C182 instructor pilot reported a near midair collision with a C172 on approach to plu.

Attributes
ACN	1354331
Time
Date	201605
Local Time Of Day	1801-2400
Place
Locale Reference	PLU.Airport
State Reference	WA
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Make Model Name	Skylane 182/RG Turbo Skylane/RG
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Other RNAV (GPS) Runway 35
Flight Plan	IFR
Aircraft 2
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Visual Approach
Person 1
Function	Pilot Not Flying Instructor
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Flight Instructor Flight Crew Commercial
Experience	Flight Crew Last 90 Days 90 Flight Crew Total 10100 Flight Crew Type 9000
Events
Anomaly	Conflict NMAC
Miss Distance	Horizontal 200 Vertical 200

Narrative:

We were on an IFR training flight controlled by seattle approach on 120.1. I was the cfii in the right seat. My student (aircraft owner) was flying the airplane. We were on the GPS approach to runway 35 at plu. The controller cleared us for the approach and switch to advisory with a traffic call to the west. The setting sun made it impossible to spot the traffic. Shortly after the call; I spotted the other aircraft below and to our right about 200 feet; close enough to see the tail numbers. It had apparently crossed under us and was flying a pirate GPS approach. We had switched to advisory and they called a 'straight in approach; traffic permitting.' I advised on the radio that we were passing on their left (to avoid collision.) we passed the skyhawk and continued to the missed approach and contacted seattle approach. The seattle controller questioned the incident and we advised of the skyhawk flying a GPS approach without clearance.

Original NASA ASRS Text

Title: C182 instructor pilot reported a NMAC with a C172 on approach to PLU.

Narrative: We were on an IFR training flight controlled by Seattle Approach on 120.1. I was the CFII in the right seat. My student (aircraft owner) was flying the airplane. We were on the GPS approach to runway 35 at PLU. The controller cleared us for the approach and switch to advisory with a traffic call to the West. The setting sun made it impossible to spot the traffic. Shortly after the call; I spotted the other aircraft below and to our right about 200 feet; close enough to see the tail numbers. It had apparently crossed under us and was flying a pirate GPS approach. We had switched to advisory and they called a 'Straight in approach; traffic permitting.' I advised on the radio that we were passing on their left (to avoid collision.) We passed the Skyhawk and continued to the missed approach and contacted Seattle Approach. The Seattle controller questioned the incident and we advised of the Skyhawk flying a GPS approach without clearance.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.