PA-28 flight instructor reported a throttle malfunction that prevented the power from being reduced.

Attributes
ACN	1504154
Time
Date	201712
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	VFR
Component
Aircraft Component	Throttle/Power Lever
Person 1
Function	Instructor Pilot Not Flying
Qualification	Flight Crew Commercial Flight Crew Flight Instructor
Experience	Flight Crew Last 90 Days 41 Flight Crew Total 610
Events
Anomaly	Aircraft Equipment Problem Less Severe

Narrative:

Took student to practice area. After steep turns we configured for slow flight. Student reported that he couldn't retard power any more for the maneuver. I took command of the airplane and could advance throttle but not pull back to idle at all. It was catching on something. At this point I could force it but what if it broke. I immediately headed towards ZZZ. Along the way I tried to baby it back but nothing. So knew I would have to set up for a power off landing. With ZZZ being 3800 ft runway and now with a real world situation I knew it had to be perfect for a zero overrun. With that in mind I decided to [advise] and head to ZZZ1. I called tower and did thus so. Abeam the numbers I cut the mixture and cut mags and did a successful landing.

Original NASA ASRS Text

Title: PA-28 flight instructor reported a throttle malfunction that prevented the power from being reduced.

Narrative: Took student to practice area. After steep turns we configured for slow flight. Student reported that he couldn't retard power any more for the maneuver. I took command of the airplane and could advance throttle but not pull back to idle at all. It was catching on something. At this point I could force it but what if it broke. I immediately headed towards ZZZ. Along the way I tried to baby it back but nothing. So knew I would have to set up for a power off landing. With ZZZ being 3800 ft runway and now with a REAL world situation I knew it had to be perfect for a zero overrun. With that in mind I decided to [advise] and head to ZZZ1. I called tower and did thus so. Abeam the numbers I cut the mixture and cut mags and did a successful landing.

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.