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Since there are different dimensions of aircraft in various approach categories, I wonder what changes pilots in different aircraft have to make when following the PAPI lights. I know that 3° (for example) for one aircraft is the same for all other aircraft, but different categories might have different touchdown points when following the same glideslope due to the MEHT.

Since the PAPI is fixed, what differences are then when flying it in two different aircraft? Does the size of the aircraft change the flight path in any way? Will I always touch down at the same spot?

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The lights themselves are fixed, there is no way ATC or the airport operator can simply adjust them to suit the next inbound aircraft. If the pilot needs to they can fly the approach with 3 whites/1 red, but AFAIK that isnt really necessary in most situations (maybe for 747s and other longer aircraft).

It's worth remembering that the PAPI cannot provide precision to the foot. Having 2 white lights only means you are anywhere between about 2.8 to 3.2 degrees on a standard approach, which can be a considerable range when you're a few miles out.

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  • $\begingroup$ Thank's for the reply @Ben. I was more concerned for the operational point of view though. I'll try to explaine me better; since the PAPI is fixed what difference can I note flying it with two different aircrafts? Does the size of the aircarft change the path in anyway? Will I touch at the same spot anyway? $\endgroup$ – Andrea Ghilardi Aug 19 '18 at 11:34
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To expand a bit on @Ben's answer and considering your comments.

All airplanes generally fly roughly the same final approach slope ~3° at most airports and all aircraft will follow the PAPI lights if they are there. Depending on the size of the aircraft, its current load and the environment, the aircraft may vary its speed and pitch to fly this approach correctly. For example a 747 will come into a runway way faster than my Piper Cherokee but will fly the same approach path (more or less).

PAPI generally provides ~3° approach path but more specifically it provides obstacle clearance and a safe approach slope and if there is an ILS system the lighting will coincide with the approach system. Although some airports can have considerably steeper approaches.

  1. General siting considerations. The PAPI must be sited and aimed so that it defines an approach path with adequate clearance over obstacles and a minimum threshold crossing height. If the runway has an electronic landing system glide slope already established, the PAPI is installed as described in paragraph 502 so that the visual glide path angle will coincide with the electronic glide slope.

Flying at or above the PAPI lights will guarantee a safe approach and obstacle clearance in the area. The ILS and or PAPI systems take you to the Touch Down Zone which is where most aircraft aim on landing and where most will touch down in a regular approach. However for short runways (mostly flown in and out of my small GA aircraft) pilots may execute a short-field approach and landing which will attempt to put the aircraft down as close to the threshold as possible. In this case they may follow the PAPI close to the runway but execute a slightly different procedure when touching down.

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I fly VFR in a light sport aircraft in an airport with a PAPI system. Because the airplane that I fly has a steeper glide slope than larger aircraft, I may start out high on the PAPI, and then get low on the PAPI, all the while descending in a normal (for the airplane) approach. Also, I've been told that the PAPI at my airport is not only set up for the 3 degree glide slope, but also to aim for the landing flare at the 1000' lines. I can usually flare and land before that spot, adding to the inaccuracy in the PAPI.

When I first started flying, I would frequently get nervous when I saw three red on the PAPI. Then I discussed it with my CFI and he explained to me how flying the PAPI glide slope is not going to be as useful for me as it would be for other aircraft.

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