Narrative:

One hour after departure; the engine began surging and dropping 100 RPM. This type of airplane normally cruises around 2400 or 2500 RPM and any variation is abnormal. As the instructor pilot and PIC; I normally check the mixture setting in case of engine roughness to help correct any mistake by the pilot flying. The mixture had been set for peak RPM at 3;000 MSL and again at top of climb. Adjusting the mixture to a more rich setting did not help; and the surges slowly increased with periodic power losses of 300 RPM. Turning on the carburetor heat seemed to have no effect other than the usual 100 RPM decrease in power. The full rich mixture setting seemed to help slightly; but the surges continued. I evaluated precautionary landing locations and decided that any course deviation would take us away from the only nearby fields that were clear of trees and water. The closest suitable airports were beyond power off range. I considered reporting to ATC but the frequency was too busy to spend any time waiting to get a word in edgewise. I decided I would need to change the transponder code if we attempted a landing. I performed a configuration flow to verify fuel and ignition settings. Further adjustments to the engine controls led to a power loss of more than 1;000 RPM. Immediately applying carburetor heat restored most of the power. After this; when I turned off the carburetor heat; the engine nearly shut down again after a few seconds. This happened two or three times; which seemed to confirm we had an induction problem rather than a fuel problem or random surging. After that; I left the carburetor heat on; moved the throttle fully forward; and reset the mixture control to peak RPM to try to increase the temperature. I also pulled firmly on the carburetor heat control for a few minutes because these controls have a tendency to slide in and sometimes seem to work better when pulled beyond the normal range of movement. The engine surging stopped with full carburetor heat. We had nearly full power and had lost only 100 feet of altitude. I tried reporting to ATC at that point and did not get a response until the second or third attempt. I advised that we were experiencing carburetor icing and seemed to have it under control. After several minutes; I turned off the carburetor heat and we were able to continue the flight with full power restored. I ensured the carburetor heat was turned back on before each change to the throttle setting. Closer to our destination; I monitored our ETA carefully because we were within 0.2 hours of the minimum fuel requirement; based on preflight visual inspection with a fuel gauge; the airplane's hour meter; and performance tables in the flight manual. When we were handed off to the tower frequency; I calculated that we would land with exactly 0.5 hours fuel remaining; and I reported 'minimum fuel' on our initial call to the tower. At the time; the left fuel gauge indicated empty; and the right fuel gauge indicated more than 1 hour fuel remaining. The landing was uneventful. Upon parking we found the left tank had no measurable fuel; and the right tank had 10 minutes of usable fuel. On the return flight; I added 7% to the fuel flow rate calculation which proved to be accurate; and there were no further problems. Contributing to the multiple near-shutdowns of the engine on this flight were lack of safety equipment; instrumentation; and accurate operational information. The airplane was not equipped with injection type induction; carburetor ice detection; engine gas temperature indication; fuel flow rate indication; or a fuel totalizer. As a result; there was no early warning of the induction problem or potential icing conditions; no way to precisely set the mixture or to achieve peak egt for carburetor heating; no indication of higher than normal fuel flow; and no way to accurately measure fuel remaining while in flight. The airplane's manual contains a carburetor icing emergency procedure that was inadequate to prepare me for this situation. It simply says; 'a gradual loss of RPM and eventual engine roughness may result from the formation of carburetor ice. To clear the ice; apply full throttle and pull the carburetor heat knob full out until the engine runs smoothly.' in this situation; the engine's surging and abrupt loss of RPM did not fit the description of a gradual power loss. Contributing to the delay in communicating safety-critical information to ATC were frequency congestion; having only one radio installed in the airplane; and both pilots being task saturated to prevent an unsafe engine failure. Human performance was optimal in this situation. Workload management was handled correctly with task delegation and prioritization. Risk management techniques were applied correctly; although I need to re-evaluate fuel management risks in the aircraft category of the pave checklist.

Related reports:

Google
 

Original NASA ASRS Text

Title: C152 instructor pilot reported dealing with a rough running engine that finally responded to carburetor heat.

Narrative: One hour after departure; the engine began surging and dropping 100 RPM. This type of airplane normally cruises around 2400 or 2500 RPM and any variation is abnormal. As the instructor pilot and PIC; I normally check the mixture setting in case of engine roughness to help correct any mistake by the pilot flying. The mixture had been set for peak RPM at 3;000 MSL and again at top of climb. Adjusting the mixture to a more rich setting did not help; and the surges slowly increased with periodic power losses of 300 RPM. Turning on the carburetor heat seemed to have no effect other than the usual 100 RPM decrease in power. The full rich mixture setting seemed to help slightly; but the surges continued. I evaluated precautionary landing locations and decided that any course deviation would take us away from the only nearby fields that were clear of trees and water. The closest suitable airports were beyond power off range. I considered reporting to ATC but the frequency was too busy to spend any time waiting to get a word in edgewise. I decided I would need to change the transponder code if we attempted a landing. I performed a configuration flow to verify fuel and ignition settings. Further adjustments to the engine controls led to a power loss of more than 1;000 RPM. Immediately applying carburetor heat restored most of the power. After this; when I turned off the carburetor heat; the engine nearly shut down again after a few seconds. This happened two or three times; which seemed to confirm we had an induction problem rather than a fuel problem or random surging. After that; I left the carburetor heat on; moved the throttle fully forward; and reset the mixture control to peak RPM to try to increase the temperature. I also pulled firmly on the carburetor heat control for a few minutes because these controls have a tendency to slide in and sometimes seem to work better when pulled beyond the normal range of movement. The engine surging stopped with full carburetor heat. We had nearly full power and had lost only 100 feet of altitude. I tried reporting to ATC at that point and did not get a response until the second or third attempt. I advised that we were experiencing carburetor icing and seemed to have it under control. After several minutes; I turned off the carburetor heat and we were able to continue the flight with full power restored. I ensured the carburetor heat was turned back on before each change to the throttle setting. Closer to our destination; I monitored our ETA carefully because we were within 0.2 hours of the minimum fuel requirement; based on preflight visual inspection with a fuel gauge; the airplane's hour meter; and performance tables in the flight manual. When we were handed off to the tower frequency; I calculated that we would land with exactly 0.5 hours fuel remaining; and I reported 'minimum fuel' on our initial call to the tower. At the time; the left fuel gauge indicated empty; and the right fuel gauge indicated more than 1 hour fuel remaining. The landing was uneventful. Upon parking we found the left tank had no measurable fuel; and the right tank had 10 minutes of usable fuel. On the return flight; I added 7% to the fuel flow rate calculation which proved to be accurate; and there were no further problems. Contributing to the multiple near-shutdowns of the engine on this flight were lack of safety equipment; instrumentation; and accurate operational information. The airplane was not equipped with injection type induction; carburetor ice detection; engine gas temperature indication; fuel flow rate indication; or a fuel totalizer. As a result; there was no early warning of the induction problem or potential icing conditions; no way to precisely set the mixture or to achieve peak EGT for carburetor heating; no indication of higher than normal fuel flow; and no way to accurately measure fuel remaining while in flight. The airplane's manual contains a carburetor icing emergency procedure that was inadequate to prepare me for this situation. It simply says; 'A gradual loss of RPM and eventual engine roughness may result from the formation of carburetor ice. To clear the ice; apply full throttle and pull the carburetor heat knob full out until the engine runs smoothly.' In this situation; the engine's surging and abrupt loss of RPM did not fit the description of a gradual power loss. Contributing to the delay in communicating safety-critical information to ATC were frequency congestion; having only one radio installed in the airplane; and both pilots being task saturated to prevent an unsafe engine failure. Human performance was optimal in this situation. Workload management was handled correctly with task delegation and prioritization. Risk management techniques were applied correctly; although I need to re-evaluate fuel management risks in the Aircraft category of the PAVE checklist.

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.