Semaphore Sam, Have you forgotten that we exist? I know that we don’t make the best coffee in the world, but its free!!!!! Anyway the next time that you are down in Kandara drop in and I will give you the long answer to your question.
The following is taken from the “FMC supplementary performance data for the Boeing B747-400 with CF6-80C2 Engines, ref DO44U702-2”.
Altitude Data
The FMC generates optimum and maximum altitude data to assist the pilot in choosing the best cruise altitude for a given trip. The FMC also uses the optimum altitude data to calculate an optimum step point. The calculated maximum altitude is used to ensure that the predicted path of the airplane remains within its performance capabilities. The Maximum altitude is the lower of the thrust limited altitude and the limits of the VNAV operational envelope.
Short Trip Optimum Altitude
The equivalent trip distance is computed by correcting the actual trip distances for the effects of climb derate selection, origin and destination altitudes, forecast winds and temperatures, takeoff gross weight, the use of thermal anti-ice protection during descent and the desired minimum cruise time.
Economy Cruise Mode Long Trip Optimum Altitude
The optimum altitude data shown in figs xx (Long Trip Optimum Altitude and Maximum Altitude ISA-ISA+10 and ISA+20) are identical as the economy cruise long trip optimum altitude is unaffected by temperature deviation.
Maximum Altitude
Maximum Altitude data applicable to the ECON mode are provided versus gross weight in figs xx for temperature deviations of ISA-ISA+10 and ISA+20.
Let me know when you are going to Cairo again and I will come along to test this out.
Cheers.
Mutt.