Q400
Flight Crew Air Transport Pilot (ATP)
Confusion
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
General Maintenance Action
Human Factors; Aircraft
On the departure briefing and shortly before takeoff; both the Captain and I verified the correct amount of fuel listed on the release was indicated on board the aircraft; and we both noticed that the fuel indicators each read within about 50 lbs of one another. Several minutes after departure; we noticed that the #1 (left side) fuel indicator drop to about 400 lbs less than the #2 (right) indicator did; and it maintained that difference throughout the climb and in cruise; with the total quantity in each tank decreasing appropriately as fuel was consumed. Once the aircraft leveled off; we ran the calculations for the reduced amount of fuel indicated; and determined that we still had enough fuel to reach our destination without burning into our reserve fuel; and there were several alternate airports along the route that we could divert to if needed. Dispatch and maintenance both agreed that proceeding to our destination wouldn't pose any safety issues; so we continued the flight.As we approached the top of descent; the #1 fuel quantity began to show about 600-700 lbs less than the #2 indicator; which triggered a caution message once the imbalance reached 600 lbs. We decided not to transfer fuel to correct the imbalance due to concerns of a possible fuel leak; and after consulting with the duty officer and determining we had enough fuel to safely reach [destination] with only the fuel in the #2 tank; we continued the flight to a normal approach and landing without transferring fuel. After landing; the #1 fuel quantity steadily increased until it very closely matched the #2 reading; and that reading remained once the aircraft was shut down.Once on the ground; maintenance inspected the airplane and determined that the #1 fuel tank actually contained about 900 lbs less fuel than indicated; which meant we had about 200-300 lbs remaining in that tank; when the 'worst case' indication on the flight deck (and our assumption) was that we had around 1100-1200 lbs remaining. The #2 tank contained the correct 1400 lbs that it indicated; so we did have sufficient fuel on board for the approach and landing; but if we'd encountered delays; there is a distinct possibility the #1 engine could have suffered fuel exhaustion. From calculating the total fuel burn after we landed; we realized that the problem had been present in [departure airport]; which means we likely departed with less than the required fuel listed on our dispatch release. Maintenance also determined that the refueling control panel on the airplane was also indicating about 900 lbs above the actual quantity in the #1 tank; so the combination of both the refueling panel and flight deck indications suffering from the same problem meant that there was no reason for the flight crew or refueler to suspect anything was wrong with the fuel quantity displayed in either location.The only way the erroneous fuel indication could have been detected was by manually checking the quantities in the tanks with a magna-stick after the airplane was fueled; but because the magna-sticks are normally only used when a fuel gauge is deferred per an MEL; and the fuel indications in the flight deck appeared correct; neither the Captain nor I had any reason to think we needed to double-check the quantity shown on the flight deck gauges; or even suspect it was incorrect until it started changing in flight. If we had known that we were actually short 900 lbs of fuel while in flight; there's no way either of us would have elected to continue the flight to our destination; and we would have either returned to our departure airport or diverted. Based on the information and resources available to the crew; maintenance and dispatch; I believe that our decision to continue to the destination was safe based on what we knew at the time; and I don't believe that there are any actions either the Captain or myself could have taken that would have revealed the extent of the failure we were actually dealingwith in flight.
Q400 First Officer reported operating their aircraft with less than cleared fuel because of a gauge error.