Using the widely accepted WGS84 ellipsoid model, this averages a nautical mile to 6,076 feet (1,852 meters), or 1. Nautical Mile: defined as one minute of arc along a meridian of the Earth.Statute Mile: the same as a standard mile as you would see driving a car.This, as mentioned before, will heavily influence the aircraft’s performance. The opposite proves to be true due to the temperature that we did not take into account for PA. Quite a bit higher than 1411 ft or 952 ft right? The sneaky thing is that if you only looked at the PA, you’d have concluded that due to high pressure, we actually expect pretty good performance. Here are some hints as to what your ballistics. So filling this in with the earlier calculated PA gives: Inputs Required by CALC denotes the measurements your ballistic calculator applications are looking for. Example 1: at 5050 feet elevation, 95 deg F air temp, 29.45 inches-Hg barometric pressure and a dew point of 67 deg F, the Density Altitude is calculated as. This is the expected temperature according to the ISA. Pressure altitude is the altitude with specified.

So the first step is to calculate what temperature we expect at 952 ft (the PA at Geneva Airport). pressureAltitude atmospalt( pressure, action ) computes the pressure altitude based on ambient pressure. Where does the 120 come from? Don’t worry, it’s simply the accepted reduction in density altitude based on the temperature difference. : Constant, molar mass average of air: M 0.0289644 kg/mol. The Density Altitude formula looks like this: This tool calculates the atmospheric pressure based on altitude and air temperature using the following formula: P P 0 e gh RT P P 0 e - g h R T. Let’s say it’s a really hot day in the summer, and the temperature is 24✬. So now that we’ve compensated for a different pressure, let’s go the extra step and take the temperature into account. To get the figure, we just subtract 370 from 1013.2 to get 643.2 HPa at 10.000 ft. At sea level there is a pressure equivalent to 10 meters of water pressing down on all of us all the time. This means that for the ISA at 10.000 ft, we experience a pressure that is 10.000 / 27 = 370 Hpa lower than pressure at MSL (1013.2 HPa). The atmospheric pressure on the other hand, reduces by 1 HPa every 27 ft on average (depending on other variables such as local temperature, the other value you might see elsewhere is 30 ft but this is slightly less accurate). So let’s say we are flying at 10.000 ft, that means that on an average day (for the International Standard Atmosphere or ISA) we are experiencing a temperature of 2 x 10 = 20✬ colder than the 15✬ mentioned earlier on the surface. The normal lapse rate for the temperature is roughly 2✬ / 1000 ft. Standard temperature at Mean Sea Level (MSL) is 15✬, and standard pressure at MSL is 1013.2 HPa (or 29.92 Hg for our American friends across the pond). Standard Pressure and temperature lapse ratesĪs we know, pressure and temperature both reduce as we increase our altitude (up to a certain point). For instance, if the pressure ratio () is known, the altitude to which it corresponds (called pressure altitude) can be calculated. It allows the engine, propellers, or rotor system to work with more air for a similar amount of effort. Similar to how a prevailing low pressure would reduce the amount of air we can work with.ĭense air (high pressure and low temperature) is good for aircraft performance, whether you are in a fixed wing aircraft or a helicopter. A higher temperature reduces the ‘amount’ of air in the atmosphere i.e density. With Density Altitude however, we add a layer of realism by also taking into account that the temperature might be really high. This is because pressure reduces as altitude increases. If the QNH is higher than 1013, the 1013 level will be above MSL.

If the QNH is lower than 1013 (like in the example below), the 1013 level will be below MSL. It shows us our altitude above or below the 1013 HPa level. ISA Temp), I get: Density Altitude 1400 +. With the PA, we take the fact that pressure is different in real life than in the models for the “standard” atmosphere, and correct for that. When I try to do mathematical using the formula, Density Altitude Pressure Altitude + 120(Outside Air Temp. So let’s start with the basics, what is it exactly? In simple terms, Density Altitude (DA) is the pressure altitude corrected for temperature, but what does that even mean? It’s simply the altitude the aircraft ‘thinks’ it is currently flying at.ĭA goes 1 step further than Pressure Altitude (PA).

Pressure altitude formula