Lest you think you can beat density altitude

[tempdoug]

Do you know the definition of Density Altitude and more importantly how to calculate it? In light of the recent viral video showing what DA can do to you I thought I would take the time to put this together as refresher course on DA. I know most people aren't out there with a chart calculating DA before we take off in the backcountry but if you ever wanted to know what it was this could help you.

DISCLAIMER: I am not a CFI! I am, however, an instructor Flight Engineer with over 5000 hrs and 14 years in Airforce multiengine jets. As an FE one of my primary duties and my bread and butter is performance data so I take it very seriously.

To calculate DA you need your Outside Air Temp and Pressure Altitude. Here's a quick step by step guide to get the numbers you need.

First of all you need an altimeter setting and field elevation to calculate your Pressure Altitude.

Pressure Altitude is defined simply as Altitude corrected for non-standard pressure. At 29.92 (standard pressure) your altimeter will read field elevation. Since it's hardly ever standard we have a chart to correct for that. Remember the old adage high to low look out below? Well here's where that comes from. Set your altimeter to read field elevation. Once you have that reading come to the following chart.

Click to make it larger



Let's say for example you are sitting at Johnson Creek and your altimeter reads 30.13 to get the field elevation of 4933 feet. To get your Pressure Altitude go into this chart from the left side at 30.1 and go right until you hit the column that reads .03 on the top as you can see the correction number is -193 feet. This is only a correction NOT YOUR PA!

Subtract 193 from 4933 and you can see your pressure altitude is 4740ft. Looking at the chart you can grasp that anything above 29.92 lowers your PA and anything below that raises your PA. Another general statement is every .1" equals roughly 100ft. So 30.00 to 30.10 lowers your PA by approx 100ft.

Now that you have your PA let's figure out Density Altitude.

Density Altitude is defined out of the Mountain Flying guide as:

Density altitude is a term that sometimes causes confusion to the uninitiated. A high density altitude is NOT a good thing. Density altitude is defined as the pressure altitude corrected for non-standard temperature variations. And while this is a correct definition, my definition is perhaps more appropriate: DENSITY ALTITUDE IS THE ALTITUDE THE AIRPLANE THINKS IT IS AT, AND PERFORMS IN ACCORDANCE WITH THIS COMPUTED VALUE.

To compute your DA use the following chart



Start at the bottom of the chart with your temp. I have a cheap digital walmart temp gauge with a sensor stuck to the bottom of my wing in my airplane just for this data.

Go straight up until you hit your Pressure Altitude NOT your Field Elevation.

Read left to get your Density Altitude. As you can see temps upwards of 35 Celsius which is 95F would take the Density Altitude in the before mentioned example at Johnson Creek to over 8000ft. Temps upwards of 90 degrees during the summer months are NOT uncommon in the Idaho backcountry during the heat of the day.

The dotted line from the bottom to the top is the 15 degree line. If you remember from way back during your training 15 degrees is standard temp. 15C is only 59 degrees F folks. Anything over that and your performance is starting to decrease on top of however high you already are.

In case you think this is just a bunch of mumbo jumbo that doesn't apply watch this video again.
http://www.liveleak.com/view?i=835_1344412426

Hopefully this is clear as mud. Feel free to contact me via PM if you want clarification on anything.
Experimental Avid Flyer C Model STOL 582 Rotax
My SPOT Page

AvidFlyer Posts: 1029 Joined: Wed Dec 28, 2005 8:22 pm Location: Nut Tree Vacaville, CA KVCB Top
[hr][/hr][h=Re: Refresher on calculating Density Altitude]3[/h]by aktahoe1 » Fri Aug 10, 2012 8:05 am

[tempdoug]

hope no one minds me putting this on here because its off of another website. If so steve delete it. Just trying to get a ball rolling for some calculating.

[Cub Builder]

Okay, so here's a question....if the airport is at a density altitude of 9,500, does the thin air affect the pilot's judgement, before flight?

We know that we don't fly over 10,000 ft (in Canada) for extended periods due to hypoxia concerns, could someone be affected at 9,500, perhaps if they have respiratory issues? My gut says yes, but I'd like to hear from others.

I sure hope not. My home airport (KLAM) is at 7171', so from May until September our DA varies between 9000' and 10,500'. Of course if it does adversely affect ones judgement, it might explain a lot.

I have seen a lot of flatlanders come in here, then abort their take off on departure thinking something was wrong with the plane. I usually give them a little talk about flying in high DA conditions and encourage the pilot to do a couple of take offs without passengers and baggage. To make things more interesting, it's a one way in and one way out airport, so departures are typically with a quartering tail wind. The good news is that if you can drag the plane off ground effect by the end of the runway, you get dumped off into the canyon with the elevation descending a good 1500 feet over the next 4 miles. Even my 160 Experimental SC feels like a dog here on a summer day when it's loaded and I'm flying with the door and windows open.

FWIW, I have owned 2 Stinson 108s. They have a reputation of being able to fly with anything you can get the doors closed around. For the most part, that's true. But, the the climb performance when loaded is pretty poor, and at high DA with a heavy load, it's scary poor, like 150 fpm climb. No room for error and no margin for rising terrain. Also really easy to get behind the power curve with a high angle of attack under those situations. You can paint yourself into a really bad corner pretty easily under those circumstances.

-CubBuilder

[aktango58]

I personally do fine for a short time at 12k.

but after 3 days at 8000 feet, (Crownpoint nm) I was altitude sick. By the time I got to Farmington driving I felt ok, and at Phoenix I was in great shape...

different for others.

temp: Thank you! good info and review.

[Darrel Starr]

tempdoug, another way to skin the cat is to download one of the free density altitude apps onto your smart phone. For iPhones, I have "DenAlt" by Marcus Staloff but other similar apps also exist. Just enter Altitude, Temperature, Altimeter Setting & Dew Point to make the calculation. If you don't have dewpoint, a good guess will not create more than a 100 to 200 ft error.

[skywagon8a]

This http://www.to-avionics.com/insight/tas.html is a neat gadget that will help with the calculations. I have one in my 185.

[OLDCROWE]

Skywagon thats pretty neat looking, would like to see more about it. I use The "E6B" screen on a my 496 quite often and find it is pretty darn handy for quick DA checks and especially with an XM subscription because it already inputs the wx data and its really neat for immiediate winds aloft monitoring.

[skywagon8a]

This http://www.insightavionics.com/tas1000.htm is the manufacturers link. I bought it for the wind indicator which is very useful when there aren't any other sources of information. Winds aloft is very useful for finding the best cruise altitude, instantly. Plus there is a lot of other information available. Sometimes it seems like information overload. You can program the information which you want to use to reduce instrument clutter. Insight makes good stuff, lasts a long time.

[12 Geezer]

I keep an E6-B for DA and a Koch chart for performance estimation in the plane - mighty revealing when it's hot out! The takeoff distance and climb rate turn out to be pretty darn close - -

[SJ]

The late Sparky Imeson has some great rules of thumb about DA in his mountain flying bible. His "50/70" rule is one I often think about: http://www.mountainflying.com/Pages/...eoff_dist.html

sj

[kiwi 180]

i've been following this thread with some interest.there has been some pretty strong opinions of the intellectual ability of the pilot-maybe with some foundation -however i would speculate that nobody here knows exactly what was going on on the day and what thought processes/decision/stress factors were involved. for instance i have no personal knowledge of the strip-what type of terrain is off either end, what the winds were and all kind of variables . obviously with the benefit of hindsight it looks like the density altitude wasnt conducive to a safe flight, however i would like to make the following comments. 1) nobody sets off to have an accident 2) as has been noted many times in the past when someone has an accident there are a lot of people who think that that pilot is way dumber than what they are and dismiss it as an event that would never happen to them.3) most accidents have a whole series of things that lead to the eventual incident.4)most people have at some stage done something that in hindsight is pretty dumb -but got away with it.
here is a question-was this accident caused by a failure to recognise density altitude? my opinion and shoot it down if you want but i think the accident was caused by a failure to have a plan!
i would speculate that if you sat at the end of the strip and before pushing that throttle in considered all the possible options before the situation is thrust upon you , you will have a better chance of making the right decision when things start getting curly.as has been pointed out- having made one decision in attempting to take off -it looks like he still had several options that could subsequently have been taken which may have had a better outcome. i would suggest that this accident has not so much to be learned from considering density altitude but stressing how important it is to stay ahead of the plane.there are so many variables that could affect us unexpectedly that if we dont actively consider them every time we go flying then we run the risk of getting caught out. this same accident could occur at sea level in an aircraft with a good load and an obstruction /wires/high terrain a few miles away and a partial engine failure.
safe flying.

[12 Geezer]

Well said - a plan for whether to abort, and if so, when and how to abort. Reading all this is a good lesson for me.

[cubdriver2]

http://www.youtube.com/watch?v=TSVC2...ature=youtu.be

Glenn

[skywagon8a]

Do you suppose that they had to change their drawers after that or are they immune to the risk? He left 50 feet behind him when he started! I have seen other Russian airplanes taking off using the same amount of runway. They seem to have different runway requirements than we do.

[Gunny]

This incident is making the rounds. Glad all survived.

http://www.aopa.org/training/article...at_pco=cfd-1.0

[mike mcs repair]

my step father said when he was teaching me..... never quit flying the plane, till ALL the pieces finally come to rest...... this guy seems to do that, as he's whacking through the trees....

[mike mcs repair]

Glad they are okay. The interesting thing I caught in the slow motion part of the video was that he did not give up flying the airplane. His hands are on the yoke at impact and appeared to continue to try to be steady. That may have played into their ability to walk away from the accident.

I just finished patching up a cub a kid wrecked he had borrowed it from a guy... he bent it good..

in taking it apart and working on it to fix crushed top deck & front, i noticed the bent instrument panel & bent key in switch... bent exactly in front of both knees...(he quit flying the plane and tucked his knees up..) That pissed the owner off more than him wrecking his plane, because he quit flying the plane.....

[Wingman1]

It's just not a good idea to load a plane down while flying hot and high in the mountains period. I wouldn't have put four people in a c180 and attempted that. Lucky crew.....

[aktango58]

When on a strip that is just long enough, it is better to use all the strip and be sure it will fly at the end, than try a bit early and it not work.

I wonder if that was the Russian pilot's thought...

[eoliverosh]

This happened today. Alt near 6,000 ft. temp near 80
Prominent politician was on board

http://www.citytv.com.co/videos/8866...-vargas-lleras

I believe density altitude had it's part on the incident.

greetings

[RaisedByWolves]

At least no one was hurt. Maybe a change of underwear was in order but not a terrible outcome

[Mikey]

Summer season is almost here and the backcountry will be calling. I just ran across this today on youtube and figured it had made it on SC.org This post deserves a bump up on the threads as a reminder to all of us.
Mikey