Reverse sensing on glideslope

[bohpilot]

When capturing a glide slope above 3 degrees eg 9 degrees the standard text reads

The false glide slopes occur at odd multiples of the true glide-slope angle (typically 3°) (i.e., at 9° and 15°). At even multiples (6°, 12°), a centered glide-slope needle occurs, but this is because of a null signal; reverse sensing is present above and below glide slopes

http://content.answcdn.com/main/content/img/McGrawHill/Aviation/f0265-01.gif

This I understand, but when you look at the associated diagram it shows the lobes at 9 and 15 degrees as 90hz above and 150hz below, the same as 3 degrees. How then do you get reverse sensing ie a fly up when it should be fly down if the lobes are the same as at 3 degrees.

Should the picture show the lobe frequencies reversed for 9 and 15..??.

Thanks

[aditya104]

Please remember the following:

Glidepath for an Aircraft for the purposes of this explanation is 3°
When the aircraft is within any 90Hz beam, the instrument needle will drop(fly down) and vice versa for 150Hz.

So, when your aircraft is flying in 9°(or 15°) zone in 150Hz beam, your needle will indicate fly up, eventhough you need to fly down in order to regain 3° glideslope. In the 90Hz beam though, your needle will ask you to fly down, which seems true but that will not guide you down to 3° glideslope as you will soon enter 150Hz beam.

Hope that it is clear to you now. I am reading ILS into this depth for the first time, so the information I gave could be incorrect and I would be pleased if someone were to correct it in that case. :ok: Btw, which book are you studying this from?

[1jz]

i guess the text explains it all.. n reverse sensing is with the 6 and 12 degrees angles.. if u closely look at the diagram, the 150hz lobe lies above these two lines n 90hz lobe lies below them.. so if flying these angles, u are on 6/12 degree false path n u deviate slightly above them then, this will keep showing u to fly UP and similary if u go below its gonna show u to fly DOWN unlike the normal 3 degree or its odd multiples..

[bohpilot]

I can understand your logic, however also in the text it states that if you are on the 6 or 12 degree lines then this will cause a flag and give no indication. So i would preusme you will not get a fly up or down indication with this flag...??

[safetypee]

Whether an instrument and/or flight guidance system will indicate the GS deviation or not, depends on how the instrument or flight guidance system is programed to use the ‘flag’ signal. Deviation and flag are usually separate signals from the receiver. It may also depend on the receiver type, digital vs analogue.

Many EFIS will not show a deviation with a flag, some remove the GS scale – perhaps the E190; perhaps other older instruments might show a (incorrect) deviation, or even not flag the signal at all; beware.
Such situations require pilot evaluation, an awareness of altitude vs range from the runway, vertical speed, aircraft configuration, etc.
Crosscheck approaches with several sources of information.