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NOTE: I fly in Canada. Also, the question is based on my experience on light aircraft like a C172.

It is known that the (standard) glideslope flown in an ILS approach is 3°. However, that doesn't seem to be the case for a visual approach. Some basic high school math tells us that an aircraft 1 NM away from the aiming point on a 3° ILS glidepath would be at 319 FT AGL, whereas I'm taught 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 approach path [2] about 4.7° - 5.6°.

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 control (once visual) way before touchdown?

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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
    Commented Aug 27, 2022 at 14:39
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    $\begingroup$ "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
    Commented 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
    Commented 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
    Commented 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. $\endgroup$ Commented Sep 5, 2022 at 1:43

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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
    Commented 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
    Commented 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
    Commented 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
    Commented 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
    Commented Sep 6, 2022 at 17:26
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There is actually a very good reason to approach at 6 degrees in a 172: more of a chance you make the runway if the engine fails.

This is true for all single engine aircraft.

The 172 has a glide ratio of 1 to 9. Arc sine 1/9 = 6.4 degrees.

Keep in mind a headwind will shorten gliding distance. Flying an approach at 65 knots in a 172 puts one right at Vbg. A little high on base leg is even safer. This is where learning to forward slip is very useful.

Although landing under power is easier (RPM can be "fine tuned" with throttle), there is no chance making the runway at 3 degrees approach unless the engine is running. Multi-engine aircraft have at least one extra in reserve.

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  • $\begingroup$ Is the engine more likely to fail during the final approach, be it 3 degrees or 6 degrees, than during the rest of the flight between when you take off and when you get to that final? If the concern is to be in the best position to make the runway should an engine fail, how is that concern met for the other 95% of the flight? $\endgroup$
    – Ralph J
    Commented Apr 5 at 18:50
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    $\begingroup$ @RalphJ Save takeoff and climb, with the "other 95% of the flight", one has potential energy in altitude to glide to a safe place. A single engine on a 3 degree final will crash short of the runway if the engine is lost. Where I learned to fly there was a 2 story building that we had to clear to land on the runway. $\endgroup$ Commented Apr 5 at 20:15
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OP's answer[1]

As I've learned after going through my flight training and further discussions with lots of people (and reading the comments and other answer on this post), the visual approach path MAY or MAY NOT be 3° depending on what is being done. The following factors (and perhaps more) come in to play:

The aiming point

When visual, one can aim at the RWY THR, 1000' RWY length marker or even before the THR. The ILS glidepath is designed to make the 1000' marker the aiming point. Therefore, depending upon how far away the visual aiming point is from the marker, the visual approach path may closely or may not at all align with the ILS glide path.

Efficiency

An ILS approach is flown at a higher speed and lower flaps setting than a visual approach (just to keep the extra drag away in case a missed approach has to be done). A C172 is usually flown at 90 KIAS without flaps and the speed is only reduced to 60 KIAS and about 20° of flaps extended once visual - at an altitude that can be as low as 200' AGL. When approaching visually, the final approach/landing configuration is set up much before. To maintain a shallower descent angle with a higher flaps setting, a higher amount of thrust is required. This need can be eliminated through a slightly higher descent angle.

Operational needs

A VFR pilot may choose a steeper approach path and an aiming point further upwind of the RWY THR to provide for altitude in case of an engine failure.

Room for error

Lastly, the most obvious factor is that it is very hard to eyeball an exact 3° glideslope and, moreover, considering a margin of error in the attempt of maintaining a reasonable descent rate, it makes sense to be a tiny bit higher than a tiny bit lower. Getting lower and lower often leads pilots - especially new ones - to end up dragging in the approach, sometimes even below VREF.


Now that I think of it, I think the last two are the reasons why my school's policy was to teach students to do it the way as I've mentioned in the qeustion.

Bottom line is, a 4° - 5.5° degree glideslope isn't unreasonable considering the basic nuiances of visually hand flying an aircraft. That being said, a person can definitely get skilled at eyeballing an approach path very close to 3° and maintaining a good descent rate with practice. At the end, visually manoeuvering an aircraft just broadens the spectrum of what can and cannot be done and also the required buffer for error.

[1] I deleted my previous answer - even though it received positive votes - as it was far more inaccurate compared to this one.

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