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My understanding is that "critical altitude" is the density altitude at which the wastegate on a turbocharged engine is fully closed, so any climb will result in a decrease in manifold pressure.

Is this different to "full throttle height"? I have heard conflicting definitions of that, such as it only applies to superchargers, or that turbocharges have multiple full throttle heights.

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    $\begingroup$ Many people consider Full Throttle Height and Critical Altitude to be the same thing. I think any answer depends a lot on the actual engine type, and is complicated by turbocharging vs supercharging, and further complicated by engines that have both a supercharger and turbocharger, or 2 speed super chargers. Intercoolers and water injection also muddy the water. $\endgroup$ – Mike Sowsun Aug 16 '20 at 16:39
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Bob Tait explains it perfectly:

Every turbo charged engine has a rated boost which is the maximum manifold pressure permitted for that engine. It is marked as a red line on the manifold pressure gauge. Because the ambient density drops with altitude, there will always be an altitude beyond which it will be impossible to obtain the rated boost. That is called critical altitude.

However the aircraft is still capable of climbing beyond that altitude but the manifold pressure available at full throttle will be less than rated boost. For every value of manifold pressure there will be some other altitude beyond which that particular manifold pressure will no longer be available. So every value of manifold pressure has its own full throttle height. Critical altitude is simply full throttle height for rated boost i.e. the red line on the manifold pressure gauge. (bobtait.com.au)

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