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Do different planes (categories A through D) aim differently in the final stages before flaring? I'm asking about precision runways.

The reasons I ask:

  • An ILS' glide slope is not coincident with the PAPI for example (when does the shift from G/S to PAPI occur?)
  • There may be differences in the touchdown zone markings
  • Airplanes come in different cockpit heights and approach speeds.

By 'final stages before flaring', I mean when the landing is manually flown in VMC, when the shift from looking down to up occurs.

  • Example: you may be on the G/S but off the PAPI when you look up, which one do you follow based on the category?

There is plenty of anecdotal information all over the internet, so I'm looking for as factual an answer(s) as possible based on experience. This post for example (pprune.org) offers contradicting advice, one quotes a document saying, "PAPI provides guidance down to flare initiation," and mostly others saying to ignore the PAPI.

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  • $\begingroup$ You can fly the ILS with glideslope right to touchdown, and most modern aircraft auto-flare, are you asking how they follow the glide slope, or how they judge when to flare? $\endgroup$ – Ron Beyer Oct 2 '18 at 17:32
  • $\begingroup$ I've added an example of what I mean, but there may be more to it, let me know if more clarification is needed, thanks @RonBeyer. $\endgroup$ – ymb1 Oct 2 '18 at 17:47
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Because of the downrange offset between the PAPI and the GS antenna, only a P3 (jumbo jet size) PAPI (14 meters eye to wheels) gives a more or less parallel path between the ILS glide slope and the pilot's eye so that they are both aligned on short final. The other PAPI categories all have a deviation between the ILS slope angle and the PAPI slope angle, so that if you stay on the ILS glideslope all the way down, at some point the eye height becomes too high for the PAPI and you will see three whites even though you are right on the ILS.

You take this into account in training to learn to resist the urge to dive to correct because the last thing you want is to increase sink rate close to the ground. So really, once you are really close in, inside the airport boundary, you don't rely on the PAPI lights. If you were on-slope in a stabilized approach configuration at Vref at decision altitude, you just have to hold the pitch attitude and speed with maybe a few small pitch tweaks based on the runway sight picture and you will cross more or less at a wheel height of 50 ft. The goal is to hear "50" from the GPWS just as the threshold is passing below, and the speed tape pointer is right at Vref. From there the touchdown point takes care of itself using the normal flare/thrust reduction procedure you use for that airplane.

This Transport Canada Staff Instruction regarding the design of PAPI installations and issues with harmonizing them with ILS approaches gives some useful background: https://www.tc.gc.ca/media/documents/ca-opssvs/AC_302-009.pdf

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  • $\begingroup$ Very insightful, thanks. I didn't know the PAPI came in sizes. $\endgroup$ – ymb1 Oct 2 '18 at 19:56
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    $\begingroup$ There's 4 categories. I made an edit with a link to a Transport Canada memo that might be of interest. I think that most pilots once they get proficient on type don't really need the lights most of the time. I never had a problem eyeballing visual approaches with no PAPI and no glide slope when I was flying RJs. They're just like small aircraft but things move a little faster. $\endgroup$ – John K Oct 2 '18 at 21:08
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I think there is some confusion in the question

An ILS' glide slope is not coincident with the PAPI for example (when does the shift from G/S to PAPI occur?)

Example: you may be on the G/S but off the PAPI when you look up, which one do you follow based on the category?

According to this answer broadly speaking, the PAPI and Glide Slope should line up as much as possible so that if you fly the approach right on slope when you look out the front you should be on the PAPI correctly. If for some reason they dont align it should be noted on the approach plate and you will need to fly the various parts of the approach according to the conditions, if you are coming in visually under VMC then you should use your visual references if you are coming in under instruments you fly the approach and keep your eyes on the panel.

You should be able to follow either right down to the runway but once you are below the clouds and inbound on a visual approach (the last segment of any instrument approach) you will use what ever visual references the airport has available to you no matter your category. There are lots of ways to judge the flair and cockpit height is simply a factor that gets addressed in type training.

Category also does not affect approach path, it will dictate approach speeds and potentially minimums but the glide slope is the glide slope no matter what you fly.

In my experience classes are sometimes crossed when flying. Generally this applies to smaller GA planes coming into larger Class C and Class B airports and ATC asks you to keep your speed up. To keep traffic flowing and provided the aircraft is capable its not unheard of to have GA planes that typically approach ~90Kts come in much faster. I flew a Saratoga into KPIT @ ~125Kts (close to the max gear speed) which puts it well into Category B. Meanwhile I have flown the same plane into other fields at the more resonable ~90Kt approach speed, well in category A.

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    $\begingroup$ I appreciate addressing the confusion, better ask than have the wrong idea. $\endgroup$ – ymb1 Oct 2 '18 at 18:22
  • $\begingroup$ @TomMcW I have updated the wording a bit to reflect. $\endgroup$ – Dave Oct 2 '18 at 19:58
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The answer to that is a combination of the specific approach procedure being flown, the aircraft approach category, the specific aircraft type, weather, winds, etc.

As to the approach itself, you’ll fly that according to the published procedure. One major difference is that in slower aircraft, you’ll have considerably more time available from the DA/MAP to the beginning of the roundout where as in category D and E aircraft, the transition to the roundout is almost immediate.

Landing techniques vary based on the kind of airframe, wing configuration and approach speeds, but for most large jets, it’s a process of arresting their descent rate around 10 or so feet above the runway and assuming a nose high attitude with a gentle descent rate to the ground until the mains contact. It’s not a full stall type of landing like a light GA airplane and more like a soft field technique. The throttles are retarded to idle about 20-30ft AGL to rapidly drain off airspeed during the roundout. Exactly when to do this is a matter of kinesthetics - pilot just feels their way down based on their judgment of height, etc. It’s a skill that takes roughly 100 hours or so of flight time to fully perfect and passed down from captains to FOs during type rating and transition experience.

In regards to using visual glideslope aids as opposed to instrument glideslope aids, in the event they are not aligned, passed the DA or MAP, that’s up to the pic and their judgment as to how to place the airplane onto the runway, obstacle clearance, etc. Both methods are fine though only instruments should be used on an approach prior to reaching the DA or MAP/DH.

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I fly in small planes, Category A. We fly the Localizer/Glide Slope needles (i.e. the ILS) at about 100 mph, keeping the needles centered, until 200 feet above the runway, and if the runway is not seen at 200 feet above the runway power is applied and we go around. When the runway is seen, power is reduced and a normal visual landing is made.

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  • $\begingroup$ Ditto. Generally the transition from instruments to visual occurs at DA on the approach. However, if you are VMC with the field in sight you can take over visually pretty much any time. This applies to any category. I understand that ILS and PAPI GS may not always coincide, but generally the difference is not insurmountable, and probably within normal pilot deviations for hand flying. $\endgroup$ – Michael Hall Oct 2 '18 at 18:16

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