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# ATPL theory questions

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# ATPL theory questions

21st Jun 2016, 10:21

Join Date: Apr 2004
Location: London
Posts: 493
Originally Posted by latecoere240
TAS : CAS +/- density error +/- compressibility error or EAS +/- density error
TAS cannot be obtained from CAS or EAS by using independent linear terms.

Originally Posted by oggers
... the proper formula I gave ...
Originally Posted by oggers
The formula is: TAS = CAS/√Relative Density ...
This formula requires a loss of distinction between EAS and CAS which may be an acceptable assumption at low speed and low altitude. As either speed or altitude increase above some threshold - see Wittingham's post here on 2016/06/03 - the assumption leads to an increasingly erroneous TAS value and it is the difference from the correct value which is referred to as, rightly or wrongly, a "compressibility error." Latecoere240 therefore probably wanted to write your formula but with the addition of a corrective term. That is, I suspect the intention was TAS = CAS/sqrt(sigma) + compressibility_error; sigma being the ratio of ambient air density to the ISA MSL value as you've given.

It is incorrect to state TAS = EAS/sqrt(sigma) + compressibility_error; EAS might as well be defined by the expression TAS * sqrt(sigma), i.e. no corrective term is needed when mapping EAS to TAS or vice versa.

It's crucial to note that this compressibility_error is a function of CAS and altitude and that the effort needed to compute TAS this way is not less than working it out directly in the first instance. The function needed to do this mapping which is mechanically effected in modern airspeed indicators, and has been since the Tiger Moth, may be found in William Aiken Jr's NACA report 837 (1946), Standard nomenclature for airspeeds with tables and charts for use in calculation of airspeed, available from the Aerade Reports Archive hosted by Cranfield University here: NACA UK Mirror report description page. Eqn 3 can be rearranged to set TAS (V) or CAS (V_c) as the subject as required.
21st Jun 2016, 11:38

Join Date: Apr 2004
Location: London
Posts: 493
Originally Posted by keith williams
... movement in any direction over the surface of the Earth will cause coriolis effect.
There are some excellent articles by the Swedish meteorologist Anders Persson on the effect. See here for a short list.
21st Jun 2016, 16:15

Join Date: Mar 2015
Location: London
Posts: 94
Originally Posted by paco
A good suggestion - I will schedule that in for the next update.
Thanks Paco, sounds good!
23rd Jun 2016, 14:18

Join Date: Sep 2013
Location: N of GIRLI
Posts: 37
Can anyone tell me how many total marks there are available for the UK ATPL Mass and Balance exam? I'm aware there are 25 questions but I assume there will be some worth more marks, just can't seem to find the totals listed anywhere.
2nd Jul 2016, 10:38

Join Date: Sep 2015
Location: with my parents
Posts: 48
Q. Why does doppler VOR have less site error compared to CVOR?
2nd Jul 2016, 20:08

Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 67
Posts: 3,748
Because the mountains (or similar fixed hard objects) don't affect the propagation. Instead of going round in a circle, the signal moves back and forth. The receiver can't tell the difference, though.

Phil
3rd Jul 2016, 09:39

Join Date: Sep 2015
Location: with my parents
Posts: 48
thanks for the input. But can you please elaborate what you just said?
3rd Jul 2016, 16:59

Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 67
Posts: 3,748
OK:

Using Doppler allows the frequency of a signal to decrease when the distance between the beacon and aircraft increases, and vice versa. The Doppler shift makes the transmitter look as if it is advancing and retreating 30 times a second. That is, the aircraft sees a varying frequency rather than varying power.

As mountains don't move, they don't affect the reception of the signals and the VOR doesn't suffer from propagation errors. Do you need an explanation of Doppler?

phil
4th Jul 2016, 03:01

Join Date: Sep 2015
Location: with my parents
Posts: 48
are you implying that just because its vari phase signal is FM and that of CVOR is AM, the site error is less. And if yes is that the only reason?
4th Jul 2016, 05:24

Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 67
Posts: 3,748
No. As mentioned above the Dopper principle removes the site errors because anything that does not move has no effect on the signal propagation. From a "normal" VOR the aircraft sees a change of power even though FM is involved. With a doppler VOR it sees a change in frequency instead.

phil
6th Jul 2016, 13:20

Join Date: Sep 2015
Location: with my parents
Posts: 48
I read somewhere that site error in doppler VOR is less because of its bigger aperture compared to CVOR. By aperture they meant the diameter of the circle in which the antennas of the doppler are arranged. Does that make any sense to you? I dont really know what tht means.
6th Jul 2016, 15:21

Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 67
Posts: 3,748
You mean wide-aperture? This averages out the local distortions which would normally be much more noticeable with the more narrowly focussed CVOR antenna (at about half a wavelength as opposed to about 5), so a theoretical tenfold reduction in site errors is possible (something to do with space diversity).

The Doppler shift comes from the relative motion of the antenna and the receiver. It is used because the wide aperture system needs Doppler to work properly, in that it creates the direction-dependent FM signal. The frequency change observed as the antenna rotates towards or way from an observer on the same circle is in proportion to the azimuth.

Phil

Last edited by paco; 6th Jul 2016 at 16:22.
6th Jul 2016, 15:55

Join Date: May 1999
Location: Bristol, England
Age: 60
Posts: 1,472
all you could possibly need on youtube
7th Jul 2016, 10:19

Join Date: Sep 2015
Location: with my parents
Posts: 48
thanks for the input alex, so thats how bigger aperture leads to less site error. Nicely explained. But I believe there's also another phenomenon here acting to reduce the site error called "Capture effect". This refers to the capability of aircraft receiver to demodulate only the strongest of the variable phase VHF signals from doppler VOR, ignoring the reflected but weaker variable phase signals. As opposed to the CVOR wherein the average of the direct and the reflected weaker signal is taken into account to derive a wrongful "average" bearing. Would like your comments on this.
7th Jul 2016, 11:33

Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 67
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The capture effect is an FM phenomenon and partly the reason for the system being designed that way. AM allows multiple signals to be broadcast on the same channel.

Phil
7th Jul 2016, 13:57

Join Date: Sep 2015
Location: with my parents
Posts: 48
so If am not mistaken, there's two effects now "capture effect" and "Bigger aperture" that is leading to lesser site error with DVOR compared to CVOR. Is that correct?
7th Jul 2016, 14:47

Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 67
Posts: 3,748
Sort of - as I see it, the site errors are minimised with the wide aperture, the Doppler effect makes that work and the capture effect just makes for a stronger signal. Simples!

Phil
7th Jul 2016, 17:20

Join Date: Jun 2012
Location: -
Posts: 1,176
The aircraft maintains the magnetic heading of 060 degrees. The declination is 8°W, the drift angle is +4°. What is the true track heading of the aircraft?

a. 056 degrees
b. 048 degrees
c. 064 degrees
d. 072 degrees

It's not ATPL level, but I am not sure 072 degrees is actually the correct answer... Shouldn't it be 048 degrees?
7th Jul 2016, 20:34

Join Date: May 1999
Location: Bristol, England
Age: 60
Posts: 1,472
Declination? Do they mean variation? And why is drift shown as +4, not 4 left or right? How confusing can you get?

If variation is 8W and drift is 4 port, yes, true heading is 052, true track is 048. Who wrote this question?
7th Jul 2016, 21:47