. The choice of flight path was described as “considerate” and “anticipatory”.

During a climb in sister aircraft HB-HOP on 6 July 2013, pilot B as commander, together with pilot A in the role of co-pilot at the time, entered the basin south-west of Piz Segnas in a similar manner to during the accident flight and flew over the ridge of the Segnes pass at approximately 30 m above ground.

During this flight, a 180-degree turn or an alternative flight path in the northern section of the basin south-west of Piz Segnas would not have been possible.

The SUST reported that the aircraft had been decommissioned by Swiss Armed Forces in 1981 and was taken over by the association of the friends of Swiss air corps museum, who started commercial flights with it in 1982 and eventually emerged into Ju-Air "responsible for flight operations, aircraft maintenance and the continuing airworthiness management organisation (CAMO)" in 1997. The SUST stated: "When the air operator began using the type Ju 52/3m g4e aircraft for civil aviation, the manufacturer or type certificate holder had long since ceased to exist. At the time of the accident, the Junkers Ju 52/3m g4e aircraft, registered as HB-HOT, had recorded approximately 10,189 operating hours."

The SUST analysed:

Technical factors

The extensive technical examinations have revealed that the Junkers Ju 52/3m g4e aircraft registered as HB-HOT had various technical restrictions. One of these was that none of the three installed BMW 132 A3 nine-cylinder radial engines were still able to reach the rpm specified by the manufacturer. It could also be proven that the aircraft was no longer capable of the flight performance specified in the operating instructions for the aircraft type. Numerous technical defects, such as corrosion damage, were also found during the investigation of HB-HOT. It was also established that various components had been inadequately maintained or replaced by reproduction parts that exhibited qualitative issues. In view of these numerous technical inadequacies, it can be concluded that, prior to the accident flight, the Junkers Ju 52/3m g4e aircraft registered as HB-HOT was not airworthy in either a physical or a formal sense. Nevertheless, the aircraft functioned in such a way that the identified technical defects did not have an effect on the accident. There is no indication that these defects influenced the actions and decisions of the flight crew. This is also proven by the fact that the crew had already flown the aircraft from Dübendorf to Locarno the day before without raising any complaints. Other flight crews did not make any corresponding complaints about HB-HOT in the weeks and months before the accident either. It is therefore possible to con-clude that the flight crews had become accustomed to the limited flight character-istics and were unable to detect the remaining inadequacies. The improper me-chanical condition of HB-HOT and its limited flight performance do, however, con-stitute factors to risk which should be eliminated in future for aircraft of the same type.

Weather

The weather forecasts which the flight crew could consult before the flight showed no signs of unusual or particularly difficult weather conditions. The investigation proved that the weather encountered during the flight was largely in line with the forecasts. Pilot B had already flown from Dübendorf that morning and had experi-enced the weather conditions. Both pilots had already crossed the main ridge of the Alps in a light aircraft immediately before the accident flight. In view of this information and the flight crew’s substantial experience, it can be concluded that the flight crew had sufficient knowledge of the weather conditions prevailing at the time. It must have been clear to the flight crew that it was easily possible to fly around areas affected by local showers or thunderstorms. The weather conditions were suitable for a VFR flight over the Alps and allowed various safe routes from Locarno to Dübendorf to be flown. In view of the forecasts and the actual weather conditions, it can be concluded that it was possible to turn back to Ticino at any time. The weather on the route did not present any surprises and was easy to assess. Furthermore, there is no doubt that the experienced, well-trained pilots were familiar with the phenomenon of a relatively high density altitude at high summer temperatures. Although, as explained above, the aircraft no longer performed as documented by the manufacturer, the temperatures prevailing on that day did not constitute a critical limitation for the planned flight.

The SUST reported: "The OFP prepared for the take-off in Locarno included a mass of 9,737 kg and centre of gravity at 1.98 m behind the wing’s leading edge. The reconstructed value for the mass at take-off from Locarno was 9,387 kg and the centre of gravity at 2.077 m. The rearmost permissible centre of gravity is at 2.060 m behind the reference line."

The SUST analysed the operational factors:

For the accident under investigation, it could be proven that the aircraft was operated with a mass below its maximum take-off limit, both during the outward flight on 3 August 2018 and during the accident flight on 4 August 2018. On both flights, however, the aircraft’s balance was behind the rearmost permissible centre of gravity specified by the manufacturer. It should be noted that if the mass and balance calculations had been performed correctly, using the documentation provided or Ju-Air’s flight planning software, the flight crew would not have been able to identify that the centre of gravity was behind the permitted limit. The reason for this lay in inaccurate raw data and the flawed design of the flight planning software. These shortcomings represent a factor that systemically contributed to the accident.

The SUST analysed the accident flight:

... the aircraft crossed the Surselva region in the municipality of Ilanz on a north-easterly heading and made a relatively tight turn to the left. This manoeuvre took the aircraft over Ruschein (canton of Grisons), where a friend of the flight attendant (ISP) lived. The ISP had sent a text message to this friend using her mobile phone a few minutes earlier, saying that the Ju 52 would shortly be flying over Ruschein. It stands to reason that this noticeable change in heading, which could be seen from the ground, can be attributed to this.

During this phase, HB-HOT was climbing and, at 2,833 m AMSL above the Nagens region, attained the highest altitude recorded on its final flight.

It stands out that, shortly after passing the Berghaus Nagens lodge, the flight crew piloted the aircraft at a speed, which, for a long time, equated to a ground speed of only approximately 140 km/h. Taking into account the headwind during this phase, the aircraft was moving at a true airspeed of approximately 180 km/h. Thus, the airspeed during the approach to the basin south-west of Piz Segnas was approximately 44 % above the stall speed. As turbulence had already occurred during the flight prior this point and it was necessary to initiate a turn in order to cross the pass, associated with a higher stall speed, this safety margin was too small. In addition, even during this phase, the aircraft tended to fly quite low to the ground at just less than 200 m above the elevation of the Segnes pass, which the crew intended to cross. This, combined with the low speed, represented a risky starting point for the continuation of the flight.

This situation did not improve, despite the speed increasing by roughly 50 km/h up to a true airspeed of approximately 230 km/h for a short time, because this increase in speed was not due to an increase in power. Rather, this resulted from a slight descent of roughly 80 m, which reduced the aircraft’s height above ground to ap-proximately 115 m when compared to the Segnes pass. The pass represents the lowest point on the mountain range that borders the basin. Due to the narrowness of the pass, terrain elevations considerably higher than that of the pass must be factored into the choice of flight path in order to safely cross the corresponding ridge.

Upon increasing the speed of all three engines by about 40 rpm, the aircraft rose by approximately 25 m to 2,767 m AMSL, resulting in a height above ground of roughly 140 m when compared to the Segnes pass. At the same time, however, the true airspeed of HB-HOT decreased to 200 km/h and the prevailing headwind steadily eased.

The analysis of the power setting and rpm of the three engines shows that they could be controlled and that the aircraft reacted according to changes in power settings. Neither the maximum permissible engine speed nor the highest possible engine speeds to be expected in view of the proven technical restrictions were achieved. From this it can be concluded that despite the relatively high density altitude and the poor condition of the engines, there was still a reserve of power.

Evaluation of the aileron deflections and the reaction of the aircraft to these control inputs proves that it was possible to control the aircraft and that it reacted accordingly during this phase of the flight.

HB-HOT entered the basin south-west of Piz Segnas and from then on, the pilots navigated the aircraft on a north-north-easterly heading in approximately the mid-dle of the valley. With this choice of flight path, the flight crew may have wanted to give the passengers a good view of the Martinsloch, a well-known geological fea-ture and tourist attraction. Due to the aircraft’s low altitude and the narrow nature of the basin, it was no longer possible to turn back or to choose a flight path other than over the crest of the Segnes pass. It is one of the basic principles of flying in mountainous areas that there must always be the option of an alternative flight path or to turn back. Combined with the aircraft’s low altitude in relation to the pass the pilots intended to cross, the flight crew’s decision to dispense with these safety-related requirements created a very high-risk situation which did not permit any tolerance for further errors, faults or external influences. This type of piloting can be seen as a causal factor of the further course of the accident.
As the reconstruction of the flight path and the wind conditions show, the aircraft began to descend over several seconds while flying past the Tschingelhörner mountain peaks. This was due to downdraughts with a vertical speed of 2 to 5 m/s. Extensive meteorological investigation proved that downdraughts of this kind were present in this area of the basin. They do not represent an abnormal phenomenon in the mountains.

As video footage shows, when the aircraft was approximately level with the Mar-tinsloch, the flight crew initiated a right turn during this descent and then made a left turn. The true airspeed was approximately 180 km/h and the difference be-tween the aircraft’s pitch and flight path angles increased to approximately 15 de-grees during the right turn.

During this phase, the power of the engines was slightly reduced, although the characteristics of the manner of control input suggests that the flight crew was in the process of synchronising the three engines. At the same time, the aircraft’s pitch attitude increased further and the descending flight path became increasingly steeper.
It is conceivable that due to their preoccupation with the engines and the view from the cockpit, which made it difficult to easily recognise the descent, the flight crew’s increase in pitch attitude was made subconsciously in order to compensate for this (see figure 21). Furthermore, the fact that the centre of gravity was beyond the rear limit facilitated the process and made the aircraft more unstable around its pitch axis, which represents a factor that directly contributed to the accident.

The aeroplane then assumed a rate of descent of approximately 6 m/s that subsequently increased further, which, based on the analysis of the flight attitude, speed and airflow conditions in the basin south-west of Piz Segnas, can no longer be attributed to a downdraught. Due to the high pitch attitude and the clearly downward flight path, it is also impossible that this descent was caused by the flight crew using the elevator control. Rather, it can be concluded that the aircraft was in a situation in which the airflow at the wing had at least partially stalled. It should be noted that a stall can occur regardless of the aircraft’s speed if the critical angle of attack for the wing profile is exceeded.

From an aerodynamic point of view, the stall can be explained as follows: HB-HOT had been caused to enter a descent by an area of downdraught. The descent in the downdraught, which was partially compensated by increasing the pitch attitude, led to a flight attitude close to the maximum angle of attack. Given this flight attitude, the additional increase in the angle of attack caused when flying into an up-draught was sufficient to lead the airflow to at least partially stall. This development would not have been expected had the downdraught continued or eased slowly. The air currents observed in the basin indicate that the aircraft was moving from an area of downdraught into an area of updraught. A change in the vertical com-ponent of the wind’s vector from a downdraught with a speed of 2 to 5 m/s to an updraught with a speed of 0 to 3 m/s was sufficient to exceed the critical angle of attack (see figures 22 and 23). As both the measurements and the airflow calcula-tion have shown, even larger shear values were easily possible. Correspondingly turbulent conditions in the mountains are not unusual and become a risk when flying close to the terrain.

It is therefore also one of the fundamental principles of flying in mountainous areas that the airspeed, and thus the energy of the aircraft, must be increased during turbulent conditions and when in closer proximity to the terrain, so that wind shear does not cause a stall, even if it would usually only do so for a short time. It must be ensured that the aircraft is not overloaded by gusts or deflection of the control surfaces, so that the calculated manoeuvring speed lends itself to be the optimum speed. HB-HOT had a true airspeed of approximately 180 km/h during this phase (when it encountered wind shear) which was roughly 55 km/h or 44 % above the stall speed in the prevailing conditions. This level of speed reserve is too small for the turbulence that is common in the mountains.
As has been proven, the flight crew did not use the engines’ available power re-serves to consistently achieve an airspeed within the design manoeuvre speed range, which under the prevailing conditions was a true airspeed of 197 km/h. If the design manoeuvring speed cannot be achieved during horizontal flight, which is possible for aircraft with a relatively large mass-to-power ratio, this speed must be aimed for when descending. This in turn requires a sufficiently large altitude reserve to be created in advance. In any case, when flying in the mountains, great attention must be paid to ensuring a safe energy level for the aircraft.

In this investigated accident, the flight crew did not follow this important principle. This is shown, inter alia, by the fact that, at a time when the aircraft was already flying too low and too slowly in the basin, they further reduced the power of the three engines. The choice of a dangerously low airspeed with regard to the flight path is therefore a further causal factor of this accident.

Video footage further shows that during this situation, which resembled a deep stall, the angle of bank to the left increased steadily. Once the bank angle reached approximately 30 degrees, there was initially a small and then a significant correc-tive deflection of the left aileron downwards, which was intended to achieve a roll to the right, or rather to counter the roll. From this, it can be concluded that the flight crew intended to stabilise the left turn at a constant bank angle and had prob-ably not really noticed the stall yet. At this time the aircraft was at an altitude of approximately 2,725 m AMSL and was therefore still at a height above ground of roughly 100 m compared to the Segnes pass.

However, the roll to the left did not slow down and the bank angle continued to increase. The ailerons were then brought into the neutral position and slightly de-flected into a position for a left turn. At the same time the nose of the aircraft began to drop. This sequence of events can be explained as follows:

- During this phase the aircraft was proven to be in a situation that could no longer be controlled or in which it was impossible to prevent the roll motion to the left, at least momentarily.

- The uncontrolled rolling motion of the aircraft occurred because the airflow was stalling on the left wing (on the inside of the turn), at least to a greater extent than it was on the right wing. This resulted in an asymmetrical distribu-tion of lift. The turbulence may also have generally had an asymmetrical effect. In the process, the right wing produced more lift than the left wing and allowed the aircraft to roll further to the left.

- Ju-Air’s Junkers type Ju 52/3m g4e aircraft were known to roll towards the in-side of the turn in the event of a stall during a turn, which leads the bank angle of the aircraft to further increase and subsequently decrease both the pitch attitude as well as the turn radius.

- The angle of attack must be reduced in order to stop the stall and bring the aircraft back under control. The way to achieve this is to reduce the deflection of the elevators and adjust the ailerons in the direction of the roll. Under no circumstances should an attempt be made to stop the roll by adjusting the ailerons in the opposite direction of the roll, as deflecting the aileron on the wing that is on the inside of the turn downwards only increases its angle of attack, making it more difficult for the airflow to reattach to the wing.

- In this investigated accident, the experienced pilots reacted appropriately and evidently tried to bring the aircraft back under control by adjusting the ailerons in the direction of the roll.

In principle, this initiated the process of bringing the aircraft back under control. It was, however, no longer possible to successfully perform this manoeuvre due to the aircraft’s proximity to the terrain, as a corresponding simulation has shown.

The sequence of events in the lead up to the aircraft’s collision with the ground in terms of flight mechanics can be explained as follows: As video footage recorded from inside HB-HOT shows, the aircraft was subject to low-frequency buffeting dur-ing its increasingly steeper flight path, which indicates that the airflow was stalling once again on the wing or horizontal stabiliser. The last photograph of the aircraft before impact shows that the wings were deflected upwards to a lesser degree than when in horizontal straight flight and that the elevators had been deflected upwards to approximately half of their full deflection. The rudder was deflected slightly to the right. At this time the aircraft was approximately 108 m above the ground. A little over two seconds later, it rolled a further 186 degrees to the left and hit the ground in a vertical flight attitude with an almost vertical flight path at a speed of approximately 200 km/h. These values also indicate that during the attempt to regain control at an airspeed of between 170 and 200 km/h, the critical angle of attack was exceeded again due to an accelerated stall. It also becomes clear that at the time when the last photograph of HB-HOT was taken, there was a pro-nounced uneven distribution of lift, which led to a roll rate of approximately 90 de-grees per second in the final phase of the flight.

Both the analysis of the engine noise and the forensic examination of the throttle levers show that the full-throttle limiter was set to ‘on’ during the last phase of the flight. This means that the flight crew had not brought the engines up to the highest possible power.

The SUST analysed the human factors:

The following examines the reasons as to why two very experienced, well-trained pilots flew the aircraft into the basin south-west of Piz Segnas in such a risky man-ner, thereby creating the conditions that enabled the accident to happen.

The investigation proved beyond doubt that both pilots were aware of the basic principles of flying in mountainous areas as described above. It is therefore impos-sible for this situation to have occurred due to ignorance. The investigation also ruled out the possibility that an incorrect altimeter reading could have deceived the pilots with regard to the true altitude of the aircraft upon entering the basin. Video recordings of the altimeter displays during the accident flight prove that the altime-ters were set to a suitable reference barometric pressure. By comparing these readings with the actual altitude flown, determined using radar data and photo-grammetric measurements, it can be concluded that the altimeters gave accurate readings based on this reference barometric pressure and that the actual altitude was, in fact, greater than that indicated by the altimeters due to the temperature change in the atmosphere. From the data available it can also be concluded that, when entering the basin at an actual altitude of 2,750 m AMSL, the crew was shown an altitude of approximately 2,650 m AMSL on the flight deck. As former Swiss Air Force pilots, the two pilots would have had extensive geographical knowledge and known the elevations of all major alpine passes, as these are taught intensively during the training to become military pilots. It can therefore be assumed that, even without using a map, the flight crew knew that the Segnes pass, which lay ahead of them, had an elevation of 2,625 m AMSL. This in turn suggests that the entry into the basin was the result of a conscious decision. It is, however, also conceivable that the crew did not pay attention to the altimeters and instead entered the basin purely on the basis of visual impressions, as is often the case when flying in mountainous areas.

This risky behaviour was ultimately the result of the pilots’ flight training coupled with their development in Ju-Air’s operational culture, which led them to become accustomed to this kind of flying. In the months and years prior to the accident flight, several safety-critical flights were documented in which pilots A and B, either individually or in some cases together, failed to comply with safety-related regulations ...

In summary, it should be noted that under the conditions prevailing on the day of the accident, it was easily possible to fly through the basin south-west of Piz Se-gnas at the appropriate altitude and reach the northern side of the Alps by crossing the Segnes pass. In addition to all of the investigations which have led to this con-clusion, this is also illustrated by the fact that around one minute prior to the acci-dent involving HB-HOT, a trainee pilot and their flight instructor on board a motor-powered Cessna C152 aeroplane were able to fly over the ridge of the Segnes pass from south to north. The crew had chosen a flight path that would have al-lowed for a 180-degree turn or an alternative flight path at any time. However, this aircraft was also piloted at an altitude above the ridge which did not comply with the rules for safe mountain flying as published in the Swiss Aeronautical Infor-mation Publication (AIP). The flight crew of the Junkers Ju 52/3m g4e involved in the accident was demonstrably accustomed to breaking the commonly accepted rules for safe flying and taking high risks, which led to the flying tactics described. This habit therefore constitutes a factor which directly contributed to the accident.

Explanatory video by SUST (Video: SUST):


Figure 2 - Flight Trajectory (Graphics: SUST):


Figure 3 - Still Image abeam Martinsloch out of video, pitch +
Incident Facts

Date of incident
Aug 4, 2018

Classification
Crash

Aircraft Registration
HB-HOT

Aircraft Type
JUNKERS Ju-52/3m

ICAO Type Designator
JU52

This article is published under license from Avherald.com. © of text by Avherald.com.
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