First of all, which height can we rely on glideslope indication?

From what I know, we can trust and use glideslope to Decision Height, not even below Decision Height.

If we use glideslope even passing Decision height, Glideslope signal can become unreliable and it can impose some problem related to signal interference.

I can't find any reference on it. It's just my inference.

Second, PAPI is for a visual glide path. What I mean is, PAPI provides a descent angle when we see at our eye height at the cockpit.

However, Glideslope provides a descent angle that the airplane's glideslope antenna passes sited angle.

But, according to ICAO Annex 14 vol 1, aerodrome design, it says that PAPI collocated with ILS should be installed that it coincidences with the glideslope indication.

So, I think below decision altitude, relying on PAPI indication will make the airplane descent even as same as glideslope descent angle.

I know that every airplane's eye height is different. And, in some airplane that has a lower cockpit than the standard operating airplane at the airport, if the pilot uses papi after descent altitude, the airplane will need to climb for papi glidepath which should be higher than glideslope.

However, is using papi after decision altitude not okay even for standard operating airplanes?

What I want to say, in every case, only following glideslope's descent angle even after the pilot can see PAPI is allowed or not.


3 Answers 3


I couldn't find anything in the FAA regs that explicitly allows or disallows usage of either the papi or the glideslope below decision height. The regulation that seems interesting to me related to this issue states that you must always be able to make a descent to landing on the intended runway using normal maneuvers and a normal descent rate. Since the glideslope will take you down to the 1000' bars on most instrument runways, I see no issue with following the glideslope signal once visual. However, especially in smaller airplanes, it may make more sense to deviate above the glideslope to deploy flaps or other drag devices and adjust your descent profile once visual with the landing area.

  • 3
    $\begingroup$ Isn't it that same GS signal that guides the aircraft to the RWY surface in a CATIII ILS approach? $\endgroup$
    – RetiredATC
    Feb 6, 2022 at 4:34
  • $\begingroup$ I read the question as related to glideslope (ILS) vs PAPI indication lights. $\endgroup$ Feb 6, 2022 at 7:06
  • $\begingroup$ The OP said the GS was unreliable below DH. My question was whether the GS provided vertical guidance to the RWY surface for CATIII ILS approaches. $\endgroup$
    – RetiredATC
    Feb 7, 2022 at 1:54
  • $\begingroup$ Although Loc is used for lateral guidance all through the landing, at a certain point rad alt takes over for controlling the roundout and flare, which is why valid rad alt is necessary for Cat II and III approaches. $\endgroup$
    – John K
    Feb 7, 2022 at 4:01
  • $\begingroup$ @RetiredATC If the ground equipment and avionics are certified to Cat III, yes, but not true for ILS generally. $\endgroup$
    – StephenS
    Feb 15, 2022 at 16:45

Coincident does not mean the same angle, it means that they cross. The decision height has little or nothing to do with the guidance signal, the DA is designed to allow for obstacle clearance even with poor altitude-instrument accuracy and some pilot/plane limitations(like response time) during a zero visibility missed approach. Often DA is not determined by the approach side but is set so that terrain and obstacles can be avoided during the missed approach procedure with a minimum assumed angle of climb and the legal maximum altitude instrument error. Both vertical and horizontal obstacle clearance is considered. Instrument systems require much greater minimum horizontal clearance than visual systems as there is more chance for aircraft deviation and less chance for emergency corrective manuevers.

PAPI is an assistive guide, not a required path. Pilots of large aircraft such as the 747 know that they have a high line of sight and they will use three white one red as a PAPI target instead of 2 white two red.

  • $\begingroup$ The fix for a high cockpit like the 747 is a 3-bar PAPI, which gives you the same angle (typically 3 degrees) to a point slightly farther down the runway. Flying 3-white/1-red will give a steeper angle (not what the 747 needs) to the same point on the runway (which is what you're hoping to avoid). $\endgroup$
    – Ralph J
    Apr 16, 2022 at 13:25
  • $\begingroup$ Yet that is exactly what they do on a normal basis. It can actually lead to a shallower angle because the point where they switch from electronic glide slope to visual slope indicator will have them at 2r-2w and they reduce the mean decent angle to achieve 1r-3w before landing. The difference in 2-2 vs 1-3 is only 0.5 degrees anyway. (From memory, not for use in critical engineering.) Also the design of new final approach slopes must consider height of the aircraft in obstacle clearances during the visual segment, in such case a steeper angle may be required for the taller aircraft. $\endgroup$
    – Max Power
    Apr 21, 2022 at 22:29

The VGSI (PAPI or VASI) should be coincident with the ILS GS, and ideally both will have a Threshold Crossing Height (TCH) of 50ft. Exceptions will be noted on the approach plate.

There may be a slight additional discrepancy due to the difference in position of the pilot’s eyes vs the the GS antenna. This is something pilots of such aircraft learn to compensate for during training.

You should not use the GS below the charted DA/DH because there is no guarantee it has been validated below that.

In contrast, you can use the VGSI from as soon as you can see it (which may be several miles away on a clear day) all the way down to the flare, which will be guided by the radar altimeter in larger aircraft.

  • $\begingroup$ You cannot use the VASI or PAPI as soon as you see it. It has been validated for obstacle clearance out to only 4NM (VASI) or 3.4NM (PAPI) with a ±10° from centerline. It is a common misconception to think if a pilot can see the VGSI lights, they are clear of obstacles. See FAA AIM 2-1-2. $\endgroup$
    – wbeard52
    Apr 16, 2022 at 4:24

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