Incident Overview
On 23 August 2000 at 08:58 UTC, an airprox occurred 18 nautical miles east of the SAFFA intersection involving two aircraft: SWR213, an Airbus A320 (HB-IJO) operated by Swissair on a scheduled flight from Vienna (LOWW) to Zurich (LSZH); and SWR3003, an ATR42-500 (F-GPYO) operated by Air Littoral on a scheduled flight from Innsbruck (LOWI) to Zurich. Both aircraft were in contact with Zurich Radar East Sector (ACE) on frequency 133.900 MHz.
Sequence of Events
SWR213 was cleared by the radar executive (RE) to descend in steps to FL150. At 08:56:40, the RE instructed SWR213: "SWR213, speed 240 knots, descend to flight level 140." The pilot of SWR213 read back: "Speed two fourty, and down to level 120, SWR213." The RE did not notice the incorrect read-back.
Meanwhile, SWR3003, initially at FL160 and approximately 10-15 NM ahead of SWR213, was cleared to descend to FL140 and later to FL130. At 08:57:45, the RE cleared SWR3003 to descend from FL130 to FL120.
At this time, SWR213 was passing FL144 descending toward FL120, having overtaken SWR3003 and now about 0.4 NM ahead due to a speed reduction. At 08:58:21, as SWR3003 passed FL128 and SWR213 passed FL135, the pilot of SWR213 transmitted "TCAS climb!" and executed a climb back to FL140. The RE then informed SWR213 that its cleared level was FL140.
Findings
- The RE had cleared SWR213 to FL140, but the pilot read back FL120.
- The RE did not correct the read-back.
- A group of 4-5 visitors was present at the East Sector position, creating a higher noise level.
- The aircraft separated vertically to 700 ft and horizontally to 0.5 NM.
- Visual meteorological conditions prevailed.
- SWR213 had visual contact with SWR3003 until the overtake, then tracked it on TCAS.
- SWR213 received a TCAS traffic advisory (TA) and reduced its descent rate, which cleared the TA, but later received a resolution advisory (RA) to climb, which was followed.
Cause
The incident was caused by a misunderstanding by the SWR213 crew regarding the cleared flight level and an incorrect read-back that was not noticed by the controller. A contributing factor was the elevated noise level at the controller's workstation due to the presence of visitors.