Incident
On 17 December 2009 at 12:40 UTC, a Socata TB-20 (registration CS-DES, call sign DIANA 435) was performing an instructional flight with an instructor and a student pilot on board. The aircraft was on the left downwind leg of the landing circuit for Runway 19 at Évora aerodrome when the Aerodrome Flight Information Service (AFIS) informed it of traffic approaching at approximately the same altitude.
The conflicting traffic was a single-engine PAC 750 XL (registration D-FGOJ) operating under a German registration. The aircraft was used for parachute dropping and was conducting a training flight following the same profile as a drop flight. The local agreement stipulated that parachute drop aircraft should descend west of the aerodrome and enter directly onto the right base leg for Runway 19 (or left base for Runway 01), contrary to the normal circuit direction. During its descent, two minutes before the incident, the crew was informed by AFIS of three other aircraft in the circuit and another approaching.
Unexpectedly, D-FGOJ appeared close to DIANA 435, forcing the latter's crew to perform a sudden evasive maneuver to avoid a mid-air collision. The aircraft came within approximately three meters of each other. The situation was so severe that the Socata pilots believed a collision was inevitable.
After landing, a meeting was held. The pilot of D-FGOJ stated that he entered the circuit without confirming the presence of other aircraft and could not contact AFIS because the frequency was constantly busy.
Analysis
The pilot of D-FGOJ was informed of three aircraft in the circuit and another about to enter. Therefore, entering the downwind leg in an atypical and unusual manner was deemed unreasonable. The fact that the AFIS frequency was very busy, as claimed by the pilot, suggesting difficulties in communications deprived him of useful information for safe flight conduct, does not alone justify inadequate or incorrect procedures. Moreover, given the nature of AFIS, the frequency could only be occupied with relevant information for aircraft in or near the circuit, including D-FGOJ.
Frequency saturation at Évora AFIS has been cited by some pilots involved in similar incidents as a contributing factor. Flight information provided by AFIS primarily serves to avoid mid-air collisions, hence the need for proper communications and avoidance of unnecessary transmissions. However, according to the Rules of the Air (ICAO Annex 2, paragraph 3.2, Avoidance of Collisions), it is important that vigilance for detecting potential collisions is not relaxed aboard aircraft in flight, regardless of flight type or airspace class.
Since D-FGOJ was conducting a training flight for parachute dropping, it was expected to follow the locally agreed profile: descent west of the aerodrome and direct entry onto the right base leg for Runway 19. However, the mission did not involve actual parachute dropping, so it was not subject to the usual constraints of an actual drop, such as keeping other traffic away from the drop area. If the aircraft did not enter the right base leg as agreed, it should have followed standard procedures for other traffic. By adopting different procedures while a significant number of aircraft were in the circuit, the crew of D-FGOJ compromised the safety of both aircraft involved.
Conclusions
The incident (near mid-air collision) occurred because the crew of aircraft D-FGOJ did not observe local procedures for entering the circuit. The mid-air collision was avoided by the prompt and effective reaction of the pilots of aircraft CS-DES.