A helicopter pilot who took off from the wrong runway at Denham was just 200ft from a mid-air crash with another aircraft.
The near-miss saw an AS350 helicopter cross just 60 metres in front of a C152 plane in the skies above Wycombe Airpark.
An inquiry by the UK Airprox board found the helicopter pilot "immediately released her mistake" following the botched take-off but became absorbed with flying her aircraft out of the area of confliction.
The report added that she was "probably rattled" after taking off from the wrong runway and this may have "caused a continued lack of concentration and mistakes".
The incident happened on Thursday January 19 earlier this year.
After she left Denham she flew south of her planned route, on a heading which took her through Wycombe Airpark and directly into the path of the C152 - a route she had "probably intended to keep clear of".
Wycombe Tower had told the aeroplane pilot that a helicopter which "wasn't speaking to them" appeared to be passing ahead of him in the airspace, so he looked out of his window where he saw the helicopter flying left to right.
But because he only saw the helicopter at the closest point of approach he "did not have time to take any avoiding action", while the helicopter pilot "had not seen the other aircraft at all".
The near-miss has been classed as a Category A risk, the most serious, because "separation had been reduced to the bare minimum and chance had played a major part in events".
The report added: "This incident should serve as a reminder to all pilots to ensure that they were in the correct frame of mind to conduct their flight.
"If she was distracted by the takeoff incident - it appeared that her navigation and lookout had been severely compromised - then it may have been wiser to land again at Denham, compose herself and then start the flight again."
The board also questioned the thoroughness of the helicopter pilot's initial flight planning, believing that she "should have planned a route which allowed for either runway departure".
It added: "The board assumed she did not know she was flying through the ATZ, but thought it good airmanship anyway to call airfields if the intention is to route close by."
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