Why are there two red lines on the G-meter on the MiG-29 aircraft? One at 7.5G, another one is at 9G. Also on the AOA scale the red line is at only 15 degrees. That is max AOA. It looks very low to me. Or the max G must not have more than the 15AOA? Now it's about the 7.5 G at 15AOA or the 9G at only 15AOA?
@George already gave a correct answer, but I'll expand it a bit and show the source.
Indeed, the lower load limit is for trans- and supersonic flight (formally M > 0.85) and the higher limit is for subsonic conditions.
The AoA redline at 15° also relates to M > 0.85. At subsonic speeds, the max AoA is 26°. I can only speculate why it is not shown on the gauge. Normally, the control system will enforce these (AoA) limits automatically.
Here are the load and AoA graphs from the MiG-29 flight manual (or more precisely, its 'Practical aerodynamics' part).
One might notice that the official supersonic load limit is 7 rather than 7.5. I'm not sure why is the gauge showing 7.5.
The curves indicate loads and AoA that are actually achievable at different altitudes (in km). The boundaries are labelled 'structural limits' (on the top) and 'control system limiter settings' (for alpha).
The text explains:
- The load applies to the mass 14200 kg, whether with or without missiles. For higher mass, the load limit is reduced by 1 g.
- The lower supersonic limit is explained by the lift losses for trim. (MiG-29 is marginally stable at low speed, but (like nearly any aircraft) is highly stable at supersonic speeds, and part of the wing lift must be used to counteract the strong tail downforce).
- At M > 0.85, the leading edge droop flaps are stowed, which limits AoA at 15°. At lower speeds, the flaps expand the AoA limit significantly. (There may be further limits; e.g. 13° with a failed SAS (stability augmentation system)).
I can’t comment on the MiG exactly, but most fighter aircraft have maneuvering limitations when heavily loaded with external stores. If I recall correctly, the F-16 is limited to ~7Gs with a loadout. A good guess is that the MiG has similar limitations; this would be verified by looking at a MiG-29 flight manual under operational limitations listed therein.