## Bush Pilot in Training Podacts -Altitude to Pressure Altitude to Density Altitude

In this episode I’m going to talk about how to work from altitude to pressure altitude to density altitude. We do this calculation in order to predict the airplane’s performance under different air pressure and temperature scenarios.

Altitude is the difference in elevation between sea level and any given point on the earth. The airport I fly out of, for example, Pitt Meadows, or CYPK, is 11 feet above mean sea level. That never changes, although you may see elevation reported as ASL or MSL.

What does change is barometric pressure and temperature, and that combination effects how an airplane performs. Higher pressure means denser air and lower pressure means less dense air. You can say that when the air pressure is lower than standard pressure the airplane “thinks” it’s flying at a higher altitude than the actual elevation, and when air pressure is higher the airplane “thinks” its flying at an elevation that is lower than it actually is flying. If you throw temperature into the mix you can figure out that a combination of high pressure and lower than standard temperature can improve performance, and high temperatures with low pressure can reduce performance, especially when it comes to taking off and climbing. Taking off on a hot summer day in mountainous terrain could be fatal if the airplane does not perform as you expect.

The elevation that the airplane thinks its flying at is “density altitude”, but to get density altitude we first need pressure altitude.

We adjust the altimeter for pressure every time we fly by setting it to either the reported pressure or setting it to a known altitude. For example, at CYPK we know that the altitude is 11 feet above sea level. If the actual air pressure is lower than standard on a given day the altimeter will read higher than 11 feet ASL even when the airplane is on the ground. If we set the altimeter sub-scale to the actual pressure the altitude will read 11 feet ASL. If we don’t know the actual pressure but we adjust the sub-scale until the elevation reads 11 feet ASL the altimeter sub-scale will then read the actual air pressure.

What this does is adjust the altimeter to read the correct altitude where we are flying for a given pressure. We’ll know when to do our 200′ checks on take off, or when we’re 700′ AGL on approach. We won’t know what elevation the airplane “feels” like it’s flying at – we’ll just know how far off the ground we are. I don’t want to make this too confusing, but the altimeter sub-scale doesn’t tell us pressure altitude – it just tells us pressure and adjusts the altimeter for the current pressure so that the altimeter reads accurately. For performance calculations we have to do the math.

It’s a fairly simple math calculation to get the pressure altitude.

Start with standard pressure of 29.92.

Subtract the actual air pressure are reported (we’ll use 29.80)

29.92-29.80= .12

Multiply .12 by 1,000 and we get 120 feet.

Add 120 feet to 11 ASL and we get a pressure altitude of 131 feet.

At 29.80 inches of mercury the pressure altitude of CYPK is 131 feet.

This isn’t a big difference, but that’s because the change in pressure we used was small, and we’re still assuming no other changes. Standard pressure assumes 15 degrees centigrade temperature. Colder air is more dense and warmer air is less dense. If we take temperature into account we come to the concept of density altitude.

If pressure altitude is elevation corrected for pressure, then density altitude is pressure altitude corrected for temperature. We do this with the E6B. Density altitude is important because a light plane can easily require 25% more take off distance with every 1,000′ of elevation gain. A hot day at a high altitude airport means density altitude can be a performance challenge.

It’s not easy to explain the E6B calculation in a podcast, so you should look at webpage to get a really good handle on this, but here’s one thing to understand: at standard temperature and at sea level pressure altitude and density altitude are the same. If you assume pressure altitude is 0 and set that at 15 degrees in the pressure altitude window (that is, line up the 0 inside the pressure altitude window with 15 degrees along the scale that reads from +50 to -50) the density altitude pointer will be pointing at 0.

If you do this and get that result you know you’re reading the E6B correctly, and you can then do calculations with different pressure altitudes and temperatures.

Let’s go back to our original calculation. We decided that with a pressure of 29.80 that CYPK’s pressure altitude was 131 feet ASL. Now let’s assume that the temperature is 30 degrees centigrade. We line up 131 feet inside the window with 30 degrees along the scale that reads from +50 to -50 . We can see that the density altitude pointer points to just under 2, which means 1,800′ density altitude.

Ok, let’s review the process with some different numbers. We’ll stick with CYPK as a starting point. We know the actual elevation is 11 feet ASL. The pressure at the time of writing is 30.07. The first step is to convert actual elevation into pressure elevation:

29.92 -30.07 = -.15 x 1,000= -150 11′ ASL – 150 = -139′ pressure altitude ASL (we’re underwater at this point!)

The temperature is 21 degrees. Using the E6B we line up -139 in the pressure altitude window with 21 degrees on the +50 to -50 temperature scale and look at what the density altitude pointer says, which in this case is about halfway between the zero and the first mark (which is 1,000′). High pressure made the airplane think it was flying below sea level while a slightly higher temperature made the airplane think it was flying higher. Let’s say density altitude on the E6B is 500′.

Let’s switch aerodromes to Prince George. At the time of writing the elevation at Prince George is 2,267′ above mean sea level. That never changes. Pressure is 30.15.

29.92-30.15 = -.23 x 1,000 = -230. Pressure altitude is 2,267 – 230 = 2,037′ MSL.

Temperature is 13 degrees. If we line up 2,037′ pressure altitude in the pressure altitude window with 13 degrees on the +50 to -50 temperature scale and look at what the density altitude pointer says we see that the density altitude looks to be about 2,300-2,400′. (In fact the wx cam at CYXS says the density altitude is 2,300′)

Let’s switch again. Lytton BC is 743′ MSL. Pressure today is 29.97. Pressure altitude is 29.92-29.97 =-.05×1,000= -50′ = 693′ pressure altitude.

Temperature is 24 degrees.

Go to the E6B and line up 693′ pressure altitude in the pressure altitude window with 24 degrees on the +50 to -50 temperature scale and look at what the density altitude pointer says we see that the density altitude looks to be close to 2,000′. In fact, the weather cam tells us that the current density altitude at CWLY is 1,800′.

The marks on the E6B are small. You often have to guess exactly where the numbers you’re using would be, and then try to line them up as carefully as you can.

If we take 693 and try to correct mathematically for temperature, without the E6B, we’d use this formula: Density Altitude = Pressure Altitude + (120 x [actual temperature – standard temperature] ). We need to remember that the standard temperature at sea level is 15 degrees, but our aerodrome elevation may not be sea level. Therefore we have to use the lapse rate of 2 degrees per 1,000 to get the standard temperature corrected for elevation.

Let’s do the math with the last set of data from CWLY. Altitude is 743 and pressure is 29.97, so we subtract 29.97 from 29.92 giving us negative .05 x 1,000 for -50, or a pressure altitude of 693. That’s the first number we need for this calculation. The next is the temperature difference.

743 is roughly ¾ of 1,000′, so its roughly ¾ of 2 degrees, or 1.5 degrees. Standard temperature, therefore, at 743 feet ASL is about 15 – 1.5 or 13.5.

OAT is 24 and standard temperature at Lytton’s altitude is 13.5.

Pressure altitude was 643

Density Altitude = Pressure Altitude + (120 x [actual temperature – standard temperature] )

Density Altitude = 643 + (120 x [24 – 13.5] )

Density Altitude = 643 + (120 x 10.5)

Density Altitude = 643 + 1,260

Density Altitude= 1,903 Remember, the weather cam said 1800 ft density altitude. I’ll comment on that more later.

Let’s run through the calculation again. Back to Prince George.

The elevation at Prince George is 2,267′ above mean sea level. Pressure is 30.15.

29.92-30.15 = -.23 x 1,000 = -230. Pressure altitude is Prince George elevation 2,267 – 230 = 2,037′ ASL. Same as we got last time.

Temperature is 13 degrees. Standard temperature at sea level is 15, so standard temperature corrected for 2,267′ ASL is 10.5 degrees (15 – 4.5 = 10.5)

Density Altitude = Pressure Altitude + (120 x [actual temperature – standard temperature] )

Density Altitude = 2037 + (120 x [13 – 10.5] )

Density Altitude = 2037 + (120 x 2.5)

Density Altitude = 2037 + 300

Density Altitude= 2,337

Again, the wx cam at CYXS says the density altitude is 2,300′

Let’s go back to our original calculation at CYPK. We decided that with a pressure of 29.80 that CYPK’s pressure altitude was 131 feet ASL. We assumed that the temperature is 30 degrees centigrade. 15 degrees standard temperature corrected for 11′ ASL is close enough to 30 degrees

Density Altitude = Pressure Altitude + (120 x [actual temperature – standard temperature] )

Density Altitude = 131 + (120 x [30-15] )

Density Altitude = 131+ (120 x 15)

Density Altitude = 131 + 1800

Density Altitude= 1931

We then did CYPK with a higher pressure – 30.07 and a lower temperature, 21 degrees.

Pressure altitude is 29.92-30.07 for -.15 x 1,000 for -150. Pressure altitude becomes -150+11 for -139.

Density Altitude = Pressure Altitude + (120 x [actual temperature – standard temperature] )

Density Altitude = -139 + (120 x [21 – 15] )

Density Altitude = -139 + (120 x 6)

Density Altitude = -139 + 720

Density Altitude= 581

There you have it. Two ways of converting altitude to pressure altitude to density altitude. Now, one last comment about the formula. Some people multiply the temperature difference by 100 instead of by 120 – it works, and sometimes gets closer to weather cam readings, but its often on the low side. Since we’re doing density altitude to be safe I’m using 120 and erring on the side of caution.

Ok, that’s altitude to pressure altitude to density altitude. Remember – start with altitude and correct it for non-standard pressure to get pressure altitude.

Correct that for non-standard temperature for density altitude.

My name is Rob Chipman and I’m a realtor and pilot based in Vancouver, BC. I AM NOT A FLIGHT INSTRUCTOR AND I AM NOT OFFERING FLIGHT INSTRUCTION! I am sharing my study notes and other things I’ve learned while getting my education as a pilot. You’re welcome to make use of this information, but do not treat it as expert advice.

I really enjoy flying, real estate and the Chilcotin. My company is Coronet Realty Ltd., located at 3582 East Hastings Street, Vancouver, BC, V5K 2A7. I have a C-150L that I own with two other pilots, based out of Pitt Meadows. Do not hesitate to contact me by email if I can help you do anything, especially if its likely to be interesting or concerns selling remote property in British Columbia.

Podcast: Play in new window | Download