A helicopter pilot did not deliberately breach air clearance rules but had lost situational awareness when he clipped a crane and caused a fatal crash, an inquest has heard.
Pete Barnes, 50, died when his helicopter hit a crane at The Tower in St George Wharf, Vauxhall, south London, and plunged into Wandsworth Road on the morning of January 16 2013.
Pedestrian Matthew Wood, 39, from Sutton, Surrey, was also killed as he walked to work. Twelve other people on the ground were injured.
Mr Barnes, who was contending with poor visibility and freezing fog, had been flying from Redhill Aerodrome in Surrey to Elstree in Hertfordshire but was diverted to Battersea heliport.
Air Accidents Investigation Branch (AAIB) inspector Geraint Herbert told Southwark Coroner's Court it was difficult for pilots to map all such obstacles around London as identified in Notices to Airmen (NOTAMS).
NOTAMS gives only a general radius within which a potential hazard is located rather than an exact GPS co-ordinate, Mr Herbert said. The crane was within a one-mile radius of the area identified.
Mr Herbert said: "You don't know exactly where these obstacles are ... you have to plot the circle, and then all the other circles (on the map)."
The court heard the fluctuating workload of pilots included the requirement to maintain 500ft clearance from any obstacles. Some obstacles were mapped while others required visual identification.
But Mr Herbert said "not hitting obstacles" is fundamental to flying.
Air traffic controllers directed Mr Barnes to a holding zone between Vauxhall and Westminster bridges while they sought permission from Battersea to divert him.
Mr Herbert told the court Mr Barnes did not take the most direct route to fulfil those directions: "What he did was something different, but it is consistent with going to the bridge."
Mr Herbert added the final movements of the aircraft might have been because he was avoiding patches of bad weather, but they also meant Mr Barnes had entered a restricted area.
Moments before the crash, Mr Barnes attempted to switch his radio from the Thames controller frequency to the Battersea frequency. Mr Herbert was pressed on whether that task distracted Mr Barnes from his flight, possibly causing the accident.
He replied: "We can't say that, because he might have just acknowledged that he was going to make the frequency change, then done the turn, then changed the frequency."
Mr Herbert added: "We have no way of telling."
The court heard cloud cover was around 700ft that morning, the same altitude as the top of the crane.
Mr Herbert said: "We did conclude that he probably was not aware of his proximity to the building at the time he turned."
Mr Barnes's failure to observe the 500ft rule was not deliberate but "a consequence of this loss of situational awareness", the AAIB investigation concluded.
Mr Herbert said air traffic controllers would not have been aware of the position of the crane, and it was the pilot's responsibility to avoid obstacles and inform controllers if he or she was unable to fulfil the required clearances.
He said: "Air traffic control does not absolve him from obeying the rules of the air."
Mr Herbert said Mr Barnes was required to remain clear of cloud, but it was likely he did not see the building just before the crash.
He said: "From the evidence we've got, we don't think he did manage to remain clear of cloud."
He added: "Even when he was actually clear of cloud, because of the nature of the cloud ... his horizontal visibility might have been restricted in one direction, even though it might have been good in another direction."
London heliport duty controller Andrew Carn told the court there was a conversation between the Thames and Battersea controllers just before 8am on the morning of the crash, requesting the diversion.
He said while diversions due to weather were rare, the conditions at Battersea were suitable for that action and authorities there were clear to accept Mr Barnes's aircraft.