A C150 pilot reports finding improperly mixed vented and unvented fuel tank caps.

Attributes
ACN	733448
Time
Date	200703
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	agl single value : 0
Aircraft 1
Operator	general aviation : personal
Make Model Name	Cessna 150
Operating Under FAR Part	Part 91
Flight Phase	ground : parked ground other : annual
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 748 flight time total : 3261.8 flight time type : 2.2
ASRS Report	733448
Events
Anomaly	non adherence : published procedure non adherence : far
Independent Detector	other flight crewa
Resolutory Action	none taken : detected after the fact
Supplementary
Problem Areas	Aircraft Maintenance Human Performance
Primary Problem	Maintenance Human Performance

Narrative:

Vented and unvented fuel caps mixed up on cessna 150. Discovered (eventually) at annual. Suggestion: emboss fuel caps permanently with a description such as: 'vented;' 'unvented;' etc; in a prominent manner.callback conversation with reporter revealed the following information: the reporter stated that on some early model cessna aircraft the fuel tanks are vented by a tube near a landing gear strut. It is possible for the tube to become plugged by mud; bugs; or other debris. When that happens the fuel tank draws a vacuum as fuel is used and at some point fuel can no longer be drawn from the tank. Having one vented fuel cap prevents the vacuum. Three types of caps exist: full vented; semi-vented and non vented. The problem is the caps are not identified and so two non vented caps can be installed allowing the vacuum problem to exist.

Original NASA ASRS Text

Title: A C150 PLT RPTS FINDING IMPROPERLY MIXED VENTED AND UNVENTED FUEL TANK CAPS.

Narrative: VENTED AND UNVENTED FUEL CAPS MIXED UP ON CESSNA 150. DISCOVERED (EVENTUALLY) AT ANNUAL. SUGGESTION: EMBOSS FUEL CAPS PERMANENTLY WITH A DESCRIPTION SUCH AS: 'VENTED;' 'UNVENTED;' ETC; IN A PROMINENT MANNER.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT ON SOME EARLY MODEL CESSNA ACFT THE FUEL TANKS ARE VENTED BY A TUBE NEAR A LNDG GEAR STRUT. IT IS POSSIBLE FOR THE TUBE TO BECOME PLUGGED BY MUD; BUGS; OR OTHER DEBRIS. WHEN THAT HAPPENS THE FUEL TANK DRAWS A VACUUM AS FUEL IS USED AND AT SOME POINT FUEL CAN NO LONGER BE DRAWN FROM THE TANK. HAVING ONE VENTED FUEL CAP PREVENTS THE VACUUM. THREE TYPES OF CAPS EXIST: FULL VENTED; SEMI-VENTED AND NON VENTED. THE PROB IS THE CAPS ARE NOT IDENTIFIED AND SO TWO NON VENTED CAPS CAN BE INSTALLED ALLOWING THE VACUUM PROB TO EXIST.

Data retrieved from NASA's ASRS site as of January 2009 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.