What happened
During an approach sequence, an air traffic controller modified the clearance for a PA-44 to ensure it followed a C172 in the landing order. This change was made after the controller observed that the PA-44 had not overtaken the C172 as previously anticipated. While the controller issued the amended instruction, the pilot of the PA-44 was distracted by an aural alert from the traffic advisory system. As a result, the pilot failed to process the specific instruction to follow the C172. Despite noticing that the visual alignment of the aircraft appeared unusual, the pilot proceeded with the original clearance, which caused reduced separation between the two aircraft.
The investigation
The investigation focused on why the amended instruction was not successfully communicated and acknowledged. It was determined that the pilot's attention was diverted by cockpit alerts at the moment the instruction was delivered. Furthermore, the investigation examined the readback process. The pilot provided an incomplete readback that omitted the new sequencing requirement. The air traffic controller did not correct this omission, as the controller believed the specific instruction to follow the other aircraft did not necessitate a formal readback.
Findings
- The pilot of the PA-44 did not identify the final portion of the amended clearance due to distraction from a traffic advisory system alert.
- The air traffic controller did not identify the incomplete readback as an error, missing an opportunity to confirm the pilot's understanding.
- A systemic issue was identified regarding the Manual of Air Traffic Services (MATS), which does not explicitly mandate that sequencing instructions be read back by pilots. This lack of explicit requirement means there is no standardized assurance that such safety-critical instructions are understood.
- The instructor pilot in the C172 maintained sufficient situational awareness to ensure that, despite the reduced separation, adequate distance was maintained to prevent a more serious incident.