Narrative:

I was conducting an instrument training flight for a current CFI; student who wished to obtain his CFI-I. I am a cfii for over 10 years. He was wearing foggles and I was instructing. Weather was good VMC conditions. We were with knoxville approach and first shot a practice localizer 26 approach to dkx with a miss at minimums. The student owned the [aircraft] we were flying; and it was a solid airplane; but with minimal IFR equipment. Knoxville approach had been advised that we were going to miss at dkx and would then request a tys 23L ILS to a landing after that dkx miss. They gave us missed approach instructions as usual; to fly westbound and climb to 3;000. We are both familiar with the area and this is the standard alternative missed instructions they give from dkx in good weather.after contacting knoxville approach after the miss and reporting our heading and altitude; they gave us a further climb to 3;500; a vector to join the final ILS approach course for the tys 23L ILS; and cleared us for the tys 23L ILS immediately. This was a bit unusual because we were at least 18 or so miles out at that point. We just figured that it must be a slow period for them. Usually; we get a lot more vectors and get turned much closer in because of larger aircraft traffic. It was good training for a CFI-I candidate though; so I sat back and monitored while he reconfigured the navigation equipment and complied quickly.we would typically; at that point; join the localizer and hold altitude until intersecting the glideslope. That would usually be at goodl intersection. However; we did not see the glideslope come alive as we expected it to; then we heard and saw the outer marker indication. On most ILS approaches; that is the FAF; but it is not so at tys. Since the glideslope was not alive; and we thought (incorrectly) that we were at the OM; we looked closely at the CDI and saw that the glideslope indicator was flagged. I just figured that it was the old navigational equipment in the airplane; so I asked him 'what are you going to do?' he correctly answered; that we are now on a localizer only approach. That was the answer I was hoping for and was correct (I thought at the time). Looking out the front window; things did not look right to me; but although I have shot that approach several times in the past; I wasn't sure why [it] didn't look right (yet).of course he was 'under the hood' so he didn't see anything outside. The tys localizer runway 23L approach allows us to descend to 1;800 after passing the FAF; which we thought we had; based on the OM indication. He started down; and things started looking worse and worse out the front window. Terrain was coming up and the runway was not in my sight yet; although we were right on the localizer. At about 2;000; I told him to take the foggles off; and initiate a climb back to 2;500. He was confused by that command; but complied. When he could see out the window; he knew why I issued that command. There was a ridge of low hills in front of us that we would not clear safely at 2;000 or lower. We did not receive any low altitude or terrain warning from ATC; although they clearly understood that it was a practice ILS training flight; and weather was clear; so I do not fault them at all for that. However; we were very much confused by what was going on with the navigation indications we were receiving. We now did both know exactly where we were on the approach course; by visual reference; and could see we were aligned with the runway but still too far out to be at that altitude. Then the glideslope came alive; so we followed it the rest of the way down and it worked perfectly and brought us to the correct place on the runway.we landed; still confused; parked the plane and went inside to debrief the training flight. After looking very carefully at the NOAA tys; ILS or localizer runway 23L approach chart; we saw that small note that said 'disregard all marker beacon indications.' what we thought was our OM; was actually the dkx OM that is very closely aligned with the tys 23L ILS approach. Had we read the chart more thoroughly; prior to take off; we would have understood what happened. However; I feel this is a very dangerous situation. Had it been true IMC low weather; and especially if we were shooting that approach as an alternate after missing somewhere else; there is a good chance that we would have missed that same note while reading the chart in the cockpit. We would have hit that ridge of hills between us and tys after thinking that OM was our FAF; which it usually is on most other ILS approaches. Keep in mind that this was 2 CFI's flying at the time. A cfii and another CFI that was working on his CFI-I and passed that check ride about a week or two later on the 1st try; and we both missed that small note on the approach plate. It was a great training lesson; but still quite dangerous in a real world situation.

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Original NASA ASRS Text

Title: GA flight instructor reported both pilots on board misinterpreted an outer marker indication and began an early descent towards terrain.

Narrative: I was conducting an instrument training flight for a current CFI; student who wished to obtain his CFI-I. I am a CFII for over 10 years. He was wearing foggles and I was instructing. Weather was good VMC conditions. We were with Knoxville Approach and first shot a practice LOC 26 approach to DKX with a miss at minimums. The student owned the [aircraft] we were flying; and it was a solid airplane; but with minimal IFR equipment. Knoxville Approach had been advised that we were going to miss at DKX and would then request a TYS 23L ILS to a landing after that DKX miss. They gave us missed approach instructions as usual; to fly westbound and climb to 3;000. We are both familiar with the area and this is the standard alternative missed instructions they give from DKX in good weather.After contacting Knoxville Approach after the miss and reporting our heading and altitude; they gave us a further climb to 3;500; a vector to join the final ILS approach course for the TYS 23L ILS; and cleared us for the TYS 23L ILS immediately. This was a bit unusual because we were at least 18 or so miles out at that point. We just figured that it must be a slow period for them. Usually; we get a lot more vectors and get turned much closer in because of larger aircraft traffic. It was good training for a CFI-I candidate though; so I sat back and monitored while he reconfigured the navigation equipment and complied quickly.We would typically; at that point; join the localizer and hold altitude until intersecting the glideslope. That would usually be at GOODL intersection. However; we did not see the glideslope come alive as we expected it to; then we heard and saw the Outer Marker indication. On most ILS approaches; that is the FAF; but it is NOT so at TYS. Since the glideslope was not alive; and we thought (incorrectly) that we were at the OM; we looked closely at the CDI and saw that the Glideslope indicator was flagged. I just figured that it was the old navigational equipment in the airplane; so I asked him 'what are you going to do?' He correctly answered; that we are now on a LOC only approach. That was the answer I was hoping for and was correct (I thought at the time). Looking out the front window; things did not look right to me; but although I have shot that approach several times in the past; I wasn't sure why [it] didn't look right (yet).Of course he was 'under the hood' so he didn't see anything outside. The TYS LOC Runway 23L approach allows us to descend to 1;800 after passing the FAF; which we thought we had; based on the OM indication. He started down; and things started looking worse and worse out the front window. Terrain was coming up and the runway was not in my sight yet; although we were right on the localizer. At about 2;000; I told him to take the foggles off; and initiate a climb back to 2;500. He was confused by that command; but complied. When he could see out the window; he knew why I issued that command. There was a ridge of low hills in front of us that we would not clear safely at 2;000 or lower. We DID NOT receive any low altitude or terrain warning from ATC; although they clearly understood that it was a practice ILS training flight; and weather was clear; so I do not fault them at all for that. However; we were very much confused by what was going on with the navigation indications we were receiving. We now did both know exactly where we were on the approach course; by visual reference; and could see we were aligned with the runway but still too far out to be at that altitude. Then the glideslope came alive; so we followed it the rest of the way down and it worked perfectly and brought us to the correct place on the runway.We landed; still confused; parked the plane and went inside to debrief the training flight. After looking very carefully at the NOAA TYS; ILS or LOC RWY 23L approach chart; we saw that small note that said 'Disregard all marker beacon indications.' What we thought was our OM; was actually the DKX OM that is very closely aligned with the TYS 23L ILS approach. Had we read the chart more thoroughly; prior to take off; we would have understood what happened. However; I feel this is a very dangerous situation. Had it been true IMC low weather; and especially if we were shooting that approach as an alternate after missing somewhere else; there is a good chance that we would have missed that same note while reading the chart in the cockpit. We would have hit that ridge of hills between us and TYS after thinking that OM was our FAF; which it usually is on most other ILS approaches. Keep in mind that this was 2 CFI's flying at the time. A CFII and another CFI that was working on his CFI-I and passed that check ride about a week or two later on the 1st try; and we both missed that small note on the approach plate. It was a great training lesson; but still quite dangerous in a real world situation.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.