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Author Topic: Cranky Inbounds Pilots due to Cranky Glide Slope Data  (Read 15773 times)

Offline Tango_Juliet_Kilo

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Cranky Inbounds Pilots due to Cranky Glide Slope Data
« on: January 27, 2010, 11:29:14 PM »
I'm a first time poster and have no technical background, therefore don't understand much about what information is sent to the pilots as they are inbound.  From what I gathered , listening to ORD tower between 4P and 5P CST, there were some false readings being given. 

While inbound pilots complained about data issues (there were specific terms used, but I cannot recall), ATC assured them there was no glide slope interference?  After about 6 straight pilots reported issues on the 28 final, ATC offered up that they believed the Runway 28 slope data issues were due to the system picking up outbound flights from 27L. 

I'm hoping that the experienced members can decode my rookie speak and shed some light on this. 

Is the glide slope adjustable based on conditions?
What sensors/warnings were the pilots referring to?



Offline jmcmanna

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Re: Cranky Inbounds Pilots due to Cranky Glide Slope Data
« Reply #1 on: January 29, 2010, 11:47:23 AM »
At that time, I believe ORD was arriving 28/27L/27R and departing 22L and 32L/T10.  Aircraft departing 22L have to pass in front of the glideslope antenna for runway 28.  It was also snowy/icy at the time.  So it was probably a combination of the atmospheric conditions and aircraft taxiing/departing runway 22L that would have caused issues.

Offline atcman23

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Re: Cranky Inbounds Pilots due to Cranky Glide Slope Data
« Reply #2 on: January 29, 2010, 03:06:39 PM »
Sounds like weather also contributed to this, as mentioned.  The glide slope is not adjustable; it is a fixed antenna that is placed near the touch down point on the runway.  It is fixed at a specific angle (usually around 3 degrees) and pilots use this for vertical (up and down) guidance to the runway.  They look at two needles in the cockpit either in a DG or other similar instrument to determine where they are at on the glidepath.  One moves side to side (laterally) for guidance to the runway centerline (the localizer) and the other moves up and down for vertical guidance.  When both needles are centered, it means the aircraft is on the glidepath.  If something interferes with the signal, the needles oscillate and that movement is not concurrent with the aircraft moving. 

If something else happens either with the localizer, glideslope or the instrument that interprets their signal, the instrument may throw a "Nav" flag indicating a failure of something.

Glideslope Antenna:


Typical VOR receiver:



Most airliners have a glass cockpit display with multi function screens that display this information differently.

Lastly, here is the localizer antenna array.  Both the Glideslipe and Localizer make up the Instrument Landing Sytem (ILS).


Offline rpd

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Re: Cranky Inbounds Pilots due to Cranky Glide Slope Data
« Reply #3 on: January 31, 2010, 08:37:10 PM »
I have never seen a DG (directional gyro) with any course guidance (nav or gs).  A DG isn't even tunable to navigation frequencies.

Study those wake turbulence charts atcman...

Offline Jason

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Re: Cranky Inbounds Pilots due to Cranky Glide Slope Data
« Reply #4 on: January 31, 2010, 09:19:24 PM »
I have never seen a DG (directional gyro) with any course guidance (nav or gs).  A DG isn't even tunable to navigation frequencies.

Study those wake turbulence charts atcman...

He probably meant an HSI which is incorporates a slaved compass card with a CDI for course guidance.

Offline atcman23

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Re: Cranky Inbounds Pilots due to Cranky Glide Slope Data
« Reply #5 on: February 01, 2010, 05:36:50 PM »
Yeah an HSI is what I was picturing and I mistakenly typed DG.  Thanks for the clarification.