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Webinar 7 & 8 Q&A

Siting Q&A

Q1How harmonization of PAPI can be achieved with ILS and what considerations do we need to account for during PAPI siting.

Rubul Kalita
1.      Identify largest (l) and smallest (s) eye/aerial (e/a) dimensions for the range of aircraft using the runway.
2.      Based on the data identified above, calculate the average (e/a) value, (l) + (s) /2
3. Location upwind of ILS origin = average e/a x cotan 3

Note; further information in CAP168 and on-line webinar recording (slide 7)
Q3One question regarding the PAPI light, is there any regulatory requirements to have these lights on both side of the runways and if its required only on one side than on which side is preferred i.e on right or left of the runway ?

Sohail Akram
PAPI on one side provides complete glideslope guidance. Convention is siting on left but on right provides equally valid guidance.Tony
Q19Typically the PAPI is sited on just the left hand side of the approach direction. However, many UK military bases adopt PAPI installations on both sides of the approach direction. What are the deciding factors and pros and cons of each installation type.

Mark Timms
See Q3. There is overwhelming evidence that operationally only one bar is required. Putting PAPI on both sides was permitted originally because pilots felt that they obtained useful roll cues from the VASI bars on both sides and wished to retain this cue. In practice there are adequate roll cues from other lighting patterns.Tony
Q21Regarding harmonisation, in many occasions the design needs the eye-to-aerial height of an aircraft which may not be in DOC 9157 Part 4 or is not a published one. Any tricks on how to estimate if the aircraft manufacturer is not cooperative?

Juan Molina
If data is not available it may be possible to estimate the eye/wheel height in the approach configuration from basic aircraft dimension data. However, this approach cannot be used for the eye/aerial value unless manufacturer’s data is available. In extremis if a decision has to be made I would suggest using an eye/aerial value that is 50% of the estimated eye/wheel dimension. Availability of data should be of concern to regulatory authorities and ICAO. Tony
Q23Again on harmonisation, ICAO Annex 14 reads "The correction factor is obtained by multiplying the average eye to- antenna height of those aeroplanes by the cotangent of the approach angle." Should this average not be somehow compensated or weighed for the number of operations expected for each type of aircraft?

Juan Molina
Annex 14 Fig.5-19 (b) includes the words “for the range of eye/antenna heights regularly using the runway” This puts the responsibility for the judgement decision in the hands of the local competent authority. If ICAO is willing to review and amend location Standards to specify a small number of location values (as in CAP168) then pilots could have location data published on their “approach charts” so that they can decide if the siting complies with the requirements for the safe operation of the type of aircraft they are operating.Tony
Q26Typically we look to install the four PAPI wingbar units at the same optical level (horizontal plane) however there is also a requirement to ensure that the units are mounted as low as possible…, i.e. allow for any transverse slope with a lateral gradient of not greater than 1.25%…, what would the panel recommend as being the overriding criteria… a level horizontal plane or mounted as low as possible.

Paul Collins
If practicable I prefer a horizontal line. Remember that the innermost PAPI unit is sited 50 feet/ 15m outside the runway edge. This dimension defines the runway strip where no objects should be above ground level. Operationally if an aircraft is going to impact a PAPI unit outside the strip it probably has problems far more serious than striking a PAPI unit.Tony
Q28We need to see how to calculate the PAPI distance from the threshold in case of Non-instrument landing and in case of ILS based on Airplane type and size?

Khaled ElOrr
Yes. This is one reason that I am advocating a revision of the application paragraphs in Annex 14. It should be clear that there are many situations where there is no precision non-visual guidance. In this case PAPI siting only requires knowledge of eye/ wheel height. In some senses this is the main application of PAPI. For precision approaches PAPi is used for the manual phase of the approach (e.g Cat.1) and it’s availability also offers a reversionary mode in the event of a failure in the coupled approach system.Tony
Q31Dear Tony

Very glad to meet you. I want to ask how could I insure that I meet OS requirements in PAPI sitting?

Mohammad Ghashghaei
See Q13Tony
Q32Many times the platform on which the PAPI lights are mounted are not at the same level of the runway. What is the best cost effective for installation of PAPI in such site situations?

Anshul Jain
There is a mis-understanding in the Industry that the PAPI Wingbar height, at its calculated location, needs to be at the same geographical height above sea level as the centreline of the runway at that point.
The source of the light for each PAPI all need to be at the same height above sea level (excepting a tolerance of 1:25 slope variation transversely).
However, this PAPI height is be the same height above sea level as the runway height at its centreline but at the threshold. This is the datum height for ALL the runway activities and all dimensions associated with runway activities are taken from this height.
As such, all Glide Slope details use the runway datum height as the baseline height for both ILS and PAPI calculations and settings.
The use of the height of the runway at the threshold in all calculations then guarantees that the correct wheel clearance and approach obstacle clearances are achieved when locating both the ILS antenna and the PAPI locations.
Consequently, the PAPI bases, normally constructed of concrete for stability reasons need to be installed at the calculated height above sea level appropriate for taking into account the height of the PAPI light source centre ( ie. the centre of the transition zone between the red & white sectors and shall include the height of the installed units + legs).
HENCE, there are several considerations which determine the final location of the PAPI Wingbar in addition to WITH or WITHOUT an ILS:
• The calculated theoretical location of the PAPI Wingbar
• The height of the top of the concrete bases
• The levelness of the concrete base both longitudinally and transversely
• Max & Min heights of legs on the PAPI
• The difference in height because of rising or falling ground at the PAPI position longitudinally from that of the runway datum height at the threshold.
• Transverse slope on the adjacent runway edges
Q33Can we have PAPI's on both sides of Runway, Left & right side as a redundancy/clarity?

Anonymous Attendee
Q35Would Tony please explain why the original tests of PAPI were conducted at 6 degrees when (most) ILS approaches are conducted at 3 degrees and for Paul, if GPS approaches become more common are they more likely to be conducted at 6 degrees?

Thomas Perry
As I mentioned in my introductory remarks for the webinar, PAPI was devised to provide a facility for flight test evaluations of city centre airports. Test pilot opinion motivated the introduction of the device into conventional approach operations. It was in this environment where PAPI was demonstrated and evaluated in extensive flight trials over a period of 3 years at many airfields in the UK and at test sites in a number of ICAO member countries. We have enough operational experience to be confident that PAPI can (and is) useable at angels well above 3 degrees. (TS) Tony
Q36PAPI siting is based on the ground height at the siting position and the THR height. Is there a max/min height between the PAPI calculated OCL (at the siting point), and the height of the runway at that point?

Hendrick Esterhuyse
See Question 32 for explanationKeith
Q39Where can we have detailed document on PAPI siting, Harmonisation?

Sudheer N
I suggest CAP 168 methodology which is compliant with Annex 14 is the most helpful source of guidance that readily is available. The paper by Chris Hedge and myself which is on the TMS webinar 7 gives useful background information.Tony
Q44Dear All,

Thanks for the class I love it.

One question.Is it correct to install a new PAPI system behind an existing PAPI system as a redundant system?

Regards from Mexico.

Oswaldo Acosta
NO … For a correct signal, the location of the PAPI is calculated according to wheel clearance at threshold; with or without an ILS; sloping gradients of the runway and land area outside the runway edges in the location of the calculated location of the PAPI’s.

We must realise that the accuracy of the PAPI signals have a direct influence on flight safety as an approach path visual aid.

If you wish redundancy, I would suggest a second PAPI WingBar located on the right hand side of the runway calculated to be installed at the correct location!
Q49The regulations state that “On a runway equipped with a visual approach slope indicator system, the beginning of the aiming point marking shall be coincident with the visual approach slope origin.” can you elaborate more on what is meant by the visual approach slope origin, and how is it determined?
The aiming point marking is located at a nominal distance from the threshold as given in Table 5.1 in Annex 14 Vol 1 Edition 8 as a factor of the Landing Distance Available (LDA)
@ 300m of 1200-2400m LDA
@ 400m for 2400m and beyond LDA
HOWEVER ; The actual visual approach slope origin will be different for a PAPI system operating WITH or WITHOUT ILS because the calculated PAPI location is different when harmonising the PAPI with the ILS as opposed to operating without an ILS.
The visual approach slope origin is determined to support he largest aircraft needs which normally operates from the runway and where the pilots eye is located in the aircraft ie. in the cockpit at the aircraft’s maximum pitch angle (attitude) for landing.
• The glide slope origin for the ILS is the ILS antenna at its calculated location adjacent to the runway according to the glide slope angle.
• The visual approach slope origin is where the PAPI’s are located
A careful review of Annex 14 reveals that several, different definitions relating to visual cues are given. This should be part of the review of Annex 14 that is now important.(TS)Tony
Q50Can you please share details of PAPI lights distance (from runway edge and threshold) according to runway types.

Naveed Aslam
Annex Figure 5-19. Siting of PAPI and APAPI states 15m +/- 1m for PAPI (Edge). Threshold distance dependent on aircraft size.
Q53When ILS installation is yet to be done, what are the pre-requisites parameters which need to be considered during the sitting calculation of PAPI for harmonization of PAPI with ILS.

What may be the reason if PAPI doesn't get visible from 3NM during good visibility

Rubul Kalita
It is not possible to site the PAPI's for harmonisation with ILS unless the location of the ILS origin & set-up details are known.
If ILS equipment is not currently installed & in operation then at least the ILS glide slope origin must be designated.

If PAPI's (no ILS provided) have been installed previously to support manual landings and ILS is now to be introduced then a review/re-calculation of the PAPI locatio must be carried out to ensure correct harmonisation. (see also the answer to Q1)

Reasons for PAPI range being less than its design specification:
1. Light sources failed eg. for a single PAPI unit eg. 2 or 3 lamp PAPI"s .. one light source failure in either will result in low brilliancy output hence its range will be compromised.
2. Night brilliance level selected by ATC during daytime operations
3. Very low & bright sun immediately behind the PAPI's (transient / rare situation.) ie. Pilot looking into the sun during approach & landing

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