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Density Altitude


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Depending on where you live, now may be the time to start thinking about density altitude again. My home airport (57AZ) is reporting a DA above 5,000 ft right now. It is 2940 MSL. Today the high is forecast to be 93ºF with a high of 95º tomorrow. Summer's coming.

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Great topic, Dr. K!

For anyone not familiar with Density Altitude (DA) and take-off distance...

temperature has a way of robbing your plane’s performance, making it possible to run out of runway before being airborne...

This even applies to powerful Mooneys like an M20J... (Patrick has become a saint for teaching us this lesson each year) :)

You don’t need to be anyplace in the south, or desert south west... you can have this challenge with a couple of friends in PA....

so.... if your not familiar, just ask...

Best regards,

-a-

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6 hours ago, carusoam said:

Great topic, Dr. K!

For anyone not familiar with Density Altitude (DA) and take-off distance...

temperature has a way of robbing your plane’s performance, making it possible to run out of runway before being airborne...

This even applies to powerful Mooneys like an M20J... (Patrick has become a saint for teaching us this lesson each year) :)

You don’t need to be anyplace in the south, or desert south west... you can have this challenge with a couple of friends in PA....

so.... if your not familiar, just ask...

Best regards,

-a-

Have to chuckle about that one. 

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On 4/8/2019 at 3:40 PM, KLRDMD said:

Depending on where you live, now may be the time to start thinking about density altitude again. My home airport (57AZ) is reporting a DA above 5,000 ft right now. It is 2940 MSL. Today the high is forecast to be 93ºF with a high of 95º tomorrow. Summer's coming.

Yep. I like to get ahead of this as well since it kills more pilots than you might think.  I wrote about this recently in my blog

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When I was at about 20 hours I went back home and trained with another CFI. Up to that point my first 20 hrs were In a piper Warrior at 2200 feet in North Dakota . I cut my teeth in the 172 back home at 4700 feet . And got about 12 hours there . 

Then I went back to work and started flying a 172 in North Dakota with another CFI . The first time I opened the throttle at 2200 feet the plane felt like it jumped off the ground instantly. The difference between 4700 and 2200 was huge .

And the temps were similar ,these two flights were about a week apart.  And as we worked our way towards winter the preformance increased substantially.  

It taught me alot about weather conditions and preformance. 

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On 4/8/2019 at 3:57 PM, gsxrpilot said:

86° in Austin today and 90° tomorrow. We're loading the truck on Wed and will be in Denver by the weekend.

@gsxrpilot-  You're probably busy loading the truck, but ya'll be careful, Paul- blizzard today and snow this weekend in Denver.  Have fun starting your new chapter.

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@steingar keep in mind our NJ Mooney pilot ran into a DA problem in our version of flatistan... actually eastern PA...

Ran out of runway... for the weight he was carrying... three guys and fuel, and an M20J.

So... fret about it enough to know the limitations of warm days in flatistan. :)

Best regards,

-a-

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6 hours ago, carusoam said:

@steingar keep in mind our NJ Mooney pilot ran into a DA problem in our version of flatistan... actually eastern PA...

Ran out of runway... for the weight he was carrying... three guys and fuel, and an M20J.

So... fret about it enough to know the limitations of warm days in flatistan. :)

Best regards,

-a-

That had a lot more to do with being overweight on a short runway than DA, but I do get the point.  Still, I think those guys would have had that problem on the coldest winter day.  I do actually give it a thought taking off from short (ish) runways.

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Not over weight..... not using all of the available runway... then the mythical 50’ tree off in the distance wasn’t so mythical...

The evidence points to...  an assumption being made... 

There is so much going on with a change of DA...

  • power produced is less...
  • lift generated is less...
  • The AOA needed to produce adequate lift is increased...
  • So the drag encountered is even more...
  • The system gets more sensitive to every extra pound added to the fuel tanks...
  • extra weight or extra AOA also increases the stall speed...

The PIC was coming up on graduation from college with an aviation related degree. The passengers were also school mates...

They had to know better.

Lesson learned... no matter how smart you are... don’t make an assumption about runway length....

Try on a couple of calculations around the edges of the envelope....

  • loaded to MGTW...
  • Set the available runway length. I use 2k’ to make case study things easy to remember...
  • put a mythical tree line out there...
  • increase the temp/DA in 10°F increments...
  • decrease available runway in chunks of 200’...

At what temp does your plane decide... NoGo...?

At what runway length does your plane decide... NoGo...?

 

We can take this idea to the next level... add a level of reality to the system...

(Assume) The POH numbers are pretty good. They are based on reality....  now, to prove that they apply to your plane....

We can actually measure our actual T/O distances using an app and a waas source...  many portable ADSB-in systems have the required waas source... (unfortunately the internal gps of an iPad isn’t accurate enough for this task)... one app that works well is called CloudAhoy...

In the end...

You will find that...

  • DA matters...
  • Even in Flatistan...
  • It counts even more, the closer to MGTW you get...
  • It counts even more, the hotter the OAT is...

The data starts to fall apart rapidly nearing the edges of the envelope... that is the big reason why we have limits to what MGTW actually is...

To make this an easy to do exercise... The POH drag your finger calculation has been simplified by somebody around here to show that it can be done....

I would like to see Mooney/MAPA bless the app approach of calculating T/O distances... it is too easy to make a fat fingered mistake using the paper charts... or slip doing the math from a chart...

This would be a great method to improve upon the know before you go idea...

Good decisions start with good data... 

PatrickF (a different Patrick) recently had posted his T/O distance calculations that he digitized from the Mooney graphs for his Long Body...  looks like a really viable way to make easy DA related Go/NoGo decisions...

PP thoughts only, not a CFI...

Best regards,

-a-

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To be quite frank, this sort of thing can bite anyone.  Fairly early on I was flying in the hills of North Carolina.  Hadn't flown around terrain, didn't think about it that much.  Came close to a bad outcome, had the temps been higher it might not have turned out well.

I wouldn't excoriate the accident pilot that much, a pity he had to wreck an airplane to get the sort of wisdom my class call gave me.  Like I said, the runway has to be pretty short to give me pause in flatistan.  That said, if the runway is too short for a hot day, its too short for a cold one.  I'm not  a big fan of pushing the envelope in a an airplane.  Strikes me as a good way to get dead.

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A 2k’runway can be good one day, and not enough on a different day.... or later in the same day.

That is what makes the DA knowledge useful.

Otherwise, what may happen...a pilot stops going places that has runways shorter than 3k’.

 

The non-linear nature between excess hp and T/O distance...

A 10% reduction in hp, can cause a 50% increase T/O run... for the same plane, everything else held the same....

A lightly loaded 310hp O, is off the ground in 800’... then climbs at 2kfpm.

the 280hp O, takes 1200’...Then climbs at about 1kfpm.

30hp is near 10% of the total hp available for the O...

 

It is really interesting what you can find about your plane’s performance using the waas source and CloudAhoy...

It really made the POH data more useable for me...

Newer POHs have more data to draw from... but the waas data collection works the same for both old and new Mooneys, short to Long Bodies....

PP thoughts only, not a CFI...

Best regards,

-a-

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47 minutes ago, steingar said:

To be quite frank, this sort of thing can bite anyone.  Fairly early on I was flying in the hills of North Carolina.  Hadn't flown around terrain, didn't think about it that much.  Came close to a bad outcome, had the temps been higher it might not have turned out well.

I wouldn't excoriate the accident pilot that much, a pity he had to wreck an airplane to get the sort of wisdom my class call gave me.  Like I said, the runway has to be pretty short to give me pause in flatistan.  That said, if the runway is too short for a hot day, its too short for a cold one.  I'm not  a big fan of pushing the envelope in a an airplane.  Strikes me as a good way to get dead.

Don't know what happened in your case, but sure, the sapping of power, thrust, and lift which comes with high density altitude is a major consideration but it is usually working in conjunction with the other  risk factors associated with mountain flying. 

I did my primary training in New England and later mover to Colorado. My friends from Colorado always joke about the eastern "baby" mountains but my experience with both has taught me that mountains are mountains and pure density altitude is only one part of thew risk equation.

Hot and cold days for runways? I'll disagree that a runway too short for a hot day is always to short for a cold day. In Connecticut, I flew out of an 1800' long runway. We avoided it on the hottest days. In Colorado, a number of flying FBOs had a policy of not permitting certain rentals when the temperature went past 90F. The 10,000', 7,000', and 4,800' runways were just fine otherwise.

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I guess my thinking is if a runway is too short for a hot day, it's too short.  To my thinking even if one can take off on a cold day, its pushing the envelope too far.  What if the airplane doesn't give you maximum performance that day?  You could wind up in the trees.  You could wind up dead.

The Westerners like to give us Easterners a hard time about our mountains.  Truth is our make weather, lots of it.  I hate tangling with them, so I suspect the IR will help.  That's the difference, in the East you need an IR to fly around our rocks, in the West you don't dare fly IFR around them (over perhaps).

But I agree, mountains are mountains.  There is a strip I've landed in the Canaan Valley in WV, its at about 3K feet.  And yes, I worry a lot about density altitude there.  But WV ain' uno flatistan.

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9 minutes ago, steingar said:

I agree, mountains are mountains.  There is a strip I've landed in the Canaan Valley in WV, its at about 3K feet.  And yes, I worry a lot about density altitude there.  But WV ain' uno flatistan.

Mountains are not mountains. My field elevation is 3,000 feet and in the summer density altitude gets well above twice that. People in the west based around real mountains may have a field elevation of 7,000 ft. Peaks in the west get near 15,000 ft. A 3,000 ft "mountain" is not the same. 

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2 hours ago, KLRDMD said:

Mountains are not mountains. My field elevation is 3,000 feet and in the summer density altitude gets well above twice that. People in the west based around real mountains may have a field elevation of 7,000 ft. Peaks in the west get near 15,000 ft. A 3,000 ft "mountain" is not the same. 

I'm sure that has killed a few people who thought, "it's only a 3000 ft little hill. Nothing to worry about" as a normal mountain downdraft led to a CFIT. 

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