Flight Crew Air Transport Pilot (ATP)
Confusion
ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
ATC Equipment / Nav Facility / Buildings; Human Factors; Procedure
I was PM when I checked in with CLE Approach; they said that a few aircraft that had landed before; had dipped down on the glideslope (GS) to maintain visual with the runway. ATC told us that they were going to clear us for the ILS 24R and that they wanted us to fly the GS all the way down to runway so they could check out the GS. The Captain and I thought this was a weird request but we obliged. We were cleared to 4;000 feet and cleared for the approach. As the GS was captured we followed it down from 4;000 [feet] at CIVAC. At CIVAC ATC said to fly the GS; fly the GS. So we did and at JEDSU ATC asked if we were on the GS and I said yes. He said impossible because JEDSU is 3;000 feet. He then asked if we had runway which I responded yes. ATC cleared us for a Visual Approach and at that moment the GS went full scale up; we reported that to ATC and nothing else was said. CLE must not realize that the final fix and GS intercept is far off.
When we checked in with CLE Approach; they had a request. They told us to expect ILS 24R and asked if we could please fly the glideslope all the way down. They said that previous aircraft had been dipping below the glide slope near the FAF. They said they believed the aircraft were doing this to stay below clouds to maintain visual with the runway. The FO and I were perplexed. We had never heard of such a request. Why would aircraft do that on an ILS approach? This raised some flags. We were then cleared to 4;000 feet and cleared for the approach. At 4;000 feet we were on glideslope a few miles from CIVAC. The airport was in sight. The minimum altitude to CIVAC was 4;000 [feet;] so I selected Nav mode and Alt mode to stop descent until passing CIVAC. The glideslope was passing below us at this time. It seemed as if the controller saw us going above the glidepath because he again told us to please stay on the glideslope that they needed to make sure it was working properly. Since we were by now passing CIVAC I started back down to the glideslope. Approaching JEDSU we switched to Tower. I was hand flying now. The Tower asked if we were on the glideslope. The FO and I looked at each other; puzzled. We had the airport in sight; exactly on glideslope. I told the FO to tell him we were exactly on glideslope. The Tower responded that that was impossible because we were supposed to be at JEDSU at 3;000 [feet.] He asked if we had a visual on the runway; the FO responded yes. At that time the command bars abruptly commanded fly up. I flew up to the command bars. Tower cleared us for a Visual Approach. I told the Tower about what we had just experienced. I told them that the sudden change of glide slope probably would have disconnected the autopilot had it been on.This was either a malfunctioning glideslope or more likely a temperature change that caused the glideslope to cross under the minimum altitudes. This phenomenon must not happen much at CLE; since it was obvious that the controllers did not know what they were dealing with. If they did; they would not have asked us twice to maintain the glideslope so that they could check it. I have never experienced this in over 25 years. Only once in the simulator; and it only resulted in a 30 foot deviation. Unfortunately; that did not leave much of an impression. I should have maintained the minimum altitudes all the way to the FAF. I did this initially; however Approach kept asking us to please stay on the glideslope.I will always brief the minimum altitudes and monitor them on ILS approaches. In the simulator training scenario that I experienced; there was only a 30 foot loss: however; the possible deviation was much more substantial in this event. It left much more of an impression. I will watch for this the rest of my career; however; I may never see it again. I have never seen the glideslope take me below crossing altitudes before. The Sim Instructor had taught me a dive and drive technique; verifying every altitude on the approach with the pilot monitoring; and resetting the altitude selector for every fix. That was very tedious; but worked great in a quiet simulator. I can't see using that technique at an airport like DTW; where the communications are constant on final approach. I will set the missed approach altitude when cleared for the approach and monitor the crossings like we do on a descend via arrival. If we do not comply with a minimum crossing I will then select Nav and Alt until we pass the fix; then continue down to the glideslope when past the minimum altitude fix. I will brief and or teach this to pilots that I fly with on the line; during IOE and during line checks.I think it would be good to put a small note about this risk and the requirement to cross check crossing altitudes in the ILS training section of the OM. Most pilots read this section of the OM once a year preparing for their training. It would be a good reminder. When one flies thousands and thousands of hours; and the glide slope always meets the minimum crossing altitudes; it's easy to let one's guard down.
CRJ700 flight crew is asked by ATC to follow the glideslope all the way down for the ILS 24R at CLE as a test. This results in crossing JEDSU below the glideslope intercept altitude of 3;000 feet.