HUMIDITY & DENSITY ALTITUDE

(How Humid IS Your Day?)



This presentation has been donated to the SAFE Library for the personal use of SAFE Members. Questions, or Permission for any other intended use, should be directed to the author: Alan Davis, [email protected]



Educators will find notes to help in the presentation of this information in the notes format section under some of the individual slides.

HUMIDITY & DENSITY ALTITUDE

(How Humid IS Your Day?)

Do you know if you are safe to fly?

Alan C. Davis Master CFI – Emeritus/Gold Seal FAASTeam SAFE

Humidity & Density Altitude

 What were you told when you learned to fly?

  How do you determine the effects of humidity?

What do we have “now” for information on “moisture” and performance?

 Where CAN you go for information about the effects of humidity on Density Altitude (DA)?

Humidity & Density Altitude

 What were you told when you learned to fly?

– – – Standard answer?

 Humidity displaces air in a given mass, thus making the air less dense.

CFI Answer?

  It decreases performance? (How much????) If performance is

that

marginal, don’t go!

Remember these?

Humidity & Density Altitude

 How do YOU determine the effects of humidity? (There’s no wrong answer here, so speak up.) – – Where do you go?

What do you use?

 Have you ever felt a little unsure?

Humidity & Density Altitude

  What do we have “now” for information on “moisture” and performance?

– – Takeoff charts – but what do they tell us?

 How much ground roll – in “some” cases.

Landing charts – but what to they tell us?

 How much ground roll – in “some’ cases.

– Freezing/Icing cautions.

 What, if any, amount of icing we can fly into.

What happens after we get off the ground and into the air – and before the ice?

Humidity & Density Altitude

 Where CAN you go for information about the effects of humidity on Density Altitude?

– That’s what the rest of this talk is about.

– Has anybody actually “seen” any data in any POH (Pilot Operating Handbook or PIM – Pilot Information Manual) about the effects of humidity?

– What ARE we to do?

Humidity & Density Altitude

  First, we need to understand the “Standard Day”, since performance charts are based on it.

What is the definition?

– – – –

Standard day conditions

means standard ambient conditions as described in the United States Atmosphere 1976!

Ever read it? I sure haven’t!

Temperature=15 o C, specific humidity=0.00kg H2O/kg dry air, and pressure=101325 PA In other words, 59 o F, 29.92” of mercury, and

0% humidity

Humidity & Density Altitude

 What do charts give us?

– Some give us standard day numbers with various calculations to perform when not standard.

– Some give us sets of standard and non-standard temps, and we get to …… interpolate, and then do other various calculations.

– Do ANY of them address humidity?

 The only ones I have seen address temperature, some have baro corrections, and wet grass (surface ONLY).

Humidity & Density Altitude

 Speaking of that, we learn Soft Field takeoffs, don’t we?

– Ever seen performance charts for a “soft” field?

–  Why not?

The only performance we “almost” have for the technique is actually for “rough” fields (which uses the same technique), and which are “dry”.

 And what about a Short Soft Field technique?

 But I digress! Back to the topic at hand.

Humidity & Density Altitude

   Would you like to know what happens to density altitude, and thus the performance of your aircraft, with humidity?

Credit where Credit is due – Barry Schiff and an article in AOPA magazine got me going.

The “source” web site: www.beaufort.usmc.mil/weather/wx_calculators.html

(unfortunately, it is no longer up and available – but stay tuned!)

Humidity & Density Altitude

  Next I will show you information derived from it.

But first, a caveat…… Per the web site,

“This calculator is for educational purposes only and should not be used for flight planning.”

– Why?

Humidity & Density Altitude

   I worked with the site for various altitudes and temps to get the information that follows.

Again, however, the same caveat applies – NOT “official” so not to be used to determine..

You don’t need to see a lot of numbers – just what the general information means.

Humidity & Density Altitude

 Humidity affects Density Altitude more at lower altitudes than at higher ones. A surprise?

– SL – 80 F – 29.92 – DA went up approximately 3.2% for every 10% increase in humidity – 32% at 100% humidity!

– Same was true, approximately, for 90 F and 100 F  90 F Dry = 1978 DA  100 F Dry = 2584 DA 100% = 2500 100% = 3410

Humidity & Density Altitude

 What does it do at higher altitudes?

– 3000 – ranges from 6.5% at 60 F to 14.5% at 100 F –  90 F dry = 5625 90% = 6288  100 F dry = 6216 100% = 7115 5000 – ranges from 4.2% at 60 F to 11.1 % at 100 F  90 F dry = 8046  100 F dry = 8627 90% = 8607 100% = 9580 – Significant increases in DA at an altitude where performance is already compromised by just the dry DA due to temp!

Humidity & Density Altitude

 So, what DO we do with this information?

– Keep in mind that humidity does have a significant relationship to performance at

all

altitudes.

– – Humidity can increase performance DA by 11% at higher altitudes to 32% at sea level.

Use the higher values, in the absence of “useable” performance data as your guide.

Humidity & Density Altitude



AND,………..

– When in doubt, fall back on the old CFI adage, “If the performance figures for the airport are that marginal, for either takeoff of landing, without the humidity applied, then

just don’t go!

Humidity & Density Altitude

   Now here IS a site that is up and available, and which uses Temperature & Dew Point to get to the same point: – – http://wahiduddin.net/calc/calc_da.htm

This is information developed by Richard Shelquist about how it was all derived: – and also on his site, for you mathematicians in the crowd, is an article  http://wahiduddin.net/calc/density_altitude.htm

Again, the caveats, so that we stay straight with the FAA and the manufacturers:

“This calculator is for educational purposes only and should not be used for flight planning.”

Humidity & Density Altitude

 Thank you for your attention!

 If you have any questions, I would be happy to try and answer them. But please remember, I am NOT an engineer, and I am NOT a weather expert – any more than any of us are in this business.

HUMIDITY & DENSITY ALTITUDE

(How Humid IS Your Day?)



This presentation has been donated to the SAFE Library for the personal use of SAFE Members. Questions, or Permission for any other intended use, should be directed to the author: Alan Davis, [email protected]



Educators will find notes to help in the presentation of this information in the notes format section under some of the individual slides.