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>Airlines consolidate heavy freight with light freight to get the ideal mix that lets them charge the companies with heavy goods by weight and the companies wit

>Airlines consolidate heavy freight with light freight to get the ideal mix that lets them charge the companies with heavy goods by weight and the companies with light freight by volume. The result is called “free margin,” and is one of the best kept secrets in the air cargo industry. This assumes that the cost of fuel doesn't increase with the mass on the aircraft. It does. But the cost of fuel doesn't increase with the volume of the payload because the fuselage is statically sized. In other words, they aren't losing money if they are shipping around a bunch of lead bricks that only take up 15% of the available cargo space.

jerf 11y ago
"In other words, they aren't losing money if they are shipping around a bunch of lead bricks that only take up 15% of the available cargo space." They may not be literally losing money, but it's a pretty clear-cut opportunity cost if there was anything they could do with that volume that produced profit meaningfully more than $0. I wouldn't care to stand in front of the airline's financial officer and try to make your argument.
- hydrogen18 11y ago
  The opportunity cost you are paying would be in not having enough flight capable planes, not in having the cargo hold half empty by volume.
repiret 11y ago
The fuel use increases with weight, but not as much as you might think, for two reasons: 1. Even at max weight, the cargo is only half the mass of a 747; put another way, you only decrease the mass by 50% when going from full cargo to empty. 2. Reducung weight reduces induced drag (the drag that you get in exchange for lift) but leaves parasitic drag unchanged. I don't know for sure, but I'd be willing to bet the parasitic drag dominates.
- spectre256 11y ago
  Pilot here. Induced drag dominates at low airspeeds (i.e. takeoff and landing). At higher speeds parasite drag does dominate, so for most of the flight its what matters the most.
- hydrogen18 11y ago
  I don't know the specifics of the 747 mass characteristics, but you are correct in saying that a good chunk of the mass is the plane itself. Not the cargo. But of the plane's mass, a huge amount of that is just fuel. Whenever they take on fuel, they measure it in thousands of pounds (in the US at least). So by reducing the cargo weight, you reduce the fuel weight as well because you can takeoff with less fuel. In other words, removing a pound of cargo can reduce the total weight of the plane by several pounds. For extremely short trips this would be irrelevant, because fuel costs are dominated by takeoff, landing, and waiting for clearance for those two things.
  - repiret 11y ago
    > In other words, removing a pound of cargo can reduce the total weight of the plane by several pounds Thats only true if a significant amount of fuel is used to overcome induced drag when in cruise. If most of the fuel is used to overcome parasitic drag, then reducing your weight doesn't significantly affect how much fuel you need to carry.

jerf 11y ago

"In other words, they aren't losing money if they are shipping around a bunch of lead bricks that only take up 15% of the available cargo space." They may not be literally losing money, but it's a pretty clear-cut opportunity cost if there was anything they could do with that volume that produced profit meaningfully more than $0. I wouldn't care to stand in front of the airline's financial officer and try to make your argument.

hydrogen18 11y ago
The opportunity cost you are paying would be in not having enough flight capable planes, not in having the cargo hold half empty by volume.

repiret 11y ago

The fuel use increases with weight, but not as much as you might think, for two reasons: 1. Even at max weight, the cargo is only half the mass of a 747; put another way, you only decrease the mass by 50% when going from full cargo to empty. 2. Reducung weight reduces induced drag (the drag that you get in exchange for lift) but leaves parasitic drag unchanged. I don't know for sure, but I'd be willing to bet the parasitic drag dominates.

spectre256 11y ago
Pilot here. Induced drag dominates at low airspeeds (i.e. takeoff and landing). At higher speeds parasite drag does dominate, so for most of the flight its what matters the most.
hydrogen18 11y ago
I don't know the specifics of the 747 mass characteristics, but you are correct in saying that a good chunk of the mass is the plane itself. Not the cargo. But of the plane's mass, a huge amount of that is just fuel. Whenever they take on fuel, they measure it in thousands of pounds (in the US at least). So by reducing the cargo weight, you reduce the fuel weight as well because you can takeoff with less fuel. In other words, removing a pound of cargo can reduce the total weight of the plane by several pounds. For extremely short trips this would be irrelevant, because fuel costs are dominated by takeoff, landing, and waiting for clearance for those two things.
- repiret 11y ago
  > In other words, removing a pound of cargo can reduce the total weight of the plane by several pounds Thats only true if a significant amount of fuel is used to overcome induced drag when in cruise. If most of the fuel is used to overcome parasitic drag, then reducing your weight doesn't significantly affect how much fuel you need to carry.