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ATPL theory questions

Old 27th Sep 2012, 10:30
  #141 (permalink)  
 
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Hi Anders,

I edited my previous entry about ,,main cloud base'' being the same as cloud ceiling:

Ceiling is by definition more than SCT, but it turns out that main cloud base is the higher layer if there are multiple layers, and the higher one is thicker.

I pm you a thread from another forum about this very question.

Last edited by Da-20 monkey; 27th Sep 2012 at 10:49.
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Old 27th Sep 2012, 11:31
  #142 (permalink)  
 
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Anders,

my interpretation of the respective group is the forecast temperature for 1500H UTC is 18 C and the forecast temperature for 1800H UTC is 16 C.

ICAO Annex 3 states forecast temperatures, though not an element of a standard TAF, may be included in accordance with regional air navigation agreements. If they are given in a TAF it is recommended to indicate the maximum and minimum temperatures for the forecast period.

The format of the temperature group in your example, however, is not in line with ICAO Annex 3 nor the WMO Manual on Codes in their current versions. I take it the example is from a question bank and might be outdated.

If the temperatures in your example were indeed maximum and minimum temperatures at 1500H and 1800H this would have to be indicated today as "TX18/1500Z TN16/1800Z", with "TX" standing for maximum and "TN" standing for minimum temperature.
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Old 27th Sep 2012, 16:46
  #143 (permalink)  
 
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Ok, that makes sense. Thanks for the responses.
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Old 18th Oct 2012, 09:47
  #144 (permalink)  
 
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Gen Nav questions

Hi

there are a few questions that are confusing me right now with regards to gen nav.

these are departure and convergency. I never seem know which formula to use and when. I understand the 2 formulas. Ch long x sine mean lat and ch long cosine lat. can someone give an example of when I should use both of these.

Right now I'm looking at q' 5566 in the atpl online system.
Given: Position 'A' N60 W020, Position 'B' N60 W021, Position 'C' N59 W020. What are, respectively, the distances from A to B and from A to C?

Why do I use the departure one for this question and not the convergency?

This is probably a pretty stupid question but its totally confusing me.

Thanks in advance
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Old 18th Oct 2012, 10:30
  #145 (permalink)  
 
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Hi,

Use the departure formula for your example, because it is for calculating distance between two point on the same latitude.

You use the convergency formula for calculating change of direction of a great circle track. Not for calculating distance, unless perhaps the average latitude and the convergency is given, in which case you could calculate the change of longitude.

To put it simple, departure and convergency has little to do with each other. One is for distance, the other for degrees.
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Old 18th Oct 2012, 11:47
  #146 (permalink)  
 
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In simple terms if the question is mentioning the words distance or position it is highly likely to be a DEPARTURE related question, going east/west along a parallel of latitude.

So in your example,which funnily enough I worked through with a student yesterday. A to B is a departure question. So Dep = Ch Long (mins) x cos lat.
1 * 60 = 60 * cos 60 (.5) = 30 nm. However A to C is not departure as the positions are on the same meridian. It is testing do you know at 1 degree of Latitude is 60 nm so A to C = 60 nm.

Now if the question mentions the words like great circle, rhumb line, initial or final track highly likely to be a CONVERGENCY problem. I always teach my students tips similar to the ones mentioned.

Last edited by RichardH; 18th Oct 2012 at 12:06.
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Old 18th Oct 2012, 18:14
  #147 (permalink)  
 
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Pressure error question

TOTAL PRESSURE ERROR

"As an aircraft moves through the air, a static pressure disturbance is generated in the air, producing a static pressure field around the aircraft. At subsonic speeds, the flow perturbations due to the aircraft static pressure field are nearly isentropic and do not affect the total pressure. As long as the total pressure source is not located behind a propeller, in the wing wake, in a boundary layer, or in a region of localized supersonic flow, the pressure errors due to the position of the total pressure source are usually negligible. Normally, the total pressure source can be located to avoid total pressure error."

"As an aircraft moves through the air, a static pressure disturbance is generated in the air, producing a static pressure field around the aircraft."

What is this static pressure field around the aircraft i would of thought this was just the free air around the aircraft, the air which is not impacted by the aircraft which would be then dynamic pressure right?

It then goes on to say: "at subsonic speeds, the flow perturbations due to the aircraft static pressure field are nearly isentropic and do not affect the total pressure."

I understand that isentropic basically means the same all round though i can't grasp what this paragraph mean either can anyone simplify it?
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Old 18th Oct 2012, 20:11
  #148 (permalink)  
 
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Basically there is a thin layer around the aircraft in which it feels like static pressure - a good way to demonstrate is put your hand out of the window in a moving car and put it flat against the door, you will feel no dynamic pressure, only static. Hence, static ports are usually small holes on the side of the fuselage.

To measure the PITOT or toal pressure, the Pitot tube must stick out far enough so it it outside this static layer. Remember that PITOT (total) pressure is dynamic pressure PLUS static. For altitude read out you need static pressure, for airspeed you need dynamic, dynamic pressure is created by the movement through the air.

Last edited by RTN11; 18th Oct 2012 at 20:14.
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Old 18th Oct 2012, 20:12
  #149 (permalink)  
 
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"As an aircraft moves through the air, a static pressure disturbance is generated in the air, producing a static pressure field around the aircraft."
Some thoughts..
As you probably are familiar with the bernoulli effect (Pt=Ps+Pd), the distribution of dynamic and static pressure over a moving surface depends on many variables (aoa, airflow angle, camber, material, etc.) f.ex. at the stagnation point of a wing the dynamic pressure will be 0 and the static equal to Total! So the a/c form can interfere with the ''normal'' airflow and that will cause pressure disturbances, mostly increased static pressure because of the deceleration of the air molecules that come in contact with the airframe . At subsonic speeds the air is thought to be isentropic, meaning that it can adiabatically change state (Kinetic to Potential energy without heating loss and vice versa). At supersonic speeds and above (I think even transonic), the air can not longer be seen as an isentropic, incompressible fluid and that means it will produce an error that will affect Total pressure readings!

Hope I didn't mess it up!
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Old 19th Oct 2012, 09:30
  #150 (permalink)  
 
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Thanks Richard and Da-20 monkey that explains things better. as you can see by my user name I am an ex PTC student and I am now doing atpl's on my own for the time being. It is hard trying to figure out everything on your own

thanks again
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Old 19th Oct 2012, 19:58
  #151 (permalink)  
 
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... PITOT (total) pressure is dynamic pressure PLUS static. [F]or airspeed you need dynamic [] pressure [] created by the movement through the air.
Total pressure in a compressible medium is not dynamic + static pressures.

When air (M ≤ 1) is brought to rest isentropically at the head of the pitot probe, the total pressure will be given by a function such as (6) in the graphic on this Glenn Research Center page, Isentropic Flow Equations

The subject of (6) in the above is Ps/Pt (s=static; t=total) and will always take a value equal to or less than unity. Hence, Pt ≥ Ps or Pt - Ps ≥ 0. This latter quantity, the difference between total and static pressure, is often said to be dynamic pressure. It is in fact something else called impact pressure. If you look at (5) in the above reference, which is the definition of dynamic pressure (defined in this form by Euler and not Bernoulli), you can see the difference in the behaviour of these two functions.

Airspeed indicators map impact pressure (not dynamic pressure) to calibrated airspeed. The function which provides that mapping assumes an isentropic process, meaning as given by Jetpipe that the process leading to a total pressure is an adiabatic and reversible one, i.e. heat is neither added nor removed and no dissipative effects occur. Obviously if these conditions are not satisfied the airspeed value will be erroneous. For these reasons it is quite important that both the pitot probes and static ports are designed, and located, to ensure minimum dissipation and heat addition.
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Old 24th Oct 2012, 20:00
  #152 (permalink)  
 
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Hi Jetpipe



"As you probably are familiar with the bernoulli effect (Pt=Ps+Pd), the distribution of dynamic and static pressure over a moving surface depends on many variables (aoa, airflow angle, camber, material, etc.) f.ex. at the stagnation point of a wing the dynamic pressure will be 0 and the static equal to Total! So the a/c form can interfere with the ''normal'' airflow and that will cause pressure disturbances, mostly increased static pressure because of the deceleration of the air molecules that come in contact with the airframe . At subsonic speeds the air is thought to be isentropic, meaning that it can adiabatically change state (Kinetic to Potential energy without heating loss and vice versa). At supersonic speeds and above (I think even transonic), the air can not longer be seen as an isentropic, incompressible fluid and that means it will produce an error that will affect Total pressure readings!"

Why can the air no longer be seen as an incompressible fluid at supersonic speeds and how come it starts to lose heat at supersonic speed?
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Old 27th Oct 2012, 18:51
  #153 (permalink)  
 
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The air IS a compressible fluid but at low speeds the friction heating is almost negligible so the isentropic equations for incompressible fluids are quite so precise... Check this wiki-page about Total air temperature !

If you need a more academic answer, I think Selfin is the man to ask!
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Old 30th Oct 2012, 19:24
  #154 (permalink)  
 
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Selfin

PITOT (total) pressure is dynamic pressure PLUS static. [F]or airspeed you need dynamic [] pressure [] created by the movement through the air. Total pressure in a compressible medium is not dynamic + static pressures.

When air (M ≤ 1) is brought to rest isentropically at the head of the pitot probe, the total pressure will be given by a function such as (6) in the graphic on this Glenn Research Center page, Isentropic Flow Equations

The subject of (6) in the above is Ps/Pt (s=static; t=total) and will always take a value equal to or less than unity. Hence, Pt ≥ Ps or Pt - Ps ≥ 0. This latter quantity, the difference between total and static pressure, is often said to be dynamic pressure. It is in fact something else called impact pressure. If you look at (5) in the above reference, which is the definition of dynamic pressure (defined in this form by Euler and not Bernoulli), you can see the difference in the behaviour of these two functions.

Airspeed indicators map impact pressure (not dynamic pressure) to calibrated airspeed. The function which provides that mapping assumes an isentropic process, meaning as given by Jetpipe that the process leading to a total pressure is an adiabatic and reversible one, i.e. heat is neither added nor removed and no dissipative effects occur. Obviously if these conditions are not satisfied the airspeed value will be erroneous. For these reasons it is quite important that both the pitot probes and static ports are designed, and located, to ensure minimum dissipation and heat addition.

When air (M ≤ 1) is brought to rest isentropically at the head of the pitot probeWhat does isotropically mean?


For these reasons it is quite important that both the pitot probes and static ports are designed, and located, to ensure minimum dissipation and heat additionDoes the location affect heat addition because the pitot probe is located far out from the aircraft skin and the static probe is built into the aircraft?


Cheers

Last edited by akafrank07; 30th Oct 2012 at 19:25.
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Old 6th Nov 2012, 14:18
  #155 (permalink)  
 
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Hey guys, need some help again.

Question 1:
On a CVOR the phase difference between the AM and FM signals is 30. The VOR radial is?
a. 210
b. 030
c. 330
d. 150

Question 2:
For a conventional VOR a phase difference of 090 would be achieved by flying from the beacon.
a. West
b. North
c. East
d. South

I put down b. on Q1, which was incorrect, and c. on Q2 which was correct. According to the answers the correct answer for Q1 was c. 330. The only thing I can come up with is that its in the wording of the question, because they are asking for the difference between the AM and FM signals, and not the difference between the FM and AM signals. Could it be that simple, or am I missing something else?

Last edited by Anders S; 6th Nov 2012 at 14:22.
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Old 6th Nov 2012, 22:18
  #156 (permalink)  
 
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Did question one include the words TO or FROM?

The 30 degree phase difference will occur on the 030 radial FROM the VOR, which is also the 210 radial TO the VOR.
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Old 8th Nov 2012, 09:11
  #157 (permalink)  
 
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Nope, the question is basically copy/pasted.
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Old 8th Nov 2012, 10:01
  #158 (permalink)  
 
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In my copy of CQB15 the correct answer is marked at 030. Is it perhaps just a case of your reference material being wrong?
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Old 13th Nov 2012, 14:31
  #159 (permalink)  
 
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Quite possibly, it wouldn't be the first time.
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Old 14th Nov 2012, 11:44
  #160 (permalink)  
 
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gen nav question HELP!

Hi can someone please explain this question to me. I don't really understand how it isn't a track change of zero. Its following the same line of latitude but the longitude is changing. wouldn't this mean that its just following a straight line towards the east?

The following points are entered into an inertial navigation system (INS). WPT 1: 60N 30W WPT 2: 60N 20W WPT 3: 60N 10W The inertial navigation system is connected to the automatic pilot on route (1-2-3). The track change when passing WPT 2 will be approximately:

4 decrease
9 increase
zero
9 decrease

Some help here would be great thanks






Am I right in thinking that I need to find the conversion angle?
I.E. find the convergency and half it? that way I know that for every 5degrees it will decrease by that number?

Last edited by PTCstudent123; 14th Nov 2012 at 12:33.
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