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  • Andreas

On the PAPI, but clear of obstacles?

Imagine a clear night “type A” IFR approach to an airport in mountainous terrain. As the approach lights and airport environment come into view, you spot the Precision Approach Path Indicator (PAPI). After the minimum, you align yourself on the PAPI profile. What about transitioning to PAPI guidance a little earlier? In this article, we will examine the obstacle clearance provided by such a system. We focus on the PAPI, as it is the most common type of visual approach slope indication system [1].


Figure 1: The PAPI can be a useful tool but has some caveats…

Visibility of the PAPI

A PAPI is typically visible from about 3-5 NM during daylight and about 20 NM during nighttime [2]. If you do the math, you will find that during a type A approach at minimum visibility, the PAPI will come into view relatively late in the approach. However, on a clear calm night the temptation might be there to switch “early” to the PAPI.


Beware…


Ironically, the chances of seeing the PAPI while being outside of the PAPI obstacle assessment zone increase at night…


Technology

Traditionally, PAPI’s have been built using incandescent lights. In recent years, several systems have been modified to LED technology, improving the visibility [3]. This video has been produced by the FAA, comparing LED PAPI approaches to incandescent PAPI approaches [3]. Notice the ease of interpretation of the LED technology…


PAPI angular arrangement

A PAPI will consist of four light housing assemblies (LHA’s) with individual angular settings to provide the desired colour-coding. Normally, the installation is on the left side, but it may be on both sides or right side only [4].


Figure 2: PAPI colours for different vertical positions [4]

The vertical setting of the lights is such that the pilot observes the applicable colours as indicated in Figure 2 above. The angles for a typical 3° approach slope are given inFigure 3 below.


Figure 3: PAPI angles for a 3° approach slope as specified in [5]

Note that the PAPI as an optical approach aid has fundamentally different siting criteria than a radionavigation-based approach. 


Airports will do their best to match the PAPI indication with the instrument approach profiles, but this will always be a compromise and optimized for a certain aircraft size…This is further elaborated in the article referenced in the last paragraph.


PAPI lateral position

A typical setup is shown below: The normal position is on the left side of the runway and the light housing assemblies are separated by about 9 m. The values may differ for short runways.


Figure 4: Typical PAPI location with respect to the runway [5]

The PAPI installation might look relatively small from the flight deck, but once you happen to stand right next to one, you will appreciate its size…


PAPI obstacle assessment

Like any other “navigation system”, the PAPI comes with some limitations. A crucial one is the obstacle assessment area. It turns out that the rules are different, depending on the airport location…


What do ICAO SARP’s specify?

If we dive into Annex 14, we find the following information [1]:

  • An obstacle protection surface shall be established when it is intended to provide a visual approach slope indicator system.

  • New objects shall not be permitted above the obstacle protection surface unless such an object would be shielded by an existing immovable object.

  • Existing objects shall be removed except if shielded by an existing immovable object or when a study indicates no adverse effect on operations.


Crucially:

For existing immovable objects above the obstacle protection surface that could adversely affect operations, the following options are given [1]:

  • Raise the approach slope

  • Reduce azimuth spread of the system, so that the object is outside of the beam

  • Displace the axis of the system by max. 5°

  • Displace the system upwind of the threshold


How does this obstacle protection surface look like in ICAO terms?


Figure 5: Obstacle protection surface for visual approach slope indicators [1] (edited for readability)

Figure 6: ICAO PAPI obstacle protection surface dimensions [1] (edited for readability)

Key parameters such as slope and lateral dimensions of the obstacle protection surface are stipulated above.


Implementation in Europe

The rules in Europe are closely aligned with the ICAO SARP's as shown below:


Figure 7: PAPI obstacle clearance surfaces acc [5] (edited for readability)

So, in more practical terms the European rules provide an obstacle clearance surface of about 8 NM and 8.5° divergence (each side) for a code 4 instrument runway (more than 1800 m runway length).


Not quite the same in the US:

According to the FAA Aeronautical Information Manual (AIM), the PAPI will provide obstacle clearance when within 3.4 NM of the threshold and within +/- 10° of the extended centerline [2].

However: Recently, there has been some confusion in the US regarding notes on the charts concerning PAPI obstacle clearance:


US-catch #1: The units

As mentioned above, the FAA AIM specifies a distance of 3.4 NM (4 SM) for the PAPI obstacle clearance surface [2]. In contrast to that, the AC 150/5340-30J mentions a distance of 4 NM… [4].


US-catch #2: The modified obstacle assessment

Significant confusion has been created after new obstacle assessment rules have been introduced by the FAA in the Order 8200.1D and Engineering Bulletin EB-95 [6]. The first modification involved an increase of the assessment distance to 8 NM for commissioning-type inspections [6]. The FAA rules do not require airport operators to mitigate obstacles outside of the 4 NM area [6]. If such obstacles exist, this will typically result in a note on the approach chart. This has confused several operators, as documented in the ASRS report ACN: 1903843 [7].


The modified commissioning obstacle assessment method stipulated in EB-95 and FAA order 8200.1D may result in a note mentioning a distance beyond which the PAPI does not provide obstacle clearance. This does not modify the obstacle clearance of existing installations.


The second modification involves an angular expansion of the obstacle assessment area. As the light beams of the PAPI typically do not have a perfect “sharp” edge and the PAPI is not installed on the runway centerline but rather with an offset, a new Light Signal Clearance Surface (LSCS) has been established:


Figure 8: FAA PAPI Light Signal Clearance Surface [6]

For obstacles found to penetrate the LSCS up to 4 NM, the airport operator must take appropriate action, such as installing baffles, removing the object etc. [6]. For LSCS penetrating objects further away than 4 NM, the airport operator has to inform the FAA flight inspection team about their existence, which will typically result in a note on the approach chart [6].


PAPI and IFR approach slope (in)-compatibility

The article above covered the obstacle assessment and siting criteria outside of the immediate runway area. The PAPI system holds more secrets when you get closer to the threshold… The interested reader is redirected to this article which covers the PAPI geometry on short final (see Approaching the threshold: PAPI/ILS alignment and MEHT)


References

[1]

ICAO, «Annex 14, Aerodromes,» vol. 1, 9th ed. 2022.

[2]

FAA, «Aeronautical Information Manual (AIM),» 2024.

[3]

FAA, «PAPI approaches,» FAA, 2014. [Online]. Available: https://www.youtube.com/watch?v=pqLjZjYlHTs&t=406s&ab_channel=FederalAviationAdministration. [Zugriff am 2024].

[4]

FAA, «AC-150/5340-30J: Design and Installation Details for Airport Visual Aids,» FAA, 2018.

[5]

EU commission, «Regulation 139/2014: Aerodromes,» 2023.

[6]

FAA, «EB-95: Additional Siting and Survey Considerations for Precision Approach Path Indictor (PAPI) and Other Visual Glide Slope Indicators (VGSI),» 2017.

[7]

NASA, «ASRS Report ACN 1903843,» 10 2022. [Online]. Available: https://asrs.arc.nasa.gov/search/database.html. [Zugriff am 10 02 2024].

[8]

EU commission, «Commission regulation 965 (EU-OPS),» 2023.


 

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