Following the GS

[Brett Hull]

Gotta chime in here. If it is not legal to use the GS below 200' on a CAT I approach, then please explain how it can be legal to continue below minimums to 100' above the TDZ with only the approach light system in sight. Uh, what do you plan to use for vertical guidance between 200' and 100'?

The Force?


That's what I use, anyway.

 

[cale42]

This is one of the scariest threads that I've ever read on here.. I can't beleive there is anyone out there with an instrument rating that doesn't know you can't use the glideslope below DH on a standard Category I approach.


If you are using the approach lights to go from DH to 100'AGL you ought to be using the approach lights as your vertical guidance, you have now transferred from an instrument to a visual approach.

In terms of using the glideslope all the way to the runway lets think about how a glide slope works and we will realize that never makes sense. The glide slope projects a bar of radio frequency that is a certain height above the ground at any point in the approach. The height above the ground of the glideslope above the threshold is the TCH that is noted on charts, usually 40 to 50 feet. Keep in mind in most aircraft the glideslope antenna is on the top of the aircraft. Well if your glide slope receiver is 8 feet off the ground (on a 172 say) that means at TCH on glide you are likely 30-40 feet off the ground at the threshold. However if your glideslope antenna is 100 feet off the ground (on a 747 say) than on glide indication at the threshold would put you 50-60 feet into the ground. So if the glideslope were used to guide planes all the way to the groundit would have to be adjusted for each type of aircraft.

but this is all a mute point for most of us because YOU CAN'T USE THE GS FOR LEGAL GUIDANCE BELOW THE DH.

cale

 

[Crusty]

I hope guys are not going visual from DH to 100'; bad practice. Stay on the G/S, it is accurate and will keep your name out of the papers. Every airline I have been with has trained this way.

Even though the expense has not been incurred to "certify" every ILS to touchdown they are still accurate in most cases. My ops specs authorize full autoland (CAT I mins) to any ILS unless specifically not authorized. I have done this on many occasions at many airports over the years and never seen the G/S out of the crosshairs before touchdown or touched down out of the zone. IT WORKS and it is approved!

 

[avbug]

Below the published minimums for a give category of ILS, the glideslope is not approved for primary guidance. You may use it, but you are then responsible for your own obstacle avoidance.

You are certainly able to continue below DH to a lesser altitude using only the red terminal bars, but terrain and obstacle clearance has then become your responsibility. Is the glide slope signal reliable? Yes. No. Maybe so. In all liklihood, yes. But It's not flightchecked for reliability, and not approved.

Does the fact that it's not legal below DH have anything to do with threshold crossing height and aircraft type? No.

Should one simply transition to visual guidance in low visibility approaches when operating below DH? No, I don't believe so. Continue to use the glide slope, but do so at your own peril; don't use it as the primary source of vertical guidance except where one has alternate approval to do so.

If one holds OpSpecs to the contrary, then these represent an addition or ammendment to regulation, and are regulation applicable to a particular operator. This is peculiar doctrine; peculiar to one operator and to no other. Therefore as general policy, where any operator might have operations specifications permitting the use of vertical guidance below DH, it is not applicable to operators not holding this specific approval.

What is one to do below DH? One should be holding a stablized descent rate, with a stabilized power setting, and a stabilized airspeed. One generally has some type of visual approach slope indication (which is more often than not, uncoincidental to the electronic descent path; they may be on different angles, and certainly almost always terminate at different locations). One should remain at or above both the electronic glide slope and visual glide paths until touchdown. Personally, whichever is higher, I remain above it, unless doing so might result in an unusual change in approach path, power setting, or descent rate. In that case, I continue stabilized using the appropriate visual cues and electronic and visual guidance.

Where ever not flight checked, electronic vertical guidance below the DH is not authorized.

I believe I used the example of the RNO ILS and Silver ILS before. When flying this approach, I've found that vertical guidance is good; it's published good well below standard minimums of 1100 HAT, as it's published with lower minimums for certified users (Silver ILS). However, for those not approved for use of the lower minimums, any continuation below DH to the runway must be reliant on vertical guidance media other than the electronic glide slope. If at any time the operator is unable to do that, a missed approach should be immediately initiated.

It's important to note that Decision Height, or Decision Altitude, is not the minimum altitude to which one may go on an approach. It's the point at which a decision must be made to go missed or continue, if visual cues are not in sight. In some cases, an aircraft going missed, having made the call at DA/DH, may touch the runway during the missed proceedure. In all cases, one should never go below the glideslope.

 

[atldc9]

Hey Cale, What? Next time you watch an autoland, you might want to think that the airplane IS using the GS for vert nav below the DA. We are authorized and routinely perform autoland approaches to CAT 1 ILS approaches. Op's Spec's for Autolands for other than CAT II/III apply. The only limiting factor is that there are no notes on the plate that would prohibit use of the GS below a spcific altitude, i.e. GS unusable below XXX, or autocoupled approaches NA below XXX.

By the way, which airplane has the GS antenna on the top? Every one I have ever flown had it on the bottom.

Cessnadriver,
Cat II/III op's spec approval comes from Wash, not your local POI, so they start basically the same, but end up different for each operator. Basically, Cat II operators can get approval for as low as 1200 RVR and a 100' Decision Height (note: not Decision Altitude, which is MSL, Decision Height is based on radio altitude flight checked for each individual runway). Cat III operators can get approval for as low as 300 RVR with no DH at all. It just depends on your equipment and your training program. When I flew the DC-9, those airplanes were hand flown CAT II to 1200 RVR with a freaking PDI, but on the B-717, anything under 1800 RVR is an autoland. Under our current procedures, any go around from a CAT II/III approach is initiated with the autopilot on. If you start the go around low enough, you might actually touch the runway, but only for a second before it blasts out of there. We actually do single-engine CAT III autolands and go arounds. Pretty straight forward actually.

 

[prpjt]

If you are using the approach lights to go from DH to 100'AGL you ought to be using the approach lights as your vertical guidance, you have now transferred from an instrument to a visual approach.
cale

Disregarding the GS and using the approach lights as vertical reference can and has put planes into the ground. Here's a scenario that I had about an hour and a half ago.

The wx is VV100 and 3/4 vis here. It's dark. At DA (584 for my approach) I glanced up. I saw 4 white strobes peering through the clouds and darkness. Back on the guages and down to 484. At 500 I look again and see green threshold lights and a couple of runway lights, landing... back on the guages.

Transfer between instruments and outside all the way to touchdown, staying on glide until about 20agl then from then on it's 100% visual.

Are you recommending that at DA a pilot should use 4 white dots in a sea of black for verticle reference? Hope not.

Oh, and I don't have a GS antenna on top of my plane.

 

[Dr Pokenhiemer]

We are authorized and routinely perform autoland approaches to CAT 1 ILS approaches.

And this brings us ALL THE WAY back to the original question of whether the glideslope is accurate below DH. I never said that I or that I recommend doing this unless specifically authorized and train to do, so I stand by my original answer that "Yes--it is accurate." Don't know where all that other stuff came from about vertical guidance from DH to 100' agl. The problem is that too many people try to read too much into something. A glideslope gives you reverse sensing if you fly an ILS inverted. Will I do it? No way!!!

 

[prpjt]

Don't know where all that other stuff came from about vertical guidance from DH to 100' agl.

From our resident CFI cale42:
This is one of the scariest threads that I've ever read on here.. I can't beleive there is anyone out there with an instrument rating that doesn't know you can't use the glideslope below DH on a standard Category I approach.

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.

 

[Donsa320]

Prpjt

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


Thanks,

D.C.

 

[Flibmeister]

It's also worth noting that controllers are not required to protect the Glideslope Critical Area unless the weather's below 800/2. When weather's that good or better, there could be airplanes, tractors, etc., in the critical area that will adversely effect the integrity of the signal.

Many pilots seem to think that, in such circumstances, advising the controller that they're making a coupled approach will assure that the critical area remains clear. It doesn't-- it simply triggers a requirement that the controller inform the pilot that the critical area isn't protected.

 

[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?