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Comment by omegant | Hacker News Reader

Also four engines are a must if you operate long haul from hot and high airports like Mexico, Bogota or Quito. Due to engine failure performance requirements a twin engine needs to cut back almost half the traffic load(cargo and passangers) to be able to take off within the tables.

Once they have to retire 747 and A340 airlines din't really know what to do in those airports. New hot and high versions of the twin engines are being studied, but they have failed to meet the expectations previously.

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I love how planes like the 747 can attach an extra engine to ferry it around.

https://www.flightradar24.com/blog/wp-content/uploads/2016/0...

The airlines now ferry around extra fuel from airport to airport as well. Some logistics smarty-pants figured out the airlines can save money this way. (Basically doing arbitrage with regional differences in fuel costs.)

Is there an attachment point on the other side as well? Being super-cynical, I can't help but wonder if these attachment points were also added just in case existing fleets of 747s were pressed into service as long range bombers...

I was once in the back of a 747, on the left side, when I noticed that the inboard flap section was jiggling about, even though it was retracted. I tried to remember if that was normal, and was about to get up and look at the other side, when the captain announced that any turbulence we might notice was on account of the extra engine under the wing, being ferried as a replacement for a failed one.

A question, for anyone who knows about these things: when an engine is being ferried like this, are the fan and compressors locked in place, or allowed to spin in the airflow? I would have thought that locking them would impose unnecessary loads on those parts and create extra drag and turbulence, while allowing them to rotate would require lubrication?

Some information here: http://www.airliners.net/forum/viewtopic.php?t=421613

Really interesting.

I'm no expert but based on these photos [1] it looks like they're only transporting the engine core. What we normally call an "engine" on an airliner is really a relatively small engine core with a huge fan on the front. The white donut-like object is probably blocking airflow through the core to minimize drag.

[1]: https://www.flightradar24.com/blog/how-qantas-ferried-an-eng...

There are many military versions of the 747. Check out the wiki page.

> 747 CMCA – This "Cruise Missile Carrier Aircraft" variant was considered by the U.S. Air Force during the development of the B-1 Lancer strategic bomber. It would have been equipped with 50 to 100 AGM-86 ALCM cruise missiles on rotary launchers. This plan was abandoned in favor of more conventional strategic bombers.

Huh, well I guess that answers my question. I can't see anything about this being used as a mounting point for heavy bombs (which is where I was going with that) but I guess you can't build a large flying platform without at least considering how to strap weapons to it.

No, there is one mount just on one side and the engine also has to be prepared for ferrying. There are quite a few planes which can do this, sometimes the engines have a pod put around them, in this case I think the fans are removed to reduce drag.

In all cases, the engine isn't working. Think of it more like putting an engine on your car roof rack.

I read the parent's comment as suggesting the attachment points could be used for bombs or missiles, rather than additional working engines.

Ah, OK. I read it as upgrading it to a six engine bomber and carrying the load internally - that would require significant airframe modifications though so probably a silly idea.

Given the rest of the Air Force fleet I can't see that a 747 with one bomb would the that useful, but it's one more airframe I suppose. It would also need to be fitted with the arming and release mechanisms so quite a lot of additional work involved there too.

Yeah, that was my thought. I bet you could fit an awfully big bomb under there... or a long range tank with enough fuel in it to keep you in the air for a day or more.

747 was developed in the 60s when it was clear that ICBMs were the future of long-range bombing. I doubt they were thinking of military applications with this. The only long-range bomber missions envisioned by that time were super-sonic deep penetration designs (e.g., see XB-70 bomber), and even those were abandoned in the face of improving anti-aircraft missile tech (see: when the Soviet Union shot down a U-2).

I dunno, something the size of a 747 is probably already flirting with the limits of diminishing returns. It's not unlike the rocket equation. Fuel is very heavy, so adding more fuel means you burn more fuel, meaning you need to carry more fuel...

The first 747 is at the Museum of Flight in Seattle and it is actually set up for inflight refueling. Quite the versatile platform! You can see this at the very back in the 3d tour (or in person, it's a great place):

http://www.museumofflight.org/aircraft/boeing-747-121

I bet there is an attachment point and another engine on there to balance things out.

I would be surprised if Boeing considered or purposely designed in some long range bomber capabilities into the 747. Maybe though as part of the sales pitch to countries that would like to add military capability in a low key sort of way...

Don't forget the MD11, also a favourite of the cargo operators due to having plenty of spare thrust for hot and high ops. Although it's also a truism that high altitude locations don't generally have a lot of heavy things to export. AFAIK Quito's main outgoing cargo is cut flowers...

Mexico main cargo are avocados for Europe, Bogota fruis and flowers too. There isn't enough cargo offer for the demand that exists now a days. We are leaving cargo behind every single flight between these airports and Madrid.

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