This article describes a real-life safety concern which was discussed at a meeting of the EUROCONTROL Safety Improvement Sub-group (SISG). Details have been changed to preserve anonymity.
Hometown is a relatively large airport with more than one runway. At Hometown, stop bars are always used. This fact has never really been up for discussion; everybody (more or less) believes it is a good idea to have an extra safety layer.
About one year after the introduction of a new runway, the number of runway incursions increased; nine occurrences were reported for one runway only. All the incursions were classed as category D incursions (no other aircraft involved), and there were many similarities. All flight crews had received line up clearance, they were all using an intersection to do so without asking or receiving permission, and they all crossed a lit stop bar.
Typically the pilots were taxiing eastbound on taxiway Uniform for departure southbound on runway 19L. Instead of lining up at the runway end, they used intersection W6. Although the stop bar was switched off at the runway end, it was still lit at the intersection. (See diagram below).
Why did the pilots not react to the stop bar? ICAO Annex 2 para 126.96.36.199.3 states that an aircraft shall stop and hold at all lighted stop bars and may proceed further when the lights are switched off (including clearance to proceed).
The investigators discussed probable causes:
- Expectation bias - the pilots did not expect a stop bar to be lit, because they had received clearance to line up (though at another intersection!).
- The turn on to intersection W6 is 150 degrees. This may make the stop bar difficult to detect.
- When the pilot starts the turn, the position is close to the threshold. This may give the pilots the false impression that they are using the correct intersection.
- ICAO Annex 2 - Rules of the Air: Chapter 3 Section 3.2.2