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It turns out Boeing 747 can carry an extra engine under its left wing in case there's need to transport an engine somewhere far away. The engine is attached between the body and the left inner engine and isn't operational during such flight.

What implications does that have on the airplane operation? I guess drag is increased notably and also drag increases only for left wing and so the plane should tend to well, turn left in case engine thrust is equal on both sides of the plane so now engines under right wing have to produce more thrust or something like that. Also the engine weighs quite a lot so less cargo may be put into the plane. Also fuel consumption increases and flight range decreases accordingly.

Is that all? What are the consequences of attaching an extra engine under Boeing 747 wing?

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    $\begingroup$ You mean the left wing engines need to produce more thrust to offset drag, correct? This would normally be done with rudder trim, rather than thrust, anyway. $\endgroup$ – Zeiss Ikon Jul 23 '18 at 14:47
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    $\begingroup$ No you've pretty much covered it. There is likely a Flight Manual Supplement for spare engine transport that has performance charts for departure, climb, cruise etc with a penalty applied. There are obvious costs to doing this, but they are seen to be cheaper than shipping the engine on its own. $\endgroup$ – John K Jul 23 '18 at 14:51
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    $\begingroup$ This is a perfect one for @Terry he may have even flown in this config before. $\endgroup$ – Dave Jul 23 '18 at 15:04
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    $\begingroup$ There may be some other implications like increased approach speeds, higher stall speeds, changes to emergency operations if one engine goes out on the left side, etc. Terry will probably know. $\endgroup$ – Ron Beyer Jul 23 '18 at 15:33
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    $\begingroup$ @JohnK it's the cost of extra fuel needed on a regular flight as compared to having to fly another aircraft on the route just to transport the engine. Yeah, it's going to be cheaper. $\endgroup$ – jwenting Jul 24 '18 at 5:51
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Operationally you would be more restricted in terms of trimming. Back when we had manual loadsheets there would be a demarcated area '5th pod operations limit'. Only the gods of the loadsheet deptartment would be allowed to do the loadsheet for these flights. I think Qantas would put a loadsheet man on board for these flights but I could be mistaken.

The flight would be re-timed as the usual 90 minute turnaround time would not be sufficient. I believe 2 hours on top of the regular turn was the norm.

An additional fuel stop was usually required as the fuel-burn was atrocious with that big thing hanging off. The weight penalty was compensated by reducing cargo.

I was working in Singapore in the 80s and we usually had Qantas doing this a few times a year. The engines were ex-SIN .. I don't know if they were overhauled in SIN or if they came into SIN on another freighter. At the time there were not many B747 freighters operating into Australia. I believe Flying Tigers had one but that was from US to Australia to SIN.. It did not do SIN-Australia.

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