ATPL theory questions
Joined: May 2017
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From: Bahrain
Thanks , it was so useful
umm another question
When an aircraft employs static vents either side of the fuselage to minimize pressure errors it is
called:?
A.Static equalization
B. Pitot correction
C. Static balancing
D. Dynamic balancing
actually this is the first time hearing about static equalization
umm another question
When an aircraft employs static vents either side of the fuselage to minimize pressure errors it is
called:?
A.Static equalization
B. Pitot correction
C. Static balancing
D. Dynamic balancing
actually this is the first time hearing about static equalization
Joined: Dec 2015
Posts: 507
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From: France
Could anyone explain the difference in the usefulness of geocentric and geodetic (or geographic) latitude ?
I have remembered the 11.6 minutes maximum difference due to earth being oblate, and the difference in the definition, but I really don't see the point of geodetic latitude.
I have remembered the 11.6 minutes maximum difference due to earth being oblate, and the difference in the definition, but I really don't see the point of geodetic latitude.
Joined: Nov 2000
Posts: 4,328
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From: White Waltham, Prestwick & Calgary
It's because the earth isn't round - if it was, we would just use geocentric. Instead we have to draw a tangent at the horizon, then a line 90 degs to that, and the angle where it meets the horizontal at the centre of the Earth is the geodetic. The difference for our purposes is negligible, and if the earth were the size of a billiard ball you wouldn't notice any difference between it and the others on the table.
I don't see much point to it (for the exams at least), except maybe to say that the mathematical model inside the GPS (based on the World Geodetic Standard established in 1984) is different enough for you to be careful if your track takes you near to the boundaries of airspace or danger areas.
I don't see much point to it (for the exams at least), except maybe to say that the mathematical model inside the GPS (based on the World Geodetic Standard established in 1984) is different enough for you to be careful if your track takes you near to the boundaries of airspace or danger areas.
Joined: Apr 2004
Posts: 734
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From: London, GB
To amplify Paco's response, the orthogonality of geodetic coordinate axes simplifies the Euclidean norm. Using an orthogonal system is equivalent to setting the angle(s), between principal axes, in the law of cosines to 90 degrees. That is, coefficient F in the metric tensor is zero when geodetic latitude is used instead of geocentric latitude, see equation 34 et seq in Thomas, P.D. (1952). Conformal Projections in Geodesy and Cartography. Special Publication No 251. US Department of Commerce.
A further disadvantage of using skew coordinates (i.e. geocentric latitude) is that each point on the height axis has a different latitude, except at the equator and poles.
A further disadvantage of using skew coordinates (i.e. geocentric latitude) is that each point on the height axis has a different latitude, except at the equator and poles.
Joined: Dec 2015
Posts: 507
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From: France
To clarify :
I'm assuming the geodetic coordinate axes at point P on the surface of the earth are :
- The perpendicular to the plane of tangency at P (up or down)
- On the plane of tangency a north vector
- A third vector determined with right hand rule
Whereas the geocentric axes at the same point would be
- A "vertical" on a line from center of the earth to P (up or down)
- A North vector
- An East or West vector
At this point I'm wondering why this second system of axis has a complicated Euclidean norm.
Maybe the North vector remained the same as in the first system of axes, then it means that the system of geocentric axes is not orthogonal (there is a small residual angle)
Is it correct up to this point ?
If yes, the other option would be to modify the North vector, to make it perpendicular to the line from the center of the earth. Then the North vector would go either into or out of Earth, and this is obviously a problem because it means that if you're travelling north or south at constant altitude then you actually have a vertical speed in this system of axes...
Regarding your last line : Up to reading your message I thought that GPS coordinates were actually the coordinates of the point, on the surface, of which we're vertical. But it's quite evident that it wouldn't work that way.
I'm assuming the geodetic coordinate axes at point P on the surface of the earth are :
- The perpendicular to the plane of tangency at P (up or down)
- On the plane of tangency a north vector
- A third vector determined with right hand rule
Whereas the geocentric axes at the same point would be
- A "vertical" on a line from center of the earth to P (up or down)
- A North vector
- An East or West vector
At this point I'm wondering why this second system of axis has a complicated Euclidean norm.
Maybe the North vector remained the same as in the first system of axes, then it means that the system of geocentric axes is not orthogonal (there is a small residual angle)
Is it correct up to this point ?
If yes, the other option would be to modify the North vector, to make it perpendicular to the line from the center of the earth. Then the North vector would go either into or out of Earth, and this is obviously a problem because it means that if you're travelling north or south at constant altitude then you actually have a vertical speed in this system of axes...
Regarding your last line : Up to reading your message I thought that GPS coordinates were actually the coordinates of the point, on the surface, of which we're vertical. But it's quite evident that it wouldn't work that way.
Joined: Apr 2004
Posts: 734
Likes: 10
From: London, GB
I'm assuming the geodetic coordinate axes at point P on the surface of the earth are ...
Whereas the geocentric axes at the same point would be
- A "vertical" on a line from center of the earth to P (up or down) ...
- A "vertical" on a line from center of the earth to P (up or down) ...
At this point I'm wondering why this second system of axis has a complicated Euclidean norm.
... I thought that GPS coordinates were actually the coordinates of the point, on the surface, of which we're vertical.
Joined: Dec 2015
Posts: 507
Likes: 1
From: France
Selfin, you seem very knowledgeable 
Another question or two about gen nav :
Why would twilight be shorter at the equator ? It seems logical to me that in winter, twilight should be longer and shorter in summer : earth's inclination about the ecliptic gives a "pseudo latitude" that's higher in winter and less in summer, providing higher insolation.
It's just as the ecliptic was a fictitious equator.
Does it work like that ?
A question in the database asked the date of the perihelion.
Why would this be of any importance when talking about gen nav ? The same course that states that perihelion/aphelion are of no importance to seasons and that their two values are very close (earth's orbit has very little excentricity)
Thanks
Another question or two about gen nav :
Why would twilight be shorter at the equator ? It seems logical to me that in winter, twilight should be longer and shorter in summer : earth's inclination about the ecliptic gives a "pseudo latitude" that's higher in winter and less in summer, providing higher insolation.
It's just as the ecliptic was a fictitious equator.
Does it work like that ?
A question in the database asked the date of the perihelion.
Why would this be of any importance when talking about gen nav ? The same course that states that perihelion/aphelion are of no importance to seasons and that their two values are very close (earth's orbit has very little excentricity)
Thanks
Joined: Apr 2015
Posts: 1
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From: netherlands
Polish ATPL PERFORMANCE
Does anybody know where I can find graphics that fit best with the "new" graphic questions in the Polish ATPL performance exam? I use aviation exam and mainly the SEP graphics match with the real test questions but the rest not. Does anybody know a better matching question bank? Thanks a lot!
Joined: Apr 2004
Posts: 734
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From: London, GB
Why would twilight be shorter at the equator ? ... It seems logical to me that in winter, twilight should be longer and shorter in summer
A similar derivation is in WM Smart's Textbook on Spherical Astronomy (art 33, p 51). Corrections for altitude are available in the UK Air Almanac, p 68 of 2017 edition; see also fig. 3 on p 69 for twilight duration in the Arctic depending on latitude and time of year. Despite the restricted latitudes in those figures the duration of twilight, on the condition of a sunrise existing (viz., upper limb touches the horizon), is shortest near the equinoxes.
Why would [the perihelion] be of any importance when talking about gen nav?
you seem very knowledgeable
Joined: Dec 2015
Posts: 507
Likes: 1
From: France
Can anybody explain this ?
https://i.gyazo.com/735e4490a9888a16...ddc5296a5f.png
I'm doubting this is the correct answer.. I don't see why it would be desirable to spiral towards the NDB
https://i.gyazo.com/735e4490a9888a16...ddc5296a5f.png
I'm doubting this is the correct answer.. I don't see why it would be desirable to spiral towards the NDB
Joined: Jan 2011
Posts: 660
Likes: 20
From: England
The following two questions which are/were in the CQB require students to distinguish between HOMING and TRACKING. I have no idea where they got these definitions from (probably a 1950's RAF navigation course).
Question 1.
Which statement is correct for tracking towards an NDB in an area with constant wind and constant Magnetic Variation?
Correct answer given by the database:
The Relative Bearing of the NDB should be equal (in magnitude and sign) to the experienced Drift Angle
Question 2.
Which statement is correct for homing towards an NDB in an area with constant wind and constant Magnetic Variation?
Correct answer given by the database:
The Relative Bearing of the NDB should be kept 000 deg;
These two questions have caused much head scratching among students in the past and will probably continue to do so in the future.
Question 1.
Which statement is correct for tracking towards an NDB in an area with constant wind and constant Magnetic Variation?
Correct answer given by the database:
The Relative Bearing of the NDB should be equal (in magnitude and sign) to the experienced Drift Angle
Question 2.
Which statement is correct for homing towards an NDB in an area with constant wind and constant Magnetic Variation?
Correct answer given by the database:
The Relative Bearing of the NDB should be kept 000 deg;
These two questions have caused much head scratching among students in the past and will probably continue to do so in the future.
Joined: Nov 2000
Posts: 4,328
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From: White Waltham, Prestwick & Calgary
Homing is adjusting for wind by keeping the needle on the 0. This results in a phenomenon called the curve of pursuit (otherwise called bird-dogging) which takes you away downwind from an airway centreline - it was a common problem for bomb aimers, and why tracking is best, where you offset for wind and fly a straighter course.
Joined: Apr 2004
Posts: 734
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From: London, GB
Paco,
Thanks for the heads up on the path being a pursuit curve. The derivation from first principles in terms of groundspeed coordinates was not going well!
The solution in airspeed coordinates appears to be given on the Wikipedia page for a radiodrome here, pushing the date back to at least the 1730s.
Thanks for the heads up on the path being a pursuit curve. The derivation from first principles in terms of groundspeed coordinates was not going well!
The solution in airspeed coordinates appears to be given on the Wikipedia page for a radiodrome here, pushing the date back to at least the 1730s.
Joined: Jan 2011
Posts: 660
Likes: 20
From: England
I have just read the reference and two things occur to me.
1. The reference describes an object which is chasing another object which is itself moving. This is not the case with the navigation beacons.
2. If the hare is running directly away from the dog (which sounds like what any sensible hare would do) or directly towards the dog (a mad march hare possibly), the dog's path will be a straight line. This is of course the equivalent of the beacon being directly upwind or downwind of the aircraft.
1. The reference describes an object which is chasing another object which is itself moving. This is not the case with the navigation beacons.
2. If the hare is running directly away from the dog (which sounds like what any sensible hare would do) or directly towards the dog (a mad march hare possibly), the dog's path will be a straight line. This is of course the equivalent of the beacon being directly upwind or downwind of the aircraft.
Joined: Apr 2004
Posts: 734
Likes: 10
From: London, GB
Keith,
In the airspeed coordinate system the beacon has a velocity vector opposite to the wind vector.
Is it not correct that when the beacon is finally reached the aircraft velocity vector is parallel to the wind vector, in the opposite direction?
In the airspeed coordinate system the beacon has a velocity vector opposite to the wind vector.
Is it not correct that when the beacon is finally reached the aircraft velocity vector is parallel to the wind vector, in the opposite direction?