Teaching the 747 in classrooms, or differences for some airlines instruction on contract, I came across "real differences" between 747 airplanes. You want an extra 10,000 kilos, ok, for that, you need to put bigger bolts, bigger tyres, or install "beefed-up" components.
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Some as an example were extending the pitch trim "green band". Other were the flap load relief. Originally, the light weight 200s had their trailing edge flaps hydraulically retracting from 30 flaps to 25 flaps when IAS was excessive until speed was reduced again. That feature, for additional weight increases included flap load relief from 25 flaps further to flaps 20 retraction.
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So, weight increases... required extra $$$ to pay to Boeing engineering.
But I have to smile on one increase that my airline got, in about 1995.
They had installed a small auxiliary forward center wing tank for extra fuel.
Now they wanted a TOGW increase from 371,945 to 377,842 kilos.
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So Boeing engineering did the increase for... some $350,000...
How did they do it...?
They printed a new page for the AFM, the AOM and MX Manuals...
Did they change anything to the airplane...? NO...
Quite expensive for a few pages... Seattle has high printing costs.
Makes me smile.
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Happy contrails