How does Pressure altitude applie to a altimeter?
Hi,,Pressure altitude is the altitude corrected for nonstandard pressure!!
If I understand correctly,,if I go out and put 29.92 in the kolsman window, that gives me
pressure altitude!!(also 29.92 is the standard off the ISA
Okay so what if I go out in the kolsman window and put the current altimeter setting
in from the awos,,,is that still called pressure altitude!! I have a really hard time explaining how
pressure altitude relates to aviation and using it on performance charts with the density altitude(which is pressure altitude corrected for nonstandard tempature)!
could someone explain the pressure altitude scenario,, and ad any reference books that may
help with it!! thanks
Comments
Here is a good article for you http://www.langleyflyingschool.com/Pages/Flight%20... and http://www.aopa.org/asf/publications/inst_reports2...
If pressure at sea level is normally 29.92 but today you set 28.92 your performance would be as if you were at 1000 foot, if you had 30.92 your aircraft wold perform as if you were 1000 foot below sea level.
In fact in you were to set 29.92 in the window and th pressure was 30.92 you would show -1000 feet on the readout. Thus the expression, "high to low look out below". Since you normally set the pressure in the window to the current settings the altitude shown on the readout is correct and you don't have to do math calculations to know your real altitude. But yes pressure does have an affect on performance. Density altitude has one as well. Density and pressure altitude will be equal if the actual temp at your altitude is "standard" for that altitude.
You are correct. When you set the altimeter to 29.92, you are reading the pressure altitude (PA).
If you use the current altimeter setting instead, the indication is your height above mean sea level (MSL), which can then be used to calculate your height above ground level. The only time PA and height above MSL are equivalent is when the current reported altimeter setting happens to be 29.92. This doesn't happen all that often. If the aircraft is on the runway and you have set the current pressure into your altimeter, you will be reading the height of the airport above sea level.
Pressure altitude is primarily used for two things (a) all aircraft flying above Flight :Level 180 (18,000 feet) must set their altimeters to 29.92 and (b) it is used for calculating performance because the density of air is directly related to air pressure.
Density altitude (DA) is PA corrected for temperature. This is the altitude the aircraft (primarily the engine) "thinks" it is flying at performance-wise. The higher the density altitude is, the less dense the air is, hence the less horsepower an engine will produce. The less horsepower the engine produces, the lower the performance of the aircraft is. Less horsepower = greater takeoff distance, slower climb rate, and lower maximum airspeed. All performance charts are based upon DA.
Pilots Handbook of Aeronautical Knowledge, Chapter 10 (Performance): http://www.faa.gov/library/manuals/aviation/pilot_...
Additional information that may be helpful:
1) apply, not applie (spelling)
2) Kollsman, not kolsman (spelling, and proper names are always capitalized)
3) temperature, not tempature
4) AWOS, not awos (acronyms are capitalized)
5) add, not ad (spelling)
6) 8 other punctuation and capitalization errors
If you wish to become a professional pilot you ought to correct this deficiency in your knowledge.
Pressure altitude is the read-out given with 1013.25 hpa,1013.25mb,or 29.92inches mercury(all the same amount of pressure just different units)in the sub-scale window.
That's about it.Flight level is also derived from the same pressure datum.
Ben about covered it, it's not that hard. The real performance issue is density altitude, that's where the airplane thinks its at.