Fuel in the tank but running on empty [Updated]

Yesterday I went up with my instructor to shake off my winter cobwebs (and instead ended up in IMC having a lesson on the Garmin 530 and autopilot, but that’s for another post) and had a very near fuel emergency.

Both tanks were filled to the collar before we took off in the 172SP, plenty of gas for our some holds at OCN, an approach at KOKB and return on the ILS into KMYF. We weren’t the only ones with the same plan and ended up having to do two more holding patterns that planned while the other plane shot the approach. It was during one of the turns that we noticed that the fuel gauge showed the left tank getting low, and the right tank still near full. We resolved to switch from ‘both’ to the right tank after the approach but in all the activity in the missed approach we forgot.

Not long before KMYF the “L Low Fuel” light came on and the gauge was sure enough very near to the bottom. We switched to just the right tank and carried on. As we got closer and closer the fuel imbalance remained, the left tank gauge was getting lower and lower. Although we didn’t say it to each other I think we were both already planning our dead stick landing to a nearby golf course. On final we switched the tanks back to ‘both’, landed and taxied back to parking.

After we were done we inspected the tanks – the right tank was as full as it had been at the beginning. Neither of us could see any fuel in the left tank at all, I don’t know how many minutes of ‘usable’ fuel there was left but I think we were pretty lucky that we made it down without an emergency.

I haven’t heard yet what the problem was (the plane is already back out of the shop) but I don’t understand how, if fuel from the right tank wasn’t flowing at all, the engine kept running when we switched the fuel to the right tank only. Surely it should have quit?

At the time I didn’t think much of it, but the more I think about it (and tell the story) the more I think we dodged a nasty bullet.

[Update:] From the plane owner: I had Sorbi take the plane off the line and check the fuel flow from both tanks. I had a concern that perhaps the fuel valve was not working right. Pablo took the fuel lime off the engine and had fuel flow from left, both and right positions and found that fuel flowed from all three positions in comparable quantities. When he went to check, both tanks had equalized and had the same amount of fuel in them. The only thing we felt that could contribute is that if you were making a significant number of right turns where the fuel would travel to the right tank.

We did do a lot of right hand turns, probably held over OCN for 5 right-turn holds so maybe that was it?

2 thoughts on “Fuel in the tank but running on empty [Updated]”

  1. Could also possibly be if you were flying slightly out of balance, fuel could slowly drain from left to right, seems fairly unlikely though.

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