Falling Left Tank at FL280: Leak or Gauge Fault?
A two-pilot turboprop is on a night IFR freight flight at FL280. About 1 h 50 min after take-off the left fuel quantity gauge reads 340 lb and the right 620 lb, with total fuel flow showing 400 lb/h per engine and no crossfeed selected. Two minutes later the left gauge reads 315 lb. The fuel system has a single crossfeed valve; the AFM permits crossfeed only for balancing, both engines running, with normal indications. Despatch fuel was 1,600 lb. Destination is 35 minutes away with RVR 1,200 m, temperature -2 C, snow and braking action medium; the alternate is 20 minutes further, +4 C in rain. Both engines show normal Ng and ITT. The loadmaster on the jump seat reports a faint kerosene smell that the pilots cannot confirm. OAT at FL280 is -42 C. Both fuel quantity systems were checked within tolerance at the last 100-hour inspection five days earlier.
1. Using the figures, decide whether this is a fuel leak or an indication fault, and give the evidence for and against each possibility.
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Evidence for a leak
- The loss is continuous, not stepwise: roughly 25 to 30 lb in two minutes, about 750 lb/h. Quantity gauges usually fail in steps, drop to zero or fluctuate; they rarely track a smooth continuous rate.
- 750 lb/h is far above the left engine's 400 lb/h. If the engine were genuinely burning that much, Ng and ITT would have moved; they have not. So the fuel is leaving the aircraft, not the exhaust.
- The right tank is stable at 620 lb, so the loss is isolated to the left side rather than a common indication or totaliser error.
- The kerosene smell is independent corroboration that no gauge fault can explain.
Evidence for an indication fault
- No visible vapour or mist, though at FL280 a leak would be a fine spray that is hard to see.
- The left gauge could have failed progressively, which is uncommon but possible.
Deciding factor: independent confirmation. Compare the totaliser, both tank gauges and the fuel flow integration at the same moment, and note the time. If the totaliser is also falling in agreement with the left gauge, the fuel is genuinely going. If the totaliser still shows the planned fuel, suspect the gauge. Given the smell and the rate, treat it as a leak: a false leak costs a diversion, a missed leak costs the aeroplane.
2. What are your immediate actions in order, and why is selecting crossfeed the wrong answer?
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- Confirm and quantify over ten minutes, not two: totaliser, tank gauges, fuel flow, time noted.
- Do not select crossfeed. Opening the crossfeed valve connects the two engine feed lines so either engine can draw from either tank. If the leak is downstream of the left tank (line, valve or the crossfeed valve), crossfeeding pressurises that leaking line from the good right tank and pumps the remaining good fuel overboard, and you lose the ability to isolate the leak. The AFM authorises crossfeed only for balancing with both engines running and normal indications.
- Isolate. Switch off the left boost pump if fitted. Close the left firewall fuel shut-off valve and shut the left engine down only if the leak is in the engine feed line; if the rate is unchanged after isolation, the leak is from the tank or vent and shutting down will not stop it.
- Divert. A suspected fuel leak is a 'land as soon as possible' condition, not 'as practicable'. Select the nearest suitable aerodrome now, not the planned destination.
- Plan single-engine before committing: missed-approach climb gradient, landing distance, and fire cover on arrival. Advise ATC as PAN with the shortest routing requested.
3. Assuming a 750 lb/h loss from the left tank, calculate what fuel will be available and state which aerodrome you can actually plan to use.
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Left tank: 315 lb ÷ 750 lb/h ≈ 25 minutes to empty.
If the aircraft lands within 25 minutes, the right tank still holds about 620 lb, giving 620 ÷ 400 lb/h ≈ 1 h 33 min of single-engine fuel from that tank. Destination is 35 minutes away; adding a go-around and a 30-minute final reserve gives about 65 minutes needed against 93 minutes available. On paper, destination is reachable.
That is not the same as destination being usable:
- The margin is only about 28 minutes, all of it in the right tank, with no ability to transfer fuel.
- There is no fuel to reach the alternate 20 minutes beyond destination, so the destination would have to be treated as a destination with no alternate, which the weather (snow, medium braking action, RVR 1,200 m) does not support.
- Any holding, vectoring or a go-around eats the margin quickly.
The correct plan is the nearest suitable aerodrome reachable within about 20 minutes with good weather and a long runway, landing with fuel still in the right tank. If none exists, destination may be accepted only if a landing is assured on the first approach; in that case declare PAN, request the shortest routing and fire cover, and brief a straight-in approach with no go-around.