ATPL Ground study material: books, DVDs, computer programs etc.

[fabiensf]

Have a look at the sticky at the top of the main page:
http://www.pprune.org/forums/showthread.php?t=131649

For the PPL you can either use the Trevor Thom or Jeremy M. Pratt series.
For the ATPL , depends which school you intend to go to but seems like the Oxford or Bristol notes are the best.

[dream747]

Apologies for bringing this thread up again. So far I've only manged to come across many useful finds after searching but they are all mainly JAA or FAA.

Are there good books for the CASA or NZ CAA PPL/CPL? Are there many differences in the syllabi of the OZ and NZ PPL compared with the JAA, FAA or UK CAA for that matter? Or would reading the JAA books be suffice to help in the others?

Thanks for any responses!

[scroggs]

Three posts have been deleted which attempted to either directly advertise stuff or to link to Ebay auctions of equipment. To repeat: advertisements are not permitted on the open forums, in any form. Please respect our rules.

Scroggs

[Mohit_C]

Hello guys,
I've only recently started my ATPL integrated course and need some help. We've been going on in class that temperature and pressure decrease as altitude increases but the density also decreases. Apparently the decrease in density doesn't have anything to do with the decrease in temperature which was what I would expect because: p=m/V Volume depends on pressure and temperature therefore the density depends on pressure and temperature.
So my question is why does density decrease with altitude?

Also another question.
Q. If you're in a hot atmosphere:
a) The pressure is less than ISA.
b) The pressure is more than ISA.
c) The density is less than ISA.
d) The density is more than ISA.
Please explain the answer.

Thanks a lot.

[neilia]

Basically, the decrease in pressure (giving reduced density) with altitude is a more dominant effect than the decrease in temperature (giving increased density), net result: reduced density.

[Mohit_C]

Originally Posted by neilia

the decrease in temperature (giving increased density)

Can you explain why?

[neilia]

Why reducing temperature increases density? Well, that's the basic relationship that exists. Reducing temperature reduces energy levels in the molecules, they move less and occupy a smaller volume of space. The same number of molecules in a smaller volume of space = increased density.

[Mohit_C]

Ah right. What a stupid question ! I forgot to include this in the previous post but why is the decrease in pressure a more dominant effect than the decrease in temperature?

And could you answer the second question in my first post?

Thanks a lot.

[Keith.Williams.]

FIRST QUESTION
If we go back to the basics:

Boyle's law says that for a fixed mass of gas the Pressure x Volume is constant if temperature is constant.

This means that if we half the pressure we will double the volume. But doubling the volume means that the same mass is of air fills twice the volume. This will half the density. So each time we half the pressure we half the density

If we go from ISA mean sea level to 35000 ft, the pressure falls from 1013.25 mb to about 218 mb. This means that the pressure at 35000 ft is 218/1013.25 = 21.5% of its mean sea level value. So if temperature had remained constant, the density at 35000 ft would be about 21.5% of its sea level value. This represents a 78.5% reduction in density.

But as altitude increases, the temperature decreases. So we must also consider the effect of this temperature decrease.

Charles' Law says that Volume / Temperature is constant if pressure is constant. This means that if we half the temperature we will half the volume. This will double the density.

But Charles was talking about the absolute temperature not the temperature in degrees Celsius.

At ISA msl the temperature is about 15 degrees Celsius, which is 288 degrees Kelvin. This reduces by about 2 degrees for each 1000 ft increase in altitude. So at 35000 ft the absolute temperature is 288 -(35 x 2) = 218 degrees Kelvin. So the temperature at 35000 ft is 218/288 = 75.7% of its sea level value.

This means that if the pressure had remained constant the density of the air would have increased to 1/0.757 = 1.32 of its sea level value. That is a 32% increase in density.

So as we climb from ISA msl to 35000 ft the reducing pressure tends to reduce density by about 78.5% while the reducing temperature tends to increase density by about 32%.

From these figures it can be seen that the reducing pressure is the more powerful effect. So as altitude increases the air density decreases.


SECOND QUESTION
If we increase temperature at any given elevation, the air will expand. This will reduce its density.

The air will tend to expand in all directions, but downward expansion will be limited by the surface of the earth. So the majority of the expansion will be outwards and upwards. This means that as the temperature increases at any given elevation, the upward expansion of the air will increase the mass of air that is above that elevation. Static pressure at a given elevation is caused by the weight of the air above, being pulled down by gravity. So by increasing the mass of air that is above a given elevation, increased temperature will increase the static pressure at that elevation.

So increasing temperature at any given elevation will decrease the density and increase the pressure. It is this increased static pressure which causes altimeters to under read when the temperature is above ISA, and to over read when temperatures are below ISA.

[Mohit_C]

Keith.Williams. thanks a lot for the entire first part, it makes so much sense to me now. Just one little thing, can you rephrase the last paragraph.."It is this increased static pressure which causes altimeters to under read when the temperature is above ISA, and to over read when temperatures are below ISA"?

Because if you increase temperature, you say that density decreases but static pressure increases...and these are the two answers which it could be in the multiple choice question.

+++EDIT+++ Now this is confusing me even more...the actual total pressure is unchanged, so if static pressure decreases, dynamic pressure increases. Is the trick in the question being that the actual total pressure is unchanged?

[Keith.Williams.]

When I said that

"It is this increased static pressure which causes altimeters to under read when the temperature is above ISA, and to over read when temperatures are below ISA"

I meant exactly that.

Imagine that you are out in space and you can see the entire earth and its atmosphere. Now imagine that the air up to some selected elevation is all red and that above it is all blue. If we increase the temperature of the air it will expand, causing the whole atmosphere to become thicker. If we look again we will see that this expansion has caused the junction between the lower red air and the higher blue air to move upwards. This means that more of the air is now above our selected elevation. So the pressure at this elevation will be increased. But the only reason that this has happened is because the air expanded. This expansion has reduced the density of the air.

So at our selected elevation the increased temperature has decreased the density and increased the pressure.

When your studies move on to Altimetry you will find that they over-read in temperature colder than ISA and under-read in temperatures warmer than ISA. The reason for this is as explained above.

Your second question is deffective in that it has two correct options.

Your reference to total pressure being constant is also incorrect. In order to convert an ISA atmosphere into a warmer atmospher, we must add some heat energy. This extra energy will increase the total pressure.

[Mohit_C]

Oh ok, I get it now. Thanks a lot Keith.Williams. I'll come back if I have other questions.

[Mohit_C]

I have a question regarding speeds and what is read in the anemometer. Suppose an aeroplane is flying at 200 knots, the anemometer would read 200 knots. But now if we have a headwind of 20 knots, would the IAS (what is read in the anemometer) ready 180 knots or still 200 knots? Would the TAS be 180 knots which is the same as ground speed?

I know that the TAS would also take into account the instrument, position, density and compressibility errors but I want to get an example say at SL.

Thanks.

[Keith.Williams.]

To keep the discussion as simple as possible let's ignore the niceties of instrument error and pressure sensing error, so that the ASI shows Indicated Airspeed.

Let's also stick to ISA msl to eliminate density errors, so that the IAS = TAS.
Now imagine that we have an aeroplane that is fitted with an Airspeed Indicator (showing Indicated Airspeed), a TASmeter (showing TAS). And a GROUNDSPEEDmeter (showing groundspeed) We also have an anemometer on the gound showing the speed of the air flowing over the ground.

The aeroplane is standing still on the runway facing into a 20 knot headwind.

The Airspeed indicator and TASmeter will read 20 knots.
The GROUNDSPEEDmeter will read zero.
The anemometer on the ground will read 20 knots.

Note that because of the 20 knot headwind, our airspeed is 20 knots higher than our groundspeed.

Now we accelerate to 180 kts relative to the ground.

The GROUNDSPEEDmeter will read 180 knots because that is our speed relative to the ground.

The anemometer on the ground will still read 20 knots because the wind has not changed.

The Airspeed indicator and the TASmeter will read 200 knots because that is our speed relative to the air. (We have 180 knots groundpseed + 20 knots headwind = 200 knots airspeed).

The key to all of this is the fact that the Airspeed Indicator and the TAS meter are measuring speed relative to the air. But the GROUNDSPEEDmeter is measuring the speed of the aeroplane relative to the ground. And the ground-based anemometer are measuring speed of the air relative to the ground.

[Mohit_C]

Ok I undestand this quite well now. Now suppose we're on ground and we have a 20 knot tailwind and stationary. I'll try and repeat exactly what you did, just correct me where I'm wrong:
The TASmeter and Airspeed Indicator would show zero as there is no -20 knot value, the GROUNDSPEEDmeter would also read zero and the anemometer on the ground would read 20 knots.
If we accelerate to 180 knots relative to the ground, the TASmeter and Airspeed Indicator would read 160 knots, and the GROUNDSPEEDmeter would read 180 knots. The anemometer on the ground would remain unchanged as we suppose that the wind hasn't changed.

[Keith.Williams.]

Taking your comments in turn.

"Now suppose we're on ground and we have a 20 knot tailwind and stationary.

The TASmeter and Airspeed Indicator would show zero as there is no -20 knot value"

The speed of the air flowing over the aircraft is now 20 knots from tail to nose. If the Airspeed Indicator and TASmeter were able to register this they would show -20 knots. They may not be able to show negative values, but this is what the airframe is actually experiencing. So as far as the airfame is concerned it is moving through the air at -20 knots (ie 20 knots backwards).


"the GROUNDSPEEDmeter would also read zero"

Yes this is true because the aircraft is not moving over the ground.

"and the anemometer on the ground would read 20 knots".

This would be true if it could sense only the speed of the air. But if it could also sense the direction, and it could indiacte negative values it would read minus 20 because the wind has reversed direction compared to our initial situation.

"If we accelerate to 180 knots relative to the ground, the TASmeter and Airspeed Indicator would read 160 knots"

This is correct. They would have gone from -20 knots to +160 knots reflecting an acceleration of 180 knots.

"and the GROUNDSPEEDmeter would read 180 knots.

This is also correct.

"The anemometer on the ground would remain unchanged as we suppose that the wind hasn't changed".

Yes it would still be reading -20 knots if it had been able to do so.

The important point to note is that in terms of lift and drag the aircraft responds to its speed relative to the air.

If we assume that lift-off speed is 170 knots, then with

20 knot headwind lift-off will be at 170 knot airspeed and 150 kt groundspeed.
20 knot tailwind lift-off will be at 170 knots airspeed and 190 kt groundspeed.

The real acceleration required is relative to the ground, so comparing the headwind and tailwind cases shows that we need more acceleration in a tailwind. This increases the length of the take-off run required to reach lift-off speed.

[Mohit_C]

Thanks a lot Keith. Williams, it has helped me a lot.

[Mohit_C]

Another question. Throughtout all this time, I have been taught that the TAS has corrected the instrument, position, compressibility and density errors and I do understand them except for the density error. Why exactly does a lower density (higher density altitude) cause the TAS to be higher than the IAS/EAS?

I know that the instrument error is that of precision and defects from factory, position error is that atmospheric pressure intakes can have turbulance and compressibility error which occurs at usually above 250 knots and basically takes readings of the compressed air pressure rather than total pressure (which makes the readings increase).

[Alex Whittingham]

The IAS is an indication of the number and energy of molecules hitting the front of the pitot tube and therefore, by implication, the wing. If the air density is less, ie less molecules in a given volume, then the same IAS can only be achieved by flying faster, a higher True Air Speed.

[Keith.Williams.]

An Airspeed Indicator is simply a differential pressure gauge with its dial marked off in knots instead of PSI.

It gives an airspeed indication that is determined by the dynamic presssure that it is sensing.

To do this it captures Total Pressure (which is static pressure plus dynamc pressure) then subtracts static pressure from it. In this way it isolates the dynamic pressure inside its capsule.

Any change in dynamic pressure will expand or contract the capsule and it is this movement which determines the position the needle on the dial.

This means that every time it senses a given dynamic pressure it will give the same airspeed indication.

Dynamic pressure = 1/2 Rho Vsquared where Rho is air denisty and V is TAS.
If we ignore the niceties of instrument error and pressure sensing error then Airspeed Indicators are calibrated so that IAS = TAS at ISA mean sea level.

Now lets imagine that we climb at constant indicated airspeed. This means that we will be climbing at constant dynamic pressure.

But as we climb, the air density, Rho decreases, so we need more Vsquared to give the same dynamic pressure.

At 40000 ft for example, Rho is about 1/4 of its sea level value. So Vsquared must be 4 times its sea level value if dynamic pressure is to be constant.

This means that at 40000 ft the TASsquared is 4 times the IAS. Which means that TAS is twice the IAS.

If we try putting different values for Rho into the equation we can see what must happen to V to keep Dynamic Pressure constant.

For example

Dynamic Pressure = 1/2RhoVsquared

let's start with Rho = 1 and V = 20

1/2 x 1 x 20squared = 200.

So we have 200 units of dynamic pressure.

If density reduces to 1/2 then to get the same 200 units of dynamic pressure we need

1/2 x 1/2 x 28.28squared = 200.

If density decreases to 1/4 then to get the same dynamic pressure we need

1/2 x 1/4 x 40squared = 200