Narrative:

We departed on a day VFR cross country training flight with an estimated 30 of 50 gallons on board our extended range C-172 'P' model skyhawk. The right tank was physically touchable indicating fuel nearly 3/4's up the chain. The left tank was just below finger level. Immediately after leveling out upon departure (2000' MSL) we set a cruise power setting between 23 and 2400 RPM and leaned the mixture. We flew to the eastern shore of maryland and conducted a single touch and go with an immediate northwesterly departure direct back to base. On return leg a glance at the right fuel gauge indicated 1/2 tank and the left indicated 1/4. Another check of the gauges crossing the bay enroute back to base indicated tanks approaching empty. Around 2500 ft MSL on descent to stay below class bravo airspace the student throttled back to 1500 RPM. I suggested he bump the throttle back up and that 2000 RPM would suffice for the descent. After noticing the engine wasn't responding to his application of power I took the flight controls. I jockeyed the throttle in and out never getting more than 1500 RPM's. The engine appeared to be still running though idling. As we were being monitored by ATC while flying through the area; I contacted approach and informed them of our rough running engine and of our possible need to attempt an off-airport landing. After adjusting the throttle and applying carb heat and having no effect on the engine approach suggested ZZZ which we were unable to make; then a private grass strip which we're able to set down on. Upon landing and shutting down a visual inspection of the left tank revealed no fuel and the right tank only showed a trace amount. There was no visible sign of fuel leaks. Each sump drain was clear and fuel revealed no impurities or the presence of water. Unless further inspection reveals a leak in the tank or some other mechanical anomaly; I might be forced to accept pilot error in determining the amount of useful fuel on board prior to our take-off. Normally I have my students top off their tanks before departing on a cross-country. Time constraints played a role as we were running late and didn't have a lot of extra time to complete the flight so elected not to give an extra safety margin that I didn't believe was required for the flight. Estimating about 30 gallons I was thinking I had in excess of 3 hours of fuel available and when the engine was turned off we had been flying for an hour and a half. The student as well as the instructor received a very valuable lesson in pre-flight fuel management; stresses associated with rushing to get a flight in; and an emergency type power off landing that fortunately had positive results and a good ending.

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Original NASA ASRS Text

Title: C172 Instructor reports departing on a cross-country flight with a student pilot and an estimated three hours fuel on board. The engine quit after an hour and a half and the reporter was able to make it to an available grass strip.

Narrative: We departed on a day VFR cross country training flight with an estimated 30 of 50 gallons on board our extended range C-172 'P' model Skyhawk. The right tank was physically touchable indicating fuel nearly 3/4's up the chain. The left tank was just below finger level. Immediately after leveling out upon departure (2000' MSL) we set a cruise power setting between 23 and 2400 RPM and leaned the mixture. We flew to the eastern shore of Maryland and conducted a single touch and go with an immediate northwesterly departure direct back to base. On return leg a glance at the right fuel gauge indicated 1/2 tank and the left indicated 1/4. Another check of the gauges crossing the Bay enroute back to base indicated tanks approaching empty. Around 2500 FT MSL on descent to stay below class Bravo airspace the student throttled back to 1500 RPM. I suggested he bump the throttle back up and that 2000 RPM would suffice for the descent. After noticing the engine wasn't responding to his application of power I took the flight controls. I jockeyed the throttle in and out never getting more than 1500 RPM's. The engine appeared to be still running though idling. As we were being monitored by ATC while flying through the area; I contacted approach and informed them of our rough running engine and of our possible need to attempt an off-airport landing. After adjusting the throttle and applying carb heat and having no effect on the engine Approach suggested ZZZ which we were unable to make; then a private grass strip which we're able to set down on. Upon landing and shutting down a visual inspection of the left tank revealed no fuel and the right tank only showed a trace amount. There was no visible sign of fuel leaks. Each sump drain was clear and fuel revealed no impurities or the presence of water. Unless further inspection reveals a leak in the tank or some other mechanical anomaly; I might be forced to accept pilot error in determining the amount of useful fuel on board prior to our take-off. Normally I have my students top off their tanks before departing on a cross-country. Time constraints played a role as we were running late and didn't have a lot of extra time to complete the flight so elected not to give an extra safety margin that I didn't believe was required for the flight. Estimating about 30 gallons I was thinking I had in excess of 3 hours of fuel available and when the engine was turned off we had been flying for an hour and a half. The student as well as the instructor received a very valuable lesson in pre-flight fuel management; stresses associated with rushing to get a flight in; and an emergency type power off landing that fortunately had positive results and a good ending.

Data retrieved from NASA's ASRS site as of April 2012 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.