Compensating for Density Altitude

azpilot said:
At what density altitude does this 90% rule of thumb kick in? 5000'? 7000'? 9000'?

I think part of the reason this might be confusing is that 'high density altitude' means different things to different people. If you learn to fly in CO, 7000' DA is just normal; if you learn in New Hampshire, it's unheard of.
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At 5,000 ft density altitude a normally aspirated engine has about 90% of its sea level power rating. Your call.
 
azpilot said:
At what density altitude does this 90% rule of thumb kick in? 5000'? 7000'? 9000'?

I think part of the reason this might be confusing is that 'high density altitude' means different things to different people. If you learn to fly in CO, 7000' DA is just normal; if you learn in New Hampshire, it's unheard of.
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Lycoming defines high alt at being 5000 msl, if I'm reading the engine manuals correctly.
 
Kritchlow said:
Isn’t that factored into the performance charts?
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Are performance charts available? Can the aircraft climb with full flaps at certificated MGOW at a particular density altitude? How many other questions are there? Why does the rule of thumb continue to be talked about when training for high density altitude airport operations? How long does this response need to be before you’ll ignore it?
 
Clark1961 said:
Are performance charts available? Can the aircraft climb with full flaps at certificated MGOW at a particular density altitude? How many other questions are there? Why does the rule of thumb continue to be talked about when training for high density altitude airport operations? How long does this response need to be before you’ll ignore it?
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Don’t know if the charts are available. Ask the aircraft owner. He should know.
But if they’re not, I wouldn’t blindly launch using a 10% buffer rule of thumb.
 
Kritchlow said:
Don’t know if the charts are available. Ask the aircraft owner. He should know.
But if they’re not, I wouldn’t blindly launch using a 10% buffer rule of thumb.
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Some of the early POHs are pretty bare-bones. Nobody advocates launching blindly. Why would you think that is part of training?
 
Clark1961 said:
Some of the early POHs are pretty bare-bones. Nobody advocates launching blindly. Why would you think that is part of training?
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I didn’t think it was part of the training, but he asked the question and it was given here as a rule of thumb.
 
Clark1961 said:
Are performance charts available? Can the aircraft climb with full flaps at certificated MGOW at a particular density altitude? How many other questions are there? Why does the rule of thumb continue to be talked about when training for high density altitude airport operations? How long does this response need to be before you’ll ignore it?
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i dont think any aircraft are supposed to, required to or expected to take off and climb with full flaps!
why did you say that?

yes, performance charts are available,, you should have some in the plane you are going to fly.
i have a poh for a 75 c150, a 78 c172 and for my 66 pa28.
they all have take off performance charts for gross weight take off from various da altitudes up to 10000 ft, except the c150 just 7500 ft.
 
I had the mighty 182 up to 13,500 briefly today. She was making a whopping 15-16” MP. Hahaha.
 
denverpilot said:
I had the mighty 182 up to 13,500 briefly today. She was making a whopping 15-16” MP. Hahaha.
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For those of us who are out of date.... let me see here... most get 29”??? Decreased approx
1” per 1000’. So... aren’t you doing okay?
Again, it’s been a while so I could be off base here.
 
Kritchlow said:
For those of us who are out of date.... let me see here... most get 29”??? Decreased approx
1” per 1000’. So... aren’t you doing okay?
Again, it’s been a while so I could be off base here.
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Yeah it’s fine. Just pointing out how much it drops off. It’ll do over 30” at sea level. Normally aspirated just poops out up there. But it trues out higher so it’s about the same as at 9000-10000 which is kinda the cruising sweet spot for fuel consumption and going somewhere at the same time.

Most humans poop put up there too. Living at 6800’ I don’t have much problem going skiing or hiking or flying up above the rocks at 13,500’ but for most folks visiting, I highly recommend they don’t do the “30 minute” thing above 12,500 without supplemental O2. They’ll at least usually end up with a headache later as a minimum and can be worse. Staying hydrated helps a lot.

I found a way to have coffee service on board like @Clark1961 has with his fancy autopilot though. Just trim it and then open a can of iced coffee so the backseater who isn’t there, doesn’t have to pour. Hahaha.

Note: Coffee is not great for staying hydrated but for those of us acclimatized to the altitude it’s just like having a cup while driving west of town. Heh.

Second note: Iced Coffee is tasty when mountain flying.

Learned today that our strobes make noise right at 124.0 (KCOS Approach) and so does my OBS Light rheostat when turned to full brightness.

Hey @jesse was that the Lincoln Approach frequency that we kept hearing crap on in my airplane a few years ago, too? Guess I’ve finally figured out what it is. Sounded the same. Bothers both the Garmin and the King.

Turn the lighting rheostat down it gets weaker and both squelch it successfully. The strobes just pop the squelch open every time either the left or right one fire.

Psscht. Psssscht. Psscht-****sht.
 
Where I live (New York City), this time of year we have negative density altitudes.

I’d love to read any comments people have on that.
 
Rory said:
Where I live (New York City), this time of year we have negative density altitudes.

I’d love to read any comments people have on that.
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Good performance.... What else are you looking for??
 
Kritchlow said:
Good performance.... What else are you looking for??
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Good question. What I’m finding is that the rough and ready calculation formulas for the New York area right now are way off compared to what I get from ForeFlight or the NOAA site.

I guess my question is whether they are generally way off. When I use ForeFlight for airports across the US at the moment, the variation from rough calculations can be pretty significant, regardless of temperature.
 
What are rough and ready calculations?

I know many small airplane POH’s don’t go below seal level, but if you use sea level you’ll always err on the safe side.
 
Kritchlow said:
What are rough and ready calculations?
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Density altitude is an objective number and has nothing to do with your airplane handbook.

There are several formulas of varying accuracy. The formula used by NOAA, and probably by ForeFlight, is more complex than the “rough and ready” formulas. I suspect that the biggest difference has to do with humidity.

But I’d love to see comments here.
 
denverpilot said:
Yeah it’s fine. Just pointing out how much it drops off. It’ll do over 30” at sea level. Normally aspirated just poops out up there. But it trues out higher so it’s about the same as at 9000-10000 which is kinda the cruising sweet spot for fuel consumption and going somewhere at the same time.

Most humans poop put up there too. Living at 6800’ I don’t have much problem going skiing or hiking or flying up above the rocks at 13,500’ but for most folks visiting, I highly recommend they don’t do the “30 minute” thing above 12,500 without supplemental O2. They’ll at least usually end up with a headache later as a minimum and can be worse. Staying hydrated helps a lot.

I found a way to have coffee service on board like @Clark1961 has with his fancy autopilot though. Just trim it and then open a can of iced coffee so the backseater who isn’t there, doesn’t have to pour. Hahaha.

Note: Coffee is not great for staying hydrated but for those of us acclimatized to the altitude it’s just like having a cup while driving west of town. Heh.

Second note: Iced Coffee is tasty when mountain flying.

Learned today that our strobes make noise right at 124.0 (KCOS Approach) and so does my OBS Light rheostat when turned to full brightness.

Hey @jesse was that the Lincoln Approach frequency that we kept hearing crap on in my airplane a few years ago, too? Guess I’ve finally figured out what it is. Sounded the same. Bothers both the Garmin and the King.

Turn the lighting rheostat down it gets weaker and both squelch it successfully. The strobes just pop the squelch open every time either the left or right one fire.

Psscht. Psssscht. Psscht-****sht.
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yup. same frequency. though it'll happen in some airplanes here on the tower frequency all the way down to about 75 AGL. An airline pilot friend told me that the ERJ does the same damn thing in Lincoln.

Don't know enough about it to know if something is off from the broadcast or if it was just **** RF planning.
 
Rory said:
Density altitude is an objective number and has nothing to do with your airplane handbook.

There are several formulas of varying accuracy. The formula used by NOAA, and probably by ForeFlight, is more complex than the “rough and ready” formulas. I suspect that the biggest difference has to do with humidity.

But I’d love to see comments here.
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Well that’s a first for me. I’ve always calculated DA. Perhaps I’ve been doing it incorrectly. Perhaps the airline is doing it incorrectly?

Plus density altitude does not take humidity into account.... not that it doesn’t matter, it’s just too small of a number.
 
Kritchlow said:
Well that’s a first for me. I’ve always calculated DA. Perhaps I’ve been doing it incorrectly. Perhaps the airline is doing it incorrectly?

Plus density altitude does not take humidity into account.... not that it doesn’t matter, it’s just too small of a number.
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Sophisticated formulas do take humidity into account.

There can be a significant discrepancy between the numbers that come from the NOAA calculator/ForeFlight and the rough and ready calculations that we are taught. Somebody asked what I mean by "rough and ready". Look at the Wikipedia article on Density Altitude and in particular at the formulas in that article on calculating Density Altitude. One thing that should be clear from looking at the formulas is that pilots are taught a formula that is significantly simplified.

The other day, I spent some time calculating Density Altitude using the simple formula that pilots are taught, and in many cases there was a significant discrepancy between the results of this formula and the results of an NOAA or ForeFlight calculation. Also interesting was the fact that the error was not consistent between airports, which is why I wonder whether humidity may be a bigger factor than we are told. Perhaps that's wrong, but it would be interesting to know exactly what is going on.

P.S. For those who are interested, and don't know, NOAA does have a Density Altitude calculator accessible via the internet. I suspect that ForeFlight uses the same formula.
 
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Rory said:
Sophisticated formulas do take humidity into account.

There can be a significant discrepancy between the numbers that come from the NOAA calculator/ForeFlight and the rough and ready calculations that we are taught. Somebody asked what I mean by "rough and ready". Look at the Wikipedia article on Density Altitude and in particular at the formulas in that article on calculating Density Altitude. One thing that should be clear from looking at the formulas is that pilots are taught a formula that is significantly simplified.

The other day, I spent some time calculating Density Altitude using the simple formula that pilots are taught, and in many cases there was a significant discrepancy between the results of this formula and the results of an NOAA or ForeFlight calculation. Also interesting was the fact that the error was not consistent between airports, which is why I wonder whether humidity may be a bigger factor than we are told. Perhaps that's wrong, but it would be interesting to know exactly what is going on.

P.S. For those who are interested, and don't know, NOAA does have a Density Altitude calculator accessible via the internet. I suspect that ForeFlight uses the same formula.
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I’ve never used Foreflight. I’ve used an E6-B and a POH, and now I hit a send button and the numbers come back to me.
Regardless, it may not be perfect but close is better than nothing at all.
 
Kritchlow said:
I’ve never used Foreflight. I’ve used an E6-B and a POH, and now I hit a send button and the numbers come back to me.
Regardless, it may not be perfect but close is better than nothing at all.
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I'm not criticizing you or questioning whether your method works on a practical level.

I raised a question about something that I observed with respect to negative density altitudes and that, when I investigated it further, showed a significant discrepancy between the results of the formula we are taught and what appear to be the real numbers, and indeed a discrepancy that differed from airport to airport.

I'm not talking about a discrepancy of two or three feet. I'm talking about a discrepancy that is routinely two or three hundred feet and can be more.

If the discrepancy doesn't interest you, cool.

For those who do use ForeFlight or choose to check out NOAA's density altitude calculator, you are in for a surprise when you compare what pilots are taught with the numbers you will see from ForeFlight or the NOAA, and an even bigger surprise when you do calculations for several different airports and find that there is no obvious pattern.
 
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Rory said:
Sophisticated formulas do take humidity into account.

There can be a significant discrepancy between the numbers that come from the NOAA calculator/ForeFlight and the rough and ready calculations that we are taught. Somebody asked what I mean by "rough and ready". Look at the Wikipedia article on Density Altitude and in particular at the formulas in that article on calculating Density Altitude. One thing that should be clear from looking at the formulas is that pilots are taught a formula that is significantly simplified.

The other day, I spent some time calculating Density Altitude using the simple formula that pilots are taught, and in many cases there was a significant discrepancy between the results of this formula and the results of an NOAA or ForeFlight calculation. Also interesting was the fact that the error was not consistent between airports, which is why I wonder whether humidity may be a bigger factor than we are told. Perhaps that's wrong, but it would be interesting to know exactly what is going on.

P.S. For those who are interested, and don't know, NOAA does have a Density Altitude calculator accessible via the internet. I suspect that ForeFlight uses the same formula.
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What is the range of water vapor content in the atmosphere? 0.25 to 3%? For high density altitude calcs the practical range is what? 0.25 to 1.5%? The molecular weight of water is 18. The molecular weight of air is about 28.6 so water vapor is roughly 60% the density of air.

If we were calculating a density altitude of 10,000 ft then water vapor would contribute about about 60% of 1% of that or 600 ft. The water vapor content might vary +/- 0.5% or about 300 ft. Note that as density altitude decreases so will the uncertainty range decrease. The possible variation caused by water vapor content is not significant in a density altitude calculation. Certainly it affects the precision of the calculation but aircraft performance prediction for our flibs is not impacted.
 
Clark1961 said:
What is the range of water vapor content in the atmosphere? 0.25 to 3%? For high density altitude calcs the practical range is what? 0.25 to 1.5%? The molecular weight of water is 18. The molecular weight of air is about 28.6 so water vapor is roughly 60% the density of air.

If we were calculating a density altitude of 10,000 ft then water vapor would contribute about about 60% of 1% of that or 600 ft. The water vapor content might vary +/- 0.5% or about 300 ft. Note that as density altitude decreases so will the uncertainty range decrease. The possible variation caused by water vapor content is not significant in a density altitude calculation. Certainly it affects the precision of the calculation but aircraft performance prediction for our flibs is not impacted.
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If you are right, cool.

Since you seem interested, perhaps do some density altitude calculations for a number of airports using the rough and ready pilot method, then do them from ForeFlight or NOAA and tell us what accounts for the discrepancies, especially the significant discrepancies between airports. One of the things that is interesting about this is that the discrepancies between airports are not linear.
 
SkyHog said:
Anyone mention the 2 knot approach speed increase at high DAs yet?
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LOL!

Rory said:
I'm not criticizing you or questioning whether your method works on a practical level.

I raised a question about something that I observed with respect to negative density altitudes and that, when I investigated it further, showed a significant discrepancy between the results of the formula we are taught and what appear to be the real numbers, and indeed a discrepancy that differed from airport to airport.

If the discrepancy doesn't interest you, cool.

For those who do use ForeFlight or choose to check out NOAA's density altitude calculator, you are in for a surprise when you compare what pilots are taught with the numbers you will see from ForeFlight or the NOAA, and an even bigger surprise when you do calculations for several different airports and find that there is no obvious pattern.
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Define: Significant.

If you’re not seeing differences of above 500’ between them, for most calculations involving aircraft performance, they’re not significant differences.

Unless you’re good enough to match the aircraft test pilot’s performance numbers to within 100’ for takeoff and landing distances on every takeoff and landing.

One of my instructors has early students add a safety margin at first of 50% more runway for all calculations. His reasoning... their skillset isn’t anywhere near refined enough yet to assure they won’t easily eat up most of that margin on any given takeoff or landing.

And if the point of running the numbers is to make sure THIS airplane and pilot are going to have adequate runway to do what THIS airplane and pilot are attempting to do TODAY, and the pilot doesn’t add a safety factor for their own skill level, the entire point of the process is missed.

So yeah, you haven’t shared the numbers, but I bet they’re insignificant for most of us. Pilot error will easily be five orders of magnitude bigger than the DA number error between various methods.

If one enjoys life, and wants to continue living, one shall have a pretty healthy safety margin above that minor mathematical error for all takeoffs and landings.

Leave the “V1 cut continue gives us 35’ over the airport fence” stuff to the pros... the vast majority of us don’t live in that coffin corner space for our safety calculations of aircraft performance. If we do, we’re either flying to a very high standard into places that demand it, or we’re fooling ourselves.

I other words, my airplane POH because of the STOL kit says I can get the thing down and stopped in 300’ at sea level in standard conditions. I’m not going to go hunting for an actual 300’ space to attempt it in. When I hit my numbers and get off by a turnoff at 400’ on my usual 7000’ runway, I’m pretty happy.

A few hundred feet of error in DA isn’t going to make a hill of beans in real world difference in that. Even 500’ of error is barely making a mark on the rest of the numbers.

The only place it is even remotely interesting is here on the Internet. In the plane, I’m not pushing margins so hard it will ever matter. Pushing margins is a great way to become a statistic.
 
Rory said:
If you are right, cool.

Since you seem interested, perhaps do some density altitude calculations for a number of airports using the rough sand ready pilot method, then do them from ForeFlight or NOAA and tell us what accounts for the discrepancies, especially the significant discrepancies between airports. One of the things that is interesting about this is what causes the discrepancies between airports, because they are not linear.
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I’ve done all the gas density calcs I ever want to do. Lol. I’ve written gas density calculation routines for commercial software. (It’s not a case of ‘if I’m right’) I just wrote out the percentages to let folks know the impact of the variation in water vapor content.
 
denverpilot said:
LOL!



Define: Significant.

etc, etc, etc.
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Not only did I not say anything about safety, I said specifically in the very post you quoted that the method works on a practical level and that if the discrepancy isn't of interest, cool.

Some people are interested in physics, and better understanding formula, and some aren't.
 
Rory said:
Not only did I not say anything about safety, I said specifically in the very post you quoted that the method works on a practical level and that if the discrepancy isn't of interest, cool.

Some people are interested in physics, and better understanding formula, and some aren't.
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So you’re on an aviation forum trolling, when there’s plenty of physics forums? :) Not to mention weather forums... tons of those where people argue over how to calibrate their garden temeperature sensors to tenths of a degree Fahrenheit or smaller.
 
denverpilot said:
I found a way to have coffee service on board like @Clark1961 has with his fancy autopilot though. Just trim it and then open a can of iced coffee so the backseater who isn’t there, doesn’t have to pour. Hahaha.
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Canned coffee is just wrong. Sorta like filetofishpaste and plankopork...
 
denverpilot said:
So you’re on an aviation forum trolling, when there’s plenty of physics forums? :) Not to mention weather forums... tons of those where people argue over how to calibrate their garden temeperature sensors to tenths of a degree Fahrenheit or smaller.
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Let me get this straight. When I calculate density altitude using the common private pilot formula, there's a significant discrepancy between the number I get and the number that ForeFlight gives, and that discrepancy proves not to be linear between airports, and asking about it here is in your view trolling.

Okey Doke.
 
Rory said:
Let me get this straight. When I calculate density altitude using the common private pilot formula, there's a significant discrepancy between the number I get and the number that ForeFlight gives, and that discrepancy proves not to be linear between airports, and asking about it here is in your view trolling.

Okey Doke.
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Can you quantify 'significant'? I think the trolling comments are stemming more from your argumentative tone than your message content. You haven't given any numbers. You haven't given 'the common private pilot formula' so I don't know what numbers you are even comparing. If you look at performance charts and then your 'significant' discrepancies, are they enough to push you up or down a row? One would typically add a margin for error on the safe side anyway, so, based on that are the discrepancies still significant?

I just used my 'private pilot formula' to calculate my local DA and compared that with a NOAA calculator and also the local ASOS and the numbers were within 100 feet. ASOS rounds down to the nearest hundred I think so based on that they all matched.

Lighten up and don't get so defensive and I don't think you'll be accused of trolling.


FYI, the one I use is:
PA = (29.92 - current pressure ) * 1000 + field elevation (or set altimeter to 29.92 and read)
density altitude = OAT - standard temp * 118.8 + PA
(!remember standard temp varies based on field elevation)
 
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Clark1961 said:
What is the range of water vapor content in the atmosphere? 0.25 to 3%? For high density altitude calcs the practical range is what? 0.25 to 1.5%? The molecular weight of water is 18. The molecular weight of air is about 28.6 so water vapor is roughly 60% the density of air.

If we were calculating a density altitude of 10,000 ft then water vapor would contribute about about 60% of 1% of that or 600 ft. The water vapor content might vary +/- 0.5% or about 300 ft. Note that as density altitude decreases so will the uncertainty range decrease. The possible variation caused by water vapor content is not significant in a density altitude calculation. Certainly it affects the precision of the calculation but aircraft performance prediction for our flibs is not impacted.
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60 feet
 
Rory said:
Let me get this straight. When I calculate density altitude using the common private pilot formula, there's a significant discrepancy between the number I get and the number that ForeFlight gives, and that discrepancy proves not to be linear between airports, and asking about it here is in your view trolling.

Okey Doke.
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As someone else said, lighten up. It’s not a “significant discrepancy”.

Okey Doke, indeed.

If anything it ends up a rounding error for real world applications to flight performance calculations, because it’s unlikely any of us can fly a spamcan that accurately.
 
Clark1961 said:
Read it again. With gusto this time.
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Okay. Help me out in your math. 60% of 1% of 10,000 ? I wouldn’t be the first time I’ve read something incorrectly but I don’t see it.
 
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