On switching tanks for landing, small aircraft has rough engine. Switches back.

Attributes
ACN	252533
Time
Date	199304
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : hef
State Reference	VA
Altitude	msl bound lower : 2000 msl bound upper : 2000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : iad tower : hef
Operator	general aviation : personal
Make Model Name	Small Aircraft
Flight Phase	cruise other
Flight Plan	None
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : commercial
Experience	flight time last 90 days : 15 flight time total : 1300 flight time type : 300
ASRS Report	252533
Person 2
Affiliation	government : faa
Function	controller : local
Qualification	controller : non radar
Events
Anomaly	non adherence : published procedure other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : detected after the fact other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I flew from bader field, nj (aiy), to manassas, va (hef). Approximately 15 mi southeast of manassas airport, in preparation for landing, I switched fuel tank to the tank which indicated the most full. Approximately 3 mins later the engine sputtered and I began losing altitude. I ran the auxiliary fuel pump which is a hand wobble pump, which had no effect. I switched back to the original fuel tank and the engine returned to normal operation. Not knowing the cause at this time, or whether it would recur, I called manassas tower and notified them that I had a rough engine and requested a straight in approach. They referred me to iad for radar vectors. Iad approach asked if I needed assistance. I declined and told them I had solved the problem. I landed at manassas without further incident. Upon examining the aircraft after landing, I discovered that the gas cap was missing on the left main tank, and the fuel in that tank had been siphoned out completely. I had approximately 1 hour fuel left in other tanks. Subsequent calls to bader field maintenance personnel indicated that they had found the missing gas cap on the ground at bader. I don't remember the details of my preflight, however, I had checked each tank for fuel quantity and apparently left the left main G cap on the top of the wing.

Original NASA ASRS Text

Title: ON SWITCHING TANKS FOR LNDG, SMA HAS ROUGH ENG. SWITCHES BACK.

Narrative: I FLEW FROM BADER FIELD, NJ (AIY), TO MANASSAS, VA (HEF). APPROX 15 MI SE OF MANASSAS ARPT, IN PREPARATION FOR LNDG, I SWITCHED FUEL TANK TO THE TANK WHICH INDICATED THE MOST FULL. APPROX 3 MINS LATER THE ENG SPUTTERED AND I BEGAN LOSING ALT. I RAN THE AUX FUEL PUMP WHICH IS A HAND WOBBLE PUMP, WHICH HAD NO EFFECT. I SWITCHED BACK TO THE ORIGINAL FUEL TANK AND THE ENG RETURNED TO NORMAL OP. NOT KNOWING THE CAUSE AT THIS TIME, OR WHETHER IT WOULD RECUR, I CALLED MANASSAS TWR AND NOTIFIED THEM THAT I HAD A ROUGH ENG AND REQUESTED A STRAIGHT IN APCH. THEY REFERRED ME TO IAD FOR RADAR VECTORS. IAD APCH ASKED IF I NEEDED ASSISTANCE. I DECLINED AND TOLD THEM I HAD SOLVED THE PROB. I LANDED AT MANASSAS WITHOUT FURTHER INCIDENT. UPON EXAMINING THE ACFT AFTER LNDG, I DISCOVERED THAT THE GAS CAP WAS MISSING ON THE L MAIN TANK, AND THE FUEL IN THAT TANK HAD BEEN SIPHONED OUT COMPLETELY. I HAD APPROX 1 HR FUEL LEFT IN OTHER TANKS. SUBSEQUENT CALLS TO BADER FIELD MAINT PERSONNEL INDICATED THAT THEY HAD FOUND THE MISSING GAS CAP ON THE GND AT BADER. I DON'T REMEMBER THE DETAILS OF MY PREFLT, HOWEVER, I HAD CHKED EACH TANK FOR FUEL QUANTITY AND APPARENTLY LEFT THE L MAIN G CAP ON THE TOP OF THE WING.

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.