Why are visual and ILS approach angles different for a Cessna 172?

Question

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 3^o 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 3^o 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 5.6^o (or min. 4.7^o 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]?

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[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.

Answer 1

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.

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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.

Answer 2

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.