Glideslope versus Papi??
 

Did we ever ascertain why the Papi did not aligne itself with the glideslope?
Whilst flying the 'slope'to 200' upon transition one finds oneself in the 'red'
on the papi...Why is this so? Papi's are usually utilized on nonprecision runways for 'slope guidance,and yet they don't aligne on the ILS runways..
I know there's a question of gear crossing height,but if one did the Autoland
the gear still crosses the end sufficient to flare etc.
Any solutions??
cheers:ok:

Not sure but EBCI - Brussels Charleroi is a fine example!!!

EBCI is probably setup for the millitary fast jets there. It's the same in Eindhoven (EHEH) it's stipulated on the Jeppesens.

Below 200 feet you'll hear people say the PAPI are useless. Only way around it is to fly the aircraft visually and down onto the markers.

MK

well, here's Airbus own explanation, quote FCOM 3.04.34:
Eye to wheel height on approach is 25 feet and minimum recommended wheel clearance over the threshold is 20 feet. Do not follow Precision Approach Path Indicator (PAPI) guidance below 200 feet when PAPI Minimum Eye Height over Threshold (MEHT) is less than 45 feet

Everything clear now?
:cool:


live 2 fly 2 live

As I understand it, it's because PAPI shows the angle from your eyes to Papi while G/S shows angle from G/S antenna on board to on ground. While this is a small error between the two's 10 NM away, the error becomes bigger and bigger, and at 200 ft it is believed to be too big for a perfect height above treshold.

Thus, the recommendation not to use it.

Dani

A TRE told me once to stick with the glideslope and not the papi's as the glideslope is monitored and the papi's is not. (lol that kinda rhymes!)

Seem to recall (from I forget where) that they are set up for a 747, so everyone else seems to be in three reds.
Any 747 jocks care to confirm/deny?

For UK, PAPIS should be sited so that they match as close as possible ILS G/S. So PAPI MEHT should take this into account. Frequent ground checks of PAPIS should be carried out to ensure they are within tolerance. Also flight checking of PAPIS is a requirement.

UK CAA CAP168 Chapter 6 details the requirements.

Yes, good ol' CAP168. In Chapter 6 Appendix 6 is the set up procs for PAPI. But it stipulates that the origin of the slope for guidance should be co-incident with the G/S origin.

Double But, unless you are flying the exact a/c that the designers had in mind it seems unlikely you'll get an on slope indication.

Treble But, how can a set of lights positioned as required, be suitable when the electromagnetic beam is co-located but the a/c's aerial isn't dangling from the rear view mirror along with the Xmas tree air freshner and the fluffy dice?:hmm:

If during a CAT l App you become visual at 200', transfer to the PAPIs would seem the next step, but if there is no correaltaion to the G/S for non-design a/c then what's the value. If you get 4 reds do you climb? If you get 4 whites do you push?

Come CAA PAPI chappy give us a clue!

Sir George Cayley

I disagree, transferring to the PAPI's is not the next step!

A good recommendation is to do nothing - the aircraft was following the loc and g/s so do nothing and give your eyes a chance to take in the picture, maintain the picture.

This 'non-design aircraft' malarky is a red herring and might account for a variation of 5 ft or so difference at touchdown - insignificant.

PAPI's are not designed to gice accurate guidance below 200ft - tha is ahy you have Cat 2/3 operations!

I had a discussion with a European National Authority inspector on this issue. His reply- paraphrased - was thus:

"ILS glideslope is primary. It is flight checked regularly to more exacting tolerances than PAPI's. The airlield has some autonomy to positon the PAPI's as seen fit for the majority and/or the most limiting of their traffic; i.e. if there are many widebody movements then the PAPI might be set to give the correct THCH for their wheels. THis will have to take account of pilot eye height. It follows that smaller a/c will, when flying 2W 2R, will touchdown slightly longer but should have no stopping problems as the RWY is long enough for their larger brethren. If a RWY is served by an ILS then the PAPI's should ideally only be switched on if the G/S is U/S."

(It used to be that with 3 bar VASIS a medium jet flew 1W 2R and the big boys flew 2W 1R, if memory serves me correctly). It would be nice to have it published that the PAPI's are set for heavies and so mediums can expect to fly 1W 3 R below 500'. I too considered that PAPI's, as VASIS, are not certified below 200'. Thus I was confused to be told by a base trainer that he wanted 2W 2R down to 50'!
Further, why do some airfileds have PAPI's left & right illuminated at the same time?

They're called Double PAPI and I feel sure that your company's Route Guide will tell you about such things.

Ozexpat: Please expand. No our route guide does not mention them. I'm still curious as to their worth, especially as the ones I've seen are on an ILS RWY, illuminating at the same time as the G/S. Seems like a waste of amps/volts to me.

TopBunk - Advice to do nothing is good but non-type/red herring comment is incorrect. I would contend that the eye height between say a CRJ and a 747 is both significant and more than 5 feet.
In fact when converting to the 747, my BTC advised to look for 3 whites one red to maintain the path for most runways.

Lighting checks
 

PAPIS are flight checked along with all runway lighting every 6 months. This includes alignment c.f. radiating glideslope where applicable. Intensity, transistions and angles are tested. The quality of lighting units, obstacles or even long grass etc can affect results.

Military fields tend to have PAPIS set at 2.5 deg not 3 deg.

loopyloo:

Please read previous note. How did your BTC know what touchdown point the PAPI's were set up for? If there is an ILS, then the G/S is primary. If nothing, then Mark 1 eyeball is required. Using either PAPI or G/S Mark 1 eye ball is always the final arbitrater, but the G/S should be accurate. Airmanship at all times.
Flying something so vast as a B747 it would seem a touch presumptious to say 3W 1 R, if on a limiting RWY. Sounds like a guess to me, rather than an exact science.

FWIW in Canada the PAPI will be switched off when the ceiling is below 500ft I assume to avoid this "hunt the correct glideslope syndrome" at low altitude on a poor viz approach. This certainly wasn't the case when I worked in the UK.

PAPI should be set for a critical type of the airplane for one particular RWY, that’s why! :ok:

PAPI is an extremely accurate angular approach path indicator. Where it is installed with a precision electronic GS, PAPI should match that GS or publish a deviation.

A discrepancy between the visual and electronic GS may result from the aircraft eye/GS aerial distance being different to that assumed by the PAPI installation. Allowances for eye/wheel height are made as above, and in extreme circumstances a second ‘long body’ set of PAPIs can be installed further from the threshold for large aircraft e.g 747 – double PAPI. Both sets of PAPI’s should never be displayed at the same time.
An alternative for any discrepancy is that at very low altitude (normally below 100ft) the ILS GS is beginning to flatten out (reducing angle) due to the parabolic effect of the aerial location and transmission method (reflection) – GS above the aircraft, PAPI shows low; or possibly that the aircraft autoland system is raising the nose and flattening the approach angle with respect to the PAPI GS, PAPI shows high.

There is a long standing myth about PAPI accuracy, possibly due to the bad reputation of VASI, which was inaccurate below 200ft, and also that this fact was perpetuated by some PAPI installations being called PAPI/VASIs.
PAPI has been demonstrated in land-based ‘carrier’ type approaches and a wide range of specialist military applications – multiangle, two segment (civil and space shuttle), and autonomous resupply - all high accuracy systems at low altitude.

Normally a PAPI installation will attempt to match a one light change over (RR/WW to RRR/W) to the ILS deviation scale i.e. 1 light per dot. This is not always possible and some installations have a ‘tight’ or ‘slack’ beam where the transition angles are closer or wider than normal. Steep approaches usually have a tighter beam to compensate for the change in GS geometry.

The Canadian action is interesting – is this due to myth, or due to the potential to chase the very accurate beam (or overly ‘tight’ beam) at low altitude?

Only Cat 2 / 3 ILS beams are monitored; some Cat 1 beams are decidedly inaccurate below 200ft.
PAPI should be used with the ILS; the ‘sharp-transition’ light change-over enables any sudden sink to be identified quicker than with ILS – and it’s a head up display!
PAPI can ‘go out of alignment’ due to several reasons, most of which are checked by ATC e.g. first switch-on on a frosty morning, ice or frost may distort the beam, or a unit has been hit by a grass cutter!

Each PAPI light is mounted on a single pole - any one of these poles can sink, or rise, due to earth tremour. A pole can also sink after heavy rain that locally water-logs the soil. I've seen all sorts of problems like that, over the years since PAPI was introduced here.