What happened
On September 29, 2014, an Embraer 170-200 was performing a landing approach to runway 07L at Frankfurt Airport (EDDG). While at an altitude of approximately 1,600 feet, the crew received a TCAS Traffic Advisory (TA) indicating a conflicting aircraft positioned behind them, approximately 600 feet above.
As the aircraft descended to 1,400 feet, the TCAS system issued a Resolution Advisory (RA) commanding a descent. In response to the advisory, the crew disconnected the autopilot and continued the descent. At 1,000 feet, the system issued an instruction to adjust vertical speed, followed by a TCAS inhibit alert as the aircraft reached the minimum operational altitude for the device. Throughout the encounter, the crew maintained visual contact with the conflicting traffic and subsequently notified Air Traffic Control (ATC) of the event.
The investigation
The investigation examined the flight data and cockpit voice recordings. Analysis of the Digital Voice and Data Recorder (DVDR) confirmed that the TCAS RA began at an altitude of 1,145 feet AGL and concluded at 1,000 feet. The investigation also noted that the TCAS transitioned to inhibit mode at approximately 900 feet, consistent with the device's altitude-based operational characteristics.
Findings
- The primary cause of the TCAS RA was insufficient separation between aircraft performing parallel approaches to runways 07L and 07R.
- The encounter was exacerbated by the significantly higher speed of the second aircraft approaching runway 07R.