Date: 201606
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Cessna 152
Operator: FBO
Operating under: Part 91
Flight plan: IFR
Mission: Training
Flight phase: Cruise
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Engine
Component problem: Malfunctioning
Reporter organization: FBO
Reporter function: Instructor; Pilot Flying
Reporter qualification: Flight Crew Flight Instructor; Flight Crew Commercial
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Separated Traffic; Flight Crew Took Evasive Action; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

Narrative: I and my student went to IFR cross country. When we were [in cruise] RPM started to fluctuate. We used Carb Heat because we suspected we picked up carb ice. But RPM kept fluctuating and carb heat didn't fix the problem. We flew [quite a distance; but] kept losing Altitude very slowly. We had notified ATC that we had carb ice and needed to land ASAP but didn't [announce an] emergency. We wanted to believe it was carb ice. [After landing] we waited about 2 hours searching for some carb ice and carb heat information online. There's no information that carb ice never melt for 20-30 minutes after applying carb heat. We did very thorough run up check and checked carb heat for few times on the ground before [departing again]. We took off and we were hoping that nothing happens this time. And used carb heat all the time when we were cruising. We leaned mixture properly. Rpm started to fluctuate again. Dropped down to 1400-1500 even we set 2500 RPM. We were using carb heat before any rpm stoppage of engine roughness. I had to decide either continue or go back to [departure airport]. I didn't want to take chance and [notified] ATC of our problem and [returned to departure airport]. I tried to lose minimum altitude but we couldn't maintain altitude and glided. There was a runway we could see straight in but tower asked us to join left downwind for runway 15 even though I have told him I cannot maintain altitude. I have told him again; he ignored and tower controller gave jet planes clearance for takeoff and then finally he said clear to land. When we tried to turn base he said cancel landing clearance extend downwind. We had no option at the lower altitude. What if we extend downwind and engine suddenly quit we cannot make back to runway. I [notified ATC of] emergency [situation] and landed airplane on runway 15. So here's what I want to know. I want to make sure what was the cause of rpm drop; it was almost full power mixture was properly leaned. Outside temperature was about 30 degree. It's rental plane they just say its carb ice. Then why carb heat didn't fix the problem for 20-30minutes. Why tower controller just ignored me and took care of IFR departure traffics. I notified him 3 times that I can't maintain altitude. He didn't even answer about it until I [confirmed] emergency.




Synopsis: A flight instructor reported experiencing engine power problems in a C152 with a student on an IFR cross country flight that may have been caused by carburetor ice. The instructor became concerned by the lack of response and support by ATC.