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Cargo airlines are known to regularly use planes designed primarily for passenger travel.

Why don't they almost exclusively use aircraft designed specifically for cargo?

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    $\begingroup$ What do you mean with "specialized cargo aircraft"? Special aircraft like an Antonov An-225, or cargo aircraft like a Boeing 747 freighter variant? $\endgroup$ – Bianfable Dec 3 at 14:31
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    $\begingroup$ Thanks for clarifying! The 747 was however primarily designed for cargo. That's why they have the upper deck with the flight deck above the nose, which can be fully opened. $\endgroup$ – Bianfable Dec 3 at 15:02
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    $\begingroup$ IIRC the 747 was designed to do both, to provide a large and economical passenger aircraft in the short term and a very effective freighter in the long term. The assumption of the designers was that subsonic passenger travel would not be around much longer. Supersonic air travel turned out to be just too damn expensive and impractical though, so the 747 stuck around in passenger as well as freight roles. $\endgroup$ – Peter Green Dec 3 at 15:47
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    $\begingroup$ interestingly, the 747 is the result of Boeing's losing bid for what is now the Lockheed C-5 Galaxy. $\endgroup$ – radarbob Dec 4 at 3:01
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    $\begingroup$ This question is spiritually similar to "when Google took off, why did they opt to use an array of commodity PC hardware rather than high-performance computers?" $\endgroup$ – Artelius Dec 5 at 1:36
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Price, commonality, and size choice.

A very large proportion of an aircraft's cost is in design and certification. It's followed by the engines, the avionics, and the fuselage. Not much cost can be removed from an aircraft by removing the passenger-specific elements of design. That is done in freighter versions.

Freighter models like the 747-8F already realize most of the possible savings. Second-hand airliners are cheaper still, and they're not hard to convert. Rip out the interior, install cargo securing systems, and fly.

Commonality with airliners means airliner-derived freighters are easier to service and repair anywhere one needs to. And, as Dohn Joe rightly adds, get pilots for; cargo airlines are both a training ground for passenger pilots and a career path of its own.

Finally, there are more airliner models than freight-only aircraft models, so there's more choice of the right payload and range capability.

This reuse of passenger airliners for freight is a great example of efficiency. Passenger airlines tend to have very high utilization, putting lots of cycles on their airframes and lots of fuel through them, so they care for better fuel efficiency, availability, and passenger experience (or at least their expectations when booking, considering the 9-abreast 787) delivered by newer airframes.

Cargo airlines run looser schedules, because their clients don't care much for their flights leaving on time (air freight is usually offered in 2-5 days), which shifts the optimization focus from fuel efficiency towards capital costs. They are able to use up the rest of the airframe's life in a less-critical job, where the oversight is a bit more forgiving and delays due to lower availability are more acceptable.

The An-225 is special in that it occupies a class above every other freighter in capability. It does the jobs nothing else can, but, until recently, the only airframe built has been able to serve the whole world.

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    $\begingroup$ Another aspect of commonality is workforce. When airlines retire a certain type, e.g. the DC-10, then a cargo airline has plenty of pilots to choose from. In the case of the DC-10, apart from the US Air Force, FedEx is the larger operator. So, a cargo airline may save the cost of pilot training, since converted airliners come with pilots included. $\endgroup$ – Dohn Joe Dec 4 at 13:06
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Pure freight operations tend to fly significantly fewer cycles than passenger or combi aircraft. And you don't care too much how ratty looking it is as long as it's reasonably reliable. This makes older used aircraft a lot more attractive from a business case perspective. You have airlines that buy combination pax/freighter aircraft new, "combies" but these are still primarily pax aircraft that are able to carry paying freight rather than go partly empty on a route with a low load factor at certain times of the day.

The best source of used aircraft is pax aircraft being retired by airlines, before they're fully worn out, when they upgrade their fleets. Many normal passenger airliners have freighter conversion kits available by Supplemental Type Certificate for large cargo doors. So a old MD-11 that is at 70% of its airframe life and can be had really cheap, still has quite a lot of life left in it if it carries on as a pure freighter at maybe only 4 or 500 cycles per year. With the modest cost to convert to a freighter (cargo door and interior) it makes for a really attractive business case.

If you paid the big bucks for a new pure freighter (as opposed to a combi, which is still primarily a passenger a/c), the capital costs would kill you; you have to fly the heck out of it to make enough cash flow to justify the capital costs and freight operations just don't have the frequency.

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    $\begingroup$ Why do cargo planes fly fewer cycles? Is it just the time spent loading them is more, or is there less demand for them to fly? $\endgroup$ – user3067860 Dec 4 at 14:15
  • $\begingroup$ Nice answer. Is 4 or 500 a typo? Maybe 400 or 500? $\endgroup$ – Richard Erickson Dec 5 at 14:37
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    $\begingroup$ @RichardErickson it's written as you'd read it: "four or five hundred", though not as in "four, or five hundred" - it really means "(four or five) hundred". $\endgroup$ – osuka_ Dec 5 at 15:59
  • $\begingroup$ @user3067860 I suspect that is because they mostly fly longer routes. 4 vs 1 hour can matter a lot for passenger flights; but when most cargo is sent on N-day delivery, it's cheaper to just fly to a hub and have a truck carry it the rest of the way instead of loading onto another aircraft for a shorter hop. $\endgroup$ – Dan Neely Dec 5 at 16:25
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    $\begingroup$ Yes I was being conversational, so it should be read as if it was being spoken. $\endgroup$ – John K Dec 5 at 16:49
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There is no such thing as aircraft designed specifically for cargo.

The types like C-5 Galaxy, C-17 Globemaster, An-124 Ruslan, C-130 Hercules or A400M Atlas are not designed for just any cargo, but for carrying cargo to the front line in a war, with ability to land on a strip prepared in a day by a small team of battle engineers and option to para-drop cargo and troops. That's a lot of very special military requirements a commercial operator flying between properly equipped airports has absolutely no use for, and all of them reduce fuel efficiency and increase maintenance costs. That's why nobody ever ordered a Lockheed L-500, the civilian version of Galaxy.

For operating between reasonably equipped airports, the only difference between cargo and passengers is that passengers want windows, and some equipment in the cabin, and cargo needs bigger door to be efficiently loaded. So there are no specific cargo designs, just aircraft built or converted for cargo, without (most) windows, but with big door for easy loading.

The exceptions are:

  • The small fleet of An-124 Ruslans and the sole An-225 Mrija for the occasional exceptionally heavy or large cargo that does not fit in a 747F, which wouldn't exist if the aircraft were not cheaply available when the Soviet Union collapsed.
  • Special outsize cargo carriers A300-600ST, B747-400LCF and A330-743L and former B-377-SG, which are all just conversions of normal airliners with significantly wider fuselage for very bulky cargo.
  • On the smaller scale, there is some market for carrying cargo to remote or poorly developed areas. This is partly covered by ex-Soviet-military An-12s, but there is an occasional L-100 to be seen as well. It's still relatively small market.
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    $\begingroup$ ... nobody ... ordered a Lockheed L-500 - And lucky they did not. That airplane went through rough teething. For example, I remember that for years cargo was limited to about 1/2 of its original spec until they could fix all the wing boxes. More than one airframe had an engine fall off in flight. No kidding. Read The C-5A Scandal. Military contracting, what a concept! $\endgroup$ – radarbob Dec 4 at 3:13
  • $\begingroup$ @radarbob Are there any still flying? $\endgroup$ – Cloud Dec 4 at 13:08
  • $\begingroup$ still flying. I believe they're up to "M" model. ... Not all bad!! In 1974 an ICBM was launched from a C-5 It was mostly to send a message to the Soviets; That's one helluva military-unique requirement I'd say. $\endgroup$ – radarbob Dec 4 at 18:37
  • $\begingroup$ It looks like there will be new production of the civilian C-130, the LM-100J lockheedmartin.com/en-us/products/lm-100j.html $\endgroup$ – trognanders Dec 4 at 23:47
  • $\begingroup$ @trognanders, yes, there are and will be some civilian L-100 (C-130). For the lower capacity range there are still remote places where the very short take-off and landing and ability to operate without much ground support have their use. It is still small market compared to airport-to-airport cargo using more typical airliners. $\endgroup$ – Jan Hudec Dec 8 at 20:10
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Another factor is that airplanes (like almost anything else) become cheaper when you buy them in bulk. That means a massive capital investment up front. FedEx, for instance, was ready to buy 10 A380s at one point, but backed out due to production delays

FedEx Express, the express package delivery unit of U.S. shipping company FedEx, became the first customer to terminate an order for Airbus’ flagship plane when it scrapped an agreement to buy 10 A380-800F aircraft.

Instead, FedEx ordered 15 Boeing 777 Freighter aircraft and took options to purchase 15 more. The previous agreement with Airbus included an option for another 10 A380 planes, a FedEx spokesman said, but those options are now invalid.

FedEx said it expects to take delivery of four Boeing 777s in 2009, eight in 2010 and the remainder in 2011. At a list price of \$232.5 million to \$240 million each, the order will be worth at least \$3.48 billion to Boeing.

It's worth noting that FedEx Express flies their fleet almost constantly, which means they're getting a lot of value out of buying brand new. For most other airlines (like, say, National Airlines, which flies only on-demand air cargo) it makes better economic sense to rehab a passenger aircraft.

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These days airlines often transport passengers by day and cargo by night. Seats are mounted on pallets, they can all be removed in about half an hour. Then the cargo goes in. The margins for passengers are shrinking but those for packages go up.

The Type of aircraft is called combi, one Example would be Boeing 747-400M, there are more current types.

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    $\begingroup$ Wow, I've never heard of this. Do you have a picture or source for this? $\endgroup$ – Bianfable Dec 3 at 16:13
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    $\begingroup$ I've flown overnight flights before and I've certainly felt like I was packed in like cargo, this explains so much!! :D $\endgroup$ – FreeMan Dec 3 at 16:17
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    $\begingroup$ I wouldn't say "often", and very few of the aircraft were designed for quick changeover -- the only ones I've found were the 727-100 and the 737-200. $\endgroup$ – Mark Dec 4 at 3:58

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