Date: 201508
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Weather elements / visibility: 20
Light: Daylight
Aircraft: Piper Single Undifferentiated or Other Model
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Descent
Airspace: Class G ZZZ
Aircraft component: Fuel System
Component problem: Improperly Operated
Reporter organization: Personal
Reporter function: Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Commercial
Human factors: Training / Qualification
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Diverted
Contributing factors: Procedure; Aircraft
Primary problem: Procedure

Narrative: I [planned to fly] 3 hours; using 13.1 gallons. Sea level cruise GPH is 5; so it was 4.36 outbound. The return flight took longer and the fuel gauge was getting low. According to calculations 40 minutes prior to arrival; I would have 1.7 gallons upon landing; plus 1 gallon in the header tank. The wing tank (only one) is 18 gal. Previously; I had flown the plane until the float gauge went out of view and the tank had 2.1 gallons in it once the gauge was no longer useful (not quite zero; just the float is not visible). 5 minutes after the float disappeared; with 7 minutes left until arrival; I reached out in flight to the wing sump and pushed the drain and fuel came out. 30 seconds later the engine began losing RPM. I immediately nosed forward; which restored fuel flow; turned toward [a private field] and executed an emergency landing without incident. The engine maintained power through descent and while taxiing; though I knew nose up pitch/go around would interrupt the fuel flow.The problem was one of 'usable fuel.' In prior instances; the float disappearing indicated 2 gallons in the wing tank + 1 in the header. That test was on a colder day; with more nose forward flight. I was cruising at 2;200 RPM to conserve fuel; with 27C OAT; thus; nose slightly up; which would not allow the fuel to enter the drain to the header tank. I have also put 18.1 gallons in that tank before; which may be either a) a problem with the airport's fuel dispenser b) on a colder day with greater fuel density or c) a colder day with nose forward pitch in cruise and I was down to the header tank (Stupid; if that was the case. From my memory; I did not run it below the float disappearing). This airplane; being so old; doesn't have reliable 'usable' amounts. Further; flight training makes usable black and white. X capacity; Y usable. End of story. Don't drain it lower. No explanation. For those that go into the unusable area; it may work one time; and not another; due to flight attitude and a whole host of other events. Further; fuel starvation is viewed as black and white also. The engine quits or it doesn't. I had just had a conversation with a pilot who starved a comparable model of fuel; and he described how the engine will repeatedly sputter and rev up for minutes on end. Had I not had that conversation; I would have thought the problem was something other than fuel starvation.




Synopsis: Pilot of a single-engine Piper aircraft attempted to fly a three hour out and back flight but had to divert on the return leg when the engine started losing power due to fuel exhaustion.