I know this is the homework assignment, but this is so damn charming I'm going to blog about it anyway. This represents the form of a sectional variation, wherein the variation is not of just a single pattern, but of a larger section. In the case of tis air the section is a group of 3 phrases. The first two phrases make up a parallel double period, with 3 half cadences (it modulates to the dominant on the second, but doesn't always sound like a PAC, and I construed it as a half cadence in the grander scheme) and similar material before a final PAC at measure 16. The final phrase reiterates the second half of the double period, again concluding on a resounding PAC. Each phrase is 8 measures long, making each section very symmetrical. The first two sections (Air and 1st variation) sound very similar, the variation being the addition of further arpeggiation of the chords with a constant sol being harmonized. The second variation mirrors the first one, until the final phrases desceneds towards the PAC when we have a furious trilling in the middle line. This figure is repeated again as we have the repetition of the second phrase. Now we move on to what sounds like new material, however it's still the same progression and identical section structure, the third voice drops out and the soprano line seems to drop the melody for simple arpeggiations. The lower voice maintains the melody line. The next variation inverts this, as the melody once again belongs to the soprano and the lower line arpeggiates out the chords. This section once again ends on a full PAC. Now we have one final variation, which is incredible, as we begin the harpsichord fireworks, whizzing up and down the keyboard with 16th note runs. When we run downwards, a run is returned up. We play back and forth with this motion, all the time maintaining our chord progression and our strict sectional form. This all ends on a PAC, as they all do, and all those runs ritard slowly as we approach our final descent to the PAC.
...sorry, wanted to go with the airplane reference.