Published on
On Monday 23 September, two aircraft came closer to each other than the separation minimums stipulate. The incident occurred while approaching the Zwanenburgbaan (18C) at Schiphol.
LVNL is conducting its own investigation into this incident and has reported this incident to the Dutch Safety Board, the Human Environment and Transport Inspectorate and the Public Prosecution Service in accordance with the investigation process.
The occurrence took place during the day's first inbound peak, when inbound aircraft land on two runways - in this case the Polderbaan (18R) and Zwanenburgbaan (18C), and outbound aircraft take off from one runway, the Kaagbaan (24).
A Boeing 737-900 (B739) is approaching Schiphol Airport from the east and descends to flight level 50 (approx. 5000 ft or 1500 m) as the pilotes are preparing to land on the Zwanenburgbaan (18C). The flight in question is scheduled to land after a Boeing 737-800 (B738) approaching from the north. The two flights are instructed to intercept the Zwanenbaan (18C) localiser in sequence.
The B738, the first of the two flights, is instructed to reduce its airspeed from 220 knots to 180 knots to maintain sufficient separation from the aircraft in front, an Embraer 190 also approaching the Zwanenburgbaan (18C). Noticing that this may cause a loss of separation between B739 and B738, the controller instructs B739 to reduce to 220 knots and turn right to heading 285 for the sake of "spacing", i.e. to create more distance between the aircraft.
30 seconds later, B739's airspeed has barely dropped, prompting the controller to estimate separation minimums would be breached if continued on this heading. ATC subsequently instructs B739 to slow down further to 180 knots, just like B738, and divert further to the right to heading 300. B739, approaching from the east, thus diverts to the northwest to stay behind B738, which is heading southward.
With both flights at approximately the same altitude, B739 ultimately catches up with B738 ahead despite the aforementioned instructions. The controller therefore decides to fly B739 all the way north on heading 360. B739's pilot correctly reads back the controller's instruction on the same frequency.
Following this instruction, B739 does indeed embark on a new heading, but instead of flying right on heading 360, the flight turns left on heading 260. Since this causes B739 to fly precisely in the direction of B738, horizontal separation between the two aircraft rapidly diminishes. The controller notices this situation and immediately orders B739 to return back to heading 360. This prompts another brief moment of confusion and the controller instructs the pilot to continue on their current westward heading, adding: "avoiding action". Although the standard separation minimum of at least 3 nautical miles (approx. 5.5 kilometres) or 1,000 feet (approx. 300 metres) has already been breached at this point, the southbound B738 has already crossed in front of B739, ruling out a collision between the two aircraft. Minimum horizontal separation between these two flights was 1.9 nautical miles (3.5 kilometres), while minimum vertical separation was 200 feet (61 metres).
After the avoiding action, separation is restored and B739 continues its approach, landing safely on the Zwanenburgbaan (18C).
Shortly after B739 landed, the pilot contacted LVNL to discuss the situation. The pilot indicated that, flying on heading 300, they expected instructions to proceed to the southwest towards the runway to follow soon. Although giving a correct readback at the moment, they assumed that they had misheard the clearance and that heading 260 corresponded more closely to their mental image than heading 360.
The investigation was reported to the airline concerned and internally to the controller on duty. LVNL investigates occurrences like this and uses them as an example to maintain awareness about human factors. In this case, the procedures and safety barriers did their job and remedied the situation.
