On 7 July 2015, a Lockheed-Martin F16CM (96-0085) being operated by the US Air Force on an instrument procedures training mission also intended to function as a post-maintenance airworthiness function flight from and returning to Shaw AFB and a privately operated Cessna 150M (N3601V) which had just departed Berkeley County Airport were involved in a mid-air collision in day VMC. The Cessna was disabled and crashed nearby killing both occupants and impact damage to the F16 prevented continued flight so its pilot ejected, sustaining minor injuries, and the aircraft crashed and was destroyed by impact forces and a post-crash fire. The F16 pilot transmitted a MAYDAY after ejecting.
An Investigation was carried out by both the NTSB and the USAF. The Investigations were based primarily on the recordings of ATC radar and ATC radio communications with the F16; the Cessna 150 was not in communication with any air traffic service and its pilot had not filed a Flight Plan. The Crash Survivable Memory Unit (CSMU) and the Seat Data Recorder (SDR) from the F16 were recovered and their data were used to confirm that there had been no relevant airworthiness issues with that aircraft. The weather conditions were benign with good in-flight visibility and were not a collision risk factor.
It was recorded that the 34 year old F16 pilot had approximately 2,383 hours total flying time which included 624 hours on type and 1,511 as a ground based pilot of the MQ1B and the MQ9. The 30 year old Cessna 150 pilot had gained his PPL just over six months previously and had 244 total flying hours of which 239 hours were on type.
It was established that the collision took place in Class 'E' airspace which had a base of 700 feet agl (800 feet QNH) and that having taken off from a non-towered airport, the Cessna was not - nor was it required to be - in contact with any ATS provider. The F16 which had earlier in the sortie (for which an IFR flight plan had been filed) made two practice approaches at Myrtle Beach, was in transit from there to Charleston for further practice approaches before returning to Shaw AFB. The pilot had contacted the Charleston radar control frequency when approximately 45 miles to the northeast of the airport at 6,000 ft QNH. After requesting radar vectors for an approach to runway 15 at Charleston, the pilot was instructed to fly a 260° radar heading and descend to 1,600 feet QNH which was the Minimum radar Vectoring Altitude (MVA). The resulting track and altitude meant that the F16 would pass about 2 miles south of Berkeley County Airport and be 500 feet above the standard circuit altitude there. The Radar controller was "controlling other aircraft at the time" but characterised her consequential workload as "light" and "routine". As the descent of the F16 to 1,600 feet occurred, there "were no conflicts or other aircraft along its route".
Less than a minute after the F16 had reached 1,600 feet, the Cessna 150, which had just taken off from runway 23, appeared on radar with a Mode C transponder ID showing 200 feet QNH "approximately 16 miles to the east of the F16". The radar controller "assumed that the Cessna 150 intended to stay below 1,000 feet QNH within the Berkeley Airport circuit to practice landings". She "did not advise the F16 of the C150's presence […] it had already broken the controller's assumption by climbing above 1,000 feet" by which time it had taken up an east south easterly track away from the circuit and established a steady rate of climb at about 400 fpm. Two minutes after appearing on radar, and with 1½ minutes to go before the collision, the Cessna was passing 1,000 feet QNH and the F16 was at 1,600 feet QNH "roughly 7 miles northeast of the Cessna 150's position". The first illustration below shows the converging tracks annotated with the altitudes of both aircraft at one minute intervals. The second shows expanded detail of the collision vicinity with times, altitudes and radio calls to and from the F16.
The converging tracks of the two aircraft prior to their collision at 1600 feet at 1100:55. [Reproduced from the USAF AIB Report]
Radar data indicated that as the two aircraft converged to within approximately 3.5 nm laterally and 400 ft vertically one minute before the estimated time of the collision, a visual and aural Conflict Alert was triggered at the radar controller's position and "about 3 seconds later" with the aircraft now approximately 2 miles apart and "closing in towards each other at 300 knots" and shortly after the Cessna 150 had passed through 1,200 feet QNH climbing, the controller issued a traffic advisory to the F16 - "traffic, twelve o’clock, two miles, opposite direction, one thousand two hundred indicated, type unknown". This transmission was the first mention of proximate traffic given to the F16 pilot and occurred "2 minutes and 25 seconds after it had first appeared on radar".
When the F16 pilot immediately responded that he was looking for the traffic, the controller added four seconds later that he should "turn left if he did not see it". The F16 pilot "did not hear that instruction because he was fixated on finding the other aircraft and asked the controller to confirm the distance" to which the response was to “turn left, heading 180, immediately” if the traffic was not seen. This instruction was immediately acknowledged and with the traffic still not seen, a standard rate 30° banked turn was commenced with the AP remaining engaged to facilitate a continued visual scan. The left turn placed the F16 on a collision course with the still unseen Cessna and as the F16 passed through 150° degrees in the turn, the controller added "traffic passing below you, one thousand four hundred". A few seconds later, the two radar returns merged as the collision occurred at 1,600 feet QNH.
It was found subsequently that the F16 pilot had finally sighted the Cessna when it was "less than 500 feet away, less than a second before the collision and had pulled full aft stick in an attempt to avoid collision" without success. Immediately prior to the sighting, he reported having been looking below the level of his aircraft.
After considerable modelling using the available data, the USAF Investigation concluded that "It is highly unlikely that either pilot involved or any pilot in a similar situation could have seen the other aircraft prior to the controller's traffic advisory".
Detail of the annotated ground tracks immediately prior to the collision (MA1/MA2 are the F16/C150 aircraft, MP1 is the F16 pilot and 'MC' is the Charleston Radar Controller). [Reproduced from the USAF AIB Report]
The main concerns of both Investigations centred on the radar controllers approach to de-confliction and the fact that neither of the aircraft involved had made full use of the available support to the well-known limited effectiveness of "see-and-avoid" in Class 'E' controlled airspace where no separation is provided between IFR traffic and VFR traffic.
The Radar Controller
- The decision to place the F16 "on a flight path projected to pass close to the departure end of the runway at a non-towered airfield on an intersect with normal departure routes" had "required significant vigilance to avoid the development of a conflict".
- The first traffic advisory was given to the F16 only about 40 seconds prior to the collision which was inadequate. It was considered that the initial assumption that the observed (Cessna 150) traffic would remain in the Berkeley Circuit was inappropriate since it had indirectly compromised late initial traffic advisory.
- The decision to turn the F16 left was made on the assumption that the F16 could and would "turn on a dime" whereas at the airspeed involved, the radius of any turn would have been considerable even if, as the controller reported having expected, the qualification "immediately" she had used in the final avoidance instruction had resulted in a steeper turn with the AP out. It was considered that to achieve last minute avoiding action, an additional degree of urgency should have been added to the call, possibly 'expedite'.
- It was considered with hindsight to be clear that "any turn of the F16 to the south would increase the likelihood of collision". It was noted that although turning the F16 to cross in front of the Cessna 150's track "was not contrary to FAA guidance for air traffic controllers, it was the least conservative decision, as it was most dependent on the F-16 pilot's timely action for its success".
- It was noted that had the controller instead "issued a right turn or climb, this would have avoided the conflict". The controller stated that she had not given a climb because "the altitude indicated for the Cessna's radar target was unconfirmed".
Use of collision avoidance aids
There were 'systems' available which may have improved or enhanced both pilots' situational awareness of other aircraft and potential conflicts with them but they were not used. The Cessna 150 pilot had not, by the time of the accident, sought flight-following from ATS Charleston, although he was still sufficiently close to his departure airport to have been monitoring the CTAF for awareness of aircraft in the vicinity of Berkeley as recommended by the FAA and it was considered, "based on statements from his flight instructor and from his logbook entries, which both cited past experience communicating with ATC" that he would probably have been intending to contact ATC at some point during the flight. It was also noted that the F16s sensors, as configured, did not provide its pilot with information on the proximity of the Cessna 150. It was nevertheless considered that whilst the support which both of these 'systems' could have provided to all parties involved "would have increased their situational awareness and aided each pilot with the difficult task to see and avoid other aircraft, there is insufficient evidence to conclude this, by itself, would have prevented the collision".
It was noted by the NTSB Investigation that there are "inherent limitations" to see-and-avoid which include "limitations of the human visual and information processing systems, pilot tasks that compete with the requirement to scan for traffic, the limited field of view from the cockpit, and environmental factors that could diminish the visibility of other aircraft". In this connection, the USAF Investigation noted that the sequence of actions required to react to an initial sighting of a proximate aircraft, typically adds up to 12.5 seconds - see the illustration below. It also noted that "when conducting a visual scan, or clearing, the procedure is to focus on 30 degree wide portion of the sky, focus on the ground about 4-8 miles away, then move up and scan a few inches above and below the horizon, scanning for at least 2 seconds in each sector". Because of the relatively low speed travelled by aircraft such as the Cessna 150, its pilot "would be required to scan a large area of the sky (approximately 180 degrees) to find potential conflicts" and a complete scan "would take more than 15 seconds to complete". The effectiveness of such a scan could easily be reduced by anything that interrupts the direct line of sight, "ranging from aircraft structures to a bug splatter on a windscreen". It was considered that "at the extreme edges of influence, these can either block a pilot’s vision or trap the eye, drawing a pilot’s focus away from scanning". In the specific case of the Cessna 150 pilot's chances of spotting the F16, it was found through modelling that the prominent wing strut on the Cessna 150 would have obstructed the pilot's view of the F16 "from approximately 1 mile away until immediately before impact".
Response time from initial visual acquisition of a potential conflicting aircraft. [Reproduced from the USAF AIB Report]
It was also noted that the size of a conflicting aircraft is likely to be very small and the constant relative bearing which characterises a real collision risk target makes early detection even more difficult. The USAF Investigation provided an illustration of this which is reproduced below.
The distance-speed-time(seconds) graphics of an F16 and a Cessna 150. The distance/time are shaded at the approximate time of the first traffic information given to the F16 pilot. Note that the depicted aircraft sizes would appear as shown if the illustration is viewed from 63.5cm. [Reproduced from the USAF AIB Report]
The Human Factors perspective on the collision for the F16 pilot was explicitly examined by the USAF Investigation. Two human factors relevant to the mishap were identified, 'Inaccurate Expectation' and 'Fixation'. Inaccurate Expectation applied because it was usual to receive initial traffic information at 20nm rather than 2nm separation and when receiving it at 2nm, the F16 pilot was forced into “aggressively” working to visually acquire and avoid the other aircraft. It was noted that "individuals in time-critical situations are susceptible for skill-based inaccuracies (and) therefore when unexpected situations force individuals to rapidly process and react, there is increased potential for error". Fixation, "when the individual is focusing all conscious attention on a limited number of environmental cues to the exclusion of others" occurred when the F16 pilot did not hear the controller's first "turn left" instruction whilst undertaking the demanding task of visual scanning. It was noted that "it is not uncommon for task fixation to occur during highly automatic or learned behaviour", especially during a time-critical situation.
The USAF Investigation concluded that the Cause of the accident was "a combination of the Controller issuing a radar vector directing the F-16 Pilot to immediately turn left onto 180° and placing the F16 and the Cessna 150M on a collision course and the fact that neither pilot was able to see and avoid each other in time to prevent a collision".
Three 'substantial' Contributory Factors were also identified:
- The Controller directed the F16 to fly on a course and at an altitude that would increase potential for conflicts by passing in close proximity of the departure routes of an uncontrolled airfield.
- Neither pilot used the available systems which may have increased situational awareness.
- The Controller did not provide the F16 pilot with traffic information in time for them to identify the conflict before the same controller issued the immediate left turn instruction.
The NTSB Investigation determined that the Probable Cause of the accident was "the approach controller's failure to provide an appropriate resolution to the conflict between the F-16 and the Cessna".
A Contributory Cause was identified as "the inherent limitations of the see-and-avoid concept, resulting in both pilots' inability to take evasive action in time to avert the collision".
Safety Action taken after the accident both by the NTSB and locally by the Berkeley County Airport operator was aimed at raising collision risk awareness and, in the light of the limitations of see-and-avoid, promoting the full use of all available support systems especially, in the case of VFR aircraft, requesting flight-following by ATC in Class 'E' airspace.
The Final Report of the NTSB Investigation was adopted and published on 15 November 2016 and the Final Report of the USAF AIB Investigation was completed on 24 October 2016 and released on 9 December 2016.