Pilots of a Saab 340 turboprop taxied across a lit stop-bar, onto an active Helsinki runway, after misinterpreting air traffic control instructions, Finnish investigators believe.
The incursion forced the crew of an ATR 72-200, cleared to land on runway 22R, to abort the approach at a height of just over 100ft after the pilots saw the intruding aircraft.
Finland's Safety Investigation Authority says the Saab - operating for Nordflyg by Latvian carrier RAF-Avia - had been cleared to taxi to holding point WD, and been slowing as it approached a red stop-bar 80m before.
The crew had to contact a different tower controller to proceed, as traffic operations on Helsinki's parallel runways 22L and 22R were split to handle the afternoon arrival peak.
"One landing," this controller informed the crew, referring to the approaching ATR, and the first officer read back: "Landing."
Although the first officer was "uncertain", says the inquiry, the captain understood the communication as a clearance to line up - entering the runway despite the stop-bar, confirmed as lit by surface-movement radar.
"The pilots remembered having seen the yellow runway guard lights but not the red stop-bar," says the inquiry.
Blue1 flight 218, operated by a Golden Air, had already been cleared to land. But the Saab crew, when switching to the other tower controller, initially selected the wrong radio frequency and might have been unaware of the ATR's approach.
Arriving from Oulu with 71 on board, the ATR had descended to 108ft before its crew saw the Saab and aborted the landing, just before a tower controller ordered a go-around.
While investigations into the 29 December 2011 event found "shortcomings" in the Saab crew's communications - notably incorrect or missing read-backs - absence of cockpit-voice data from the Saab hampered attempts to analyse the pilots' cockpit co-operation. As well as failing to clarify possible misunderstandings in air traffic communications, the pilots disagreed on whether the approach path was visually checked for traffic during runway entry.