This is from my DFC90 manual. I think when you set the ALT to “0” the autopilot is also trying to capture that altitude. When I am flying my SR22 (which is always over the mountains here in Northern CA and Southern OR, I am constantly thinking about what I would do in a loss of engine power. My process would be:
1. Sync the HDG bug on my PFD by pressing the lower right knob.
2. Set my DFC90 to HDG. IAS and use the dedicated IAS knob to set 88.
3. Make sure AP is pushed.
Now I am flying straight at an IAS of 88. I would expect my plane to briefly gain some altitude to go from 120 to 88. Engine out, 120 to 88 will happen fast. The DFC90 will now “sacrifice altitude to maintain IAS of 88.”
I can now use my heading bug to fly (glide) to a safe are for a CAPS pull if I can not get an in air engine restart.
R