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enter image description here
(airplane-pictures.net) Airbus A330 no engine option.

Why are empty pylons weighed down?

  1. The plausible answer is to maintain the correct center of gravity. But why not just load ballast pallets in the forward cargo compartment, like those used on tail heavy planes on ferry flights?

  2. On airliners.net there is a theory (plus debate) that states another reason is to relieve the load on the wingbox, which I would imagine would also decompress the landing gear (I don't see how).

  3. Lastly, I remember on a TV documentary it was stated that the reason is so the wing would not deform upwards (again I don't see how).

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    $\begingroup$ No Engine Option, hee hee. $\endgroup$ – Koyovis Aug 22 '17 at 5:53
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    $\begingroup$ To simulate the actual mas distribution on the wing. It's not only the weight that is important, its location affects various characteristics like modal frequencies in the wing and the load distribution between the wheels. $\endgroup$ – AEhere supports Monica Aug 22 '17 at 6:19
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    $\begingroup$ @AEhere: With the same center of gravity, the wheels see the same weight. But the load distribution on the cargo floor might be out of specs. $\endgroup$ – Peter Kämpf Aug 22 '17 at 6:31
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    $\begingroup$ Why is the center of gravity so important? The plane is hardly going to take of in its current state... $\endgroup$ – fgysin reinstate Monica Aug 22 '17 at 11:50
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    $\begingroup$ And everybody says "NEO" means "New Engine Option"... $\endgroup$ – FreeMan Aug 22 '17 at 12:16
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Your reason 1 is correct. Without the ballast the aircraft would become a taildragger.

Why not pallets? This would produce the same center of gravity location, but a different mass distribution. Clearing this configuration even for being loaded and pushed around is more effort than simply placing the ballast where the mass of the engines would go. Now all clearances are valid and the aircraft can be handled much as one with the engines in place.

Next, engines are only installed shortly before delivery or first flight. The cost of capital is too high to install them any sooner.

enter image description here
(airbus.com) Example of clearances that would be affected by non-standard mass distribution.

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  • $\begingroup$ How is the changeover from engine to ballast performed? Same question goes for swapping an engine.. $\endgroup$ – pericynthion Aug 22 '17 at 8:03
  • $\begingroup$ It would be interesting to verify with a calculation that shows the shift in cg that occurs when an engine is removed, and that it moves behind the landing gear location, but I suspect the data required isn't all available. Perhaps the issue only occurs when both engines are removed, which would allow one to be removed, and then ballast attached, before the other is removed? $\endgroup$ – Penguin Aug 22 '17 at 11:44
  • $\begingroup$ Hanging concrete blocks from the pylons is much more preferable and easier. I remember when I was an intern at Boeing in the summer of 2000 they did this all the time on the 747-400 final assembly line when moving the aircraft prior to engine installation. $\endgroup$ – Carlo Felicione Aug 22 '17 at 13:22
  • $\begingroup$ @Richard: The engines are installed only right before delivery (or first flight, whatever comes first) and never removed. Capital cost of engines is too high to hang them on the unfinished aircraft. Even the painting of the nacelle is done with a dummy engine. $\endgroup$ – Peter Kämpf Aug 23 '17 at 4:38
  • $\begingroup$ @Peter. Engines are removed during their service life to replace the parts that have a fixed life, such as discs and spacers, to ensure structural integrity of these critical rotating parts. Also, when the engine degrades to the point it has no EGT margin, it will be removed for an overhaul, then reinstalled. Engine MROs exist because of this maintenance work. Typically the maintenance that can be done on wing is with the external accessories, pumps etc, though some adjustments of balance weights and borescope blending of some blade stages may be possible. Regards. $\endgroup$ – Penguin Aug 23 '17 at 10:38
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You're correct- it is to maintain the center of gravity within limits and to prevent the possibility of tipping over.

As for why weights in pylons and not ballast, this method is quite simple. You can simply hang the weight of the engine on the pylon and the aircraft will behave as it there is an engine (weight and c.g. wise). In case you want to load ballast, first you have to calculate the ballast to to be loaded based on the location.

In some cases, the cargo compartments may not take the load at all- for example, in case of A320 neo, each PW1000G weighs around 2.8 tons, while the forward compartment is limited to 3.4 tons.

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    $\begingroup$ But wouldn't the fwd compartment have a longer moment arm? $\endgroup$ – ymb1 Aug 22 '17 at 14:00
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    $\begingroup$ @ymb1 Yes, but not by much. In the linked page, you can see that the distance between the forward cargo compartment and engine is not much. $\endgroup$ – aeroalias Aug 22 '17 at 14:03
  • $\begingroup$ +1 and very nice info, I chose PK's answer because of mentioning clearances, thanks again :) $\endgroup$ – ymb1 Aug 22 '17 at 17:19
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You want to handle the aircraft as it was initially designed. This means that even though you could have proper CG location with internal ballast, you might fall out of design elsewhere, like in mass distribution. For example, consider the location of the engines vs the landing gear. There definitely were a lot of design considerations in choosing how to support the aircraft on the landing gear. It appears the gear are inboard of the engines (or where they normally would be) in the image shown. By instead moving all the weight inside the plane, you would be creating a different bending stress distribution, as all the engine mass, originally outboard of the gear, is now very much so inboard.

As a dramatic (more fun) example, imagine a scenario in which the ground crew, having fun with the plane, somehow managed to load the ballast in the correct CG location, but all the way out on the wing tip. I hope you could see that the wing wasn't originally designed with the necessary structure to support that weight all the way at the tip. The tips would droop down to the floor (if the wing didn't totally fail), and then scrape all the way to the next hangar area... :) Imagine then tossing the keys of your bent or drooping A330 to your dumbfounded buyer. "She's all yours".

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That is to keep the center of gravity sufficiently forward of the main gear to prevent the aircraft leaning backward on its tail while the weight of installed engines is not present.

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