Collision between Pawnee tug and Cirrus glider

Citation

(2003), "Collision between Pawnee tug and Cirrus glider", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 4. https://doi.org/10.1108/aeat.2003.12775dab.011

Collision between Pawnee tug and Cirrus glider

Collision between Pawnee tug and Cirrus glider

Keywords: Aircraft, Safety

The Cirrus was the seventh glider to be aerotowed during the afternoon at Aston Down gliding club. Initially, the launch was aborted after about 6ft of tow due to wing instability caused by the crosswind. A success ful launch was however subsequently carried out, at 1,510 h with the Cirrus glider releasing from the tug at approximately 2,000 feet agl up-wind to the west of the field. The Pawnee tug aircraft then returned to the field and towed two gliders further up to 2,000 ft (Figure 1).

Figure 1 Aston Down airfield – showing glider landing areas, tug arrival routes and estimated tracks prior to collision

Witnesses on the ground, both at the launch point and in adjacent fields, saw the tug aircraft and the Cirrus glider as they returned to the field and positioned for their respective approaches. The glider was seen to fly a right hand circuit for an approach to land on the southern grass area parallel to the runway centreline. The tug aircraft appeared to join the landing pattern on a left base leg with the intention of crossing the extended centreline of approach to turn closer into wind and land diagonally across the southern grass area as it had done before. The Pawnee and the Cirrus glider approached each other at a height estimated by witnesses to be between 150 and 400 ft. The glider was turning to line up to land with slight right bank applied. The tug aircraft was carrying out a 20° banked continuous level turn. Neither pilot took measures to avoid collision before the two aircrafts collided. After the collision, both aircraft descended out of control. Both pilots were fatally injured on impact with the ground.

Wreckage examination

The wreckage of the two aircraft lay in ploughed fields approximately half a kilometre to the south of the intended landing area, and were separated from one another by a distance of approximately 160m, the tug being to the east of the glider. The Piper Pawnee tug aircraft had struck the ground almost vertically, causing total disruption and break-up of the nose structure and forward fuselage back as far as the instrument panel. It had rotated through approximately 90° from its initial impact position before collapsing down onto its left side and coming to rest, indicating that the aircraft was falling in a spinning motion to the left prior to impact with the ground. All of the wreckage of the tug lay within its ground impact area, and it was evident that nothing of substance had become separated from the aircraft prior to its impact with the ground. The tow rope lay separately on the ground, adjacent to the tow hook on the aircraft's tail section, which was in the "released" condition. The post impact position of the rope and the condition of the cable operated hook mechanism suggested that the latter had opened, and released the rope, as a result of forces imposed on it during the impact sequence.

Clear witness marks were left in the ground by each of the wing leading edges during the initial impact. The impression produced by the left wing however, was not in its expected (symmetrically opposite) position relative to that from the right wing; instead, it lay approximately at right angles to it, consistent with the whole of the left wing having folded upwards relative to the fuselage, so as to lie against the fuselage side, prior to ground impact. Both of the primary lift struts, which support the left wing were bent upwards having failed in a compression buckling mode. Evidence of heavy pre-impact damage was noted on the leading edge regions of these struts in the area where they had buckled. The smaller stabilising struts which support the main lift struts in this same area also exhibited evidence of heavy pre-impact deformation and buckling, with associated red paint transfer (smearing) overlaying the white paint finish. Furthermore, localised regions of red paint transfer were found on the lower surfaces of the inboard sections of the main lift struts of the left wing, in the area of the pre-impact buckling, and on skin panels immediately adjoining the leading edge root. A small isolated area of red paint transfer was also noted on the left side of the cockpit roof.

Both main lift struts on the left wing, together with their associated stabilising struts, had suffered extensive damage during the collision with the glider which compromised their ability to carry the imposed flight loads. As a direct consequence, these struts failed in a compression buckling mode, allowing the left wing to fold upwards at its pinned attachments to the fuselage. Such a failure sequence would have caused the aircraft to roll violently and irretrievably to the left and enter a steep spiralling dive to the left. The aircraft's controls were examined in situ and no evidence was found of any pre-impact defect or malfunction. It was established that the flaps were set to the mid position. The four bladed wooden propeller had fragmented during the ground impact, and it was not possible to make any reliable assessment of engine power at impact from their remains. However, it was apparent from collision damage observed on the glider's remains that the propeller was rotating under power at the time of the collision.

The greater part of the glider, comprising the entire fuselage together with the fin and rudder, the whole of the right tailplane and right wing (the latter broken into two pieces), and the greater part of the left wing, lay within the glider's ground impact area. The rest of the aircraft was scattered over a separate region (measuring some 150–60 m) to the south east of the glider's ground impact point, and approximately 50m to the south of the tug wreckage. The glider had struck the ground in a very steep nose down attitude, slightly beyond the vertical, resulting in the complete disruption of the nose and cockpit. The aircraft had come to rest inverted, having turned slightly from the initial impact marks. This indicated rotational momentum to the left (viewed from above) prior to impact. Both wings remained securely attached to the fuselage and each had produced clear leading- edge impressions in the ground during the impact. It was apparent from these impressions that the wings had been in their correct relationship to one another at impact (i.e. neither wing had folded). The impression left by the left wing however, terminated (short) at the fractured outboard end of the wing, confirming that the outboard section of left wing had separated from the glider prior to impact with the ground. The right outer wing had broken away from the inboard wing section, but lay over the top of its initial impact mark on the ground, and it was evident that the right wing had been intact prior to impact with the ground.

The rear fuselage had broken apart during the impact at a position approximately mid way between the wing and the tail. The tail section, comprising the rear fuselage, fin and rudder, the tailplane centre section and right tailplane (the latter still attached by its hinge point to the top of the fin), was laying separately adjacent to the inboard right wing. A small segment of the inboard left tailplane lay on the ground closed by the tail section, but most of the left tailplane was missing. Only two areas of pre-impact damage, both at the tail of the aircraft, were found on items of glider wreckage found at the impact site. The first comprised distinct chordwise cuts, characteristic propeller strikes, at the points of separation of the small piece of inboard left tailplane. The second was a lozenge shaped area of paint transfer (smearing) on the lower fuselage left side, which extended rearwards and partially up onto the fin and rudder. This region extended longitudinally from a point approximately 1m ahead of the rudder hinge line aft to a position just behind the hinge line, and contained distinct areas of green, blue, and red paint transfer, together with more generalised smearing of an indeterminate grey colour. The direction of the smearing was uniform at approximately 12° to the fuselage longitudinal axis.

No evidence was found to suggest any pre- impact defects or malfunction of the glider's controls except for the fracture and separation of the left aileron at the point where the left outer wing panel had broken away and separated prior to the ground impact. Witness marks were found on the wing surfaces, adjacent to the airbrake/spoiler recesses on each wing, suggesting that the air brakes had been extended (partially, at least) at the time of impact. The pattern of smearing at the rudder hinge line suggested that the rudder trailing edge had been deflected approximately 13 cm to the right at the time the marks were produced. The Cirrus rudder surface, however, is hinged on its right side and the forces acting upon it during the paint transfer process would have tended to push the rudder across to the right. The position of the rudder indicated by the marks, therefore is likely to be spurious.

Conclusion

The glider was carrying out a right hand circuit and approach to land on the grass area parallel and adjacent to Runway 03 at Aston Down. This was in accordance with the guidance material published by the gliding club. The tug aircraft was returning to the airfield on a left base to cross the extended centreline of Runway 03 and land diagonally on an approximate heading of 340 across a grass area to the south of Runway 03 threshold. The flight path crossed the final approach path of the glider and was at variance to the procedures published by the club.

The evidence suggests that both aircraft were serviceable prior to impact. Whilst turning from opposing directions onto final approach towards a common landing area, both aircraft collided. The collision caused the break up and separation of the glider's left outer wing and aileron, and then loss of its left tailplane due to successive propeller strikes by the tug aircraft. This probably caused it to yaw to the left and fly on momentarily before pitching steeply down into a spiralling left hand descent to the ground slightly to the left of its original track. The collision also deformed the tug aircraft's left wing strut, which failed in compression overload. As a direct consequence, the left wing folded upward and the tug immediately followed a ballistic trajectory with an increasingly steep nose down spiralling descent to the left before impacting the ground at a short distance down track from the collision point. Whilst neither aircraft would have been obscured from the view of the opposing pilot, if either pilot had been concentrating on the landing area moments before the collision, then the other aircraft would have presented a small quasi-static image in his peripheral field of view. In such conditions, the opposing aircraft is not likely to have registered visually until the instant of the collision or fractionally before it.

The Rules of the Air detail, which aircraft has the right of way when aircrafts are on converging tracks and/or involved in making an approach to land. In order to determine their respective responsibilities in such circumstances, however, the commander of an aircraft has to see and be aware of conflicting traffic. In this case, it is clear that neither pilot saw the other until it was too late to avoid a collision. Operations involving approaches from opposing directions to common or closely adjacent landing areas can be considered inherently vulnerable to accidents of this kind.

The following three recommendations are made as a result of this accident.

Recommendation 2002-26. It is recommended that the British Gliding Association (BGA) advise gliding clubs, who use aerotows as a means of launching gliders, to review their procedures with a view to ensuring that appropriate separation between powered aircraft and gliders is maintained under all operating conditions and that best use is made of external lighting on tug aircraft to enhance their conspicuity.

Recommendation 2002-27. It is recommended that the BGA review the use of radio procedures to be used by tug aircraft and those gliders fitted with radios, with a view to improve the awareness of all pilots involved in gliding operations, in the presence of other aircraft in the vicinity of airfields involved in glider operations.

Recommendation 2002-28. It is recommended that the BGA oversee the introduction of a standard core syllabus for the training of tug pilots and ensure that all BGA approved tug pilots, who wish to maintain their tug pilot status, carry out their bi-annual PPL-SEP rating renewal with a CAA authorised BGA tug instructor.

ReferenceAAIB Bulletin 2/2003