Fuel temperature and tankering in the stratosphere
A jet is cruising at FL370, SAT −56 °C, TAT −26 °C. The fuel in the wing tanks has been at −38 °C for two hours. The AFM fuel system limit is a minimum fuel temperature of −43 °C at the tank, and the fuel freeze point (Jet A-1) is −47 °C. The crew is considering a further climb to FL390 where SAT is forecast −60 °C, to take advantage of better winds.
1. Assess the fuel temperature margin and the effect of climbing to FL390.
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Current fuel temperature −38 °C, limit −43 °C, so margin is 5 °C. Freeze point −47 °C, so the fuel is 9 °C above its freeze point. Climbing to FL390 drops SAT by about 4 °C. The fuel temperature lags SAT and will trend toward the new SAT over time, so the margin will shrink. If the fuel reaches −43 °C, the AFM limit is exceeded - this is a limitation, not a recommendation. The crew should not climb further unless they can confirm the fuel temperature will remain above −43 °C, for example by descending into warmer air or routing to a warmer area. Monitor fuel temperature continuously and be prepared to descend.
2. What is the difference between the AFM fuel temperature limit and the fuel freeze point, and why does it matter?
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The freeze point (−47 °C) is a property of the fuel - the temperature at which wax crystals begin to form. The AFM limit (−43 °C) is an operating limitation set with a margin above the freeze point to prevent waxing in the fuel system, filter blockage and flameout. You must respect the AFM limit, not the freeze point: reaching −43 °C means you are already operating outside the approved envelope even though the fuel has not frozen. The 4 °C buffer is the safety margin, so action must be taken before the limit is reached, not at it.