Collision

Citation

(2001), "Collision", Aircraft Engineering and Aerospace Technology, Vol. 73 No. 3. https://doi.org/10.1108/aeat.2001.12773cab.004

Collision

Collision

The accident occurred when a Cessna Fi50J and a Yak Aerostar collided in the circuit at North Weald Airfield. The weather at North Weald was observed as 200/15 knots, few at 3,000ft with unlimited visibility. The pilot of a Yak 50 aircraft, was a member of a seven aircraft formation aerobatic team operating from North Weald. Following a comprehensive pre-display briefing, the team conducted a pre-display rehearsal over the airfield at North Weald. When the display rehearsal was completed, the Yak, in company with another, departed for Little Gransden, Cambridgeshire, where the second Yak was based, and for the pilot to collect an item of display equipment. The Yak departed for North Weald and was shown on radar recordings to transit from Little Gransden Airfield direct to the West of Harlow and then route around to the South of the town, heading East towards North Weald. The owner of the Cessna had travelled to North Weald and took off on Runway 20 (right-hand circuit) and departed to the South East from the end of the down-wind leg. The aircraft then travelled West, passing to the South of North Weald, and joined the circuit on the downwind leg at 16.19 hours.

At 16.20 hours the Yak pilot, approaching the circuit from the West, called North Weald Radio "Aerostar initial ER break to land". The radio operator in the tower saw the aircraft at about circuit height and North Weald Radio responded, "Roger One aircraft late downwind". The Yak pilot confirmed he had the Cessna in sight by responding, "Visual with him and ER 30 seconds to initial". The term initial was used by the formation aerobatic team, of which he was a member, when 1 n mile on the extended centreline from the airfield to signify the IP before running in for the break. The Cessna at this point was approximately 1.5 n mile South of the Yak, which continued on an easterly track to pass in front of the Cessna. Shortly after the last transmission the Yak was seen by the radio operator to enter a dive, levelling at about 100ft which was held momentarily before entering a climbing turn. This manoeuvre was carried out in the vicinity of the pilot's house. Due to the distance from the aircraft the radio operator could not determine whether the turn was to the left or right, however a witness close to the aircraft track who also observed the manoeuvre saw the aircraft make a left turn. He then looked away.

At about that time the Cessna made a right turn onto base leg. Using the last seven radar returns and allowing for the prevailing wind speed it was possible to calculate the average ground speed of the Cessna as 87 knots and that of the Yak as 124 knots. The Yak climbed back up to approximately circuit height and transmitted to North Weald Radio, "Aerostar 4 is initials for the break and the other traffic is?" Thinking that the Cessna pilot would have heard this call the radio operator waited for him to respond but, on not hearing any transmission, looked up to try and locate both aircraft but could not see them.

An off-duty air traffic control officer saw the Cessna on a constant easterly heading at about circuit height. He watched the Yak initially in straight and level flight on a South westerly heading make a gentle left turn using some 5 to 10 of bank as if turning towards North Weald. It was also at about circuit height with engine power sounding as if at a low to medium power setting, with the aircraft nose slightly above the horizon. Just before the two aircraft came together the Cessna made a very rapid roll to the right to a steep bank angle and, about one second later at 16.21.44 hours the two aircraft collided. The right-hand wing of the Cessna detached and the aircraft descended rapidly in a spiral to the right, impacting the ground in an open field. The Yak also made a steep descent, impacting in another field, 150 metres South of the Cessna. The two occupants of the Cessna and the occupant of the Yak received fatal injuries in the impacts.

Air traffic communications at North Weald comprise an aerodrome air/ground service with licensed radio operators passing information to, but not controlling, aircraft. Although there is no requirement to record RT traffic, such a recording was routinely carried out. At about this time the tape in the recording system was changed and the joining call from the Cessna was not recorded, but the call was made and acknowledged by the radio operator.

Wreckage and analysis

The two aircraft impacted the ground relatively close together. The close proximity of the two main wreckages suggested that both aircraft had immediately become uncontrollable after the collision. With the loss of the complete right wing this was an unavoidable consequence for the Cessna. The major collision damage to the Yak appeared to be confined to the right tailplane and elevator. Whether this would render the aircraft totally uncontrollable is not known but it appears likely. Although the propeller slash in its right wing continued to the aft fuselage, it was a shallow cut and was positioned aft of the pilot's seat and hence pilot incapacitation as a result does not appear likely. Given the low altitude at which the collision occurred there was little time for the pilot to abandon the aircraft by using his parachute and no such attempt appeared to have been made.

The two aircraft, while sustaining major damage, had struck each other in a relatively "glancing" manner inasmuch as a more positive impact could reasonably be expected to have resulted in much more airborne fragmentation than actually occurred. Although most witnesses described the Yak striking the left wing of the Cessna, there was no evidence of impact on that area. The damage to the right wing and strut, which caused it to detach, came from below and behind. Macroscopic and forensic examination of all the witness marks left by one aircraft contacting the other strongly suggest that the aircraft were effectively "belly-to-belly" and generally heading in the same direction at the moment of impact. Marks on the Cessna right wing and strut were matched with the tailwheel and lower rudder of the Yak: black marks on the Yak elevator lower surface fabric were matched to Cessna tyre rubber and, as mentioned, the propeller slash on the underside of the Yak right wing. It is concluded that the Yak pilot also tried to take avoiding action by banking very rapidly to the left, even though this was not recalled by the eye-witnesses.

Analysis confirmed that the Cessna pilot, having completed his flight to the South East of North Weald, had rejoined the established right-hand circuit for Runway 20. This he had done from the up-wind end of the down-wind leg in accordance with the recommended airfield joining procedures. The Yak pilot, having arrived from the West had declared his intention to break to land and prefixed this with the word "initial". The term initial was used by the formation aerobatic team, of which he was a member, when 1 n mile on the extended centreline from the airfield to signify the IP before running in for the break. This location is not laid down in the North Weald AOM. While it is not known if the Cessna pilot was familiar with the significance of the term "initial" as applied to North Weald, it is probable that he understood the term "break" as being the manoeuvre described here.

The run and break manoeuvre was adopted by the armed forces in wartime as a means of permitting high speed aircraft to land at an airfield with a minimum of delay for both tactical and operating reasons. The aircraft would maintain its high flying speed down the dead side of the duty runway at a pre-determined height until at a certain point the pilot entered a climbing turn, reducing speed and joining the circuit pattern downwind for landing. Associated with the manoeuvre was an initial point (IP) at a specific distance from a position on the airfield or a time which the pilot reported to ATC commencing the break in order that other pilots or ATC were aware of the aircraft position and intended flight path. The procedure and any reporting points as well as any heights to be flown were promulgated in the relevant airfield documentation made available to those involved in flight operations. Visiting pilots could either establish prior to departure the procedure to be adopted when carrying out a run and break at an airfield or on the RT prior to arrival. The call of "initials" or "30 seconds to initials", informed ATC and other traffic that the aircraft was at the IP or had 30 seconds to run to the IP. Visual acquisition of the approaching traffic was therefore assisted by the geographical position reported. The term "initial" or "initials" has no defined meaning in civilian ATC phraseology. Such a call can only convey a specific position if it is promulgated in documents available to pilots and ATC or communicated on the RT. This was not the case at North Weald.

There are two main possibilities of why the Cessna pilot did not see the Yak until just before the collision. If he had not seen the Yak crossing in from left to right, he would have looked to the right in order to try and locate other aircraft in the area of the extended centreline of Runway 20. He might have assumed that he had missed seeing it and the break was complete with the Yak downwind behind him. As he made the turn onto base leg, adjusting his position in relation to the runway, most of his attention would have been directed looking out to the front or to the right. It is unlikely that he would have seen the Yak make the low level left turn and climb from below as he was seated on the left side of the aircraft and the manoeuvre was probably obscured by the nose and right side of his aircraft. Alternatively, if he had seen the Yak pass from left to right, he might have seen it make the dive and adopt a flight path, which would take him on a base leg to run in for the break. Again, concentrating on positioning and configuring his aircraft for landing, he would not have expected the Yak to appear from his left.

The Yak pilot, on completing the left climbing turn at the end of his dive, made a steep climb, taking him quickly to the height at which the collision occurred. The long nose of this aircraft does limit forward visibility significantly in a climbing attitude, although the pilot was able to maintain a lookout either side of the nose and clear the airspace in front by turning from side to side. The cockpit support structure and the position of the sun may further have degraded his ability to locate the other traffic. The call "Aerostar 4 is initials for the break and the other traffic is?" indicates that he had lost sight of the Cessna and the speed and gentle left turn of his aircraft, as described by the witnesses, suggest he was looking for other traffic to his left before commencing his restated intention to perform the break manoeuvre. Furthermore, he could not afford to fly too far eastwards without encountering the gliding activity on the dead side of the airfield. The Cessna at this point was to his right and slightly below.

The conclusions include the fact that the Yak pilot declared his intention to carry out a "run and break" with a time frame of "30 seconds to initial". It was thus possible for others listening on the frequency to be aware of his intended flight path, which was to descend and make a right turn in order to align his aircraft with the runway before carrying out the run down the runway and then break right to join the circuit downwind. By making the left turn and descending to such a low height, he deviated from the flight path and placed his aircraft in a position relative to the Cessna, which was not expected by the Cessna pilot or by the radio operator. The collision occurred because the pilots of both aircraft did not see the other aircraft in sufficient time to take effective avoiding action.

The airfield operator had commissioned a safety review by an external consultant in January 1998. They have indicated their intention to undertake a further review, which will include "consideration of the operating procedures, the airfield operating manual, levels of air traffic service and ways in which, in conjunction with our tenants, safety and good airmanship be positively encouraged and maintained".

Reference

AAIB Bulletin, 12/200