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NOTE: I fly in CANADA. Also, the question is specific to light aircraft like a C172.

It is known that the (standard) glideslope flown in an ILS approach is 3o to the T point[1]. However, that is not the case for a visual approach. Some basic high school math tells us that an aircraft 1 NM away from the T point[1] on a 3o ILS glidepath would be at 319 FT AGL, whereas I'm supposed to be at around 600 FT AGL (or, "ideally" 500' AGL- the same as when turning crosswind) when turning on (approximately) a 1 NM final from base in a VFR traffic pattern- making my "glideslope"[2] about 5.6o (or min. 4.7o if at 500').

What is the reason for this difference in glideslope between a visual and ILS approach?

An answer at this PPRuNe post says that the angle was chosen as it supposedly worked well for the flight computer to be able to AUTO LAND the aircraft. However, that begs the question: why enforce this even in a CAT 1 ILS approach where the pilot takes manual (and visual too, perhaps?) control way above the T point[1]?

[1] For the sake of simplicity, I use "T point" to interchangeably refer to the aiming point AND the touchdown point.

[2] I vaguely recall reading somewhere that the term "glideslope" only makes sense in the context of ILS but I use it to refer to my visual glidepath angle anyway again for the sake of simplicity.

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  • $\begingroup$ Why do you think you should fly such a steep final in a VFR pattern? I was trained to roll out on final exactly on the VGSI, where one exists. $\endgroup$
    – StephenS
    Aug 27, 2022 at 14:39
  • $\begingroup$ "whereas I'm supposed to be at around 600 FT AGL when turning on (approximately) a 1 NM final from base in a VFR traffic pattern- making my "glideslope"[2] about 5.6o." According to whom? Please provide a reference. $\endgroup$
    – Pilothead
    Aug 28, 2022 at 1:04
  • $\begingroup$ @Pilothead According to all the instructors at my school. The airport elevation is (rounded off to) 200’, so we’re asked to be at (about) 800’ at the beginning of final. I will cross check tomorrow and get back to you. $\endgroup$
    – PapaMike99
    Aug 30, 2022 at 6:36
  • $\begingroup$ @Pilothead I Cross-checked with my instructor who says the ideal height at 1NM final is 500' AGL, and another student whose instructor says approx. 600' AGL. Added an answer based on the conversations. $\endgroup$
    – PapaMike99
    Sep 4, 2022 at 20:13
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    $\begingroup$ Are you sure you want to always put yourself in a situation where you are 100% reliant on the engine to make the runway during VFR flying in a light airplane? I was taught to have the power at idle during VFR final approaches in single-engine light aircraft. This results in a glide path steeper than the ILS glide slope. $\endgroup$
    – quiet flyer
    Sep 5, 2022 at 1:43

2 Answers 2

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After discussing with a fellow student and my own thinking, I've come to better understand the situation and have an answer. Not the ultimate or THE correct answer, but definitely a logical explanation.

Visually approaching in a C172 at a higher than 3 degree glideslope provides better VISIBILITY especially when far away from the runway (1 NM is actually far away). The pitch/AOA required to maintain 60 KIAS makes it hard to be able to see outside. As we come closer to the runway, we come closer to a point where the 4.7 (or 5.6) degree approach path intersects with the 3 degree approach path so from (or shortly after) short final, the visual approach picture closely coincides with an ILS approach picture. Also as Max says in their answer, planes like C172 being light, the [slightly] higher approach path doesn't pose a problem to the energy management while we round-out and flare.

When we do an ILS approach in a C172, our primary reference becomes the CDI/HSI as well as marker beacons, so it's safely possible to maintain the 3 degree glideslope.

Not marking any answer as the CORRECT answer as of now because all the explanations I've learnt of so far point towards different ways of looking at the question and explores different reasons for doing what I do.

P.S. Apologies for using the words "glideslope", "glidepath" and "approach path" interchangeably.

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    $\begingroup$ I'd be careful about your visibility assumption. I'm not flying a mile-long final in a 172 at 60 kts. The common practice, that aligns with Cessna recommendations including the 172 POH, would not involve being at 60 Kts until nearly in the flare. The "usual" guidance is about 90 in the downwind, 80 in the base, turning to final at 75, and plan to cross the threshold around 65. Whether I am flying visual or the ILS, I am not flying at 60 kts a mile from the runway in a 172. A stable, decelerating approach at normal angles does not compromise forward visibility in any way. $\endgroup$
    – Max R
    Sep 5, 2022 at 3:14
  • $\begingroup$ @Max Makes sense! The POH does say (and my previous instructor did too) that the final approach speed doesn't have to be aimed for until you're actually on FINAL. Also, even though the speeds you mentioned are the same that many people follow, we here in BC, Canada are taught to be at 70 KIAS (flaps 20, 1500 RPM) on base. In fact, we configure the plane and establish this in the window between being abeam the aiming point and turning base at 45 degs from the RWY THR $\endgroup$
    – PapaMike99
    Sep 10, 2022 at 23:46
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Several factors and thoughts here.

  1. I think you might be flying a very light aircraft with very slow approach speeds that support steeper approaches, also meaning less vertical descent energy to be absorbed in the round out/flare. As you move to heavier/faster aircraft you’re going to need to be staying fairly close to 3 degrees. In my aircraft a descent steeper than 3-4 degrees is going to absolutely require one of two things. I’m either going to be flying the final well below 1.3x V(s) or I am going to float a long ways and need a very long runway, like literally 1,000 more feet.

  2. Remember that VASI/VGSI systems are most usually also 3 degrees. Turning in from one NM at 600 feet is putting you well above the VASI visual slope. Again, that may be fine for a slower/lighter aircraft but if you’re flying something like a 182 or Bonanza, it’s really, really hard to get that plane on the ground unless you are very deliberate about matching the right speed to the right descent angle.

  3. I very much disagree with your quote PPRuNe quote about auto land. The 3 degree convention for ILS distantly predates any modern auto-landing systems.

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  • $\begingroup$ Thanks for the answer Yes, you're right- I fly a Cessna 172. I have been trained to not look at the PAPI lights but try to achieve the "right picture of the RWY" while judging height on approach. Apart from that, I take it from your answer that the 3 degree angle was decided to make the NAVAID universally usable by all aircraft types (since both a C172 and a B747 can approach t a 3 degree glideslope), right? I haven't done any instrument approaches in the C172 yet, so I don't have a lot of understanding at this point. $\endgroup$
    – PapaMike99
    Aug 27, 2022 at 4:55
  • $\begingroup$ @PapaMike99 Remember that landing is the ultimate energy management maneuver. I have to bring my vertical energy to zero at touchdown and my horizontal energy to zero before the end of the runway. A light aircraft at 60Kt GS is descending at 600 FPM in order to go from 600 AGL to 0 AGL. 600FPM x 1900 Lbs (vert) and 60Kts x 1900 Lbs (horz) is well within the control authority available in a 172 on 2,000 feet of runway. 600FPM x 600,000 Lbs (v) and 150Kt x 600,000 Lbs (h) is a profoundly different problem to solve, the technical term for these kinds of energy is “an ass-ton,” LOL. $\endgroup$
    – Max R
    Aug 27, 2022 at 18:08
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    $\begingroup$ @PapaMike99 a loaded 747 would need a flare longer than the entire runway to fully arrest a 6 degree descent at 150-ish Kts. At some point, aeronautical engineers, landing gear structural engineers, runway strength civil engineers, land planners who had to buy runway land, found 3 degrees to be about the right compromise of safe approach speeds, reasonable flap and elevator sizes, reasonable brake design, reasonable land use, etc. $\endgroup$
    – Max R
    Aug 27, 2022 at 18:14
  • $\begingroup$ makes sense for an airliner. But don’t Cessna’s also approach at 3 degrees on ILS? If so then why? 3 degrees would put a 172 much lower than what we’re taught here in BC, Canada! $\endgroup$
    – PapaMike99
    Aug 30, 2022 at 6:40
  • $\begingroup$ @PapaMike99 it would be impractical to have a ground-based ILS system configurable to specific aircraft, and creates the possibility of the airport being mis-configured for the landing, not the airplane. We already have non-precisions approaches using WAAS GPS that would allow you to determine your own vertical profile at or above the minimum altitudes. But by not relying on a ground-based ILS, you become subject to higher minimums. So if you wanted to fly in steeper than the ILS, you’d request the GPS approach and manage your own altitude, within the limits of the published approach. $\endgroup$
    – Max R
    Sep 6, 2022 at 17:26

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