Green arc tachometer increase with altitude

[tehmightypirate]

So, in the G1000 172S the green arc on the tach maxes out at 2500 RPM. Unless you go above 5,300 feet, then its 2600 RPM. Again, at 9,700 ft it increases to 2700 RPM. I've always just accepted this but only recently asked why. Google had no answers so maybe you can shed some light on this.

Why does the "normal cruise RPM" displayed on the G1000 increase with altitude?

I feel this should be super obvious and something from aviation 101 that I'm forgetting.

 

[EdFred]

% of max power.

If you set your RPM for 2500 as 75% max power at sea level, how much of your max power are you using at 5000' while at 2500RPM ? Less than 75%. So, you can increase the RPM, and still be at or below 75%.

 

[tehmightypirate]

% of max power.

That would make sense wouldn't it. I shall begin face-palming.

 

[EdFred]

That would make sense wouldn't it. I shall begin face-palming.

Cue the Picard!

 

[AggieMike88]

It's been a long day when the eyes read the title and the brain goes, "the green arc gets bigger as he climbs?"

I need a nap

 

[MAKG1]

FWIW, the earlier 172s have the exact same issues, but the green arc does not show them.

In a 172N, the green arc tops out at 2700 RPM, but the POH "recommends" no more than 75% power, which at sea level on a standard day is 2500 RPM.

 

[Larry in TN]

What does the AFM say for RPM limitations in the Limitations chapter?

 

[tehmightypirate]

What does the AFM say for RPM limitations in the Limitations chapter?

Just what I posted above; it states that the green arc (NORMAL OPERATING RANGE) on the tach changes with altitude and states that the maximum RPM is limited to 2700 RPM.