AD of the Week: AD 2005-19-20 — A 20-Year-Old Warning for Today's Fuel Switch The New Piper Aircraft, Inc. (PA-28-160/161/180/181)

‍If you've got a PA-28 modified under Petersen Aviation's STC SA2660CE to burn auto fuel, this one's worth a hangar conversation. During post-installation checks, inspectors caught a mismatched fuel fitting in the supplemental fuel pump plumbing leaking fuel directly into the engine compartment, precisely the type of failure that results in an engine fire and subsequent loss of control.

‍Field Notes The STC swaps in two electric boost pumps to support autogas operation. Petersen's follow-up Service Bulletin SB98-1 added a fuel flow bypass using anAN894-6-4 bushing, a straight-thread fitting designed to seal against a flat boss using an O-ring.

The problem? It was mated to an AN826-6 tee, which relies on a standard flared-cone surface to seal and completely lacks the mating surface for that O-ring. The result is a mechanical illusion: the straight threads bottom out and feel tight, but there is zero actual sealing pressure on the cone surface. Two of these cross-threaded hazard points sit in critical spots: one at the gascolator, and one feeding the top fuel pump inlet.

‍Why It Matters This AD is a 20-year-old cautionary tale that is suddenly highly relevant again. Petersen's autogas STC was an early, FAA-approved attempt to movegeneral aviation piston fleets off standard aviation gasoline and ontosomething cheaper and more accessible, conceptually identical to the goaldriving today's 100LL-to-unleaded transition under the EAGLE initiative.

What this AD demonstrates is that changing the fuel chemistry isn't usually the hardest part of the equation; it's managing the fuel system hardware down stream of it. A mismatched hardware fitting, not the autogas itself, created the fire risk here.

As G100UL, UL94, and other unleaded aviation fuels work through fleet-wide rollout today via new STCs, this AD serves as a vital reminder for mechanics and operators alike: Any fuel system modification, new pumps, revised plumbing, or altered seals, demands the exact same scrutiny as the fuel itself, regardless of how solid the underlying regulatory approval looks on paper.

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