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PAPI guidance below 300 ft

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PAPI guidance below 300 ft

Old 15th Jun 2015, 12:45
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PAPI guidance below 300 ft

During flight calibration testing of visual approach slope systems, the relevant technical manual used by technical staff when these systems first were introduced, stated that these systems were not intended for use below 200 ft. The reason given was the aircraft should have been stabilised on a visual approach well before reaching 300 feet and that it was not possible to ensure the light signals gave accurate readings below that height. This was due to siting of the light boxes to one side of the runway giving rise to a lateral error.

It is common to see pilots using these systems for guidance well below 200 feet. Not only that, but pilots but still calling if the signals show below or above on slope when below 200 ft resulting in superfluous "support" calls that are both annoying and distracting.

There must be a cut-off height on short final where visual signals become invalid. Can anyone quote that height from an official document?

Further to that. Most international runways (in Australia, anyway) served by PAPI, have a published mean eye height over the threshold (MEHT) eye height over the threshold for long bodied aircraft types. Typically 74 feet. This invariably means a smaller type (737, A320 et al) using the ILS electronic glide slope indication will see three or sometimes four reds on a PAPI that is calibrated for long body types (B777 and similar length). If a smaller type is now using PAPI guidance solely, (ILS inoperative or not installed) does this mean that using the normal two reds and two whites will of necessity place that type higher over the threshold than desirable?

If that statement is true, then what combination of red/white visual signals should be tracked in order to produce the normal 50 feet wheel height over the threshold for (say) 737/A320 types. Appreciate quotes from relevant official documents.

Last edited by A37575; 15th Jun 2015 at 13:02.
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Old 15th Jun 2015, 12:53
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superfluous

I like the idea that there is a great deal of information coming out of the PNF.

Certainly things like: you are ugly and you smell funny is truly in the world of superfluous, but high or low, fast or slow ain't too bad.
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Old 15th Jun 2015, 19:19
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Don't confuse PAPI with VASI
The PAPI is a light array positioned beside the runway. It normally consists of four equi-spaced light units color-coded to provide a visual indication of an aircraft's position relative to the designated glideslope for the runway. An abbreviated system consisting of two light units can be used for some categories of aircraft operations. The international standard for PAPI is published by the International Civil Aviation Organisation (ICAO) in Aerodromes, Annex 14 to the Convention on International Civil Aviation, Volume 1, Chapter 5. National regulations generally adopt the standards and recommended practices published by ICAO. An earlier glideslope indicator system, the visual approach slope indicator (VASI) is now obsolete and was deleted from Annex 14 in 1995. The VASI only provided guidance down to heights of 60 metres (200 ft) whereas PAPI provides guidance down to flare initiation (typically 15 metres, or 50 ft).
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Old 15th Jun 2015, 22:06
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I cannot produce direct quotes from any documents, but I have read that PAPI was produced for low vis ops. My last employer discouraged its use below 3-400 feet saying aiming point was the way to go, however when I was employed in the UK we used it and I continued to do so with anything else that was available.

If a smaller type is now using PAPI guidance solely, (ILS inoperative or not installed) does this mean that using the normal two reds and two whites will of necessity place that type higher over the threshold than desirable?
Yes if you consider touching down a little longer is not desirable, which would be the case on a short runway but not necessarily so on a long one.
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Old 15th Jun 2015, 22:29
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Look into MEHT ie on a 767 with a MEHT of 53ft you would expect accurate signals all the way down however if the MEHT is something like 73ft you would expect the PAPI to give three red in the last 200-300ft as you drive to your aiming point
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Old 16th Jun 2015, 01:22
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A37575

The closest I can cite is what is contained in ICAO Aerodrome Standards:

5.3.5.37 The angle of elevation settings of the light units in a PAPI wing bar shall be such that, during an approach, the pilot of an aeroplane observing a signal of one white and three reds will clear all objects in the approach area by a safe margin.
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Old 16th Jun 2015, 01:24
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Originally Posted by A37575
If a smaller type is now using PAPI guidance solely, (ILS inoperative or not installed) does this mean that using the normal two reds and two whites will of necessity place that type higher over the threshold than desirable?
The only time a PAPI will have an MEHT of over 50ft will be on a runway that is served by long-bodies, which in turn means the runway itself is quite long. A smaller type will obviously cross the fence at greater than 50ft but will have more than ample runway to pull up on because the runway can take long-bodies.
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Old 16th Jun 2015, 07:46
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The design guidance can be helpful for the discussion...
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Old 16th Jun 2015, 11:11
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This discussion was thrashed to death a couple of years ago. The general consensus is that at 200' on a stable approach on glide path you should be concentrating on the touchdown point not some fairy lights.
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Old 16th Jun 2015, 11:40
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Originally Posted by Capn Bloggs
The only time a PAPI will have an MEHT of over 50ft will be on a runway that is served by long-bodies, which in turn means the runway itself is quite long. A smaller type will obviously cross the fence at greater than 50ft but will have more than ample runway to pull up on because the runway can take long-bodies.
True, but you need to be mindful of runways that are shortened for works but still have the 74' MEHT.
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Old 17th Jun 2015, 06:53
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Thanks for the instructive replies.
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Old 17th Jun 2015, 07:10
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RAT is right!
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Old 18th Jun 2015, 19:26
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PAPI Origins

You might be interested to know how PAPIs became a replacement for VASIs.

In the mid 1970s PAPIs were used as a visual aid for use by (principally) aeroplanes and a helicopter (HS748, BAC 1-11 and Wessex 2) operated by the Blind Landing Experimental Unit (BLEU) at RAE Bedford to assist pilots flying normal (3 degree), steep and two-segment approaches. Thanks to an initiative shown by the two airfield lighting boffins, JJ and TS, ICAO agreed that PAPIs might be employed as an alternative to VASIs on civilian aerodromes.

As one of the BLEU pilots (the clue is in my PPrune name), I used them on many occasions, with a chap on the ground adjusting them to a range of different approach slopes in the time it took us to fly a circuit. One trial we conducted on a clear day with our flight being monitored by highly accurate kine-theodolites involved each of three pilots flying three approaches to Bedford's long runway: one with PAPIs, one with VASIs and one with no visual light aids. The most accurate approach was that flown using PAPIs and the next most accurate was that using neither PAPIs nor VASIs. With VASIs it was all too possible to 'bounce' in and out of the relatively wide approach slope corridor.

We also conducted trials of the first (and only) British-designed GPWS, and when demonstrating its operation at Bedford, i.e. diving towards the runway, on a day when the visibility was not especially good, I used guidance from PAPIs to help me demonstrate to the aviation Press standing behind me how the GPWS functioned 'in a timely manner' whilst ensuring that we remained over the aerodrome. Interestingly, the Press were more fascinated by the PAPIs than by the GPWS!

Although ordinarily you would not expect to expect to use PAPIs below 200ft above touchdown elevation since by that time on a manually-flown approach you should have acquired sufficient visual cues to have confidence that you could follow-through to a safe touch-down, we noticed - as you might expect - that they still provided a measure of guidance below that height. This, however, was not and should not be relied upon as primary guidance for reasons already stated by other respondents.

After leaving Bedford I enjoyed (!) a spell in the MOD where, when a general question was asked if anyone had any ideas as to how finance that had been arranged for the introduction of Tornado but not spent due to delays might be used, I proposed and had accepted a suggestion that industry might be persuaded to invest in produced a few sets of PAPIs for use at Northolt, to facilitate accurate approaches over built-up areas, and at a Lightning station to reduce the risk of tail-strikes, again by helping to promote stable approaches.

After that, the idea caught on and now PAPIs are used across the globe. Incidentally, our proposal was that a set should comprise eight boxes with four on either side of the runway co-located with the glide slope origin. If you've flown an approach with this arrangement you will have observed that the gap between each group of four helps define that origin, and of course for the occupant of the right-hand seat the right-hand group is more in line with his/her view of the runway.

And this all began 40 years ago!
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Old 19th Jun 2015, 02:08
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OTHERS REFERENCES

Once on a PAPI, TWO REDS/TWO WHITES then below 2-300 feet its more important to keep a constant VERTICAL SPEED, typically around 700 fpm coupled with the aiming point AT A CONSTANT POSITION ON YOUR VIEW OUT THE WINDOW rather than chasing the PAPI.This is my instructional method, has worked fine over the last 26 years.
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Old 19th Jun 2015, 05:01
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PAPI considered not useable below 200ft
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Old 19th Jun 2015, 07:33
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Yet I heard of one 'superior' operator who taught base training so as to maintain PAPI to 100'. Dubious technique?

Last edited by RAT 5; 19th Jun 2015 at 08:59.
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Old 19th Jun 2015, 10:10
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DUBIOUS

Following a PAPI below 300 feet from the threshold elevation may or may not be dubious to some, however the really important information to the pilot is the IVSI rate of descent which can vary depending upon actual groundspeed on the day but is TYPICALLY about 700 fpm for most jets.If this is monitored and adhered to it is very likely that you will arrive just above the correct touchdown point in the "zone" so to speak and what happens next is dependent upon your flare technique, another issue altogether.Chasing the PAPI below 2-300 agl is not a good idea in most cases.I have conducted 100s of hours of base training in the real jet airplane and this mirrors what we have previously practiced in the SIM and it seems to work quite nicely in general..........
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Old 19th Jun 2015, 11:04
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Because PAPI is a system with a focused, projected, and ‘sharp transition’ beam, the limitations of its use depend more on the airfield installation, aircraft type, and its purpose.
Theoretically it can be used as low as it is possible to see the lights, depending on weather and task. Thus for a visual / night approach in a helicopter, PAPI could provide guidance all the way down to the pad (the PAPI boxes could be laterally angled towards the centreline).
In practical terms the height bands defined by the vertical angular difference in the colour changeover, which change with altitude – tightening as altitude reduces, dictate usability together with the aircraft speed and control response.

Airfield (runway) use depends to some extent on the range of tasks.
The discussion of MEHT relates to the centre of the beam, but might overlook a small range in height depending on the angular difference in the point of colour changeover. Adjusting these settings (within limits) could provide acceptable accuracy for a greater range of aircraft types. There is of course a point where larger aircraft would not meet the safety requirements and an alternative PAPI set would be required.
There is also some variability in an installation’s lateral displacement, angular setting, and a particular design's angular beam spread.

These aspects focus predominantly on the safety clearances at the threshold, but in low vis the system is used more a (spot) check of the primary guidance, opposed to a full guidance system. These checks (a quick glance) can be as low as it is practical to take time to look and the range of visibilities, e.g. one or two spot checks after Cat 2 visual contact.
For non-precision low vis approaches, the PAPI could become the primary guidance system after visual contact, and the system could again be used as low as practical. However, there should be a gradual transition from using PAPI for guidance to that of a spot check of the final visual approach and landing.

Changing the glideslope angle also affects the range of height between colour changeovers, thus a steep approach using standard changeover settings might initially appear easier to fly, but at lower altitudes it provides more viable – flyable guidance.

PAPI provides a highly accurate beam defining the glidepath, with low ambiguity in deviation relative to the beam centre than the older systems (VASI). It is important to consider how the system should be used, in what situations, and aircraft type. It is not necessary to have a hard cut-off / min-use height, but crews should have knowledge of how the system works and the difference between a guidance system and an aid to check / monitor the late stages of the approach.
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Old 20th Jun 2015, 11:34
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JESUS

Holy moly, that is a long winded way of saying...........actually im not sure what your point is.Whats wrong with looking out of the window at your AIMING POINT and keeping it in the same position in the window view coupled with the odd look inside to check AIRSPEED and SINK RATE?........whatever happened to basic flying skills.....JESUS
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Old 20th Jun 2015, 11:56
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PAPIs

There are some good replies here. Below your DA/MDA the approach should be visual using the correct RoD and visual aspect of the runway, not using the PAPIs.

Because of the way the PAPIs are set up a pilot who flies the PAPI to touch down will end up with a flat long approach from DA with power on to hold the speed. All of which will make for a long landing with too much power.

It is perfectly normal and correct for smaller commuter aircraft to be in 3 reds and one white below DA/MDA on a correct visual approach. It can be seen from the helpful diagram posted above that it is perfectly safe and clear of obstacles. There used to be very helpful diagrams in an older version of the UK CAP168 which are unfortunately not there in the current edition.

Some charts publish LDA for a particular runway and with very good reason also state a reduced LDA if landing beyond the PAPI.

In short I agree with the advice above.
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