On 20 July 2014, a Boeing 737-800 (SU-GDY) on a non-scheduled passenger flight from Cairo to Copenhagen and maintaining 4000 feet in day VMC under Copenhagen radar control passed very close to a privately operated Cessna 172 (OY-AKH) which was en route from Bornholm to Roskilde at the same altitude and which had previously entered the Copenhagen TMA without clearance and with only a mode 'A' transponder identification transmitting.
Noting that the air traffic services in the area where the conflict occurred were delegated to a Danish ANSP, the Investigation was delegated to the Danish AIB by the Swedish Accident Investigation Authority (SHK).
It was noted that the 66 year-old 172 pilot had a PPL and a total of 52 hours flying experience and was using a moving map displayed on a tablet fixed to the (occupied) front passenger seat control column, with which he "had limited experience" as his main navigation aid, although "a paper VFR sectional chart was available in the cockpit". The tablet display "included certain altitude restrictions concerning Malmo and Copenhagen TMA, Copenhagen Area and the presentation of the FIR boundary between Sweden and Denmark". The 43 year-old commander of the 738 was the holder of an Egyptian ATPL and had 12000 hours total flying experience which included 6000 on type.
It was established that having reached and levelled at 4000 feet, the 172 pilot had made contact with Sweden Control prior to entry into Swedish airspace and had been given a transponder squawk. He had then read this back incorrectly and, in the absence of controller challenge, had set it incorrectly - although without mode 'C', which did not appear on radar screens until after the investigated event. ATC radar subsequently observed the error and re-issued the correct code which was then set. As the aircraft approached Swedish airspace without having requested a permission to enter the controlled airspace of Malmo TMA, the controller - in the absence of any recollection that the altitude had been requested or given - had "presumed" that it "was flying below the lower limit of Malmo TMA" in Class 'G' airspace which was "common practice among pilots conducting VFR flights from Bornholm to other airports in Denmark". It was surmised that the pilot had presumed that his exchange with Sweden Control amounted to a clearance to enter the Class C airspace of the Malmo TMA - which it did not. It was further surmised that such an assumption (and the nature of the controllers response to the aircraft initial call - just to allocate a squawk and not to query the absence of Mode C) may have been a consequence of one frequency handling both TMA traffic and traffic in the Class G airspace below it, which was a different system to that used in Danish airspace where different frequencies handled traffic within and below the Copenhagen TMA.
Shortly before the 172 passed into the Copenhagen TMA from the Malmo TMA still at 4000 feet, "the passenger....became sick and vomited in the cockpit and on the pilot’s tablet" which "caused some distress in the cockpit and the pilot’s focus diverted from the operation of the aircraft" so that his "pre-planned descent, to an altitude below the lower limit of Copenhagen TMA, was delayed".
Meanwhile the Boeing 738 inbound to Copenhagen was maintaining 4000 feet on a projected track converging with that of the 172, which was closing on a westerly track, at angle of approximately 25 degrees. The 738 pilots stated that they had first observed the other aircraft "at a horizontal distance of 8 nm in their 2 o'clock position at the same altitude". A TCAS TA without altitude indication was then presented on their NDs at a range estimated by the available evidence of 2nm. No avoiding action was taken and the 738 crossed tracks with the 172 "at a radar presented horizontal distance of 0.0 nm and seemingly at the same altitude" and with the relative radar tracks showing that the 738 "was overtaking the 172 almost directly from behind at with a speed of approximately 175 knots". The 738 pilots' subsequently stated that the other aircraft had "passed in front of them" at a range of 2nm and had been a Cessna 172. It was considered that the track closure shown on radar prior to crossing would have made the 172 "appear as an almost static object in the horizontal plane" as observed from the 738 flight deck. It was noted that "a static object is harder to detect for the human eye than a moving object" and that "a pilot, who loses visual contact with a 'static' aircraft usually finds it harder to re-establish visual contact with such an aircraft than with a 'moving' aircraft". It was concluded that "seemingly, a misperception by the pilots of the 738 of (their) horizontal separation (from the 172) influenced their decision on not making an appropriate avoiding action".
Recorded radar data was found to have presented the minimum horizontal separation between the two aircraft as 0.0 nm which "indicated a separation of less than 0.05 nm, as a separation of between 0.05 nm and 0.149 nm would have been presented as 0.1 nm". It was considered by the Investigation that the accuracy with which an altitude of 4000 feet was being maintained by both aircraft was likely to have been greatest in the case of the 738 since the 172 had no autopilot and there was no reliable data to demonstrate otherwise.
The 172 pilot subsequently advised that he had first seen the 738 shortly after the closest proximity and the radar recording of the aircraft track showed that at this point, its groundspeed had suddenly "increased from 110 knots to 150 knots and then decreased back to 110 knots" over a 33 second period and that "during the same time interval and according to the radar presentation, it had turned approximately 30 degrees to the right followed by a similar turn in the opposite direction back to the original course". Since the temporary increase in groundspeed was well above the speed attainable in level flight, it was concluded by the Investigation that it had occurred during a rapid descent from 4000 feet towards the base of the Copenhagen TMA at 2500 feet. Immediately after this 'apparent' descent, the 172 pilot advised Sweden Control that he would be changing frequency to Copenhagen Information where, having checked in and advised and confirmed that he was at 4000 feet, was aware of the need to descend to below 2500 feet before reaching the Danish mainland on his current track. A new squawk was given and correctly read back but incorrectly set. The error was seen and corrected, after which the Mode 'C' altitude appeared as 2000 feet on the radar display at Copenhagen Information and was presented continuously and correctly for the remainder of the flight. The Investigation noted the claim of the 172 pilot – that Mode 'C' had been selected on his transponder throughout the flight – but was unable to find any evidence of a fault in this function.
The near miss had occurred in Swedish airspace delegated to Denmark for control purposes and after an unauthorised entry by the 172 into the Copenhagen TMA for which Copenhagen Control is the ATC authority. The controller responsible for this airspace advised having observed the track of the 172 without an altitude label, but had "perceived it to be an uncontrolled VFR flight flying below Copenhagen TMA" and for that reason had not considered it relevant to issue traffic information on it to the 738. It was noted that both the Swedish and Danish ATM systems had the capability to automatically couple the stored ATC flight plan of an aircraft with the radar track of the same aircraft. Such coupling required, amongst other requirements, that "the pre-assigned transponder code in the stored ATC flight plan was identical to the transponder code reply presented in the radar track and the radar track contained a Mode C altitude readout". Once all conditions were met, the aircraft label (registration or call sign) would then appear on the screen. In addition, it was also possible for a controller to achieve coupling between a radar label and a radar track without an altitude readout being present.
In respect of the applicable generic right of way rules which had applied in case of the investigated conflict, it was noted that the following 'General Rules' detailed in ICAO Annex 2 Chapter 3 had applied to the 738 but had not been followed:
- Converging: When two aircraft are converging at approximately the same level, the aircraft that has the other on its right shall give way.
- Overtaking: An overtaking aircraft is an aircraft that approaches another from the rear on a line forming an angle of less than 70 degrees with the plane of symmetry of the latter, i.e. is in such a position with reference to the other aircraft that at night it should be unable to see either of the aircraft’s left (port) or right (starboard) navigation lights. An aircraft that is being overtaken has the right-of-way and the overtaking aircraft, whether climbing, descending or in horizontal flight, shall keep out of the way of the other aircraft by altering its heading to the right, and no subsequent change in the relative positions of the two aircraft shall absolve the overtaking aircraft from this obligation until it is entirely past and clear.
- An aircraft that is obliged by these rules to keep out of the way of another shall avoid passing over, under or in front of the other, unless it passes well clear and takes into account the effect of aircraft wake turbulence.
The formal statement of Factors relevant to the event identified the following:
- During the sequence of events, ATC and TCAS of the 738 did not receive mode C information from the transponder of the 172.
- Due to the passenger becoming sick, the focus of the pilot of the 172 was diverted from flying the aircraft.
- Without authorisation, the 172 penetrated Copenhagen TMA.
- A seeming misperception by the pilots of the 738 of the horizontal separation influenced their decision on not making an appropriate avoiding action.
Safety Action taken as a result of the event was documented as follows:
- The Danish and Swedish ANSPs introduced the Prefix speed “N” in the radar label of aircraft using Mode A transponder function only seen by controllers, in order to prevent them mistaking the aircraft altitude indication for the aircraft speed indication.
- The Danish and Swedish ANSPs and the joint subsidiary of the Danish and Swedish ANSPs met with the Swedish Transport Agency and the Danish Transport Authority in order to conduct an information campaign regarding airspace rules and regulations for VFR pilots.
- The joint subsidiary of the Danish and Swedish ANSPs reviewed the EUROCONTROL initiative European Action Plan for Airspace Infringement Risk Reduction in order to ensure that “Best Practices” were adhered to.
- The Danish and Swedish ANSPs implemented an Airspace Intrusion Warning (AIW) on 22 January 2015. This is a ground (radar) based tool which warns the controller about an unauthorized penetration of a defined airspace volume. One critical input to this system is the aircraft transponder transmitting Mode C information. If the system predicts an imminent threat of intrusion or an actual intrusion is detected, an alarm is presented to the controller.
With the status of Suggestions, The Danish AIB stated that they would like to encourage:
- VFR pilots to conduct the en route part of their flight according to published VFR cruising levels, e.g. 3.500 or 4.500 feet, even when flying below transition altitude (TA).
- VFR pilots to at any time during flight activate the Mode C function of the aircraft transponder (if so equipped).
- All pilots to take into account the ICAO “Rules of the air”, Chapter 3 “General Rules” during all phases of flight.
The Final Report of the Investigation was published on 28 November 2015. No Safety Recommendations were made.