# Presentation: 931977

## Water skiing

## Density Altitude

- Density Altitude and it’s effect on Aircraft performance & flight operations
- Prepared for the
- 	Savannah Squadron of the CAP
- 	GW-075
- 	By
- 	Capt Roger H. Crane, CFII

## Agua Dulce Airpark (L70)2660’ Elevation – 4600’ RW – 1% Up Slope to the NE 

## Agua Dulce Airpark (L70)2660’ Elevation – 4600’ RW – 1% Up Slope to the NE

## Fallbrook Airpark (L18)708’ Elevation – 2160’ RWDrop offs both ends – No Over runs

## What is Density Altitude?

- Density Altitude is Pressure Altitude Computed with or corrected for temperature.
- You can’t see it.
- You can’t smell it.
- You can’t taste it.
- You can’t feel it.
- You can’t hear it.
- So what does it do?

## Density Altitude

- IT Kills Pilots
- and
- Bends Airplanes!

## Density Altitude

- The normal density of the air mass is computed on a standard day.  There is a decrease in air density as elevation increases.  Pilots compute that in 1,000 foot increments.
- High density altitude is an adverse condition for aircraft during takeoff, landing and climbs, caused by the heating of the air which in turn becomes thinner or less dense.
- On a hot day, a sea level airport could have air density equivalent to 4,000 feet.

## 101 Primer on Altitude

_**Absolute Altitude**_ – *The vertical distance of an aircraft above the terrain. (AGL)*
_**Indicated Altitude**_* – That altitude read directly from the altimeter when set to the current altimeter setting*
_**Pressure altitude**_* – The altitude indicated when the altimeter is set to 29.92” (the standard datum plane)*

## 101 Primer on Altitude – Continued

_**True Altitude**_ – *The true vertical distance of the aircraft above sea level (the datum plane) – the actual altitude.  Most often expressed as MSL*
- Density Altitude – This altitude is pressure altitude corrected for temperature variations.
- This is the altitude that directly effects the performance of your aircraft.

## Humidity 

- Water vapor is lighter that air, therefore it’s content in the atmosphere reduces air density and has an adverse effect on the performance of aircraft.  Because there is no workable formula to compute the effect of water vapor it is generally ignored other than the statement that _**you should expect a decrease in overall performance in high humidity conditions.  **_

## The Basics

- The FAA Standard Day
- Is computed from Sea Level (datum plane) and stated to be:
- An atmospheric pressure of 29.92 Inches of Mercury
- An ambient atmospheric temperature of 15 Degrees Celsius or 59 degrees Fahrenheit.
- Do we ever have standard day?
- How many days a year do we have a standard day?
- How many days a year do we have a below standard day?
- How many days a year do we have an above standard day?

## Mercurial Barometer

## A non-standard day is?

- A non-standard day is what we wake up to & enjoy almost on a daily basis.  The change in atmospheric pressure and temperate result in a constant change in air density, also known as,
- Density Altitude
- The density altitude may be lower or it may be higher than our “mythical” standard day.

## Why is this information important to Pilots?

- Density Altitude affects all aspects of our aircraft performance.
- Available engine horsepower
- Takeoff roll.
- Takeoff climb over obstacle.
- Climb Performance.
- Cruse Performance.
- Landing Performance.

## How Low Density Altitude affects Performance

- On a cold day, with a temperature under 59 Degrees Fahrenheit or colder, and
- An atmospheric pressure of 29.92 inches of Mercury or higher.
- Result in a LOWER DENSITY ALTITUDE
- Better overall performance, however
- Fuel consumption will suffer due to higher drag from the denser air mass and higher volume of fuel to keep the air/fuel ratio correct..

## How High Density Altitude affects Performance

- On a hot day, with a temperature over 59 degrees Fahrenheit, and
- A atmospheric pressure of 29.92 inches of Mercury or lower
- Result in a HIGHER DENSITY ALTITUDE
- Aircraft performance is degraded
- Exceptions:
- Cruise performance improves up to about 8,500 feet density altitude
- Fuel consumption improves

## N3173E – The Good News

- STC’d for a 180 HP Lycoming engine.
- That is a 12% increase in HP
- STC’d for 250 pounds increase in gross weight
- That is a 11% increase in gross weight
- Propeller changed?  Unknown pitch, probably a cruise pitch propeller
- Better performance you think?  Lets take a look

## N3173E The Not So Good News

- CG Moved forward 1 inch due to heaver engine weight.
- Resulting in
- Increase in induced drag
- Slower speed
- Increased angle of attack
- Increased up elevator

## More not So Good News

- No performance data provided
- No takeoff data provided
- No landing data provided
- No range data Provided

## Enter the Denalt Computer & Koch Chart

## Have I got you thinking now?Well what the hell, you always wanted to be a TEST PILOT, Right?

- Remember the airplane cannot think.  It only knows Density Altitude.  Do you?
- Whether you do or not the airplane does.  Therefore it will fly and perform to the ambient air mass in which it is contained regardless of what you want it to do.

## A few rules of thumb

- Average adiabatic lapse rate is 3.5 degrees Fahrenheit or 2 degrees Celsius per one  thousand feet of altitude up to approximately 18,000 Feet.
- True airspeed increases approximately 2% of indicated airspeed per thousand feet.
- Pressure decreases approximately 1 inch of                  mercury for each 1,000 feet of altitude.
- At 18,000 feet the air is approximately one half as dense that at sea level

## Rules of thumb

- At approximately 7,500 feet MSL the maximum power a  normally aspirated engine can develop or provide is approximately 75% power.
- Clouds “Suck”!

## Now back to N3173E and Density Altitude with a Scenario

- Claxton Airport (CWV)
- 5012 Foot runway.  Trees 634’ beyond the departure end of runway 9
- Elevation 112
- Wind above the trees 0315
- Wind on runway L&V
- Altimeter setting 29.60 *
- Temperature  110 Degrees F – 43 Degrees C
- Pressure Altitude is 412
- Density Altitude 4,000 Feet
- Large fair weather cumulus in the area

## Claxton Airport

## Approach to landing at the airport	

- Fair weather Cumulus over approach end of runway 9
- Slight problem with airspeed control due to convection  from Cumulus Cloud.
- As you near the ground you perceive your speed to be too fast and reduce power and pitch up to control airspeed and start a high round out to bleed off airspeed
- Just below tree line, rapid sink rate occurs and decay of airspeed.
- Pilot adjusts with pitch as he still senses the speed is too fast and to check the sink rate.
- Plane continues to descend and the pilot applies full back pressure and experiences a “hard” landing.

## Ouch	

- Well it must have been a good landing.  We walked away from it

## Departure

- Surface wind L&V.
- No overhead Cumulus cloud as they moved south and there is currently a clear sky over the airport between the clouds.
- Full fuel and at gross weight.
- Departed from fuel pumps and used the taxi way farthest west.  Approx 3700 feet of runway for runway 9.
- Ground roll – Book 965, Denalt 1252.
- Climb 72 KIAS Book 669, Denalt 553
- Scenario: Ground Roll 1,108, Climb 611 FPM.

## Liftoff

- After liftoff there is 3226 feet to the tree line
- Climb speed 72 KIAS or 108 FPS forward motion.
- True airspeed is (+2% per 1000 feet) (Density altitude is 4,000 feet) 78 KTS or 117 FPS
- 3226 / 117 = 28 Seconds until tree line.
- ROC is 611 FPM.  Altitude at tree line 285 feet or 221 feet above trees (62 feet)
- But wait!

## The pucker factor

- What about that 15  knot wind from 030 coming over the trees?
- That wind would come pouring over those trees that parallel the runway on the north side causing a “down slope” effect.
- 15 Knots times 1.5 is 22.6 FPS or but to be fair, we’ll give it a 9 knot speed for 13.5 FPS.
- That amounts to 378 FPM down

## Down Slope effect

## The Bad News

- The bad news is that our 611 FPM climb is now only 233 FPM.
- At the tree line we now have 108 feet AGL. That’s 44 feet above the trees if they have not grown.
- Well ***maybe*** we do.
- Remember that Cumulus cloud that moved away?
- Without that Cumulus cloud the air mass could very well be a SINKING air mass.
- And then remember the adverse factor of humidity?  The one thing Georgia has lots of and we cannot compute it’s effect.

## Things to Ponder

- Aircraft Engine – How many hour are on the engine.  How much horse power is it really putting out.
- Engine Thrust alignment.
- Wheel pants.
- Parasitic drag
- Paint
- Rigging
- Pilot proficiency

## Conclusions

- So, I’ll leave you with the problem of deciding whether the aircraft cleared the tree line or clipped the top and cart wheeled into the woods.
- I also want you leave you with some good practices when flying on hot, humid days.

## Good Practices 

- Do check Density Altitude.  Know what it is and compute your aircrafts performance.
- Remember that with a Density Altitude of 5,000 feet you can only produce approximately 80% horsepower .
- USE all of the available runway.  Do not make intersection takeoffs.
- Consider the winds and the terrain, including man made structures.
- Consider cloud formation and their effect on the air mass, (convection)

## More Good Practices

- The aircraft which was used to obtain our performance data was as perfect aircraft as the factory could make it and the pilot was a professional.
- Pilot proficiency. How often do you fly?  When did you last make a short field take off or landing.
- Remember that your aircraft is not new.  The engine hours and condition. The paint, antennas, rigging, wheel fairings and other subtle changes all effect performance.
- 3 takeoff and landings in 90 days will not keep you proficient.
- Fly the Down wind side of the valley or terrain.  That’s the side where the wind is moving up and over.

## Some guys have all the fun.  But tell me do you need a Sea Plane rating for this?