Find Pressure Ratio at Cruising Altitude Barometric Altimete

Find Pressure Ratio at Cruising Altitude
Barometric Altimeter setting = 30.42 in.Hg
Cruising Indicated Altitude = 20,500 ft
Outside Air Temperature at altitude = -22.32 deg F
Atmosphere Table 2.1

Answer options

Solution

Further to what one answerer had mathematically stated: calculation of Pressure Altitude is done by calc. the difference bet. the current altimeter setting and the standard altimeter setting. The difference is then converted into feet based on the given 1\" Hg = 1000\' (which is refered to as the Standard Pressure Lapse Rate.). Accordingly, if the current altimeter setting is 30.12\" Hg, we would then calc. as follows: 30.12 minus 29.92 = .2

Using the Standard Lapse Rate, .2 is converted to 200 feet. So then if we want to determine the pressure alt. for an airfield we must add or subtract the 200 relative to the airport\'s elevation. If we want to calc.the pressure altitude of an intended altitude that we want to cruise at, we must add or subtract the 200\' relative to the planned cruising altitude. The question of adding or subtracting the CORRECTION VALUE is determined by whetehr the current air pressure is higher or lower than standard. Since pressure decreases with altitude ---and the current altimeter setting is higher than standard, we should subtract the 200\'.

Thus if the a/p elev were 3427\' we can then say that the airport\'s pressure altitude is 3227\' for the purpose of determining take-off performance. And if the planned cruising altitude were 6500\' we know that when we level off with 6500\' indicated on the altimeter, the a/c will burn fuel and produce an indicated airpseed as if it had levelled off at 6300\'.

Just as an added quibble: While PRESSURE ALTITUDE provides a correction of pressure, it does not take into consideration the AMBIENT TEMPERATURE OF THE AIR---a factor that on a hot day when performance of the a/c is degraded---is far more critical.