On 7 March 2017, a Cessna 525 Citation in Class 'G' airspace came into very close proximity with an unseen Ikarus C42 in day VMC at an altitude of 3,500 feet after the pilot of the C42 had, following a late sighting, executed last minute avoiding action to effect a recorded separation of less than 100 feet vertically and less than 185 metres laterally. Both aircraft were operating VFR in good forward visibility.
The location of the CPA and the recorded prior flight paths of the two aircraft. [Reproduced from the Official Report]
An Investigation was carried out by the UK Airprox Board. Although the event occurred outside controlled airspace, a relevant R/T recording and a high quality radar recording of the occurrence was available from the ANSP which had been providing FIS to one of the aircraft involved.
It was established that the Citation had initially requested a direct routing to its destination, Biggin Hill, whilst working London Control over the English Channel and six minutes later (and 11½ minutes prior to the CPA) had been instructed to contact London Information whilst in descent to an indicated FL050 and passing FL182 whilst still in controlled airspace. On checking in with London Information, the Citation pilot "reported being on" an RNAV STAR designated for London City arrivals, despite this not being their destination and advised their intention to "leave the STAR once they had descended out of controlled airspace". The London Information FISO "confirmed that it would be a Basic Service once they left controlled airspace" (the base of which was FL 075). The Citation pilot stated that the aircraft would cross the UK coastline "in the Eastbourne area" and the FISO requested that they report doing so. Shortly after the Cessna pilot had reported passing FL75 and "turning towards the Seaford area", the FISO "allocated the London FIS Conspicuity code 1177 and confirmed it was (now) a Basic Service" and that the flight would be "transferred to Farnborough Radar as they passed Eastboume".
About 2 minutes later, the Cessna pilot reported having left FL050 for 3,500 feet QNH and 2½ minutes after that reported crossing the coast "in the descent to 2,300 feet" (an altitude which would keep the aircraft beneath the London TMA). In reply, the FISO instructed the aircraft to squawk 7000 and freecall Farnborough Radar on a frequency provided. One minute later, at 3,500 feet QNH and prior to making the initial call to Farnborough, the Citation passed very close overhead the unseen C42. The Citation pilot stated that he had been routing towards the Mayfield VOR. As the two aircraft had approached each other, it was estimated that the Citation's speed was 240 knots and the C42's speed was 55 knots - a closing speed of almost 5nm per minute.
The C42 pilot stated that they had been conducting a student training flight on stalling and in order to stay clear of congested airspace at lower altitude he had climbed to 3,700 feet QNH where he had been demonstrating to the student that only a small amount of aileron was required to maintain an accurate and constant heading. Having seen the other aircraft on an almost reciprocal track, they had "executed a steep turn to the right". The other aircraft was seen to be heading in a northerly direction "apparently with no evasive action taken". At the time of the conflict, the C42 had been "listening out" on the unmanned Deanland Airfield air-ground radio frequency and was not receiving any form of ATS. The C42 pilot had subsequently called Farnborough Radar to report the AIRPROX and had then returned to the scene in order to squawk for Farnborough and allow the location to be recorded before continuing the training detail. This pilot provided video evidence of the event to the Investigation - see the stills sequence reproduced below.
The view from the C42 approximately 2 seconds prior to the CPA. [Reproduced from the Official Report]
The view from the C42 approximately 1 second prior to the CPA. [Reproduced from the Official Report]
The view from the C42 approximately at the CPA. [Reproduced from the Official Report]
Area radar evidence submitted by the ANSP showed (see the illustration below) the two aircraft with altitude tags (the C42 had a Mode C Transponder and the Citation a Mode S Transponder) approaching each other almost head on just 12 seconds prior to the CPA (Closest Point of Approach). This was the point when the Citation had reported "crossing the coast" and been directed to call Farnborough Radar by the London Information FISO. The Investigation noted that UK Regulatory requirements prevent a FISO from utilising surveillance-derived data to provide traffic information when providing a Basic Service which since the Citation was VFR was the only type of service that could be offered. The UK CAA notes that such a Basic Service "relies on the pilot avoiding other traffic unaided by controllers / FISOs". It was noted that the fact that the Citation had been transferred by London Control to London Information whilst still inside controlled airspace had been "non-standard" since "an ATC service cannot be provided by a FISO to an aircraft inside controlled airspace".
The radar approach tracks and position of minimum separation of the two aircraft. [Reproduced from the Official Report]
The Board concluded that the Cause of the occurrence was "a late sighting by the C42 pilot and a non-sighting by the C525 pilot".
One Contributory Factor was also identified:
- The C525 pilot reported not receiving a TCAS alert (for which no explanation could be found from the evidence available).
The Final Report of the Investigation was agreed in principle on 21 June 2017 and subsequently published. The ICAO Risk Category 'A' was assigned (an aircraft proximity in which serious risk of collision existed). No Safety Recommendations were made.