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Dear Capt Lim,
Can you tell me how much does passenger weight influences fuel consumption on your typical Boeing 747 or 767 flights? Is there a formula used to estimate the total passenger weight based on how many seats are filled? Is there a per-person fuel allotment? Do you have a formula to estimate carry-on baggage weight? How carefully do you measure cargo weight?
Just curious! I just saw the Jodie Foster movie "Flight Plan" and I wonder if Jodie's character, who is supposed to be a propulsion engineer, could have used fuel consumption versus passenger manifest and cargo records to determine if her daughter was actually on board the aircraft.
Thanks for your help! This is obviously NOT an urgent question!
Regards,
Anne Garvey
Hi Anne,
Yes, passengers' weights do influence the fuel consumption of a typical jetliner - the heavier the plane, the higher the fuel consumption. For instance, on a Boeing 777-200ER, it consumes around 8,000 kg (kilograms) of fuel per hour when it weighs 280 tons and only 6,000 kg per hour when its weight is down to 200 tons. I did not watch the movie "Flight Plan", and assuming Jodie*s daughter weighs 50 kg, it would be very insignificant to affect the fuel consumption. Any variable in the air such as a five-minutes delay in descent because of air traffic or the pilot decides to select flaps earlier than normal (which would be unknown to the propulsion engineer - Jodie Foster - on the ground) would render any calculations almost impossible.
As far as I know, there is no per-person fuel allotment formula that I have come across. But there is a formula for estimating the passengers' weight. On a Boeing 777, an average adult weight is 75 kg, a child, 40 kg and an infant, 10 kg. Baggage entitlement of an adult in the economy/coach class is 20 kg, in the business class, 30 kg and first class, 40 kg. This varies slightly with airlines. In winter, extra allowances for weight are given (heavy coats). Carry-on baggages are allowed up to a maximum of 5 kg.
How carefully are cargo weights measured? Well, normally all cargoes are weighed at the truck dock then loaded on the plane in an orderly manner. The airline figures out how best to load them in the correct zones by distributing the load so that the center of gravity (CG) is within the safe limit. This is done together with the other load (passengers and luggage) through a computer that also calculates the amount of fuel necessary for a safe flight to the destination.
Let me repeat a tale I wrote in an earlier answer. In the past, when airlines were not as stringent in checking the carry-on luggage, a small jet plane did suffer a close call. In 1980, when a plane flew out of Chicago, the pilot reported that the take off acceleration was very sluggish and required more than usual power to safely take off and cruise. Suspecting something was abnormal, the pilot initiated an investigation. It was found that most of the passengers were coin collectors traveling to a convention. Their carry-on luggage contained over a ton of coins! Yes, a ton in excess is very crucial in a smaller jet plane than one on a larger one.
P.S. I will see the movie "Flight Plan" to get an idea what you are talking about when I have the opportunity.
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Flying the Plane 
