Learning to fly, or thinking of learning? Post your questions, comments and experiences here

Moderator: AndyR

User avatar
By David Wood
FLYER Club Member  FLYER Club Member
#1833618
The Localiser (Loc) transmits a signal from the far end of the runway in a narrow fan shape down the runway and up the approach, facing aircraft approaching to land. It is arranged such that the Course Deviation Indicator (CDI) in the cockpit gives a left/right indication depending how far off the centerline the aircraft is as it approaches to land.

What your text says is that a full-scale deflection of the CDI would be achieved if the aircraft was in line with the edge of the runway (ie, off the centerline) 215m from the threshold. Obviously the width of the fan widens with distance.

In practice, the pilot is obliged to go-around if the CDI indicates more than a half-scale deflection.
By A4 Pacific
#1833644
I have to ask because I genuinely don’t know. Does that mean that on shorter runways the localiser beam for full deflection is wider at distance from the field? On longer runways the beam will be narrower at the same distance? For example LCY is about 1200m, LHR about 4000m.

Because that’s not as I understood it? Is the position of the loc antenna adjusted to provide the same pattern on all runways? It’s something I’ve never really thought of.

The difference could be quite considerable couldn’t it?
User avatar
By AndyR
FLYER Club Member  FLYER Club Member
#1833708
A4 Pacific wrote:I have to ask because I genuinely don’t know. Does that mean that on shorter runways the localiser beam for full deflection is wider at distance from the field? On longer runways the beam will be narrower at the same distance? For example LCY is about 1200m, LHR about 4000m.

Because that’s not as I understood it? Is the position of the loc antenna adjusted to provide the same pattern on all runways? It’s something I’ve never really thought of.

The difference could be quite considerable couldn’t it?



That is correct.

The longer runways will in fact have a more 'sensitive' localiser on intercept for the same distance from the threshold as the beam width is narrower. There are limits, but when the localiser is checked, this is one of the tolerances it must meet, the width being co-dependent on the runway length.

Or put more pictorially, approaching from 90 degrees on to the localiser, it requires a steeper turn to get on to a narrow width LOC than a wider width LOC from when the needle first moves. I know the intercept is normally around 30 degrees, but in ILS calibration flying it is approached from 90 degrees and you quickly learn that some LOCs are rather narrower than others :D
User avatar
By lobstaboy
#1833799
AndyR wrote:
The longer runways will in fact have a more 'sensitive' localiser on intercept for the same distance from the threshold as the beam width is narrower. There are limits, but when the localiser is checked, this is one of the tolerances it must meet, the width being co-dependent on the runway length.



OK, but why? Why do it like that?
Surely it's more simple for the folk flying the localiser if they are all the same?
The definition of the required width is purely arbitrary and it could be different. Radio waves don't know how wide the runway is.
I don't get it...
User avatar
By David Wood
FLYER Club Member  FLYER Club Member
#1833809
I suspect (but this is only a guess) that it's to do with the 1/2 scale deflection limit insofar as if at minimums (usually 200') you were at the limit of a legal approach (ie, at a half-scale deflection of the CDI) and looked up to visually acquire the runway you then would be in a position in which, albeit off the center-line, you would not need to make significant heading changes in order to make a safe landing. That's my guess.
User avatar
By lobstaboy
#1833839
David Wood wrote:I suspect (but this is only a guess) that it's to do with the 1/2 scale deflection limit insofar as if at minimums (usually 200') you were at the limit of a legal approach (ie, at a half-scale deflection of the CDI) and looked up to visually acquire the runway you then would be in a position in which, albeit off the center-line, you would not need to make significant heading changes in order to make a safe landing. That's my guess.


OK thanks - that makes sense.
So what matters is the bit close to the runway, not how wide the localiser is miles away.