The Close Pass

Both the F-16 and LN-PAA were flying in accordance with Instrument Flight Rules (IFR). In practice, this means that the air traffic service is responsible for separating the aircraft from one another. For safety to be maintained, the pilots must adhere to their assigned altitude.

In 1989, civilian aircraft had no means of detecting other aircraft in the vicinity, and were therefore dependent on the air traffic service for information about any other traffic in the area. "Essential traffic" is air traffic that will come closer than the minimum separation prescribed by regulations, and which the air traffic controller is obliged to inform pilots about. In this case, the aircraft were separated in accordance with regulations by 2,000 feet of vertical separation, and the traffic was therefore by definition not "essential." The fact that the F-16 pilot nevertheless referred to LN-PAA as "essential traffic" on the radio is in itself an indication that the actual separation was far less than 2,000 feet.

The information that there were several fighter jets in the air simultaneously, and that they were flying just ahead of the Partnair aircraft, has not been mentioned by the accident investigation board. Since the crew of the F-16 was not informed by either Danish or Norwegian air traffic control about which aircraft they were encountering in the direction of Rygge military air station, confusion could naturally arise.

Radar data showing that the F-16 descended in altitude has been omitted from the report by the accident investigation board. The combination of the radio recording and the radar plot makes it probable that the close pass was not a coincidence.

Radio Communication

To fly in controlled airspace, as all IFR traffic does, pilots are required to be in contact with the air traffic service by radio. When an air traffic controller uses an aircraft's callsign, it means that the following message is intended for that specific aircraft. Since one can hear communications with other aircraft, pilots can form a mental picture of the traffic in the area.

A challenge with civilian and military aircraft in the same airspace is that civilian aircraft use VHF frequencies, while military aircraft use UHF. The air traffic service can use both VHF and UHF simultaneously. In practice, this means that civilian aircraft only hear the voices of pilots in other civilian aircraft, while military aircraft only hear other military aircraft. When the air traffic service transmits on VHF and UHF simultaneously, all pilots in the area hear what the controller says, regardless of whether the controller is addressing civilian or military aircraft. Without hearing both sides of the conversation, it is in many cases impossible to interpret which aircraft the radio communication concerns and where the aircraft is located.

The F-16 was on Copenhagen Control's UHF frequency when Partnair contacted the same controller on the VHF frequency. The F-16 pilot therefore did not hear Partnair report its altitude (flight level 220), but could only hear the controller's reply: «Partnair 394 good afternoon, radar contact, Copenhagen control.» Based on this information, the F-16 pilot had no way of knowing that the aircraft, which appeared to be coming from the same direction as Rygge, was LN-PAA.

Other Fighter Jets

Together with the Partnair aircraft, in Oslo Control's airspace, the military aircraft Viper 20 and Viper 71 were also en route to Aalborg. These were likely American F-16s that had participated in an exercise that day. The crew of the Norwegian F-16 likely had knowledge that there were other F-16s in the area. Viper 20 and Viper 71 had probably already passed over Denmark when the Norwegian F-16 (Norwegian 5007) took off from Aalborg.

F-16 Fighting Falcon

Many military aircraft have a nickname in addition to their official name. The F-16's nickname is "Viper."

«The F-16 was given its name of "Fighting Falcon" on 21 July 1980. Its pilots and crews often use the name "Viper" instead, because of a perceived resemblance to a viper snake as well as to the fictional Colonial Viper starfighter from the television program Battlestar Galactica, which aired at the time the F-16 entered service.» (F16 Fighting Falcon, Wikipedia)

Viper 20 and 71

At the same time as the Partnair aircraft was climbing towards its cruise altitude, two military aircraft were in the same airspace. These had the callsigns "Viper 20" and "Viper 71." Both were flying shortly after one another, but not in formation. During formation flight, only the formation leader communicates with air traffic control on behalf of everyone in the formation.

According to the radio log, Viper 20 and Viper 71 cross into Danish airspace at approximately 14:10. They are then approximately midway between Rygge and Aalborg, a flight that according to Norwegian 5007 took approximately 20 minutes.

«The undersigned flew on Friday 8 September 89 from Aalborg Air Station with take-off at 1420 Zulu. After one minute, the aircraft was established at FL 240 en route to Rygge. At approximately 1430Z, I detected an aircraft on the radar coming straight towards me—at approximately the same altitude. … Flew VFR into Rygge and landed at approximately 1440Z.» (Norwegian 5007, 1989, p. 1)

When Norwegian 5007 took off from Aalborg, Viper 20 and Viper 71 had likely already reached the Danish mainland.

Radio Log

Clearances and instructions from air traffic control must be read back by the pilot to confirm that the message has been understood and will be carried out. The readback functions as a contract between the controller and the pilot.

Military aircraft use UHF frequency, while civilian aircraft use VHF. What civilian aircraft say on the radio therefore cannot be heard by military aircraft, and vice versa. Air traffic control hears both military and civilian aircraft, and transmits on both VHF and UHF simultaneously so that all aircraft hear what the controller says.

The radio communications below are taken from Oslo Control (1989, pp. 1–2) and Copenhagen Control (1989, p. 1). The purpose is to show the timing of relevant events, and irrelevant parts of the communication have therefore been omitted.

(Oslo Control, 1989)

The Pass

After the encounter with LN-PAA, a confrontation arises between the F-16 pilot and the air traffic controller. In the recording of the radio communication, the F-16 pilot is heard saying: «You knew you had that airliner coming straight at us here?» After a response from the controller, the F-16 pilot continues: «Just confirm that you had him, pretty essential traffic.»

Norwegian 5007 was a two-seat F-16 with a crew of two qualified F-16 pilots. According to the Armed Forces, the pilot in the back seat had no assigned responsibilities on the trip from Aalborg to Rygge, but as a qualified F-16 pilot and instructor, he was part of the crew. He testified in the lawsuit Partnair brought against the state, and also claimed in the documentary that the F-16 maintained its assigned altitude. (Wisting, 2023)

The radio recording between the F-16 pilot and the air traffic controller clearly shows that the F-16 pilot was agitated. That he would react to an aircraft 660 metres (2,000 feet) below his own altitude seems entirely incomprehensible. The only logical explanation is that the F-16 and LN-PAA passed each other at very close range. The sharp criticism directed at the air traffic controller suggests that the F-16 pilot believed he should have been informed about the civilian, opposing passenger aircraft that the controller had on his radar screen.

The recording of radio communication between the F-16 pilot and the air traffic controller is not discussed in the accident report. The fact that two military aircraft, likely F-16s from Rygge, were flying towards Aalborg at the same time as the Partnair aircraft has been completely omitted. In this way, the accident investigation board bypassed everything that could have put the spotlight on the Armed Forces.

Motivation

A central question is why the F-16 crew apparently departed their assigned altitude and flew towards LN-PAA. The answer may lie in the fact that they believed they were encountering a colleague.

The F-16 crew likely knew that other F-16s—probably American aircraft that had participated in an exercise—were en route to Aalborg in the opposite direction. When the F-16's radar detected a target in the direction of Rygge, the crew may have assumed that this was one of those aircraft. Among fighter pilots, it is not uncommon to carry out a close pass of another fighter—a "stunt" that both parties are trained for and prepared to handle. In such a scenario, a close pass would have been considered harmless by the crew, since they expected that the pilot of the opposing aircraft was a colleague accustomed to such encounters in the air.

The problem was that the target on the radar was not an F-16, but the Partnair aircraft LN-PAA—a civilian passenger aircraft with 55 people on board. Neither the Danish nor the Norwegian air traffic controller informed the F-16 crew that the civilian aircraft was in the area, and the F-16 pilot could not hear Partnair's radio conversation with the controller since it took place on VHF. The misidentification thus became catastrophic.

Since the F-16 is equipped with its own radar, this may have given the crew a false sense of security that they had the situation under control—which may have made it easier to break the safety rule of maintaining their assigned altitude. Given that the F-16 pilot switched off the aircraft's transponder, the aircraft would in practice have become invisible in the radar system of the civilian air traffic service.

When the F-16 pilot says on the radio «You knew you had that airliner coming straight at us here?», the controller replies: «Norwegian 5007 will you confirm that you are flight level 240.»

The controller's question may suggest that he did not have an up-to-date altitude reading on his radar screen, but perhaps only a prediction from the radar system. If the F-16's transponder had been switched off, the controller would only have had the last known altitude to rely on.