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Why are PAPI lights installed 300m from the threshold? What are the conditions that define the position of the PAPI lights from threshold?

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US DoT Order JO 6850.2B contains installation criteria for visual guidance lighting systems. The following is taken from that order.

For siting PAPI along the runway, in case an (ILS) electronic glide slope is present, the lights are located such that the PAPI visual approach path coincides with the one produced electronically as much as possible.

In case of runways without an electronic glide slope, the following criteria are considered for locating the PAPI:

  • Threshold Crossing Height

The TCH is the height of the lowest on-course signal at a point directly above the intersection of the runway centerline and the threshold. The minimum allowable TCH varies according to the height group of aircraft that use the runway, ... The PAPI approach path must provide the proper TCH for the most demanding height group that uses the runway.

  • Glide Path Angle

The visual glide path angle is the center of the on-course zone, and shall normally be 3 degrees when measured from the horizontal.

  • Obstacle Clearance Surface

The PAPI obstacle clearance surface is established to provide the pilot with a minimum clearance over obstacles during approach. The PAPI must be positioned and aimed so that no obstacles penetrate this surface.

PAPI

Image from US DoT JO 6850.2B Visual Guidance Lighting Systems

Basically, in case ILS is there, align the PAPI with that; else locate the PAPI such that it satisfies TCH and OCS.

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  • $\begingroup$ Excellent answer; I knew there would be some sort of technical order out there for this. However, I would question that given the presence of an ILS GP, the PAPI will be coincident; many are not. Take the KMKT ILS 33 for example. See the "VGSI and ILS glidepath not coincident" note in the profile view. See also KGEG ILS 21, KLVN ILS 30, etc. These are just ones I have personally experienced, but there are many others. $\endgroup$ – J Walters Jan 15 '16 at 13:49
  • $\begingroup$ Practically (rather than just by the TO), there are three indications that should coincide 1,000ft from the threshold. This answer of course mentions the two in the TO: PAPI and the GS. The third is the runway markings, particularly the aiming point as per code7700.com/images/runway_aim_touchdown_markings.png $\endgroup$ – rbp Jan 15 '16 at 13:52
  • $\begingroup$ @rbp Technical specs for aiming point markers can be found in AC 150/5340-1L $\endgroup$ – J Walters Jan 15 '16 at 14:03
  • $\begingroup$ Additonal reference containing technical specs for PAPI: AC 150/5340-30H $\endgroup$ – J Walters Jan 15 '16 at 14:10
  • $\begingroup$ good references. but not my point. $\endgroup$ – rbp Jan 15 '16 at 22:04
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PAPI Jersey
(Image Source: WikiPedia)

In this picture, you see that the PAPI is placed right next or near to the touchdown zone.

ToucdownZone
(Image Source: Aviation.SE)

Since you want guidance for this area, placing them near the threshold would actually make you aim too close to the threshold and increase the possibility of touching down too short of the runway.

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  • $\begingroup$ To clarify (first picture may be confusing) PAPI are usually on the left of the runway. $\endgroup$ – mins Jan 15 '16 at 19:09
  • $\begingroup$ Overrun that runway and you go right into the drink! That's quite demanding! ;) $\endgroup$ – FreeMan Jan 27 '16 at 17:36

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