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?
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2$\begingroup$ Interest only: An uncle used to drop humanitarian aid supplies in Africa from a Hercules, altitude as close to zero as terrain and nerve allowed. Somewhere under 50 feet typical. Grain in sacks often split open. Add another sack outer - these occasionally split open. Add ANOTHER sack outer. Hooray. Triple bagged grain sacks NEVER split open. [[Not too too long before that he used to deliver stuff over Germany at night from a Lancaster, but after a while the recipients invited him down for about a 3 years break]]. $\endgroup$– Russell McMahonMay 1, 2019 at 13:38
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5 Answers
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That scenario only makes sense if your airplane stays at cruising altitude: although taxi and takeoff does use up fuel it's really the ascent to cruise that takes the most. 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.
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.
The military only air drops cargo when there's no other alternative - now you know why.
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20$\begingroup$ Precision high-altitude drops to within 50-75m using GPS-guided steerable parachutes are a thing.. $\endgroup$ Apr 28, 2019 at 22:53
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12$\begingroup$ @JörgWMittag From your source "achieving a high degree of accuracy (less than 100 yd (91 m)) requires the aircraft to fly at the lowest altitude possible, which can range from 400 ft (122 m) above ground level to as high as 1,500 ft (457 m)" That does not sound high-altitude to me. $\endgroup$ Apr 29, 2019 at 7:26
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15$\begingroup$ @AEhere That's talking about normal airdrops. The next paragraph says JPADs can achieve the same or better accuracy from greater heights, allowing the aircraft to drop the load at a much higher, and usually safer, altitude. $\endgroup$– RawlingApr 29, 2019 at 8:02
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34$\begingroup$ "Everything is air drop-able at least once", eh @MartinBonner? $\endgroup$– GdDApr 29, 2019 at 8:44
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3$\begingroup$ @Rawling fair enough, I was confused by the lack of actual or claimed accuracy for the system. From one of the wiki sources: "Unlike the lighter JPADS 2K’s 150m accuracy, the 10,000 pound capacity JPADS 10K is accurate only to within 250 meters." with remarks about approx. 25kft ASL as the drop altitude. That article could use a revision for clarity and consistency. $\endgroup$ Apr 29, 2019 at 9:15
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Looking purely at operational cost of the aircraft, yes. You save time, burn less fuel, don't have to pay for the landing etc.
But dropping the cargo makes the cargo more expensive. You have to provide parachutes (and return them after use, inspect them etc). You have to combine cargo into parachute loads. You have to package the cargo for a hard landing, getting pulled over on its side by the parachute after landing etc.
You have to use an aircraft suited for airborne dropping (i.e. with a tail ramp). Commercial cargo aircraft usually don't have one, so you have to switch to more expensive military aircraft (Hercules, C-17).
And occasionally a parachute won't work and the cargo will dig a crater.
You can also go for low-altitude horizontal extraction, but that also has its cost, and entertaining failure modes.
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1$\begingroup$ Good summary, I'd add to this many cargoes can't handle the g forces from landing, even with chutes it's still a few Gs. $\endgroup$– GdDApr 28, 2019 at 21:51
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2$\begingroup$ Note that the OP specified in the question that the plane doing the land-unload-takeoff is more fuel-efficient than the plane doing the drop. So, the question which of the two options im more fuel-efficient cannot be answered other than "it depends on how much more fuel-efficient the other plane is". $\endgroup$ Apr 28, 2019 at 22:50
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$\begingroup$ OF course you also save the cost of building a runway in the first place $\endgroup$ Apr 29, 2019 at 16:13
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$\begingroup$ "And occasionally a parachute won't work and the cargo will dig a crater." Even if it doesn't, the cargo is unlikely to survive it. Cargo not surviving is likely to drive-up the cost of transport. $\endgroup$– MastApr 30, 2019 at 6:01
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$\begingroup$ @Mast: Those Humvees falling off their parachutes in the video you linked was due to sabotage (npr.org/sections/thetwo-way/2018/05/10/610099456/…), not normal failure. Yes cargo damage is likely, but craters aren't. The link you chose doesn't support either argument. $\endgroup$ Apr 30, 2019 at 18:27
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The main advantage of landing is that the plane can then carry another cargo on the return journey. Flying an empty plane back home is extremely inefficient and halves the range of the plane.
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.
Dropping a load by parachute is fairly difficult, but loading a plane in the air is a real challenge!
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Outside of any of the difficulties associated with dropping cargo out of a plane at altitude, the answer is still 'it depends'.
For long distance flights, a large part of the initial weight of the plane is due to the fuel load, not the cargo. This fuel weight imposes a penalty on both climb- and cruise performance. It may be beneficial to land halfway and refuel, so that on both legs of the journey, less fuel has to be carried. 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).
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). However, in that case you might as well use road or rail transport. 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.
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Depends entirely on the constraints. So: "Sometimes" or "maybe".
There are a few effects that make landing more fuel efficient:
- On long trips refueling midway is fuel efficient, even if it involves landing.
- Saving a second trip by loading new cargo midway is even more fuel efficient.
- Having lower weight cargo due to absence of air drop packaging is fuel efficient.
- Saving a trip due to tightly packed cargo is fuel efficient.
Related:
- Dropping a parachute over a runway from high altitude can disrupt air traffic for a significant amount of time.
- Dropping a parachute over a runway from low altitude means having to climb again, which is not fuel efficient.
There are many (contrived?) scenarios where none of these arguments come into play - in those scenarios air drop is indeed more fuel efficient. 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.
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$\begingroup$ RE your next-to-last bullet, that is easily solved by delivering air-dropped cargo somewhere other than an airport. $\endgroup$ Apr 30, 2019 at 23:36
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$\begingroup$ @ThePhoton Absolutely. But I assumed that to be a requirement of the question. $\endgroup$– PeterMay 1, 2019 at 8:26
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$\begingroup$ I guess he did say "onto a runway", but there's no reason we couldn't build a runway someplace where there's no airport. $\endgroup$ May 1, 2019 at 14:51