PA-25 Pawnee
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 1; Flight Crew Total 540; Flight Crew Type 30
Situational Awareness; Training / Qualification
Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Flight Crew Inflight Shutdown; Flight Crew Landed in Emergency Condition
Aircraft; Human Factors
The Piper Pawnee PA-25-235 was being used as a tow plane in glider operations.   A Schweitzer 2-33 glider was in tow when the tow plane lost power approximately 100 feet off the ground.  The glider was released and both the tow plane and glider landed on the far end of the runway with no damage.  This particular PA-25-235 has been modified for towing operations with a hopper fuel tank.  Upon stopping the plane and further investigation; I noted 85%+ fuel in the plane.  When I opened the fuel tank cap; the fuel tank popped up/out approximately 1'.   After further investigation; the fuel vent line was completely plugged and had caused a complete vacuum in the fuel system.  I have no knowledge of a proper vent cover on this Pawnee and it has been flying for years from what I can tell with no vent tube cover.  I hope by filing this report that other owners of PA-25-235 with modified hopper fuel tanks ensure that they are using fuel vent tube covers.  This was the 5th flight of the day in this plane and this is a particularly difficult if not impossible item to pre-flight.
PA-25 pilot reported the aircraft lost power at 100 FT AGL when the fuel vent line became clogged.