Following the GS

[cale42]

Alright, even as I typed that post last night, I said to myself.. self don't post this, you don't know enough about what your talking about. Then I read it an hour later and said self, your going to get yourself lit up for this one.

And I did.. quite deservedly.

I will apologize to all for my ignorant comments regarding use of the glideslope in autoland situations... I know nothing about autoland and should have kept my mouth shut in the first place.

As far as from a teaching standpoint, I will have to reconsider some theories. I always learned to transition to a visual using the approach lights and disregard the glideslope. There have been some good points regarding that brought up here and I will certainly reconsider how I approach this as a pilot and as an instructor.

The one thing I will stand by is that on most Cessnas the glideslope antenna is either mounted on top of the windshield or coupled to the VOR's on the top of the fuselage or on the vertical stabilizer. However I had no grounds for extrapolating that placement to any other airplanes and should not have done so.

To all, I apologize for having to be the poster child of what a 200 hour pilot does not know and should keep his trap shut about... I'll try to do better in the future.

cale

 

[avbug]

And this brings us ALL THE WAY back to the original question of whether the glideslope is accurate below DH.

No, it doesn't. The original question did not concern accuracy below DH/DA. The original question, as posed by minitour, asked if the glideslope is typically flown to the runway, and if it's common practice, as quoted here:

Do you guys typically fly the glide slope all the way down to the runway? Flare?

Just wondering what is common practice and if there are any reasons?

I believe most would agree that common practice is to fly at or above the glideslope to touchdown. The question has arisen as to accuracy below DH. While certain anomolies can occur below DH, and below 100', the geometry of the approach suggests that small errors at one end of the stick add up to large errors at the other. If you've flown a stable, accurate signal to DH, excepting local abberations below 100 AGL, you should find a reliable signal.

The question then falls back to what is legal. Can you fly the glideslope to the runway? No. Not on a category 1 approach. (If your opspecs allow other criteria for your specific operation, that's another matter, because the regulation has been ammended specifically for you, and those opspecs have no bearing on general rule). Can you follow it and remain above it for general guidance? Yes. Do most do it? Yes. So do I.

Propjt was quick to jump in here regarding my prior comments regard flight inspections or flight checking. He notes the following reference from the AIM:

And I know I'm treading dangerously by disagreeing with this statement from avbug:
Is the glide slope signal reliable? Yes. No. Maybe so. In all liklihood, yes. But It's not flightchecked for reliability, and not approved.

From the Aeronautical Information Manual:
NOTE: Unless otherwise coordinated through Flight Standards, ILS signals to catagory I runways are not flight inspected below 100 feet AGL. Guidance signal anomalies may be encountered below this altitude.

Note that nowhere in this statement from the AIM, does the text indicate anything other than the concept that a Cat I ILS is NOT flightchecked below 100 AGL. This statement is generalized and somewhat misleading, and does NOT stipulate that glideslope is authorized below published DH, that glideslope is flight checked to DH, or anything else for that matter. Only that it is not flightchecked below 100 AGL, and says nothing about being flight checked above it. So much for the snide barb by Donsa320,who stated:

Is it not amazing how the "experts" quiet down after the real facts are posted? <grin>

Not so fast.

To visit the issue of flight inspections, or flight checks, perhaps rather than turning to the AIM, we might be better served by turning to a more authoritative source, the guidance for flight checks, found in FAA Order 8200.1A, titled United States Standard Flight Inspection Manual.

On the subject of the localizer, the manual, in paragraph 217-3207, states:

For a localizer-only approach, the published or proposed procedural altitudes shall be maintained in each segment, except the final segment shall be flown as follows: Upon reaching the FAF inbound, descend at a rate of approximately 400 feet per mile (930 feet per minute at 140 knots; 800 feet per minute at 120 knots) to an altitude of 100 feet below the lowest published MDA and maintain this altitude to Point C, which is the MAP. NOTE: See Section 301 definition of Point C for localizer only approaches.

This paragraph concerns only the localizer, without vertical guidance considered, and submits an altitude for flight check of 100' below the lowest published MDA.

Section 217.33 details flight inspection of the glide slope. A series of inspections are required for various parameters, not all of which must be done in flight. Not all of them involve flight along the entire glide slope. Determination of angle, for example, as detailed in 217.3306(a)(1)(b), may be made using data from any two points along the glide path, and interpolated. It does not need to result from flying the entire path, nor descent to a particular altitude, as cited (italics added):

(b) Altimeter and Ground Speed Method. Fly inbound. Mark checkpoints with the event mark and identify them on the recording. Checkpoints are normally the outer marker and the glide slope antenna; however, any two checkpoints separated by a known distance may be used.

Numerous parameters and tests are conducted, on, above, and below the glide path. Antenna offset, part of the flight inspection, is actually conducted with the aircraft parked on the runway, not in flight. Level runs are used for other parts of the checks, well above DH.

That the AIM submits that flight checks are not conducted below 100 AGL is inconsistent with flight inspection proceedures, but only insofar as the AIM was never intended to address them, and speaks in general terms only. Further, it only stipulates that the proceedures are not conducted below that altitude (they are), and says nothing about conducting them above that altitude.

All of that is largely not relevant in the face of the original question; do we follow the glideslope down? I have never had a check ride in which I was permitted to duck below the glidslope. Always above it, and above the visual approach indicators as applicable, until touchdown. Yes, we do follow it to touch down.

May we legally continue a category 1 approach using only electronic guidance for vertical guidance below decision height? No. Visual cues are required for continued use below that altitude. Decision height does present a limitation so far as descent; continued descent below DH is permissible during a missed approach, and using visual cues associated with the runway environment. But for a normal category 1 approach, descent below DH is not permitted without being engaged in a missed approach, or without visual cues available as applicable to the operation in question, using only electronic means for vertical guidance.

Flight checking is conducted above and below the glide path, inside and outside of the DH/DA on the glideslope centerline. However, approval for use of the glidepath below that altitude is not made for primary guidance below DH...not because of lack of signal integrity or signal inaccuracy (although this is sometimes the case), but because the proceedure has not been established for the purpose of descent below that point, nor flight checked for useage below that point, using the vertical guidance for descent.

How many accidents over the years have occured when pilots broke minimums, pushing down a little farther, a little closer, to get to the runway. Folks sneer when XXX company takes out runway lights pushing below and beyond that to which they're entitled. Without additional equipment to enable the descent by strictly electronic means, then it's not legal, nor wise.

A runway is served by ILS equipment certified to Cat III capabilities. A Cat I airplane approaches to land, flown by a Cat I pilot. While signal accuracy is not in question; the approach proceedures for that runway exist that go to touchdown, the flight is not legal to continue below DH using only the electronic means of vertical guidance, and the approach in that case has not been certified for it, under Category 1. Neither at that time has the pilot, nor the airplane. A quadruple whammy, as it were, regardless of the flight inspection that has been performed. A category 1 approach is not flight inspected for use in Category 1 below minimums, such as they may be applicable to that particular proceedure. That it may be inspected for accuracy, spurious radiation, width, etc, is not relevant.

 

[Donsa320]

Avbug

Do you get paid by the word? You managed to get so far off of the point that I have to agree with you...don't go below the glide slope after the DH. At least I think that is what you said. Now if that is not "Following the Glide Slope" below the DH then let's change the concept to "Respect the Glide Slope" after the DH if that change in semantics suits you. Jeez.

D.C.

 

[avbug]

Tell ya what, big guy. Next time I'll include pictures so you can follow along.

All better?

 

[FoxHunter]

From my limited experience G/S works fine all the way to touchdown. As a matter of fact we can use a CAT 1 ILS to do an autoland to upgrade the aircraft to CAT3B. Yes, at times this may not work out, but 90%+ it is fine. BTW, If the G/S is not normally usable to touchdown why do some runways have a note on the Jepp page that the G/S is unusable inside the MM or a certain altitude such as is the case with 18R at MEM?

 

[Donsa320]

Avbug

Tell ya what, big guy. Next time I'll include pictures so you can follow along.

All better?

Well, they say a picture is worth a thousand words. In your case I would have to agree. <grin>

D.C.

 

[snoopy_1]

Maybe if you put down the MS Flight Sim long enough and read a book, you will understand the difference.

Come on, MS Flightsim 2004 is really improving my inverted ILS approaches http://forums.avsim.net/dcboard.php?az=show_topic&forum=147&topic_id=173752&mesg_id=173752&page=

 

[corpflunkie]

according to an airways facilities friend of mine, the glideslope antenna is the same for catI, catII or catIII. The difference is the localizer antenna array. GS is accurate to the height the antenna is from the ground (10-12ft) unless there is some obstacle that may distort the beam ie the restriction at a place like KMEM

 

[Donsa320]

according to an airways facilities friend of mine, the glideslope antenna is the same for catI, catII or catIII. The difference is the localizer antenna array. GS is accurate to the height the antenna is from the ground (10-12ft) unless there is some obstacle that may distort the beam ie the restriction at a place like KMEM

I'm wondering what your friend means by that. The glide slope signal is bounced off of the ground in front of the antenna. This is to suppress any "fly down" side lobe signals that otherwise might be below the useable glide slope. Hopefully all the signal BELOW the useable glide slope will be "fly up".

Here in snow country we frequently lose the glide slope when the reflective plane gets enough snow on it to screw up the reflective angle. The monitor then trips the glide slope off 'till the snowplows clear the reflection zone. Not too handy. <g>

D.C.

 

[prpjt]

I included avbugs post in an example of why we were discussing GS between DA and 100' I took exeption to these words, " It's not flightchecked"

I took those words to infer that the GS was not tested inside the point where DA exists on the GS centerline. I don't think that is what he meant, but that's how I read it.

I'm using the word flightchecked to mean tested.The flightcheck has nothing to do with wether the signal is approved for use inside a particular point.. And the GS in fact is tested inside the DA point. The type of test for fly up signal depands on the type of Cat I approach. I was unaware that there was more than 1 type of cat I ILS.

A sincere thanks to Avbug for the reference to the FAA Order. And I'm not being a smarta$$. Thanks.

I would like to ask a question regarding some terms used in that order. Either an answer or a reference would be appreciated.

What makes a GS restricted or unrestricted.
How is it determined wether a Cat I ILS will be "for use below 200" or "not for use below 200" (those are the 2 types of Cat I ILS, and GS is tested differently)

And donsa is correct that heavy snow can render a GS useless and initiate an OTS Notam.