Forum: aviation.stackexchange.com
Tags: <fuel><efficiency>
Title: Is it cheaper to drop cargo than to land it?
Question: Is it more fuel efficient to drop cargo onto a runway from the air en route to a final destination than to land and unload it using a more fuel efficient plane?

Answers:
sents: text: That scenario only makes sense if your airplane stays at cruising altitude: although taxi and takeoff does use up fuel
text: it's really the ascent to cruise that takes the most.
text: You aren't really going to be able to drop cargo accurately from cruising altitude, so you'll have to descend pretty low, then you'll need to climb up again, and that would suck up lots of fuel and make it much less efficient a method of delivery.
text: Add to that the weight and cost of the parachute mechanisms as well as the massive protective packaging the cargo would need to survive the jolt (2-3 Gs when it hits the ground) and the whole thing becomes pretty uneconomical.
text: The military only air drops cargo when there's no other alternative -
text: now you know why.
answer_details: author: aviation.stackexchange.com/users/1791/GdD
score: 49
sents: text: Looking purely at operational cost of the aircraft, yes.
text: You save time, burn less fuel, don't have to pay for the landing etc.
text: But dropping the cargo makes the cargo more expensive.
text: You have to provide parachutes (and return them after use, inspect them etc).
text: You have to combine cargo into parachute loads.
text: You have to package the cargo for a hard landing, getting pulled over on its side by the parachute after landing etc.
text: You have to use an aircraft suited for airborne dropping (i.e. with a tail ramp).
text: Commercial cargo aircraft usually don't have one, so you have to switch to more expensive military aircraft (Hercules, C-17).
text: And occasionally a parachute won't work and the cargo will dig a crater.
text: You can also go for low-altitude horizontal extraction , but that also has its cost, and entertaining failure modes .
answer_details: author: aviation.stackexchange.com/users/11395/Hobbes
score: 34
sents: text: The main advantage of landing is that the plane can then carry another cargo on the return journey.
text: Flying an empty plane back home is extremely inefficient and halves the range of the plane.
text: Air dropping might make sense for a large number of relatively small but urgent packages with lots of destinations along a route, but even then the plane would be mostly empty towards the end.
text: Dropping a load by parachute is fairly difficult, but loading a plane in the air is a real challenge!
answer_details: author: aviation.stackexchange.com/users/40081/Robin Bennett
score: 24
sents: text: Outside of any of the difficulties associated with dropping cargo out of a plane at altitude, the answer is still 'it depends'.
text: For long distance flights, a large part of the initial weight of the plane is due to the fuel load, not the cargo.
text: This fuel weight imposes a penalty on both climb- and cruise performance.
text: It may be beneficial to land halfway and refuel, so that on both legs of the journey, less fuel has to be carried.
text: This is also why refuelling is typically done at every stop (unless poor availability or high fuel costs forces 'tankering' - landing with enough fuel left to do a return flight or the next leg as well).
text: The only use case for commercial cargo drops seems to be if you need to deliver cargo at a number of closely spaced airports (in which case the short hops would be fuel-inefficient).
text: However, in that case you might as well use road or rail transport.
text: The only remaining use case, which is unsurprisingly the only use case in reality, is to drop cargo if no other means of delivery are available - for example, conflict zones, disasters, etc.
answer_details: author: aviation.stackexchange.com/users/4108/Sanchises
score: 10
sents: text: Depends entirely on the constraints.
text: So: "Sometimes" or "maybe".
text: There are a few effects that make landing more fuel efficient: On long trips refueling midway
text: is fuel efficient, even if it involves landing.
text: Saving a second trip by loading new cargo midway is even more fuel efficient.
text: Having lower weight cargo due to absence of air drop packaging is fuel efficient.
text: Saving a trip due to tightly packed cargo is fuel efficient.
text: Related: Dropping a parachute over a runway from high altitude can disrupt air traffic for a significant amount of time.
text: Dropping a parachute over a runway from low altitude means having to climb again, which is not fuel efficient.
text: There are many (contrived?)
text: scenarios where none of these arguments come into play - in those scenarios air drop is indeed more fuel efficient.
text: These scenarios would usually involve short distances, inherently air droppable cargo, runways that see very low amounts of traffic, and machines that will stay at relatively low altitude during most or all of the trip.
answer_details: author: aviation.stackexchange.com/users/12665/Peter
score: 2

Summary of the answers:
Dropping cargo is really only useful in areas where it is the only option, i.e. in conflict zones and zones with difficult terrain. If cargo is dropped the plane canot pick up new cargo and therefore becomes continually emptier. Flying empty planes is not cost-effective. Moreover, the plane would have to descend to drop the cargo and then climb back to cruising altitude, hence burning a lot of fuel and thus another disadvantage. There are also problems related to the cargo itself: preventing it from being damaged, returning parachutes after the drop, preventing incidents on the ground from dropping loads, etc.
Air-dropping is considered more efficient only under certain circumstances: e.g. for a lot of small, urgent packages to aiports within short distance. Other than that, it is discouraged. Having to descend low to drop the cargo more accurately consumes a lot of fuel. Although some would argue that strictly speaking, it is more efficient, they need to consider that flying an unloaded plane back is much less fuel efficient all in all. The cargo becomes more expensive due to the measures that need to be taken to ensure its safe delivery during a hard landing. Also, you have to use a specific kind of aircraft and type of cruising.