Too low on PAPI?

Discussion about „there are 4 red PAPI lights, you are to low“ in landing phase.

The answer is just: No, I am not to low when I see 4 red PAPI lights.
But why? So lets start at the beginning:

First of all, let´s have a look on our example Runway, which is Runway 05 in Geneva LSGG.
Here are the main componets we are talking about:

1. Glide Slope Antenna of the ILS.
2. TDZ, where we want to touchdown with our main gear.
3. PAPI.

Let´s talk about the PAPI. and when we see red and when we see white lights: As we can see in the picture, we will have 4 reds at 2,5 degrees and below:

Now let´s talk about the PAPI and the Glide Slope Antenna of our ILS:
Thats roughly 105 meters distance from each other, or 345 feet.

What we need to do now? We need to draw two lines:

• 2,5 degrees line from the PAPI (remember „4-reds“ are 2,5 or below.)
• 3 degrees line from the ILS Glide Slope Antenna (or call it Glide Path, who cares)

WOWOW, thats to much….. Alright, let´s go into the details….

——COFFEE BREAK——

Limitations:

In every single case of scientific discussion, laboratory tests or wherever you mesure something, there is room for mistakes or let´s say inaccuracies. Some are acceptable, some not.

Where are the inaccuracies here?

• The height of the Glide Slope Antenna is not taken to account
• The height of the PAPI is not taken to account
• The position of the G/S receiving antenna and pilots position is not an exact value (but close to)
• Distance mesurement in Google Earth is not 100% accurate.

Is that acceptable?

Yes. Because this is only a general discussion. If the PAPI lights change at 110ft height or 90ft height, the general statement (No, I am not to low when I see 4 reds) will remain the same.

Conclusion:

The ILS Glide Slope and the PAPI Glide Slope is designed for large airplanes like A350 or B747 where the pilot sits 60 Meters (200 ft) or more in front of the Glide Slope receiving antenna. And yes, these guys see 2 white and two red most likely to prevent that their main landing gear plows through the front yard of the neighbours.

I talked to the gentleman who does the flight data monitoring and ask him what data he is looking for regarding the Glide Slope: FDM looks on the ILS G/S antenna only. PAPI is not a factor.

If you want to make the FDM guy happy, fly 3 degrees ILS G/S path, ignore the PAPI (or accept the 4 reds in your Gulfstream) and let the aircraft land in the TDZ. This prevents long landings and you can land nicely in the TDZ.

I am open for discussion any time 😉