Current payment tables for Aviation Bonus (AvB), Aviation Incentive Pay (AvIP) and Critical Skill Incentive Pay (CSIP) programs.
veteran.com
Can't speak to what it was in 1991.
But from 2000 - 2016 the Aviation Bonus was $25,000 per year. Now it's $35,000 per year ($2,900 per month).
Looking at the Military Pay charts.
An O-3 @ 6 years has a base pay of $7,120, plus the $2,900 take it to just over $10,000 per month. An O-6 base pay is under $10,000 @ over 14 years and breaks into over $10,000 at over 16 years.
So ya, it is possible (at least now) for an Aviation Bonus qualified individual (basically a pilot) as an O-3 to earn more per year than a non-aviation O-6.
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My daughter is an AF Major (O-4) over 8. With her career field bonus she makes a few hundred a month over the next higher grade (Lt. Col., O-5).
WW