Ukraine says its gained access to the black box flight recorders of the Ukrainian plane that crashed in Iran. A short time after the plane crashed on Wednesday, killing all 176 people on board, Iranian authorities recovered its black boxes. Both of the data recorders are damaged but their memories can be downloaded and analysed to help determine why the plane went down. Iran has rejected Canadian and US allegations that Tehran is responsible for the crash. The country's civil aviation chief says it was caused by a mechanical failure. Shamim Chowdhury explains.