Localizer front/back course and reverse sensing

[Mark Hansen]

During my instrument training, I learned that I would have "normal"
CDI sensing whether flying in the localizer front course or out the
back course.

However, At our local International airport (Sacramento, CA), one
of the runways (16R/34L) has an ILS on both 16R and 34L, although
the ILS for 34L is currently out of service.

While flying the ILS approach to runway 16R, I was to continue
straight out after a touch and go, so I planned to follow the
localizer. The D.E. with me said that I would have reverse sensing
while flying out the back course.

What!?! That's not what I learned. While flying out, I noticed that
the needle displaced further while he was 'correcting' in what I
felt was the wrong direction. This seemed to make my point, but
he then said that I was getting 'correct' sensing due to the fact
that that particular runway has an ILS on both ends, both on the
same frequency.

OK, this gave me pause - I had never noticed that before. In looking
at the ILS 16R and ILS 34L approach charts, they both are on frequency
111.1 - although they have different Identifier codes.

How does this work? I thought the Localizer antennas operated in both
directions, ahead of the antenna and behind as well. When they both
use the same frequency, are they omni-directional?

Assuming they are omni-directional, does that mean you will receive
both signals while overflying the runway?

Given that the ILS for 34L was out of service, there is no doubt
that I should get normal sensing while flying out the back course,
however both my CFII and the Designated Examiner had this reversed.

How can a lowly student be correct and his CFII and Designated Examiner
be wrong?


--
Mark Hansen, PP-ASEL, Instrument Airplane
Sacramento, CA

[Mike Adams]

Mark, I'd say you are correct and got some amazingly bad advice.

Mark Hansen > wrote:
> While flying the ILS approach to runway 16R, I was to continue
> straight out after a touch and go, so I planned to follow the
> localizer. The D.E. with me said that I would have reverse sensing
> while flying out the back course.

Nope.

> What!?! That's not what I learned. While flying out, I noticed that
> the needle displaced further while he was 'correcting' in what I
> felt was the wrong direction. This seemed to make my point, but
> he then said that I was getting 'correct' sensing due to the fact
> that that particular runway has an ILS on both ends, both on the
> same frequency.
>
> OK, this gave me pause - I had never noticed that before. In looking
> at the ILS 16R and ILS 34L approach charts, they both are on frequency
> 111.1 - although they have different Identifier codes.
>
> How does this work? I thought the Localizer antennas operated in both
> directions, ahead of the antenna and behind as well. When they both
> use the same frequency, are they omni-directional?

They are on the same frequency because only one of them is in use and on the air at a given time,
based on which runway is active.

(But "omni-directional" means in all directions - the opposite of what I think you meant above. But no,
they are not uni-directional - this is just not an issue if only one is transmitting at a time.)

>
> Assuming they are omni-directional, does that mean you will receive
> both signals while overflying the runway?
You will be on the back course when overflying the runway, based on which ILS is active. You will get
normal sensing while flying out the back course, but would get reverse sensing if you were to turn
around in come back inbound on the back course. (unless you have an HSI, but that's another story.)

>
> Given that the ILS for 34L was out of service, there is no doubt
> that I should get normal sensing while flying out the back course,
> however both my CFII and the Designated Examiner had this reversed.
>
> How can a lowly student be correct and his CFII and Designated Examiner
> be wrong?

Hang in there.They're only human, but I agree, they should understand this.

Mike

[Steven P. McNicoll]

"Mark Hansen" > wrote in message
...
>
> During my instrument training, I learned that I would have "normal"
> CDI sensing whether flying in the localizer front course or out the
> back course.
>
> However, At our local International airport (Sacramento, CA), one
> of the runways (16R/34L) has an ILS on both 16R and 34L, although
> the ILS for 34L is currently out of service.
>
> While flying the ILS approach to runway 16R, I was to continue
> straight out after a touch and go, so I planned to follow the
> localizer. The D.E. with me said that I would have reverse sensing
> while flying out the back course.
>
> What!?! That's not what I learned. While flying out, I noticed that
> the needle displaced further while he was 'correcting' in what I
> felt was the wrong direction. This seemed to make my point, but
> he then said that I was getting 'correct' sensing due to the fact
> that that particular runway has an ILS on both ends, both on the
> same frequency.
>
> OK, this gave me pause - I had never noticed that before. In looking
> at the ILS 16R and ILS 34L approach charts, they both are on frequency
> 111.1 - although they have different Identifier codes.
>
> How does this work? I thought the Localizer antennas operated in both
> directions, ahead of the antenna and behind as well. When they both
> use the same frequency, are they omni-directional?
>
> Assuming they are omni-directional, does that mean you will receive
> both signals while overflying the runway?
>
> Given that the ILS for 34L was out of service, there is no doubt
> that I should get normal sensing while flying out the back course,
> however both my CFII and the Designated Examiner had this reversed.
>
> How can a lowly student be correct and his CFII and Designated Examiner
> be wrong?
>

Where ILSs are installed at both ends of a runway and operate on the same
frequency they do not operate simultaneously. The switching used ensures
that only one can be selected at a time.

[Roy Smith]

Mark Hansen > wrote:
> However, At our local International airport (Sacramento, CA), one
> of the runways (16R/34L) has an ILS on both 16R and 34L

This is very common at major airports.

> OK, this gave me pause - I had never noticed that before. In looking
> at the ILS 16R and ILS 34L approach charts, they both are on frequency
> 111.1 - although they have different Identifier codes.
>
> How does this work? I thought the Localizer antennas operated in both
> directions, ahead of the antenna and behind as well. When they both
> use the same frequency, are they omni-directional?

There are two localizer antennas, each one just beyond the departure end of
the runway they serve. There are also two glideslope antennas, each off to
the side of the runway near the touchdown zone (approx 1000 feet from the
threshold).

Only one localizer (and one glideslope) antenna is in use at any one time.
Selection of which one is active is done by a switch in the tower. The two
systems share the same frequency (which is why only one can be active at a
time), but have distinct ident codes (for example, at HPN, the ILS-16 is
IHPN, and the ILS-34 is IOJZ).

It is educational to ask the tower to switch to the other ILS while you're
flying a practice approach (obviously only in VFR, and they'll only be
willing to do it if there's no other traffic around). You'll hear the
morse ident change to the other code, and you'll see the CDI needle reverse.

> Assuming they are omni-directional, does that mean you will receive
> both signals while overflying the runway?

Nope, they are not omnidirectional. When an ILS is installed in both
directions, the two localizer antennas are the same directional arrays that
would be used for a single ILS installation.

BTW, what *is* omnidirectional is DME. If there is a DME component
installed for the ILS, it will usually be near one end of the runway. Look
at the plates for the SMF ILS-34L and ILS-16R. Notice that HADSU is 4.1 nm
from the approach end of 34L and 3.7 DME, while on the other side, JARNU is
5.3 nm from the 16R threshold, but 7.2 DME. This says to me that the DME
ground station is about 0.4 nm in front of the 34L threshold.

> How can a lowly student be correct and his CFII and Designated Examiner
> be wrong?

Wouldn't be the first time.

[Mark Hansen]

On 9/10/2005 11:30 AM, Mike Adams wrote:

> Mark, I'd say you are correct and got some amazingly bad advice.
>
> Mark Hansen > wrote:
>> While flying the ILS approach to runway 16R, I was to continue
>> straight out after a touch and go, so I planned to follow the
>> localizer. The D.E. with me said that I would have reverse sensing
>> while flying out the back course.
>
> Nope.
>
>> What!?! That's not what I learned. While flying out, I noticed that
>> the needle displaced further while he was 'correcting' in what I
>> felt was the wrong direction. This seemed to make my point, but
>> he then said that I was getting 'correct' sensing due to the fact
>> that that particular runway has an ILS on both ends, both on the
>> same frequency.
>>
>> OK, this gave me pause - I had never noticed that before. In looking
>> at the ILS 16R and ILS 34L approach charts, they both are on frequency
>> 111.1 - although they have different Identifier codes.
>>
>> How does this work? I thought the Localizer antennas operated in both
>> directions, ahead of the antenna and behind as well. When they both
>> use the same frequency, are they omni-directional?
>
> They are on the same frequency because only one of them is in use and on the air at a given time,
> based on which runway is active.

OK, so only one would be on at any one time. That makes perfect sense.
I couldn't understand why they would have both on at the same time
anyway...

Note that when I showed the D.E that I was getting normal sensing
while flying out the back course, he said "Oh, yes, you are, but not
for the reasons you think - it's because of the 'other' localizer
antenna..."

I gotta say, this really put a damper on my Instrument check flight.
This was coming from the Chief Flight Instructor/Owner of the FBO, no
less.

>
> (But "omni-directional" means in all directions - the opposite of what I think you meant above. But no,
> they are not uni-directional - this is just not an issue if only one is transmitting at a time.)

Yes ... <sheepish grin> - I mean uni-directional.

>
>>
>> Assuming they are omni-directional, does that mean you will receive
>> both signals while overflying the runway?
> You will be on the back course when overflying the runway, based on which ILS is active. You will get
> normal sensing while flying out the back course, but would get reverse sensing if you were to turn
> around in come back inbound on the back course. (unless you have an HSI, but that's another story.)
>
>>
>> Given that the ILS for 34L was out of service, there is no doubt
>> that I should get normal sensing while flying out the back course,
>> however both my CFII and the Designated Examiner had this reversed.
>>
>> How can a lowly student be correct and his CFII and Designated Examiner
>> be wrong?
>
> Hang in there.They're only human, but I agree, they should understand this.

Thanks very much for the help.

>
> Mike


--
Mark Hansen, PP-ASEL, Instrument Airplane
Sacramento, CA

[Peter Clark]

On Sat, 10 Sep 2005 11:14:46 -0700, Mark Hansen
> wrote:

>During my instrument training, I learned that I would have "normal"
>CDI sensing whether flying in the localizer front course or out the
>back course.

Does the aircraft in question just have a VOR receiver, or does it
have an HSI?

[Bob Gardner]

Astounding. The mind reels. Don't really know how to blow the whistle on
this guy, but he needs it.

Bob Gardner

"Mark Hansen" > wrote in message
...
> During my instrument training, I learned that I would have "normal"
> CDI sensing whether flying in the localizer front course or out the
> back course.
>
> However, At our local International airport (Sacramento, CA), one
> of the runways (16R/34L) has an ILS on both 16R and 34L, although
> the ILS for 34L is currently out of service.
>
> While flying the ILS approach to runway 16R, I was to continue
> straight out after a touch and go, so I planned to follow the
> localizer. The D.E. with me said that I would have reverse sensing
> while flying out the back course.
>
> What!?! That's not what I learned. While flying out, I noticed that
> the needle displaced further while he was 'correcting' in what I
> felt was the wrong direction. This seemed to make my point, but
> he then said that I was getting 'correct' sensing due to the fact
> that that particular runway has an ILS on both ends, both on the
> same frequency.
>
> OK, this gave me pause - I had never noticed that before. In looking
> at the ILS 16R and ILS 34L approach charts, they both are on frequency
> 111.1 - although they have different Identifier codes.
>
> How does this work? I thought the Localizer antennas operated in both
> directions, ahead of the antenna and behind as well. When they both
> use the same frequency, are they omni-directional?
>
> Assuming they are omni-directional, does that mean you will receive
> both signals while overflying the runway?
>
> Given that the ILS for 34L was out of service, there is no doubt
> that I should get normal sensing while flying out the back course,
> however both my CFII and the Designated Examiner had this reversed.
>
> How can a lowly student be correct and his CFII and Designated Examiner
> be wrong?
>
>
> --
> Mark Hansen, PP-ASEL, Instrument Airplane
> Sacramento, CA

[Mark Hansen]

On 9/10/2005 4:48 PM, Peter Clark wrote:

> On Sat, 10 Sep 2005 11:14:46 -0700, Mark Hansen
> > wrote:
>
>>During my instrument training, I learned that I would have "normal"
>>CDI sensing whether flying in the localizer front course or out the
>>back course.
>
> Does the aircraft in question just have a VOR receiver, or does it
> have an HSI?

No HSI involved. Besides, we were flying out the back course. With
the HSI, it can compensate for flying in the back course (assuming
you have the front course set in the OBS). ... that wasn't the case
here.

--
Mark Hansen, PP-ASEL, Instrument Airplane
Sacramento, CA

[Tauno Voipio]

Mark Hansen wrote:
> During my instrument training, I learned that I would have "normal"
> CDI sensing whether flying in the localizer front course or out the
> back course.
>
> However, At our local International airport (Sacramento, CA), one
> of the runways (16R/34L) has an ILS on both 16R and 34L, although
> the ILS for 34L is currently out of service.
>
> While flying the ILS approach to runway 16R, I was to continue
> straight out after a touch and go, so I planned to follow the
> localizer. The D.E. with me said that I would have reverse sensing
> while flying out the back course.
>
> What!?! That's not what I learned. While flying out, I noticed that
> the needle displaced further while he was 'correcting' in what I
> felt was the wrong direction. This seemed to make my point, but
> he then said that I was getting 'correct' sensing due to the fact
> that that particular runway has an ILS on both ends, both on the
> same frequency.
>
> OK, this gave me pause - I had never noticed that before. In looking
> at the ILS 16R and ILS 34L approach charts, they both are on frequency
> 111.1 - although they have different Identifier codes.
>
> How does this work? I thought the Localizer antennas operated in both
> directions, ahead of the antenna and behind as well. When they both
> use the same frequency, are they omni-directional?
>
> Assuming they are omni-directional, does that mean you will receive
> both signals while overflying the runway?


The localizer antenna array consists in fact of three antenna
groups. Each of the individual antenna elements is slightly
directional, but radiates in principle to both directions,
front and back.

The middle antenna group radiates the signal as it is
on the center line, and the side groups radiate a signal
representing the difference of the center line signal and
the side signal. The final signal is created by the combination
of the three signals at the receiving antenna, and the time
delay of the difference signals compared to the center line
signal create the modulation depth difference for the
indicator deflection.

The sense of a localizer keeps the same when flying the same
direction, so outward on a back course shows correctly
on the indicator, and outward on the main course shows
reverse indication. Right above the antenna array the
indication may show weird fluctuations.

A HSI shows always correctly if the course selector
is set to main approach direction.

Your confusion may be from the two localizers on the
same frequency: the direction is changed when the
localizer in use is changed. It is impossible to run
two localizers on the same frequency at opposite directions
simultaneously.

HTH

--

Tauno Voipio, Avionics Expert, CFII
tauno voipio (at) iki fi

> Given that the ILS for 34L was out of service, there is no doubt
> that I should get normal sensing while flying out the back course,
> however both my CFII and the Designated Examiner had this reversed.
>
> How can a lowly student be correct and his CFII and Designated Examiner
> be wrong?
>
>

[[email protected]]

Steven P. McNicoll wrote:
>
> Where ILSs are installed at both ends of a runway and operate on the same
> frequency they do not operate simultaneously. The switching used ensures
> that only one can be selected at a time.

It does not, however, ensure that the right one is turned on. Thus it
behooves you to pay attention to the identifier, which is usually only
different by the last letter.

-cwk.

[Roy Smith]

wrote:
> It does not, however, ensure that the right one is turned on. Thus it
> behooves you to pay attention to the identifier, which is usually only
> different by the last letter.

Why do you say they only differ by one letter? In the examples I know of,
they are completely different (except for the initial I).

You are, of course, correct that you should always ident the signal to make
sure it's the right one (protects against incorrectly entered frequencies,
too). One of the nice little features of the CNX-80 is that it decodes the
morse for you and displays the ident in letters.