Collision shortly before touch-down

Citation

(2001), "Collision shortly before touch-down", Aircraft Engineering and Aerospace Technology, Vol. 73 No. 1. https://doi.org/10.1108/aeat.2001.12773aab.014

Collision shortly before touch-down

Collision shortly before touch-down

Keywords: Safety, Aircraft industry

Two Piper PA-28-61 aircraft were being used for flying training at Oxford Airport, Kidlington, each with an instructor and student pilot. One aircraft had call-sign Oxford 67, and the other Oxford 64. They collided shortly before both touched down on Runway 20. Oxford 67 was completing an instrument flying (IF) sortie with IF screens fitted and was carrying out an NDB/DME procedural approach to Runway 20. The IF screens, fitted to the student's windscreen (left-hand) and the side windows, complied with the CAA requirements for such modification. The screens comprise a set of vertical plastic louvres that are inclined towards the instructor, who retains an adequate outside view, whereas the student is obliged to fly solely by reference to instruments.

Oxford 67 was conducting circuits and landings on Runway 20 in the left-hand visual circuit and, just prior to the accident, was number two of three aircraft in the circuit. Oxford 64 had completed a "touch and go" on Runway 20 and had followed the visual circuit pattern with three aircraft ahead and one, Oxford 55, which had joined from overhead behind him. Oxford 67 was at that time on the outbound leg of the instrument procedure. The three aircraft ahead of Oxford 64 had made their downwind calls in sequence, stating their intentions, which were for the first two to "touch and go" and the third "to land". Oxford 55 called "downwind to land" when abeam the tower and was advised by ATC to "Report final. Three ahead in the circuit and one straight in not yet four miles". Oxford acknowledged the transmission. Oxford 64, who was ahead of Oxford 55, called "Downwind touch and go". The controller requested him to "Report final. Four ahead in the circuit and one straight in not yet four miles", which Oxford 64 acknowledged. Shortly after this Oxford 67 contacted the tower and called "Four to land" and was requested to "Report again at two (NM)".

At this point Oxford 55, realising that he would not be able to land from that circuit, elected to go-around early from the end of the downwind leg. Oxford 64 was turning from the base leg on to final approach and the commander, looking over his right shoulder, had visual contact with Oxford 67 above and behind his aircraft at a distance he estimated to be 2nm. Oxford 67 called "Two d to land" and was instructed to "continue". Immediately after Oxford 67, Oxford 64 called "Finals touch and go" and the controller also instructed him to "continue" which he correctly acknowledged.

The controller advised Oxford 67 that he had "One ahead for a touch and go", which he acknowledged and stated that he did not have the other aircraft in sight. The controller confirmed the position of Oxford 67 as approximately "half a mile" and then asked Oxford 64 if he had the "one ahead in sight", to which he responded that "he had one high finals". This referred to his earlier sighting of Oxford 67 over his right shoulder. From the tower the controller could see both aircraft one above the other, but due to the short distance between the two aircraft he could not be sure which aircraft was which. He asked Oxford 64 to confirm that the other aircraft was ahead. The only aircraft visible to the crew of Oxford 64 was the aircraft ahead of them in the circuit, Oxford 40 T, which was about to land. Oxford 64 therefore responded to ATC "ER negative one on the runway and ER we're about to cross the threshold". The controller continued to try and identify which of the two aircraft visible to him was Oxford 64. By this stage both aircraft were on very short finals and in close proximity to each other. He asked Oxford 64 if he was "the orange one", to which the pilot replied "Affirm". The controller, having correctly identified Oxford 64 as the lower aircraft, intended to pass him a landing clearance but instead transmitted "Six seven clear to land". This was acknowledged by Oxford 67. The commander had taken control of the aircraft and had asked the student to remove the IF screens when they levelled at the MDA, which was 500 feet above threshold elevation. He maintained control of the aircraft which had been flown at an IAS of 95 knots during the procedure and when given the landing clearance reduced speed and selected flap 40, intending to position the aircraft for the student to carry out the landing.

The controller transmitted the words "Oxford 64" but did not give any instructions or request any information before ceasing his transmission. Oxford 64 immediately called "Oxford 64 is going around", the instructor took control, looked back and up over his right shoulder and, seeing no other traffic, commenced a go-around. He applied full power, raised the nose of the aircraft and selected flaps 25. Almost immediately he saw the right wing of Oxford 67 and heard a loud bang as the two aircraft collided. In the collision, at an estimated height of 30 to 50 feet, Oxford 67 was pitched nose down and banked to the left. The commander was able to recover the aircraft to wings level and land straight ahead on or near the threshold of Runway 20. Oxford 64 corrected a turn to the right and turned towards Runway 20 while transmitting a Mayday distress call. He landed approximately one-third of the distance along Runway 20. None of the occupants of either aircraft was injured and they exited through the normal aircraft doors. The airfield Rescue and Fire Fighting Service attended immediately.

Analysis and conclusions

The collision occurred when the lower aircraft, Oxford 64, was in a climbing attitude and the higher aircraft, Oxford 67, which was descending, was just to the left of Oxford 64. The propeller of Oxford 64 contacted the right-hand flap and aileron of Oxford 67, parts of which detached from the aircraft. The tail tie-down plate, part of Oxford 67, sliced through the port wing leading edge skin of Oxford 64.

The aerodrome controller was acquainted with local procedures designed to accommodate the busy circuit at Oxford; this included use of the pin-board as an aid to the aerodrome controller's task. The circuit was a busy one, with, at one time, five aircraft in the visual circuit, of which three were on circuit training and two had rejoined. While there is a maximum limit of four aircraft carrying out dedicated circuits and landings at any one time, other traffic, both visual and those on instrument approaches, are permitted to arrive and land. The departure of two other aircraft was also being co-ordinated by the tower controller. Added to this was the instrument approach traffic carrying out the procedure. The spacing between the aircraft on the downwind leg had reduced, thereby creating a situation where the controller had only a short time to issue a late landing clearance between traffic clearing the runway and aircraft on the approach. The controller's difficulties were further compounded by his need to sequence the instrument approach aircraft into the circuit traffic. The controller, in order to maintain a visual plot of the relative position of the aircraft in the circuit, used a pin-board, which depicted the circuit pattern and aircraft locations. The updating of this by the controller reduced the amount of time spent visually monitoring aircraft positions.

The two aircraft reached a point where they were in such close proximity to each other that it was difficult for the controller to identify the aircraft by their respective call-signs and to determine which aircraft was ahead on the final approach. Having decided that Oxford 64 was the lower aircraft, it was the controller's intention to clear that aircraft to land but instead of using the call-sign Oxford 64 as intended he mistakenly used the call-sign Oxford 67.

The problems of conducting instrument flights in a busy circuit environment, where the need for constant lookout is paramount, are well-known. What may not be so readily appreciated is the speed at which seemingly large separation distances are eroded by relatively small airspeed differentials.

Included in the follow-up actions is an internal report on the accident by the Air Traffic Service (ATS) provider at Oxford, which made a number of recommendations concerning the provision of ATS, and these are in the course of implementation in association with the resident airfield users.

Circuit reporting procedures have been reviewed and a temporary operating instruction (TOI) issued by the manager ATC. This has resulted in more positive control instructions being given by controllers without restricting circuit training. A comprehensive re-write, partially covered by the TOI, is under way. Moreover, ATC limit traffic in the circuit positively to allow for current conditions and training requirements.

Regular meetings (monthly) will take place between the ATC manager, chief pilot and the head of training. The problem of traffic bunching in late downwind and base leg position leading to unplanned go-rounds had been identified during a 1999 inspection of Oxford by Air Traffic Services Standards Department (ATSSD), Southern Region Inspectorate of the CAA Safety Regulation Group (SRG).

ATS investigations of SRG, having investigated the ATC aspects of this incident, recommend that the problem should be re-addressed, with the ultimate aim being to facilitate a safe and orderly flow of traffic and to reduce the number of unnecessary go-rounds. ATSSD and Flight Crew Licensing Department of SRG intend to monitor and approve progress towards this aim. Therefore, any further recommendations by the AAIB are not appropriate at this stage.

ReferenceAAIB Bulletin (2000), October.