How Qantas Ferried an Engine on the Wing of a 747 | Hacker News Reader

How Qantas Ferried an Engine on the Wing of a 747

flightradar24.com

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Fair dues to QANTAS for milking this on social media but spare-engine ferrying was common from the dawn of the jet age.

The 707, 747, DC-8, DC-10, TriStar and VC-10 all had an underwing hardpoint for the extra engine and carrying it was fairly routine; just apply the compensations laid-out in the flight manual.

Four-engined TriStars looked funny...

It is rare nowadays due to the ubiquity of twin-engined airliners which have neither the underwing space nor the take-off engine-failure margin to ferry a spare. Instead their spare engines are stripped and sent in freighters, usually by the engine manufacturer who provide power-by-the-hour contracts to the airlines.

> nor the take-off engine-failure margin to ferry a spare.

What does this mean?

Simply, aircraft engines are designed to be able to produce a lot more than their "fair share" of the thrust needed for flight.

In the case of a twin engine aircraft, if one engine fails the other has to be able to compensate so the flight can continue safely, especially at takeoff when the plane is at its heaviest. Twin engine aircraft can easily fly with one engine out but the additional drag imposed by the ferried engine can reduce the ability for the flight to continue under the power of one engine.

Ahh, wasn't thinking of the drag aspect. Thanks!

No problem!

There are other factors too, like increase adverse yaw which would become more pronounced if the ferried engine were on the same side as the failed engine.

I seem to recall I once flew on an L-1011 TriStar as a kid that ferried an engine. It was out of Atlanta but I can't remember if we were going to Los Angeles or San Francisco. That was probably 25 years ago, so not sure which it was.

As a sidenote, the L-1011 is still one of my favorite planes. It was one of the most technologically advanced airliners ever created when it was unveiled; lots of things about it were way ahead of its time. It's a shame that the competition with the Douglas and the DC-10 nearly bankrupted both Lockheed and Douglas. Airlines played both companies against each other and nearly killed both. Lockheed ended up exiting the commercial business while Douglas merged with McDonnell.

There's a great long article on the the L-1011 published a few months ago: http://www.airlinereporter.com/2015/09/requiem-trijet-master...

The L-1011 is/was amazing. It was the first airplane to be certified for full autoland in zero visibility conditions.

fantastic read @peckrob ~ https://news.ycombinator.com/item?id=10873607

Indeed it was common. Quoting from the article:

> Qantas has used the Boeing 747’s ability to ferry an extra engine in the past, most recently in 2011. Qantas used the method often with their Boeing 707s when engines were less reliable, but the procedure has become quite rare.

Seeing a 747 with 5 engines reminds me of seeing GE's N747GE testbed aircraft in Mojave, which usually flew with a odd engine configurations. I can't remember if they used the 5th pylon at all and most pictures show an experimental engine on the inboard pylon. Sometimes a very large engine[1], sometimes a small one[2].

One time they tried to ferry the aircraft from Mojave to Victorville with only three engines (they must have removed the experimental engine for whatever reason). They attempted the take-off but had to abort. It was quite a sight anyways. They went back to the hangar and we saw it fly away with four engines a few days later.

GE now has a second 747 to play with.[3]

[1] http://www.jetphotos.net/photo/45634

[2] https://en.wikipedia.org/wiki/File:Ge-747-N747GE-020404-01.j...

[3] http://www.sieinc.com/into-thin-air-the-lofty-side-of-jet-en...

I'm guessing they would never use a 5th pylon when engine 2 is missing and replaced by a potentially misbehaving or inoperable test engine. The 5th pylon could never accommodate a running engine since it's just an external cargo mount, not a full blown engine mount with all the necessary connections.

My father works for QANTAS, has for 40 years. He said they used to do one, once a month on the older jumbos. But with the newer ones 400s, QANTAS just retired one to the desert with the 4 original engines still on it. Meaning it never needed an engine change in its entire operating life.

> The fifth pod option is restricted to Qantas’ Rolls-Royce-powered 747s

Why the restriction to RR-powered 747s?

The restriction is to 747-400 vs 747-400ER models. The former are powered by RR engines (at least, at Quantas) while the latter are powered by PW engines. The 747-400ER has not been certified with a fifth engine pod.

Qantas flies one 747-400 with GE, though. (and 4 with RR) I was thinking there might be a contractual issue with GE over this, like GE will just swap the engine on demand at their own expense at any station.

Duly noted. That one /is/ an odd one, though -- it's a 747-48E that was originally from Asiana, so it might not even have the mount for the 5th engine (as it's an optional extra). Still, you are right that it makes more sense to be related to contracts with the engine mft.

I would guess it's a flutter problem. Flutter is when an aircraft becomes dynamically unstable due to the interaction between the aircraft structure and the air around it. Certain aircraft are more prone to flutter than others such that it occurs at speeds closer to its max airspeed. The type of engine and how it's attached to the wing can have a big effect on flutter since it's a large mass that's hanging off the wing. As a result the same aircraft with different engines can have significantly different flutter properties.

https://en.m.wikipedia.org/wiki/Aeroelastic_flutter#Flutter

An oscillation builds up in the wing span to the point where the wing self destructs. Here's an video example from the development of the 747:

http://www.airspacemag.com/videos/category/flight-today/wing...

Why not use a full fairing over a complete engine?

Probably less drag to let the air flow through an "empty" engine rather than to have a large obstruction on the wing.

I don;t know. Cramming all that air down the tube of a fanless engine doesn't seem very efficient compared to the clean cone of a fairing.

Adding a bigger fairing will increase the cross section of the object, increasing the form drag. (Bigger things push more air out of their way when moving)

https://en.wikipedia.org/wiki/Parasitic_drag#Form_drag

I'm also curious about what had happened the engine that needed to be replaced.

Is the 6T engine the heaviest non-visible part on a 747?

From the article: "The disabled engine in Johannesburg will make its way back to Sydney at a much slower pace by ship."

Sorry, I meant what happened to the engine that meant it needed to be replaced. It doesn't sound like these engine replacements happen too often so I figured it was in some way noteworthy.

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