What is the standard temperature for an airport with an elevation of 4000’? The answer is this: 4 ´ 2 = 8 and 15 – 8 = 7, or 7° C. So we can see by the formula that density altitude will equal pressure altitude when the actual temperature is in fact the standard temperature for a given elevation or cruising altitude. (The origin of the standard temperature lapse rate is discussed on ) What then is the standard temperature for an airport with an elevation of 2000’? The answer is 11° C as, we know, temperature will decrease 2° C for every 1000’ increase in altitude (15-4=11). The standard temperature at sea level is 15° C, and the standard temperature lapse rate is 2°/1000’ (actually 1.98° per 1000’, but the. Okay, how you are asking “what the heck is ‘standard temperature’”. Here we must use math, and the formula is as follows:ĭensity Altitude = Pressure Altitude + (100 ´ ) In factoring in the influence of temperature on aircraft performance we shift to the concept of density altitude-that is, the higher the temperature, the higher the density altitude. While pressure altitude provides a correction for pressure, it does not take into consideration the ambient temperature of the air-a factor that, on a hot day when performance of the aircraft is degraded, is far more critical. Thus, if the airport elevation were 3427’, we can say that the airport’s pressure altitude is 3227’ for the purpose of determining takeoff performance, and if the planned cruising altitude were 6500’ we know that when we level off with 6500’ indicated on the altimeter, the aircraft will burn fuel and produce an indicated airspeed as if it had levelled off at 6300’. Since pressure decreases with altitude, and the current altimeter setting is higher than standard, we should subtract the 200’. The question of adding or subtracting the “correction value” is determined by whether the current air pressure is higher or lower than standard. If we want to determine 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. What do we do with this 200’? The answer is that if we want to determine the pressure altitude for an airport, we must add or subtract the 200’ relative to the airport’s elevation. Accordingly, if the current altimeter setting is 30.12”Hg., we would calculate as follows: 30.12 – 29.92 =. This difference is then converted into feet, based on the given that 1”Hg = 1000’-this is referred to as the Standard Pressure Lapse Rate. Conversely, if the altimeter setting were 30.92”Hg-i.e., the pressure of the air is higher than normal or standard-the takeoff performance of the aircraft would be enhanced-as if the takeoff were conducted at an airport situated 1000’ under sea level where thick dense air (not water!) would provide better conditions for lift from the wing surfaces and thrust from the propeller and engine.Ĭalculation of pressure is done by calculating the difference between the current altimeter setting and the standard altimeter setting. Therefore, if we are at an airport with sea level elevation, and the current altimeter setting is 28.92”Hg, we can predict that the aircraft during the takeoff, for example, will perform as if it is doing the same takeoff at an airport with an elevation of 1000’, and performance will be degraded accordingly. We also know that the pressure of the air normally decreases 1”Hg with an 1000’ increase in altitude. We know that standard pressure-the pressure most likely to be encountered-is 29.92”Hg (inches of mercury) at sea level. The concept of pressure altitude allows us to take into consideration any variation in the ambient pressure. Generally, aircraft performance-in terms of lift generated by the wing, and thrust generated by the engine-decreases with an increase in ambient temperature and a decrease in ambient air pressure. There are many occasions in which a pilot has to predict the performance of an aircraft-e.g., in planning a takeoff, in planning a climb over mountainous terrain, in predicting rates of fuel consumption, and aircraft performance with respect to speed. Students / Resources / Classroom / Private Pilot Groundschool /.
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