A C172 pilot reported his engine developed a surge while cruising at 8;500 FT. An emergency was declared and the flight diverted to a nearby airport. The fuel cap vent was believed to have been plugged; leading interrupted or diminished fuel flow.
Synopsis
A C172 pilot reported his engine developed a surge while cruising at 8;500 FT. An emergency was declared and the flight diverted to a nearby airport. The fuel cap vent was believed to have been plugged; leading interrupted or diminished fuel flow.
Narrative
The airplane was cruising at 8;500 FT MSL at 90 KTS for 45 minutes when I noticed a slight drop in RPM. Carburetor heat should be +10 degrees Celsius. Carburetor heat was applied and the carburetor temperature moved to +25 degrees Celsius. Within 3 minutes the plane lost all power for approximately 5 seconds followed by 3 second of returned power. This oscillating of power on-power off continued to worsen with longer periods of power off up to twenty seconds at a time I could no longer maintain altitude and put the plane into a 65 KT glide. Center was notified of the emergency. I was handed over to Approach; the closest runway was a grass strip. By the time I reached that airport the motor oscillation was much improved to the point I could just maintain level flight. An airport with a paved runway was only a few more miles and I had the altitude to make the runway. I kept my altitude as high as possible on the approach. This put me into Class C airspace at another airport; which was approved by Approach. Once the runway was made; Approach had me contact Tower. Tower cleared me to land and the airplane was taken to the FBO for repair. Carburetor heat; cycling mags; changing the mixture; switching fuel tables; pumping the accelerator did not correct problem. I believe the reason for the problem was fuel starvation. The most likely course of fuel starvation could have been plugging fuel vents; ice; or another obstruction in the fuel line. All fuel lines were cleaned and that fuel tank vents were cleaned and the carburetor was removed and checked. No specific problems were found. The plane was flown to 9;000 feet MSL with no problems and was flown 150 NM to our home base. A new vented cap for the left tank was also ordered.
NASA callback
The reporter's aircraft is a 1964 C172 for which both a Service Bulletin and an AD have been issued in order to mitigate a fuel system main tank check valve vacuum flaw in 1969 C172 aircraft. The reporter stated that his aircraft complies with AD because it has the extra tank vent and the vented cap. He is not sure why this anomaly occurred and although it is rare he believes that three elements came together to cause the check valve to stick because of a vacuum: 1. The tanks were full at takeoff 2. The check valve became plugged for an unknown reason and 3. The flight was long allowing a vacuum to form as the fuel was used. The temperature at cruise was 20 degrees; so it possible that ice formed on the main tank check valve. According to the reporter; Cessna states that this type of anomaly can occur on hot days; but that that was not the case for his flight. He also stated that during cruise flight he was operating on both fuel tanks and when the power loss occurred began alternating from left to right tank and the engine began developing sufficient power only at lower altitudes. As part of his preflight the reporter now blows into the tube in order to ensure it is unseated prior to each flight.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.