Incident Overview
During an approach sequence, an air traffic controller identified that a PA-44 had not overtaken a C172 as expected. The controller amended the PA-44 pilot's approach clearance, instructing them to follow the C172 as the second aircraft in the landing sequence. However, the pilot's attention was focused on an aural alert from the traffic advisory system when the amended clearance was issued, and they did not identify the final part of the instruction to 'follow the Cessna'. Consequently, and despite recognizing that the sight picture was 'looking a bit odd', the PA-44 pilot passed the C172 as per the original clearance, resulting in reduced separation between the two aircraft.
Air Traffic Control Instructions
The controller had issued an amended clearance requiring the PA-44 to follow the C172. The pilot's readback of the clearance was incomplete, omitting the sequencing information. The controller did not correct the incomplete readback because they considered that the pilot was not required to read back that instruction. This was a missed opportunity to resolve the situation.
Pilot Actions
The PA-44 pilot, having missed the requirement to follow the C172, did not include the sequencing information in their readback. Despite noticing the sight picture appeared unusual, the pilot proceeded with the original clearance, passing the C172.
Contributing Factors
Three contributing factors were identified in the investigation. First, the PA-44 pilot did not identify the final part of the amended instruction to 'follow the Cessna', leading them to pass the C172 as per the original clearance. Second, although a key element of the revised approach clearance, the pilot's incomplete readback was not corrected by the controller, as the controller believed the pilot was not required to read back that instruction. Third, the Manual of Air Traffic Services (MATS) did not explicitly state that sequencing instructions were required to be read back by a pilot, providing no assurance that this safety-critical aspect had been correctly understood.
Outcome
The situation awareness maintained by the instructor in the C172 ensured that adequate separation was maintained between the two aircraft as the situation developed, possibly preventing a more serious outcome.