Fuel-Flow Indication Failure and an Out-of-Balance Tank at Night
You are PIC of a twin piston aeroplane with two wing tanks, 136 US gal usable, on a night IFR flight of 3 h 10 min with a 45 min final reserve. You depart with 100 US gal and plan 22 US gal/h total at 65% power. One hour fifty minutes into the flight, level at FL100, the left fuel-flow gauge falls to zero and the totalizer stops counting; it now reads 80 gal remaining. The left engine runs smoothly at the same power setting with normal EGT, CHT, oil pressure and a steady 16 psi fuel pressure, and the left tank gauge has dropped from 47 to 36 gal over the last hour, about the expected 11 gal. The right tank reads 24 gal, giving 60 gal total. There is no fuel smell and no staining. The destination is 50 minutes ahead and an airport with an ILS is 25 minutes off track. Crossfeed is available and approved above 3,000 ft; the AFM limits lateral fuel imbalance to 10 US gal.
1. Which fuel indications do you trust, and why? State what you believe has failed.
Read the worked answer
The engine is running at unchanged power with normal EGT, CHT and fuel pressure, and it cannot do that on no fuel, so the flow is real. The left tank quantity is falling at about the planned 11 gal/h and the right tank agrees with the plan. What has failed is the left fuel-flow transmitter or its indicator. Because a totalizer usually sums the flow transmitters, it has frozen as well, so its 80 gal figure is the least reliable number on the panel. The tank gauges are float senders and are imprecise (about ±5% and attitude-sensitive), but they are independent of the failed transmitter, so they are the best available source, backed by time-at-power arithmetic. I would start a written fuel log: power setting, time and gauge readings every 15 minutes, compared against 22 gal/h.
2. Work out the fuel needed from this point for destination plus reserve, and for the diversion, and state your decision.
Read the worked answer
Remaining on the gauges is 60 gal. Destination: 50 min at 22 gal/h is 18 gal. Final reserve: 45 min is 16.5 gal. Total to destination with reserve is 35 gal, leaving 25 gal surplus, about 68 minutes of extra range. Diversion case: 25 min to the alternate is 9 gal plus the 16.5 gal reserve, 26 gal, still inside 60 gal with a large margin. Both options are safe on fuel, so I would continue to the destination but re-plan every 15 minutes and set a hard decision point: if the gauge total falls below 40 gal before the destination, divert. I would also tell ATC I have a fuel-quantity indication problem and give them a minimum-fuel figure, so that priority is available if the gauges fall faster than predicted.
3. The tanks are now 12 gal out of balance. What can you do about it, and what are the risks?
Read the worked answer
Twelve gallons exceeds the AFM's 10 gal limit, so it should be corrected. With crossfeed selected, the right engine can be fed from the left tank, so the left tank drains at about 22 gal/h while the right stays static; roughly 30 minutes of that moves the imbalance back to near zero without transferring fuel between tanks. Risks: crossfeed creates a path for one tank to feed both engines, so a selector error, or feeding from a tank that runs dry, can cause a double engine failure from a single tank; an engine that stops while on crossfeed can be hard to restart; and the crossfeed source tank is being run down faster, so its remaining quantity must be watched. I would crossfeed in short timed intervals, one engine at a time, monitor both gauges and re-check the balance before landing.