Radio Altimeter
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Radio Altimeter
Came across this question in ATPL studies:
The radio altimeter receives information from:
a) The gound within an ellipse of 30 degrees in the for and aft axis and 60 degrees in the athwartships axis beneath the aircraft
b) The ground within a 30 degree cone directly beneath the aircraft
c) The ground directly beneath the aircraft
d) The ground below the aircraft within a narrow cone angled 70 degrees down from the horizontal
The poor spelling and grammar are not mine, this is exactly how it was written
Which is the least wrong answer and why?
The radio altimeter receives information from:
a) The gound within an ellipse of 30 degrees in the for and aft axis and 60 degrees in the athwartships axis beneath the aircraft
b) The ground within a 30 degree cone directly beneath the aircraft
c) The ground directly beneath the aircraft
d) The ground below the aircraft within a narrow cone angled 70 degrees down from the horizontal
The poor spelling and grammar are not mine, this is exactly how it was written
Which is the least wrong answer and why?
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C) sounds good but sure ain't "correct" since it is to short and doesn't have the cone-part in it.
D) has a "narrow cone" which sounds good to me, but the beam would be facing too far forward.
and since A) is total crap i would go for B)
D) has a "narrow cone" which sounds good to me, but the beam would be facing too far forward.
and since A) is total crap i would go for B)
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Thanks,
I answered c) and was told that was wrong and that a) was the correct answer because the radio altimeter transmits an eliptical pattern of +/- 30 degrees in pitch AND +/- 60 degrees in roll.
Now I may only be a novice at this but they seem to have got their axes confused.
I answered c) and was told that was wrong and that a) was the correct answer because the radio altimeter transmits an eliptical pattern of +/- 30 degrees in pitch AND +/- 60 degrees in roll.
Now I may only be a novice at this but they seem to have got their axes confused.
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I worked on it seeming to be the least wrong.
a) had the axes mixed up,
b) could be right
c) the RA receives information from the ground correct as far as it went
d) just nonsense.
This left b) and c) as the only two that could be right and as I was not sure of the shape of the cone, decided to plump for the least technical.
Anyway, they told me that answer a) was the correct one - which makes me wonder whether I am mad or they are mad.
a) had the axes mixed up,
b) could be right
c) the RA receives information from the ground correct as far as it went
d) just nonsense.
This left b) and c) as the only two that could be right and as I was not sure of the shape of the cone, decided to plump for the least technical.
Anyway, they told me that answer a) was the correct one - which makes me wonder whether I am mad or they are mad.
Thanks,
I answered c) and was told that was wrong and that a) was the correct answer because the radio altimeter transmits an eliptical pattern of +/- 30 degrees in pitch AND +/- 60 degrees in roll.
Now I may only be a novice at this but they seem to have got their axes confused.
I answered c) and was told that was wrong and that a) was the correct answer because the radio altimeter transmits an eliptical pattern of +/- 30 degrees in pitch AND +/- 60 degrees in roll.
Now I may only be a novice at this but they seem to have got their axes confused.
30 degrees for and aft, and 60 degrees "athwartships", translates correctly to 30 degrees in pitch and 60 degrees in roll. Or were you of the opinion that it should be 60 degrees in pitch and 30 in roll? That wouldn't make a lot of sense.
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Considering the information given by the beasty, height above ground coupled with the fact that the aircraft can have all sorts of pitch and bank angles, a 60 degree "cone" in bank and 30 degree "cone" in pitch would fit the bill nicely.
PM
(Standard jet departure with 18 degrees pitch up and a 30 degree banked turn starting at maybe 500' - the RadAlt must be able to cope with that!)
PM
(Standard jet departure with 18 degrees pitch up and a 30 degree banked turn starting at maybe 500' - the RadAlt must be able to cope with that!)
In this sense, fore and aft is equivalent to pitch and athwartships to roll.
Note that athwartships is not an axis at all but a direction of orientation (across the ship) so 60 degrees athwartships is 60 degrees left and right, i.e., around the roll axis.
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It is poorly worded then and not very helpful is helping with the understanding.
I guess the trick is to assume anything worded like this is the right answer on the bluff, double bluff basis.
Glad I dont need this cr@p for a living.
I guess the trick is to assume anything worded like this is the right answer on the bluff, double bluff basis.
Glad I dont need this cr@p for a living.
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Maybe I'll have to get myself seriously updated on how a present-day Rad Alt works.
The 30°/60° tx/rx cone makes some sense, but I would still have thought the RA would "lock on" to the first return, which would be from straight down below the aircraft (unless you were 200 ft off the Rock of Gibraltar).
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Edit
The venom seems to be in the head, rather than the tail:
"...receives information from..."
Considering the typical RA aerial pattern, this is probably true.
It then should be capable (and presumably is), to process this information sufficiently to display the height ABOVE GROUND of the aircraft, even if the aircraft is not horizontal, either in pitch or roll.
Anybody has a straightforward answer? Or do I have to go and dive into ancient literature, or (heaven forbid) Google?
The 30°/60° tx/rx cone makes some sense, but I would still have thought the RA would "lock on" to the first return, which would be from straight down below the aircraft (unless you were 200 ft off the Rock of Gibraltar).
"Old Boffin"
****
Edit
The radio altimeter receives information from:
a) The ground within an ellipse of 30 degrees in the for and aft axis and 60 degrees in the athwartships axis beneath the aircraft
a) The ground within an ellipse of 30 degrees in the for and aft axis and 60 degrees in the athwartships axis beneath the aircraft
"...receives information from..."
Considering the typical RA aerial pattern, this is probably true.
It then should be capable (and presumably is), to process this information sufficiently to display the height ABOVE GROUND of the aircraft, even if the aircraft is not horizontal, either in pitch or roll.
Anybody has a straightforward answer? Or do I have to go and dive into ancient literature, or (heaven forbid) Google?
Last edited by ChristiaanJ; 28th Aug 2006 at 17:48. Reason: Remark added
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For heaven's sake, what's the problem? 25º to 30º copes with all manoeuvres likely in a passenger jet below 2500ft. You're unlikely to put on more than 15º of bank and if you pitched more than 20º it would be unusual. if you have a 30º coverage it just means the radalt won't unlock if you put 30º of bank on, were you planning on 60º? What is not straight forward?
Google wouldn't hurt. At the very least, if you are going to ask a question listen to the answer.
Google wouldn't hurt. At the very least, if you are going to ask a question listen to the answer.
Last edited by Alex Whittingham; 28th Aug 2006 at 21:37.
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Alex,
Sorry, I'm just an ancient, have not delved into the functioning of a rad alt for 30 years.
All I was trying to say that in a 30° bank I'd expect the RA output (i.e., the indicator) still to show true height above ground, not slant range (2 / sqrt 3 (1.15) times the true altitude).
Sorry, I'm just an ancient, have not delved into the functioning of a rad alt for 30 years.
All I was trying to say that in a 30° bank I'd expect the RA output (i.e., the indicator) still to show true height above ground, not slant range (2 / sqrt 3 (1.15) times the true altitude).
For heaven's sake, what's the problem? 25º to 30º copes with all manoeuvres likely in a passenger jet below 2500ft. You're unlikely to put on more than 15º of bank and if you pitched more than 20º it would be unusual. if you have a 30º coverage it just means the radalt won't unlock if you put 30º of bank on, were you planning on 60º? What is not straight forward?
Google wouldn't hurt. At the very least, if you are going to ask a question listen to the answer.
Google wouldn't hurt. At the very least, if you are going to ask a question listen to the answer.
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Sorry, Christiaan, I was getting a bit tetchy.
This is an old UK CAA exam question, pre-1999, in the Radio Aids subject. There is some evidence that it was added to the JAA Central Question Bank early on and placed in the 022 Instrumentation section but I haven't seen it for years so, if it did get passed across, it seems to have been ditched fairly sharply. Personally, I think it was never transferred to the JAA Question Bank, it certainly isn't in the infamous 'Italian Feedback' which dates to around 2000.
We explored the answers to this with the UK CAA examiners 6+ years ago. They confirmed the Airbus view that civilian radio altimeters had a transmission pattern of 25º to 30º. Their standard textbook for the subject, Radio Aids by Underdown, was no help.
This question persists in 'school feedback' around Europe. The answers marked correct reflect the opinion of the instructor, not the examiner. I'm guessing that whoever told chrisbl that (a) was correct was taking their answer from military references.
This is an old UK CAA exam question, pre-1999, in the Radio Aids subject. There is some evidence that it was added to the JAA Central Question Bank early on and placed in the 022 Instrumentation section but I haven't seen it for years so, if it did get passed across, it seems to have been ditched fairly sharply. Personally, I think it was never transferred to the JAA Question Bank, it certainly isn't in the infamous 'Italian Feedback' which dates to around 2000.
We explored the answers to this with the UK CAA examiners 6+ years ago. They confirmed the Airbus view that civilian radio altimeters had a transmission pattern of 25º to 30º. Their standard textbook for the subject, Radio Aids by Underdown, was no help.
This question persists in 'school feedback' around Europe. The answers marked correct reflect the opinion of the instructor, not the examiner. I'm guessing that whoever told chrisbl that (a) was correct was taking their answer from military references.
Last edited by Alex Whittingham; 29th Aug 2006 at 09:11.