A pilot is planning to fly a 200 NGM trip with a TAS of 150 kt. The aircraft burns 4 kg/NAM and initially, there is a 10 kt tailwind. For every 10°C increase in temperature, the TAS increases by 1 kt. Which of the following scenarios will result in the lowest fuel burn?
Let's calculate how much fuel will be burned in each scenario:
"Temperature decreased by 15°C"
For every 10°C increase in temperature, the TAS increases by 1 kt. So if temperature decreased by 15°C, that TAS would decrease by 1.5 kt and be 148.5 kt. Adding the 10 kt tailwind component, will give us a GS of 158.5 kt. The fuel consumption is based on NAM, we need to calculate how many NAM we would cover:
NAM = NGM x (TAS / GS)
NAM = 200 NM x (148.5 kt / 158.5 kt)
NAM = 187.4 NAM
Fuel Consumption: 4 kg/NAM
187.4 NAM x 4 kg/NAM = 749.6 kg
"Temperature increased by 10°C"
Due to the temperature increase, the TAS would increase to 151 kt. Adding the 10 kt tailwind component will give us a GS of 161 kt.
NAM = NGM x (TAS / GS)
NAM = 200 NM x (151 kt / 161 kt)
NAM = 187.6 NAM
Fuel Consumption: 4 kg/NAM
187.6 NAM x 4 kg/NAM = 750.4 kg
"Tailwind component increased by 10 kt."
The TAS is 150 kt and if the 10 kt tailwind is increased by an extra 10 kt, the GS would be 170 kt.
NAM = NGM x (TAS/GS)
NAM = 200 x (150/170)
NAM = 176.5 NAM
Fuel Consumption: 4 kg/NAM
176.5 NAM x 4 kg/NAM = 706 kg
"Headwind component increased by 15 kt."
The TAS is 150 kt and if the headwind is increased by 15 kt, there would be a 5 kt headwind component and the GS would be 145 kt.
NAM = NGM x (TAS/GS)
NAM = 200 x (150/145)
NAM = 206.9 NAM
Fuel Consumption: 4 kg/NAM
206.9 NAM x 4 kg/NAM = 827.6 kg
The lowest total fuel burn is recorded when the tailwind component has increased by 10 kt and this is the correct answer.
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