Pressure Altitude explained

Nov 17, 2017

Pressure Altitude explained

Pressure Altitude explained

If you are like a lot of pilots, you struggle with understanding pressure altitude. Lets make it real simple, "its what you get when you put 29.92 in your kolsman window." Do not get confused with density altitude, which is pressure altitude corrected for non standard temperature and humidity.

Pressure altitude is the performance altitude, if you will, for the helicopter. If your field elevation is 800' above sea level, and the barometric pressure that day is 29.92, then the kollsman window will read 29.92, when you move the altimeter to 800 before take off. 

Pressure Altitude explained

We know that the field elevation will not be changing. So when you jump in the helicopter on the ramp and set the altimeter to field elevation, the kollsman meter will show you the barometric pressure. Then you can see what the pressure altitude is by putting 29.92 in the kollsman meter and look where the altimeter is pointing.

If the barometric pressure is above 29.92, it will cause the pressure altitude to be lower. High pressure means high performance (memory aid).

Pressure Altitude explained

If the barometric pressure is below 29.92, it will cause the pressure altitude to be higher. Low pressure means low performance (memory aid). The lowest pressure ever recorded was in a hurricane!, hurricanes are bad and so is low pressure. 

If you listen to an awos, asos, or atis..... the recording will say "altimeter 29.92" for example. Instead of stating "barometric pressure is". Remember to update the altimeter by getting a current "altimeter" from one of these broadcasts every 100 miles at least. If you are flying along and get a current "altimeter" and put the number in the kollsman meter, then your altimeter will be reading accurate MSL. This is done in flight.

There is a method for figuring pressure altitude on paper, and this is likely going to be demanded of you during the oral part of the checkride. Our examiner usually will pick a high altitude airport and provide a low pressure, and have you figure out the pressure altitude and cross check the POH to see if you can hover in and out of ground effect when you get there.

Pressure Altitude explained

The higher the pressure altitude, the lower the performance will be in the helicopter, as noted when you look at your POH graph for hovering ceilings in and out of ground effect.

To figure it by hand, you take the barometric pressure provided and put it in a subtraction problem with 29.92 (putting the bigger number on top). Now drop the decimal point from the answer and add a zero. You then have a number to either add or subtract from the field elevation known. The above memory aids come to play here. If the given barometric pressure is lower than 29.92, then add your number to field elevation because low pressure is low performance. Likewise if the given barometric pressure is higher than 29.92, take your answer and subtract it from field elevation to get pressure altitude. On a high pressure day, the pressure altitude will be lower than field elevation, when on the ground at the given airport.

Check out our free audio lessons! Start with episode 141 and explore the large collection we have for you:

Pressure Altitude explained

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Gary Cleveland, Chief Pilot

Helicopter Online Ground School

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