FlyHigh423

Hello Everyone!

I have a quick question regarding critical altitude. I know how critical altitude is where the wastegate is completely closed and the engine can no longer produce its rated horsepower if it continues to climb. Any thing I have read on the issue has something alone the lines of "At this point the engine will continue to loss Manifold pressure as if it was a NA engine". Which makes complete sense to me. However, at the altitude where these critical altitudes occur a lot of times over half of the atmosphere is BELOW you. So, wouldnt the engine loss manifold pressure at a MUCH slower rate due to the atmosphere not losing nearly as much pressure per 1,000 feet from when departing form sea level?? How high above critical altitude can a typical turbo powered engine fly? Thanks for the help everyone, just could not find the answer to this anywhere, figured I would come here!

FlyHigh423

shdw

Pressure variation is relatively linear so long as you remain in earths atmosphere. Beyond that point I'm not sure. The point, however, is that you can assume 1" per thousand feet well into the flight levels with minimal error.

Fly Boy Knight

From a scientific standpoint, atmospheric pressure decreases on a logarithmic scale. This means that as you increase in altitude at a constant vertical speed, the amount of pressure change will DECREASE as you climb higher and higher. For a mathematical approximation, most scientists use the Hypsometric Equation to calculate "thickness" (Altitude between pressure levels)

From an aviation standpoint, the FAA Aviation Weather book mentions the variation in pressure decrease with altitude however, "Within the lower few thousand feet of the troposphere, pressure decreases roughly one inch for each 1,000 ft increase in altitude." - FAA Aviation Weather, Chapt 3, Pressure Variation, Altitude -

Because the -1'HG / 1,000ft approximation works pretty well for the lower troposphere, we use this approximation to reduce higher altitude weather stations down to Mean Sea Level in order to see basic weather info like high and low systems, troughs, ridges n such. Because there are not many weather stations above 10,000 FT MSL, we don't much need to worry about the "logarithmic" pressure decrease.

From an altimetry standpoint, our altimeters use this same -1'HG / 1,000ft approximation when displaying our altitude/Flight Level. We do this because if an altimeter is off, we'd rather it be off AT ALT (where there is room to spare) vs near the ground where there is no room for error. That said, because our altimeters use the -1'HG/1,000ft approximation, our altimeters have an inherent error built into them which is negligible near the surface (lower troposphere) and gets more pronounced as you climb. Because of the way our altimeters calculate "altitude," when you reach your critical "altitude," it is in reference to the altitude read off of our altimeters which has this inherent error. SOOOO... finally, we can approximate the engine losing the same amount of power (per alt change) at our C.A. as it would at the surface because our altitude is in reference to our altimeter which ASSUMES a LINEAR PRESSURE DECREASE therefore, even though it is not ACTUALLY a linear pressure decrease, because we are using an altimeter that assumes so, we assume the engine will lose the same power per altitude change at sea level as it would at its critical altitude.

JamesNoBrakes

Bingo.

Many turbocharged aircraft are "limited" by their operating handbooks, while often capable of flight above what is listed. There are several issues that come up when this altitude is exceeded, mechanical, physiological, that the pilot and plane are usually not equipped for. Traditional turbocharged aircraft (hopefully on the way out) allow you to "overboost" and exceed the maximum power at lower altitudes. Turbo-normalized is what most new "turbocharged" aircraft are these days and will only go to maximum SL pressure when you go full throttle. I think you usually see 20-25,000 as a maximum operating altitude with most turbocharged light aircraft.

That said, most combat WWII aircraft used superchargers, sometimes in unison with turbochargers, to reach altitudes above 50,000', although 30-40,000 was a bit more realistic for the top-performing aircraft. Eventually you are limited by the equipment and how fast you can spin the turbocharger and supercharger, as well as a plethora of other mechanical issues.

To reitorate, the rate is the same because it's based on the altimeter. You might see something different with GPS altitude. Realize that while the atmosphere is logrithmic, but also how little we can travel into it. Look up how many miles/km the atomosphere is considered to extend. Then compare the pressure over that distance.