AVCANADA

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.

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.

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.

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.)

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?

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.

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.

performance numbers in the POH are assuming average pilot technique

as a low time PPL I use worst case x 2

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.