Date: 202003
Local time of day: 1201-1800
Location: GEG.TRACON, WA
Light: Daylight
Aircraft: PC-12
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class C GEG
ATC / advisory: TRACON GEG
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Government
Reporter function: Approach; Instructor
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Training / Qualification; Situational Awareness; Distraction; Communication Breakdown; Workload
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: Airspace Structure; Human Factors; Procedure
Primary problem: Human Factors

Narrative: I was providing OJT (On the Job Training) on West Radar and we were in a North Flow. Aircraft X came over requesting an RNAV Approach into ZZZ and was cleared direct to a ZZZZZ. Shortly after Aircraft X was given the approach clearance and told to cross ZZZZZ at 5000 ft. The pilot read everything back correctly from the clearance and I observed the pilot crossing ZZZZZ at the correct altitude. TRACON called to request approval for Aircraft Y direct to the field and I think I zoomed my scope out to look at the aircraft. I then turned my attention to Aircraft Y and did not scan back to Aircraft X before my trainee switched them to the ZZZ Tower. Shortly after the frequency change was given another controller said 'I think he too low.' At that point I went back to Aircraft X and realized they were at 3500 ft. descending and needed to be at 3900 ft. on that segment of the approach. I told the trainee to call Tower and tell them to issue a low altitude alert. The controller said 'you want me to do it now?' I told them that Aircraft X needed to be at 3900 ft. on that segment of the approach. The lowest I observed the aircraft descend to was 3300 ft. on the approach segment before starting their climb back up.I was disappointed in myself for not scanning back and recognizing that Aircraft X was low on the approach when they were switched. My trainee is at a stage where I am trying to give him a little more autonomy and help him feel like he has ownership/responsibility for the position. Unfortunately; I let my guard down a little too much and didn't recognize the situation quickly enough. I did feel a little extra tired today and not quite as focused as I usually am.In the future I am going to stay more vigilant no matter what stage the trainee is at. In addition I am also going to hold off on changing the color of the data block to blue until I scan their altitude and give them a frequency change. From a technology standpoint I was disappointed that the Low Altitude Alert did not go off in this situation. The red font with LA (Low Altitude Alert) really grabs your attention and I think it would have helped us recognize this situation earlier.




Synopsis: GEG TRACON Controller reported not noticing an aircraft had descended below the glidepath while performing on the job training.