Near-collision between Airbus A321 and hot air balloon near Saint-Prex

Casualties unknown • Sullens, VD, CH

An Airbus A321 and a hot air balloon experienced a dangerous separation of only 0.3 NM during an instrument approach to Geneva.

What happened

On April 15, 2018, a hot air balloon, an Ultramagic T-180 with registration HB-QPY, departed Sullens for a flight with seven passengers. During the flight, the pilot encountered stronger winds than anticipated, and the batteries for the aircraft's GPS and VHF radio failed. Relying on a mobile application for altitude information, the pilot initiated a rapid ascent to navigate toward the northwest. Because the pilot believed they were below the 7,50 and feet AMSL floor of the Terminal Control Area (TMA), they did not contact other balloon operators for coordination.

Simultaneously, an Airbus A321-111, registration HB-IOC, operated by Swiss International Air Lines, was conducting an IFR approach to Geneva (LSGG). While flying under radar guidance toward the Saint-Prex (SPR) radio beacon, the flight crew spotted the balloon ahead. The crew reported the sighting and executed a 10-degree right turn to avoid the obstacle. Despite the maneuver, the two aircraft reached a minimum horizontal separation of only 0.3 NM at the same altitude, approximately 10 NM northeast of Saint-Prex.

The investigation

The investigation focused on why the separation minima of 5 NM horizontal or 1,000 ft vertical were breached. Investigators examined the equipment on both aircraft and the visibility of the balloon to air traffic control and the airliner's onboard systems. The investigation also reviewed the pilot's decision-making regarding altitude and communication during the balloon's ascent.

Findings

  • The primary contributing factor was that the hot air balloon was not equipped with a transponder.
  • Due to the lack of a transponder, the balloon's position was invisible to Air Traffic Control, preventing the issuance of traffic information or the activation of the Short Term Conflict Alert System (STCA).
  • The absence of a transponder also meant the Airbus A321's Traffic Alert and Collision Avoidance System (TCAS) could not detect the balloon or issue a Resolution Advisory (RA).
  • The collision was avoided only because the flight crew maintained visual contact with the balloon due to favorable weather conditions.

Safety action

Following a similar incident in 2012 involving a glider, a safety recommendation was previously issued suggesting the establishment of Transponder Mandatory Zones (TMZ) around Swiss control zones to ensure all aircraft are visible to surveillance systems.

Probable cause

The near-collision was caused by the hot air balloon's lack of a transponder, which rendered it invisible to both air traffic control surveillance and the airliner's TCAS, leaving the flight crew solely dependent on visual acquisition to avoid a collision.

Frequently asked questions

What happened in the 2018-04-15 AIRBUS INDUSTRIE A321-111 accident near Sullens, VD, CH?

An Airbus A321 and a hot air balloon experienced a dangerous separation of only 0.3 NM during an instrument approach to Geneva.

What aircraft was involved and where did it happen?

The accident on 2018-04-15 involved a AIRBUS INDUSTRIE A321-111, registration HB-IOC, at Sullens, VD, CH.

What was the probable cause of the accident?

The near-collision was caused by the hot air balloon's lack of a transponder, which rendered it invisible to both air traffic control surveillance and the airliner's TCAS, leaving the flight crew solely dependent on visual acquisition to avoid a collision.

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