Airprox between N67JG and SWR886 near Wil, Switzerland

Incident Overview and Aircraft Involved On 24 November 2000, an airprox occurred in Class C airspace 15 nautical miles northeast of the Wil VOR/DME. The incident involved a private Piper Cheyenne II (PA31T) with registration N67JG, operating from Regensburg to Grenchen, and a Swissair Airbus A321 with registration HB-IOJ, call sign SWR886, operating from Zurich to Brussels. Both aircraft were flying under instrument flight rules (IFR). ## Sequence of Events N67JG was descending in steps toward FL120 under the control of Zurich Radar West. At 15:23:29 UTC, the controller cleared N67JG to descend to FL120. Two minutes later, the controller observed on radar that N67JG was descending below the cleared level. When queried, the pilot of N67JG replied, “I was a little bit on the low side, maintaining flight level 120 now.” Meanwhile, SWR886 was climbing from Zurich Departure. At 15:21:19 UTC, SWR886 reported 3,200 ft and climbing to FL80. Zurich Departure then cleared SWR886 to climb to FL110 and later instructed it to maintain FL110. Departure also provided traffic information about an aircraft 1,000 ft above at the 2 o’clock position. SWR886 reported receiving a TCAS Resolution Advisory (RA). ## Separation and TCAS Response The two aircraft approached to a horizontal separation of 0.9 NM and a vertical separation of 500 ft. SWR886 received a TCAS Traffic Advisory (TA) at approximately FL100 and a TCAS RA at approximately FL108. The first officer of SWR886 established visual contact with N67JG shortly after the TA and maintained it throughout the event. The pilots of SWR886 assessed the collision risk as considerable. Following the TCAS RA, the pilot of SWR886 disconnected the autopilot and interrupted the climb at approximately FL108. The aircraft then descended at about 400–600 ft/min to FL104. The first officer reported that SWR886 was not at a constant altitude during the event, but followed a wave-like pattern caused by the descent and climb of N67JG above it. ## Findings and Contributing Factors N67JG was in radio contact with Zurich Radar West on 135.675 MHz, while SWR886 was in contact with Zurich Departure on 126.950 MHz. All pilots held valid licenses. The autopilot altitude preselect on N67JG was not engaged. The pilot of N67JG was familiar with the route, having flown it more than 200 times. During the descent to FL120, the pilot noticed that the Horizontal Situation Indicator (HSI) on the right side was displaying an incorrect compass course. The descent rate of N67JG as it passed FL120 was approximately 1,500–2,000 ft/min. The pilot recognized the error shortly after passing FL120 and immediately climbed back to FL120. ## Assessment The Zurich Radar West sector was handling high traffic density and complexity at the time. The sector was staffed according to the duty roster, and technical systems were functioning normally. The radar executive expected SWR886, still on the Zurich Departure frequency, to climb to FL110 according to the standard departure procedure, and therefore cleared N67JG only to FL120. The radar planner became aware of N67JG’s unauthorized descent below FL120 through the Short Term Conflict Alert (STCA) and immediately informed the radar executive, who then queried N67JG about its altitude. The investigation concluded that the incident was due to inattention by the pilot of N67JG. Contributing factors were the pilot’s distraction during a critical phase of flight and the fact that the autopilot altitude preselect was not engaged.