The pilot in command of a Metro 23 had requested an intersection departure on runway 23 at Adelaide airport. At the time of the incident, weather conditions were VFR and radio traffic on the tower frequency was increasing. The pilot in command of the Metro 23 had been instructed to line up and be ready for an immediate departure (visual) with a maintain runway heading restriction. However, the pilot recalled that there had been considerable radio traffic on frequency and the ADC had spoken to at least one other aircraft between the times when he had passed the departure instructions and the take-off clearance.
The co-pilot had received the departure instructions because the pilot in command had had his attention diverted to another pre-take-off task when the clearance was issued. The pilot stated that he was concerned that a B737 was awaiting departure instructions at the threshold of the same runway and he wanted to minimise any inconvenience to that aircraft. He stated that perhaps as a result of the traffic on the tower frequency and that the departure instructions and take-off clearance had been passed in two distinct transmissions, he had overlooked the requirement to confirm the maintain runway heading instruction with the ADC. The pilot in command stated that at the time of take-off he was sure that the only instruction which had been passed was "118.2 airborne, clear for take-off".
When airborne and at a safe height he had queried the co-pilot whether or not they should make a left turn as they had made a visual departure. The pilot then elected to make the left turn, and they sighted another aircraft which was tracking to intercept final for rwy 30. It then became apparent to the crew of the Metro 23 that a mistake had been made. The error was later confirmed when they transferred to the approach frequency and the controller asked whether or not the ADC had stipulated maintain runway heading in the departure instructions.
Although it remains the responsibility of the pilot in command to abide by ATC instructions, this incident has highlighted the dangers associated with ATS personnel "splitting" an instruction. If ATS had repeated the requirement to maintain runway heading as part of the take-off clearance, the crew would have been reminded of the instruction at the point of departure.