Inex-Adria Aviopromet Flight 450

Inex-Adria Aviopromet Flight 450, JP 450, was an international charter flight from Tivat in the Socialist Federal Republic of Yugoslavia to Prague, Czechoslovakia which crashed in the Prague suburb of Suchdol on October 30, 1975, at 09:20 AM. The McDonnell Douglas DC-9-32 with 115 passengers and 5 crew on board descended, under Instrument Meteorological Conditions (IMC), below defined Minimum Descent Altitude (MDA) during the final approach to Prague Ruzyně Airport RWY 25, entered a gorge above Vltava river, and was unable to outclimb the rising terrain. 75 of the 120 occupants died during the crash itself while 4 others died later in hospital. The accident remains the worst aviation disaster on the Czech Republic soil.

Aircraft
The aircraft operating the flight was a McDonnell Douglas DC-9-32, aircraft serial number 47457, manufactured in 1971, delivered new to Atlantis as before it was sold to Inex Adria Aviopromet in 1973 as.

Accident


The DC-9 was operated by Inex-Adria Aviopromet on a morning flight from Tivat to Prague with mostly Czechoslovakian tourists returning home from vacations on the Adriatic sea. Captain Miodrag Marović (40) and First Officer Rade Popov (49) had an uneventful flight with clear weather, until they entered a published landing procedure above PR non-directional beacon (NDB). Czechoslovakian ATC took over the aircraft at the border at 09:01. After identification, the crew asked for weather information. ATC responded that the total visibility in Prague is 1,500 m and on runway 25 (later known as runway 24) only 200 m. At the same time, ATC warned the crew that both the instrument landing system (ILS) for RWY 24 and the precision approach radar (PAR) were inoperative - "JP450, ILS and PAR are out of service on runway 25".

The pilot asked - "I understand, but what do we do now?". ATC responded - "You can continue to Prague or divert to another airport."

"Please wait" - the pilot replied.

After a while, the pilot continued - "So the RVR of 1,100 meters and the ILS and approach lights out of service?"

Since ATC never mentioned approach lights to be out of service, this message from the crew reveals that mental load started to build up caused by the uneasiness with the situation at the airport.

ATC responded - "The ILS is out of order. The approach lights and the runway lights are working. There are also beacons (NDBs) in operation."

Based on the information received, the crew decided to proceed according to the flight plan to Ruzyně Airport.

ATC contacted the crew again, while the aircraft was at an altitude of 1,500 m approaching PG NDB (beacon) from the southeast, and instructed the crew to continue to PG NDB then to PR NDB, and make a published, standard turn. ATC approved descent to 550 m (communication was in metres, not feet) and provided QFE pressure of 981. ATC's instruction was correctly read back by the JP450's crew. After passing overhead PR NDB the crew entered a published landing procedure. However, somewhere above the village of Vodochody the crew inadvertently missed to start the published right turn and continued in a broader turn over villages of Veliká Ves and Kojetice. (See Fig. 2). After executing a wider-than-published right turn, the crew entered the final approach for runway 25 in IMC conditions (fog). Without the ILS and PAR support the crew was placed in a challenging and stressful situation during the final approach. Stress hormones level found, post mortem, in the body of the First Officer proves the emotional stress the crew was exposed to.

In addition, while executing a non-standard right turn the crew positioned the aircraft to the south (left) of the published glide path for the final approach. This horizontal deviation from the glide path continued to increase and at the place of the impact it was approximately 0.7 nmi. (see Fig. 3)

During the turn, ATC again alerted the crew to an ILS outage, repeated the information about the QFE pressure at the airport, and required confirmation of the approach over two NDBs - "JP450, confirm approach over PR beacon and L beacon".

At 09:18 JP450 confirmed - "I understand, PR and L beacons".

This was the last transmission received from the JP450. During this communication the crew continued to descend below the altitude approved by the ATC and deviate horizontally from the glide path, flying directly towards a Suchdol’s gorge, cut by the Vltava river, and well below the airport's elevation. Unfortunately, there are no publicly available Cockpit Voice Recorder (CVR) or Flight Data Recorder (FDR) recordings, to credibly reconstruct the cockpit communication and actions of the crew. Moreover, some sources6,7 cited the official report that the CVR stopped recording some 15 minutes before the crash due to a short magnetic tape, so there is no data regarding cockpit communication in this critical part of the flight. The crew, most probably, realized their mistake after establishing a visual contact with the gorge. They selected full power, trying to climb above the rising ground of the river gorge, but it was too late. The aircraft first hit trees, 91 m below the airport's elevation, then struck a building and crashed into the residential area, leaving a 350 m long debris trace. Time of the crash was 9:20 AM. Taking into account elevation of the first impact 91 metres below the airport elevation, and the last approved descent to 550 meters QFE (above the airport elevation), the crew descended 641 m below the approved altitude.

At 09:21, when the crew did not report, ATC called them - "450, this is Prague. Confirm over PR beacon"

"JP450, can you hear me?"

"450, 450, do you hear?"

"450, I'm broadcasting blindly."

Response from JP450 never came.

Of 115 passengers and 5 crew on board, 40 passengers and 1 flight attendant survived the crash.

Conclusions
There are no publicly available investigation reports, CVR transcripts or FDR data, without which the true causes of the crash can only be speculated. It is unknown:


 * if aircraft's navigational instruments and altimeters were functioning correctly and if the crew set them and used properly,
 * if communications between the crew and ATC was clear, precise and without misunderstandings,
 * physical and emotional state of the crew and quality of the Crew Resource Management (CRM).

The crash ensued due to the crew descending the aircraft below published MDA and below altitude approved by the ATC. Precisely, taking into account the first point of impact, 91 metres below the airport elevation, and the last approved altitude of 550 meters QFE (above the airport elevation), the crew descended 641 meters (2100 ft) below the approved altitude.

Other contributing factors to the crash are:

Taking into account all known facts, this crash can be classified as a Controlled Flight Into Terrain (CFIT) type of accident.
 * flying the aircraft approx. 0.7 nautical miles (1.3 Km) to the left of the published glide path over PR and L NDBs, bringing it directly into the river gorge, which they didn't succeed to out-climb,
 * inoperative airport's ILS and PAR systems,
 * poor visibility,
 * spatial disorientation of the crew.

Unsupported theories on causes of the crash 6,7
Why the aircraft descended 641 meters lower than the ATC approved, was never clearly determined.

There were several widespread theories, but most of them were misguided, according to experts.

One of them claimed that the crew missed to set their altimeters correctly in accordance with the ATC's instruction. Crash investigation proved that the altimeters were set correctly, since the altimeter found at the crash scene had QFE pressure set at 981. Even if the crew was mistakenly descending having QNH in mind, collision with the terrain below the airport elevation (approx. 350 m Above Mean Sea Level) shouldn't have happened.

Second theory claimed the possibility that the crew confused altitude in meters for altitude in feet. However, the crew confirmed approved altitude several times, in meters. It is hard to imagine that two experienced pilots, with thousands of flying hours on the aircraft, would make such a mistake.

The inhabitants of Suchdol promoted another theory, based on alleged testimonies of survivors, that the crew mistakenly took illuminated railway track and road, under the Suchdol's gorge, for the airport. They based this theory on statements that passengers near the cockpit, shortly before the impact, have heard the pilot announcing visual contact with the runway and starting to land. If this was the case, it could be assumed that the pilot misinterpreted road/railway lights as the runway threshold lights and started landing prematurely. This theory is questionable, though spatial disorientation was a factor contributing to the crash, since an experienced crew, flying under IMC, should restrain from looking out from the cockpit (Visual Flight Rules flight) and must rely solely on their instruments (Instrument Flight Rules flight). However, bearing in mind that ILS and PAR were inoperative, it can happen that the crew was intensively looking out for a visual contact and thus missed to monitor the aircraft's altimeters and correlate read altitude to the ATC's instruction and airport's elevation.

The official report states that, during the inspection of the cockpit wreckage, it was found that "Both radio compasses on board the aircraft were set to only one beacon - PR". This fact does not explain the non-compliance with the altitude, but explains the horizontal deviation from the glide path, and testifies about the crew's acts and cooperation before the crash. On a non-precision approach, when using two beacons, the crew should have set both frequencies, so that the correct course can be followed. This is the reason why the crew did not detect, in the final approach, that they continuously deviate from glide path over PR NDB and L NDB to the south (left). Returning to the previous consideration that the crew was under stress and overwhelmed mentally due to inoperative ILS and PAR, it is highly likely that they were rather focused on looking for an outside visual contact than on instruments. This standpoint is supported by Ladislav Keller, then Czechoslovakian civil pilot and air accident expert, who stated that the insufficient setting of the radio compasses indicated that the crew did not prepare well for landing, did not cooperate properly, and missed to monitor the instruments during the final approach. If the pilots would have been monitoring the altimeters and the approach map, they would have stopped the descent the latest at the Minimum Descent Altitude (MDA), used for non-precision approach. Unfortunately, this did not happen.

Keller said: "If the crew stopped descending at decision height (Wiki author's comment: term Decision Height is used for a precision approach, while Minimum Descent Altitude term is used for a non-precision approach, which was flown by JP450), ''there would probably be no accident because the plane would fly about 90 meters above the runway threshold. However, the crew descended to an altitude of 100 meters below the runway threshold elevation. This indicates that the crew was totally disoriented, did not know their position and did not monitor the altimeter at all. There is a presumption that the pilot was not watching the instruments and looking out of the cockpit." 6,7''

External links used as references

 * 1) 1975: letadlo zmizelo v mlze, pak se ozval náraz. Zemřelo 77 lidí, article in Sedmička (retrieved from archive.org), including pictures from the site
 * 2) (in Czech)
 * 3) Article in Czech with photographs from the accident
 * 4) Accident Photos https://zpravy.aktualne.cz/domaci/foto-unikatni-snimky-uplynulo-45-let-co-prazsky-suchdol-zdev/r~0d10bfd01ab411eb9d74ac1f6b220ee8/v~nahledy/
 * 5) https://www.idnes.cz/zpravy/cerna-kronika/40-let-od-padu-letadla-v-suchdole.A150925_115617_domaci_cen
 * 6) https://www.idnes.cz/technet/technika/nejhorsi-letecka-nehoda-ceskoslovensko-suchdol.A121029_145902_tec_technika_sit
 * 1) https://www.idnes.cz/technet/technika/nejhorsi-letecka-nehoda-ceskoslovensko-suchdol.A121029_145902_tec_technika_sit