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?

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)

Answer (b) for the exams, the manufacturers say a 25º to 30º cone. (a) is more suited to military aircraft.

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.

Who told you (c) was correct?

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.

What makes you think the axis are 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.

The fore and aft axis is the roll axis so 30 degrees in fore and aft axis I would have thought was 30 degrees in the roll axis

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!)

It is poorly worded, but I think that by "fore and aft axis" they mean 30 degrees fore and aft of a perpendicular line dropped down from the aircraft (i.e., aligned with the fore and aft axis not rotated around it.) 60 degrees athwartships would mean 60 degrees left and right of the same perpendicular line.

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.

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.

Its an old question, I'm fairly sure its not in the current database. Maybe that's why they took it out.

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).
"Old Boffin"
****
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?

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.

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).

Thanks Alex,

I appreciate your responses. Hopefully it is a question not in the databank anymore. I am always suspicious when words like athwartships is used.

Did you miss the bit where he was told that the correct answer (as defined by the examiner) was A? Regardless of what the real correct answer is, surely there is some merit in discussing the examiner's correct answer?

Piltdown Man

Why do you need the Rad Alt for a departure?

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.

Alex,

You may be right. Anyway, thanks for your time.

chrisbl

Cos that's when the RadAlt says it's above 1,000', TCAS will give RA's. The EGPWS is also rather interested in your absolute height, where ever you are.

But to change tack, if Alex says that the answer is x, then the answer is x. This man knows more than I ever will and knows the answers to the campaign's questions, so please ignore my previous post.

PM

Just ran across this old one. The radio altimeter "sees" the ground in the cone described in a). It indicates the distance to the ground per c), in other words, the closest point. Which altitude is important to you?

GB