Utterly Confused

[uwochris]

Makes lots of sense now! Thanks a lot!!

This is a new way that I like to interpret Pressure Altitude (PA)... Assuming temperature and all other variables (other than barometric pressure) are held constant, PA is the altitude the aircraft thinks it's flying at, or it would be flying at on a standard day.

So, if the elevation outside was 3000 ft and the pressure was 29.92 (let's assume 15 degrees C for ease) the aircraft would think it's at 3000 ft. If there was any other pressure outside and the other variables all stayed the same, the aircraft would think it's at a different altitude. For example, if it was 30.92, because of the increased pressure/density and effect this has on performance, the plane would think it's at 2000 ft. This of course is assuming all other variables are being held constant, as PA only factors in non-standard pressure. Higher than standard pressure reduces your PA and implies better performance than would be expected on a standard day at the same elevation.

Also, on another forum, I was told that Indicated altitude is PA corrected for non-standard pressure, and NOT the other way around. Perhaps this is why I was so confused with the definition.

Have I gotten it??? Please say yes!!!!

 

[Timebuilder]

If the pressure suddenly increased, the aircraft would perform better, IF you remained at the same true altitude, but chances are you wouldn't remain at that altitude. You would make a slight climb.

Here's why. You said that you have a barometric pressure of 29.92, and a temperature of 15C, for a standard day. Your airplane is at 3000 MSL, indicating "3000" on the face of the altimeter, your "indicated altitude".

Now, you start flying toward an area of higher pressure. Unless you compensate for this by resetting your altimeter to the local pressure, which is higher, your altimeter will slowly, imperceptibly, begin to register a loss in altitude as the pressure increases, and you will slowly correct for it. You are in a VERY SLOW climb! If you did compensate by dialing up an AWOS station and putting in the "new" pressure setting, which would be higher, it would move your altimeter's hands to indicate a slightly HIGHER altitude, and you would make a slight descent to make the altimeter say"3000" once again.

The pressure went from low to high. "Low to high, clear the sky".

The same thing happens when you fly through a change in temperature. Training aircraft don't have altimeters that compensate for changes in temperature, so your true altitude on a cold winter's day will be lower than on a hot summer's day.

The other half of this rule is "High to low, look out below".

When you are confronted with these questions on the written test, you have to read them very carefully, since their construction is a little awkward. You must first understand what is being asked and then choose your answer.

Did any of that make a light go on for you?

 

[uwochris]

Thanks for that last response Timebuilder. That certainly does clear things up for me

Thanks again to everyone who helped me out here!!

 

[L.A.pilot]

Sentient Aircraft

Hey guys,

Airplanes don't think. The computers on board execute sequenced instructions, and they do not think either. They could care less about what altitude they may or may not be flying at, since they do not have feelings or emotions, either.

Pressure Altitude:

The International Standard Atmosphere (ISA) was established by scientists using measurements from around the globe. The standard includes variations of pressure, temperature, and density with altitude. These values represent an average; in reality the atmosphere varies greatly.

Pressure altitude is the altitude defined in the International Standard Altmosphere that corresponds to the actual ambient air pressure. The altimeter actually measures pressure, and is calibrated to indicate pressure altitude corrected for non-standard sea-level pressure (SLP), as set in the Kollsman window. An altimeter setting is issued by a ground station computing the actual SLP. When this is set in your altimeter, it will give you a height above that reference datum (MSL). When set to 29.92, the altimeter indicates height above standard SLP, or pressure altitude.

Why 29.92"?

The ISA sea-level pressure is 29.92" Hg (meaning that pressure of the atmosphere at sea-level is equivalent to that of a column of mercury 29.92" high). You may learn that pressure decreases 1" per 1000 ft. It is important to note that this is a very rough approximation that is not valid for higher altitudes, as the variation of pressure vs. altitude is non-linear. Indeed, if that were the case, you would encounter negative pressures above 29,920 ft (a physical impossibility). I had to laugh when I saw one of Jeppesen's latest text books containing a chart with negative pressures because somebody had applied the 1"/1000' lapse rate all the way up.

Density altitude is similar to pressure altitude: it is the ISA altitude that corresponds to the actual air density. Aerodynamically, airplane performance will be constant at a given density. However, engine performance varies with different combinations of pressure and temperature. Therefore, modern airplane flight manuals do not use density altitude, but rather components temperature and pressure (expressed as pressure altitude).

The standard ISA temperature at 3000' is 9°C, not 15°C.
The standard is 15°C at sea level and decreases about 2° per 1000', up to 36,000'. If the temperature were 15°C at 3000', it would be 6° above standard, resulting in a higher density altitude-- less performance than standard.

So if an airplane could think, assuming it was familiar with the International Standard Atmosphere, the airframe would think it was at density altitude, and the engine would think that it is really too loud and hot in here and I've got a headache.