Density Altitude for a new PPL

[bigEh]

I am a new PPL just about to start my life with the bit of freedom that a PPL gives me. Due to a recent event near where I train and the recent crash video from inside the plane's cockpit, I have been trying to find more information about high density altitude and other dangerous situations that could lead a low time pilot like myself to make a drastic mistake. I am finding it difficult to understand the full ramifications of high density altitude and how I would be able to make a go/no go decision. Obviously my experiences are limited and I can only go by what I have read and been taught. I have noticed a marked difference the Cessna's performance has undergone between early March'c colder temperatures when I started my training to today's 25C temperatures. But as I move from a 152 to a 172 and load my friends in for a flight, maybe near max weight, other than using the POH and the Koch chart, what are the ways in which I can make myself and my friends/family safest. Basically, I would like help pointing me in the direction of good research material. I am going on to my CPL soon, with my Night rating and Mountain course thrown in between. I assume I will learn more on the subject but right now, during a brief break in my formal studies, can anyone recommend any articles or or books or formulas, that could better help explain the effects of high density altitude and how to recognize a potentially fatal situation.

thanks in advance

[Colonel Sanders]

It's really very simple.

The engine, prop and wing work better with thick air, than thin.

Thick, dense air contains more oxygen and acts like a turbo-charger
to the engine. During the cold winter months, you can actually have
NEGATIVE density altitude, with so much oxygen going into the engine
that it leans out on takeoff.

Thick, dense air is easier for the prop to bite into, and allows the prop
to produce more thrust because it doesn't slip as much with thin air.
It's got more mass (of air) to accelerate backwards, to produce thrust.

Thick, dense air allows the wing to produce more lift. Remember that
fundamentally how a wing works is that it accelerates a mass of air
downwards (f=ma) and the lift is a result of equal and opposite reactions
(see a very nasty man called Sir Isaac Newton).

Note that there are turbo-charged, and non-turbo-charged versions of
many aircraft available. One might think that the turbo-charged version
(which uses a pump driven by the exhaust, to pump air into the engine
air intake) which restores the lost manifold pressure, would solve all
your problems, but it doesn't solve the prop or wing problems above,
and due to heating of adiabatic compression, doesn't entirely solve
the engine problem either, at least without a 100% effective intercooler.

You need to learn two fundamental concepts, which elude many pilots:

INDICATED airspeed, and

TRUE airspeed.

Aircraft are built so that at sea level, at +15C and 29.92 inches of barometric
pressure, and no wind, if the airspeed indicator shows 100 MPH, you are
actually moving at 100 MPH through the air mass (and over the ground).

Now, in that situation, climb up to 10,000 feet in that standard air mass
which is conveniently not moving. If you set the throttle so that you
are indicating 100 mph, you will actually be moving through the air mass
and over the ground at 117 mph. This is 17 mph of "free airspeed", so
don't think that thin air is always your enemy!

You want thick air for takeoff and landing, and thin air for cruise.

But what happens if you don't get it?

Let's continue flying in our scenario above, west towards the rockies
at 10,000 feet as the land gets higher and higher. Eventually we will
get closer to Leadville, CO which has an elevation of almost 10,000
feet.

If we join the pattern (sorry, British circuit at Leadville, and turn
final, and indicate our normal 70 mph on final, we will be travelling
much faster through the air (true airspeed) than 70 mph.

Because kinetic energy is a function of the SQUARE of your speed,
you will need lots of runway to land at Leadville. And that's with
the power at idle - we aren't even worrying about the effects of
thin air on the engine and prop (eg for takeoff).

There are some important concepts above, which should have
been taught to you during your ground school, but obviously
weren't. Re-read this posting a few times to make sure you
understand them.

Ok, next important thing: If there is any doubt in your mind,
do quick takeoff and landing calculations using the POH/AFM.

DO NOT do the stupid time-consuming triple interpolations that
your instructor taught you - take the WORST number, because
that's what you're going to get on your tired old airplane. You're
not exactly a factory test pilot, either.

[Shiny Side Up]

There are some important concepts above, which should have
been taught to you during your ground school, but obviously
weren't.

Don't assume "wasn't taught" rather than "wasn't learned". While I can appreciate our OP wanting to learn more, all of what you posted above is neatly in the FTGU covered very well, I'm not sure how much more research one might need on the subject. I had to go look to make sure its there, since questions like this repeatedly arise to the point where it makes me doubt that its there.

Typical of human nature, it required something graphic to hit this home and make it sound important for this student. I'm starting to wonder if every ground school should include a viewing of a collection of videos of such nature to hit home the points. Maybe a few Troy MacLure hits like When Head Meets Attitude Indicator and Little Airplanes, Big Trees vol.4 or some such.

Re-read this posting a few times to make sure you
understand them.

Indeed. Don't be content to float through your training, I know some of the shit sounds boring but its important to know it more for than just satisfying the paper requirements.

[cptn2016]

DO NOT do the stupid time-consuming triple interpolations that
your instructor taught you - take the WORST number, because
that's what you're going to get on your tired old airplane. You're
not exactly a factory test pilot, either.

I very much like this bit here. I always thought this was too much work for too little benefit. Taking the worst numbers from all of the relevant rows and columns is much easier. I would do this for an airplane of any age because I am very much NOT a factory test pilot.

[bigEh]

I appreciate the detailed response Colonel. Thank you for all the info. I think though, that I may have asked my question incorrectly. I was not so much asking to explain what density altitude is and how it effects the performance. I feel I have a good grasp of how density altitude works and how it will affect my aircraft. I studied extremely hard for my exams and did well. Take that for what it is worth. I worked hard to understand concepts like True Altitude and how it will change in relation to my Indicated airspeed. Shiny Side Up, I certainly know the areas in FTGU where this is discussed. Don't worry it's there haha. there are many fingerprint smudges all over those pages in my copy.
What I was trying to ask, and apparently failed at, was how more experienced pilots make the Go/No Go decision when it comes to High Density Altitude. At what point do you say no? Obviously every aircraft will be different, but I have spent all my time in a 152 and soon to be 172s, so that is primarily my concern. Is it solely based on the terrain? If I am at a high altitude but in a flat area, is my decision different than if I am in a lower altitude valley surrounded by large mountains? That seems obvious to me that it would be a different process for taking off or staying put, but assuming things....well we know how that goes.
Hopefully I have cleared up what I was intending to ask, or maybe I should get my head back into my books.
I appreciate any responses.
Cheers

[frog]

A way to do it is to do all your calculations with the worst case scenario and then add 15 % to your figures (landing and TO distance...etc.)

[ahramin]

Bigeh, if you have been taught, read, and understood all the theory of density altitude, the way to avoid being the hero in the video noted is to apply that theory when you go flying. DO the performance calculations and if you find yourself getting numbers for takeoff distance that are anywhere close to your available distance, start thinking about why that is and what you should be doing to make sure you get best performance from your aircraft. For example maybe a rolling takeoff with slow application of power isn't the best choice today.

But if you really want to make sure that you don't end up in the trees like that, start developing the mindset that if you are rolling down the runway and the plane's acting funny and doesn't want to fly: STOP. Cultivate the habit of NOT ignoring the nagging feelings that tell you something is wrong. Instead, act on those feelings.

[Shiny Side Up]

What I was trying to ask, and apparently failed at, was how more experienced pilots make the Go/No Go decision when it comes to High Density Altitude.

Ah, a more reasonable line of tact. Another way of thinking about the issue is thinking about where your airplane's service cieling is and how much working space you have between where you're planning on operating and it. Remember that the closer you get to it, d.alt wise the worse the airplane is going to perform. The worse it performs, in general the more space you're going to need to do stuff. Something that catches people (like the guy in the video) is that flat high (density) altitude terrain is safer than more rugged terrain, but that's but an illusion, well I suppose unless you're talking about ability to crash and survive, in which of course the flat is somewhat safer, but that's a different matter, supposing instead our definition of safe is "not crashing". Its all about thinking about how much room have you got to work with, and thinking about how that changes where your out will be. IF you by chance miscalculated things, you should also have figured out how you're going to know, and where your abort or turn back point is going to be, whether that be referring to a take off, or crossing a ridgeline. The fellow in the video clearly didn't figure out a go/no go point for his flight. The Colonel points you in the right direction for some numbers in that regard.

[Bushav8er]

A way to do it is to do all your calculations with the worst case scenario and then add 15 % to your figures (landing and TO distance...etc.)

Curious, if you are already, by default, using the worst case numbers...why add another 15% to them?? You'll never land or take off. Only time this makes some sense to me is if the conditions are dictating the worst figures.

[Rowdy Burns]

What I was trying to ask, and apparently failed at, was how more experienced pilots make the Go/No Go decision when it comes to High Density Altitude. At what point do you say no?

viewtopic.php?f=3&t=74972

[bigEh]

I appreciate all the replies. I think I got my question across the second time around.
Cheers for the help.
Big Eh

[Panama Jack]

A way to do it is to do all your calculations with the worst case scenario and then add 15 % to your figures (landing and TO distance...etc.)

Curious, if you are already, by default, using the worst case numbers...why add another 15% to them?? You'll never land or take off. Only time this makes some sense to me is if the conditions are dictating the worst figures.

Nobody says you have to use 15%, it is up to you. However, even the worst case figures are flown precisely by a factory test pilot using maximum performance techniques, a new airplane, and perhaps even a few runs to get the numbers trimmed down. Not exactly the circumstances of most pilots using average piloting techniques.

I fly an airliner and when we calculate our landing distances (with failures), Company Policy is to add 15% to compensate for line-pilot techniques. It is the Captain's discretion not to use this if a field is marginable, but they do warn that the risk of a runway excursion is greatly enhanced.

I am not a bush pilot and am not an expert on squeezing the last bit of performance out of a GA airplane. When I fly a 172 I also add another 15% to my takeoff and landing performance calculations. If that is a make-it or break-it for using the field, then perhaps I should consider going elsewhere or delaying my flight for more ideal conditions.

[LousyFisherman]

A way to do it is to do all your calculations with the worst case scenario and then add 15 % to your figures (landing and TO distance...etc.)

Curious, if you are already, by default, using the worst case numbers...why add another 15% to them?? You'll never land or take off. Only time this makes some sense to me is if the conditions are dictating the worst figures.

Personally as a low time PPL I use worst case x 2.
Why? So I have lots of time to make good decisions, gather information, make corrections, or abort and go-round.
My worst landing, density alt of 6500, numbers say 1600 ft, it is a 3000 ft grass strip. tailwind (which I did not know), full flaps. As I cross the fence I realize I'm going really fast, force the plane down and porpoise. Hit the gas, which in the 150 with full flaps puts the plane into a perfect landing position, realize I am only halfway down the strip, so I take the gas off and have her stopped within 300 ft. I still had 1000 feet left. If I had used the worst case, first: my numbers would have been wrong because of the tailwind; second: I would have had some very nervous moments climbing over the trees or I would have overrun by 100 ft. Neither is what I want to experience while I am flying.

You ask why add extra space yet at the same time ask how you decide what is safe?
You add enough extra space so you KNOW you are safe or you do not go in there.

HTH
LF

[Bede]

Just to clarify, performance numbers in the POH are assuming average pilot technique even though they were obtained by a test pilot, albeit with a brand new airplane. Follow the procedures listed in the POH without resorting to dumb stuff you saw on the internet, ie. dragging a plane in below Vs with full power on to make a nice 50' landing roll. (You won't gain any advantage to this since your takeoff run is almost always longer than your landing roll. Unless you're not planning to take-off again).

[Colonel Sanders]

performance numbers in the POH are assuming average pilot technique

Maybe, but remember that there was plenty of pressure
from the sales department, to put "good numbers" into
the POH. There are many examples of this, but grabbing
the POH for a 1975 C172 off the shelf, it says that with
no wind, at sea level and max gross, you can land it in
520 feet

Best of british luck trying to land a C172 in 520 feet with
"average pilot technique".

as a low time PPL I use worst case x 2

There are not many low-time PPL's that can consistently
land a 172 at max gross with no wind in even

520 x 2 = 1040 feet.

That's only 5 runway lights!

Now, back to your stupid triple interpolation so you can
correctly answer questions like these:

Landing distance required (blah blah blah):
1) 928 feet
2) 929 feet
3) 930 feet
4) 931 feet

[dr.aero]

It should be pointed out that indicated airspeed isn't really a speed at all - it is a pressure. The more you understand that, the easier it is to understand. The ONLY speed (distance/time) is True Airspeed - which is the speed at which you are moving through an air mass.

They have determined that at SL with 15 degrees C and dry air and 29.92", a total pressure of x (as measured by the pitot tube) will equal x knots of True Airspeed. You could think of it like a chart: at a certain pressure you get a certain speed, at a higher pressure you get a higher speed.

When you climb up, the static pressure and density both decrease, which decreases the pressure felt by the pitot tube for the same speed (TAS) through the air mass. That's why as you climb up (or ANY time the air is thinner) you'll have indicated airspeed less than TAS. The air could be thinner because of being hotter or because of less pressure being exerted on it. As a refresher, air pressure is a result of the air resting on top of it - as you climb, there is less air resting on top so there is less pressure.

So anytime you're not under the "standard" conditions, your airspeed indicator is essentially lying to you because it's not telling you your TAS. So we apply corrections to IAS to get TAS.

IAS corrected for position error (mounting of the pitot tube - it's not able to point directly into the airstream during all angles of attack) is called Calibrated airspeed. CAS corrected for compressibility (if you compress air you require excess pressure to do so which causes a higher pressure reading than normal) is called Equivalent airspeed - which is less than CAS. EAS corrected for temperature and pressure (density) is TAS.

Below 10,000' and 250 knots, the compressibility effects are essentially negligible, which is why you won't find a chart to convert CAS to EAS in your 172 POH!

[JungianJugular]

It's really very simple.

The engine, prop and wing work better with thick air, than thin.

Thick, dense air contains more oxygen and acts like a turbo-charger
to the engine. During the cold winter months, you can actually have
NEGATIVE density altitude, with so much oxygen going into the engine
that it leans out on takeoff.

Thick, dense air is easier for the prop to bite into, and allows the prop
to produce more thrust because it doesn't slip as much with thin air.
It's got more mass (of air) to accelerate backwards, to produce thrust.

Thick, dense air allows the wing to produce more lift. Remember that
fundamentally how a wing works is that it accelerates a mass of air
downwards (f=ma) and the lift is a result of equal and opposite reactions
(see a very nasty man called Sir Isaac Newton).

Note that there are turbo-charged, and non-turbo-charged versions of
many aircraft available. One might think that the turbo-charged version
(which uses a pump driven by the exhaust, to pump air into the engine
air intake) which restores the lost manifold pressure, would solve all
your problems, but it doesn't solve the prop or wing problems above,
and due to heating of adiabatic compression, doesn't entirely solve
the engine problem either, at least without a 100% effective intercooler.

You need to learn two fundamental concepts, which elude many pilots:

INDICATED airspeed, and

TRUE airspeed.

Aircraft are built so that at sea level, at +15C and 29.92 inches of barometric
pressure, and no wind, if the airspeed indicator shows 100 MPH, you are
actually moving at 100 MPH through the air mass (and over the ground).

Now, in that situation, climb up to 10,000 feet in that standard air mass
which is conveniently not moving. If you set the throttle so that you
are indicating 100 mph, you will actually be moving through the air mass
and over the ground at 117 mph. This is 17 mph of "free airspeed", so
don't think that thin air is always your enemy!

You want thick air for takeoff and landing, and thin air for cruise.

But what happens if you don't get it?

Let's continue flying in our scenario above, west towards the rockies
at 10,000 feet as the land gets higher and higher. Eventually we will
get closer to Leadville, CO which has an elevation of almost 10,000
feet.

If we join the pattern (sorry, British circuit at Leadville, and turn
final, and indicate our normal 70 mph on final, we will be travelling
much faster through the air (true airspeed) than 70 mph.

Because kinetic energy is a function of the SQUARE of your speed,
you will need lots of runway to land at Leadville. And that's with
the power at idle - we aren't even worrying about the effects of
thin air on the engine and prop (eg for takeoff).

There are some important concepts above, which should have
been taught to you during your ground school, but obviously
weren't. Re-read this posting a few times to make sure you
understand them.

Ok, next important thing: If there is any doubt in your mind,
do quick takeoff and landing calculations using the POH/AFM.

DO NOT do the stupid time-consuming triple interpolations that
your instructor taught you - take the WORST number, because
that's what you're going to get on your tired old airplane. You're
not exactly a factory test pilot, either.

Colonel Sanders where do you instruct?