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.
