PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Commercial
Flight Crew Last 90 Days 250; Flight Crew Total 775; Flight Crew Type 500
Situational Awareness
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Flight Crew Landed in Emergency Condition; General Flight Cancelled / Delayed
Aircraft; Human Factors
Started our flight in ZZZ2 to ZZZ1; ZZZ; ZZZ2 for our commercial cross country requirement. Had flight planned around XA:00am with the student for our departure at XH:30 pm. Flight up to ZZZ1 was uneventful; burned we burned 21 gallons going up; leaving us with 29 gallons to get to ZZZ. The flight planned showed we required 19 gallons to land at ZZZ where we planned to refuel before returning to ZZZ2. This left us with 10 gallons reserve; meaning over 1 hour of flight time at cruise. Roughly 50 miles out from ZZZ we had engine roughness and realized we had pulled the tank dry; so we swapped tanks again with no issue. We knew we had a reserve left and headed directly for ZZZ. Upon entering the pattern and turning base (2 miles) the engine sputtered again and we knew we had run out of gas. Knowing we would not make the runway we notified ATC and landed uneventfully on highway. Fire responded with no injuries or damage. ZZZ ground crew towed the airplane to the airport we had an A&P check everything over; added fuel; did a ground run; everything checked out good and we continued the flight to ZZZ2. Coincidentally when we fueled at ZZZ knowing the airplane has a 50 gallon capacity and the POH says all 50 gallons usable; from dead empty the plane only took 48.2 gallons. In conclusion; during the flight back from ZZZ1 to ZZZ we knew the head wind was more than we had factored for so we recalculated midair and that is where our mathematical error came into play. A simple addition mistake caused us to land just short of the runway that night.
Flight Instructor reported that a miscalculation of fuel requirements resulted in fuel exhaustion and an off-airport landing.