Narrative:

The temperature was between 98 an 100 degrees. Local terrain was approximately 700' MSL. During preflight I observed that fuel tanks were full and oil was 6 qts in the small aircraft. The student and I flew to the northwest of ftw to do commercial maneuvers. We did these for 2 hours then landed at ft worth alliance airport (11 mi northeast) to rest due to the heat. We departed alliance and flew to the northwest and performed commercial maneuvers for 1 hour. We were at 1900' MSL over the saginaw airport and cleared to land on runway 16L at meacham when the engine started to run rough and the rpms went from 2200 down to 1000-1300. I advised ftw tower I had a rough engine and requested runway 16R as it was closer. I was cleared to land on runway 16R. Ten seconds later the engine vibrated even more, so I advised tower I would be unable to make it to meacham and would be landing at the saginaw airport and would advise them after I landed if we were ok. I landed the airplane (I took command of the plane at the first sign of a problem) and on short final pulled both the mixture and fuel valve handles to the off position to prevent a fire. Upon landing I rolled off the runway onto the turnaround area at the end of the runway. I inspected the aircraft and noticed the right fuel tank empty and between one and two inches of fuel in the left fuel tank. Oil was 5 qts. After 30 mins of waiting the mechanic came and west/O adding fuel, started the engine and did a full power static runup for a couple of mins. He then taxied the plane approximately 1/2 mi and parked it. He stated it had a bad right magneto and that a valve may have gotten hot and stuck, causing the loss of power. This is the probable cause of the problem. I also feel that performing commercial maneuvers for 3 hours, in 100 degree heat with the mixture full rich and nearly 3000' density altitude may have caused the engine to burn much more fuel then expected. The flight lasted 3.1 hours with approximately 4 hours of fuel aboard. Even with fuel in the left wing and a gravity fed system, the fuel could become 'positional' when low and not flow down to the engine in certain flight attitudes. (Ours was straight and level when the problem occurred). Solution. Before ever flying an aircraft anywhere near the maximum endurance time be sure you know the actual fuel flow or the procedure/maneuver you'll be doing. Don't ever assume the values given in the operating handbook are correct. Also, don't just add an extra gallon per hour 'just to be safe' either...your plane may burn an extra 2 or 3 gallons per hour. If it does, where will that leave you? Maybe in saginaw, tx.

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Original NASA ASRS Text

Title: GA SMA DEVELOPES ROUGH ENGINE IN TFC TO FTW. INSTRUCTOR PLT DIVERTS TO F04. REPORTER SUSPECTS LOW FUEL BUT GND CREW INDICATES IT COULD HAVE BEEN A STICKING VALVE CAUSED BY HIGH TEMPERATURE PLUS A BAD MAG.

Narrative: THE TEMP WAS BTWN 98 AN 100 DEGS. LCL TERRAIN WAS APPROX 700' MSL. DURING PREFLT I OBSERVED THAT FUEL TANKS WERE FULL AND OIL WAS 6 QTS IN THE SMA. THE STUDENT AND I FLEW TO THE NW OF FTW TO DO COMMERCIAL MANEUVERS. WE DID THESE FOR 2 HRS THEN LANDED AT FT WORTH ALLIANCE ARPT (11 MI NE) TO REST DUE TO THE HEAT. WE DEPARTED ALLIANCE AND FLEW TO THE NW AND PERFORMED COMMERCIAL MANEUVERS FOR 1 HR. WE WERE AT 1900' MSL OVER THE SAGINAW ARPT AND CLRED TO LAND ON RWY 16L AT MEACHAM WHEN THE ENG STARTED TO RUN ROUGH AND THE RPMS WENT FROM 2200 DOWN TO 1000-1300. I ADVISED FTW TWR I HAD A ROUGH ENG AND REQUESTED RWY 16R AS IT WAS CLOSER. I WAS CLRED TO LAND ON RWY 16R. TEN SECS LATER THE ENG VIBRATED EVEN MORE, SO I ADVISED TWR I WOULD BE UNABLE TO MAKE IT TO MEACHAM AND WOULD BE LNDG AT THE SAGINAW ARPT AND WOULD ADVISE THEM AFTER I LANDED IF WE WERE OK. I LANDED THE AIRPLANE (I TOOK COMMAND OF THE PLANE AT THE FIRST SIGN OF A PROB) AND ON SHORT FINAL PULLED BOTH THE MIXTURE AND FUEL VALVE HANDLES TO THE OFF POS TO PREVENT A FIRE. UPON LNDG I ROLLED OFF THE RWY ONTO THE TURNAROUND AREA AT THE END OF THE RWY. I INSPECTED THE ACFT AND NOTICED THE R FUEL TANK EMPTY AND BTWN ONE AND TWO INCHES OF FUEL IN THE L FUEL TANK. OIL WAS 5 QTS. AFTER 30 MINS OF WAITING THE MECH CAME AND W/O ADDING FUEL, STARTED THE ENG AND DID A FULL PWR STATIC RUNUP FOR A COUPLE OF MINS. HE THEN TAXIED THE PLANE APPROX 1/2 MI AND PARKED IT. HE STATED IT HAD A BAD R MAGNETO AND THAT A VALVE MAY HAVE GOTTEN HOT AND STUCK, CAUSING THE LOSS OF PWR. THIS IS THE PROBABLE CAUSE OF THE PROB. I ALSO FEEL THAT PERFORMING COMMERCIAL MANEUVERS FOR 3 HRS, IN 100 DEG HEAT WITH THE MIXTURE FULL RICH AND NEARLY 3000' DENSITY ALT MAY HAVE CAUSED THE ENG TO BURN MUCH MORE FUEL THEN EXPECTED. THE FLT LASTED 3.1 HRS WITH APPROX 4 HRS OF FUEL ABOARD. EVEN WITH FUEL IN THE L WING AND A GRAVITY FED SYS, THE FUEL COULD BECOME 'POSITIONAL' WHEN LOW AND NOT FLOW DOWN TO THE ENG IN CERTAIN FLT ATTITUDES. (OURS WAS STRAIGHT AND LEVEL WHEN THE PROB OCCURRED). SOLUTION. BEFORE EVER FLYING AN ACFT ANYWHERE NEAR THE MAX ENDURANCE TIME BE SURE YOU KNOW THE ACTUAL FUEL FLOW OR THE PROC/MANEUVER YOU'LL BE DOING. DON'T EVER ASSUME THE VALUES GIVEN IN THE OPERATING HANDBOOK ARE CORRECT. ALSO, DON'T JUST ADD AN EXTRA GALLON PER HR 'JUST TO BE SAFE' EITHER...YOUR PLANE MAY BURN AN EXTRA 2 OR 3 GALLONS PER HR. IF IT DOES, WHERE WILL THAT LEAVE YOU? MAYBE IN SAGINAW, TX.

Data retrieved from NASA's ASRS site as of July 2007 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.