Brett Hull
Pastafarian
- Joined
- Nov 25, 2001
- Posts
- 970
Welcome to Flightinfo.com
- Register now and join the discussion
- Friendliest aviation Ccmmunity on the web
- Modern site for PC's, Phones, Tablets - no 3rd party apps required
- Ask questions, help others, promote aviation
- Share the passion for aviation
- Invite everyone to Flightinfo.com and let's have fun
Following the GS
- Thread starter minitour
- Start date
- Watchers 12
Welcome to Flightinfo.com
- Register now and join the discussion
- Modern secure site, no 3rd party apps required
- Invite your friends
- Share the passion of aviation
- Friendliest aviation community on the web
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
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
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!
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!
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.
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.
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.
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.
Last edited:
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.cale42 said:
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
Well-known member
- Joined
- Oct 19, 2004
- Posts
- 301
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!!!atldc9 said:
From our resident CFI cale42:Dr Pokenhiemer said:
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.
Flibmeister
Well-known member
- Joined
- Jun 13, 2004
- Posts
- 47
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.
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.
Last edited:
Share this page
Latest resources
-
-
-
-
-
AC 90-89C - Amateur-Built Aircraft and Ultralight Flight Testing HandbookAmateur-Built Aircraft and Ultralight Flight Testing Handbook
- Neal
- Updated:
