GS vs PAPI/NPA vs PAPI

[Tiburon peligroso]

Would appreciate opinions regarding the following;

Flying a 3 deg slope often seems to put you low on the PAPI, why?

If flying low on the PAPI, what effect would it have on the touch down point and subsequent landing distance?

When should you transit from the GS to the PAPI?

Flying a NPA, should you continue on approach profile or transit to PAPI upon visual reference?
Thanks.

[ab33t]

I have never seem a GS put you below the papi I have seen the opposite though , where the light keeps changing from red to white on the high side. Papis are synced to the required GS for that runway . Remain on the GS until TD that wil put you in the TD zone

[kijangnim]

Greetings,
PAPIs are useful when the GS is U/S and you shoot a LOC only approach.
Or when flying Visual.:ok:
When you fly a full ILS, you dont care about PAPIs :}, what matter is your instruments :ok:

[CHfour]

I usually find that the glide takes me into 3 reds (B737). Papis aren't type specific so you'll often get errors due to differing eye heights so I'd always follow the GS.

[rudderrudderrat]

Hi TP, The ILS and PAPI installations are usually set for the largest aircraft expected at the Airport. There are different eye heights to wheel heights depending on the aircraft geometry. By following the glide path where the facility is set for a 747 say, then on a smaller aircraft you may see that you are in the reds on the PAPI.

The ILS G/P should be perfectly safe (unless there is a note otherwise). The PAPIs may be disregarded below 200 ft AGL provided your aiming point is safe.

10.6 – Aircraft threshold crossing height. The aircraft threshold crossing height is the difference between the eye height over threshold and the eye to wheel height of the particular aircraft in the approach attitude. Eye path to wheel path height for particular aircraft should be obtained from the airline or the aircraft manufacturer concerned Some common nominal aircraft eye path to wheel path heights are set out below:

Eye path to wheel path height (ft) Aircraft type
12 F 50, BAE 146
19 F 100, B737-200
21 B737-400, B707, DC9
23 B727
24 A320, MD-87
26 MD-80
30 A310-300, B767
33 A300-600
38 DC 10
45 B747
Source: ICAO Aerodrome Design Manual Part 4, Appendix 6)

[safetypee]

Where there is a good match between aircraft type (ILS GS aerial / eye ht) and the PAPI installation, then a significant advantage of PAPI is that it can be used to very low altitudes – well below 200ft, during the ILS to visual transition. In addition, this feature overcomes problems of ILS GS beam instability / parabolic bending at lower altitudes (below DH).

For NPAs, providing the aircraft type wheel / eye ht meets the obstacle requirements, then PAPI is the ideal guidance system providing both deviation and some indication of deviation rate.

The apparent / actual mismatch between GS and PAPI has been discussed previously in this forum; it is generally due to aircraft size characteristics.

Because PAPI is an angular system, flying low will tend to fly the aircraft back towards the normal glideslope origin; the wheel height will determine the size of the error, which for large aircraft may be unacceptable at the threshold.

For an ILS, use PAPI as the primary approach reference after deciding that the visual conditions are sufficient to land, but don’t discard the ILS completely.

Good NPA design should align the PAPI glidepath with the visual descent point, but caution as there are some exceptions. Use PAPI when you have good visual contact, but do check the approach plates for any warnings / caution about PAPI use.

[DFC]

The PAPIs may be disregarded below 200 ft AGL


The CAT 1 ILS is certified to keep you clear of obstacles down to a minimum of 200ft above the threshold.

The PAPI are certified to keep you clear of obstacles down to the threshold crossing height.

Disregarding a landing aid that provides obstacle clearance and a stable approach would not be what I would call safe standard of operation.

The on-slope indication of PAPI is usually quite small. This can be expanded slightly so that the PAPI agrees with the Glide Slope down to the glide slope minimum height (lowest DH).

Everyone has to remember that the ILS reference height (the height at which the GS antenna in theory crosses the threshold is not established by measuring the position of the position in space where the signals provide an on slope indication. No. this height is derived by extending the straight line slope on final to DH until it crosses the threshold.

The PAPI provide suficient obstacle clearance for an aircraft with 3 red and 1 white to clear the obstacles. i.e. the obstacle clearance slope starts a good diatance before the threshold and is at a lesser slop[e than the approach path.

St Gallen in Switzerland LSZR is a great example fro showing these issues.

The ILS and PAPI both have 4 degree slopes. However, the ILS is set further along the runway than both the PAPI and the aiming point.

The ILS threshold crossing height is 48ft. This means that if the pilot follows an on-slope indication to DH then the extension of that trajectory followed by the GS antenna will cross the threshold at 48ft. There is a 100m displaced threshold so the GS antenna's trajectory will cross the start of the runway at 71ft.

The PAPI will cause the pilot's eye's to cross the threshold at 7m (23ft). Again the 100m displaced threshold causes the pilots eye to cross the start of the runway at 46ft.

You can see that in both cases the PAPI on-slope indication is 25ft lower than the ILS trajectory.

Normal operations and standard LDA apply when the aircraft crosses the threshold at 50ft.

If however the performance is based on short field operations then the threshold crossing height will be less.

It is clear that in the case of St Gallen, one can follow the ILS trajectory all the way down provided that the landing performance of the aircraft does not rely on short landing operations. The PAPI will indicate that the aircraft is high. The GS indications are not certified below DH but should not have any "significant" bends.

Following the PAPI indication will provide the required obstacle clearance. See the ILS 10 approach chart where it clearly requires the pilot to not fly below the PAPI indications due to obstacle. Clearly the PAPI indication is going to keep the aircraft clear of the obstacle. The PAPI are also going to combine with the Aiming point markers but not the ILS.

the angular difference between the PAPI on slope and the ILS trajectory at the threshold as viewed from the PAPI is about 4 degrees if my basic calculations are correct. To expand the PAPI on slope indication would be impossible. (That is why this is such a good example).

Following the appropriate PAPI in the approriate coverage area will always be safe (in terms of obstacle clearance).

Ignoring the PAPI could be (very) dangerous.

No safety minded operator in their right mind would ever recomend or condone normal operations that ignored a serviceable PAPI instalation as a general rule. The only exception being where the approved approach procedure fro that runway is going to have the PAPI showing high and other elemets are used to prevent unstable approaches.

To simply say something like "The PAPIs may be disregarded" would be giving the lawyers a blank cheque if someone hits an obstacle on approach.

[rudderrudderrat]

Hi DFC, We had notes in our Landing Performance manuals to disregard the PAPI below 200 ft AGL at certain airports because of the minimum wheel height crossing the threshold. We aimed at the ideal touch down point and disregarded the high indication on the PAPI.

From CAP 637:
8.5 The Minimum Eye Height over Threshold (MEHT), which is notified in AIP, is a reference value, calculated with respect to the promulgated approach angle for each PAPI. It is the lowest eye height over the runway threshold at which an on-slope indication will be seen. From examination of published MEHT, it may at first sight appear that for some runways, adequate wheel clearance at the threshold is not assured for all types of aircraft likely to use those runways. However, a typical eye height achieved in practice when crossing the threshold following well established 'on slope' approach would in fact be well above the published MEHT value.

[Sir George Cayley]

If there was a huge problem with ILS Glide Path signals being wildly different to PAPI approach slope indications, don't ya think we would've heard about it by now.:confused:

The maths vs reality seem to show that the margins of error are sufficient. Unless you're gonna fly a 747 into 2200m where the design a/c is a 737 :eek:

There's bigger problems than this.

Sir George Cayley

[DFC]

We aimed at the ideal touch down point and disregarded the high indication on the PAPI.




I hope that the instruction was very clear that only "high" indications were to be ignored.

A general "ignore the PAPIs" says ignore low indications also. So one could have pilot A who flies in at 3 degrees to a deeper touchdown paralleling high on the PAIPs but you could have pilot B who flies a 1 degree approach to the same touchdown - also ignoring the PAPIs - until the gear drags through the trees!! As I said - a blank cheque for the lawyers if anthing happened.

For turbine CAT operations I would expect such a measure to be very temporary in nautre and only for as long as it took the aerodrome authority to relocate the PAPI.

[Tiburon peligroso]

DFC
Would appreciate your opinion on this please.

If flying low on the PAPI, what effect would it have on the touch down point and subsequent landing distance?
Thanks
TP

[fireflybob]

Surely any indication (whether PAPI or ILS GP) that the a/c is low on approach should be acknowledged and corrected for unless specific chart notes or NOTAMs specify otherwise?

[DFC]

Tiburon peligroso,

The aircraft will be flying a flatter approach.

There are a few ways to look at this. Imagine a perfectly flat surface with a very long runway.

The first aircraft makes a 5.5 degree approach.

The aiming point is the marks but the trajectory of the aircraft is partly down and partly along the runway. Changing the trajectory at the roundout (rapid increase in drag) uses energy. Therefore assuming perfect technique the aircraft main wheels will impact the ideal touchdown point and the aircraft will at that moment have say X amount of energy pointing along the runway (X has to be absorbed by brakes etc to stop).

Next approach 3 degree slope the aiming point is the same as above. There is less downdard component to the trajectory and more of a horizontal component. Again assuming the pilot uses the perfect technique (less of a roundout - uses up less energy), the aircraft will again impact the ideal point but with more than X energy along the runway. So stopping distance will be longer.

Finally, the aircraft flies just less than horizontally to the runway. Say 0.5 degree approach angle. In this case, there is very little downward energy. Most of the energy is along the runway and with no roundout there is not using up of the energy and so we have the highest amount of along runway energy to be absorbed by the brakes.

The certification requirements are very clear at what type of approach must be used and at what speed for testing the landing performance. This prevents the test pilto from using an idle power steep approach with a big roundout to acheive very short distances.

Next think of the above 3 cases while remembering that the basic Landing Distance is measured from below the point where the aircraft is at 50ft to stop. That would be a very long distance in the last example. ;)

Perhaps the easiest way to understand the principle is to drop a stone vertically onto the ground at an impact speed of say 60Kt. Landing distance = zero.

Now shoot the same stone on an almost horizontal angle so that it again impacts the ground at 60Kt. Measure the distance from where is touches the ground until it skids to a halt. Same stone, same speed, Bigger distance.

As for the PAPI.

If they are all white then you can not tell your approach angle other than it is steeper than the set slope. You could even be in level flight or climbing. If they are all red then the same applies but lower then the set slope.

With 3 white and 1 red, one can check Annex 14 and see what the possible range of angles are which provide that indication. Same goes for 3 red and 1 white.

However, provided the PAPI are not all red or all white and not changing indication then the aircraft is travelling towards the aiming point and the aim point should be a constant angle below the horizon.

----------


Surely any indication (whether PAPI or ILS GP) that the a/c is low on approach should be acknowledged and corrected for unless specific chart notes or NOTAMs specify otherwise?


Bob,

You know that. I know that. Does the litigation expert who finds "disregard the PAPI indications" on a company instruction after an aircraft hits an obstacle care? They would laugh all the way to the bank with a clear unqualified instruction from the company to disregard a piece of safety equipment!!

Even if it did not have a direct link with the accident they could use it to show that management expected pilots to disregard safety warnings. :eek:

[rudderrudderrat]

Hi DFC. You'll find that I said "The PAPIs may be disregarded below 200 ft AGL provided your aiming point is safe." Please don't selectively quote out of context.

From Manufacturers' Manual: "Eye to wheel height on approach is 25 feet and minimum recommended wheel clearance over the threshold is 20 feet. Do not follow PAPI guidance below below 200 feet when the PAPI Minimum Eye Height over Threshold (MEHT) is less than 45 feet."

So if you are flying a B737, the ILS at LGW will put you in the 3 reds 1 white PAPI (which is set for B747) when you are below 200ft. At airfields where the MEHT is less than 45 feet, then I have to fly 3 whites 1 red (or whatever it takes) to ensure my aiming point is safe.

[DFC]

Hi DFC. You'll find that I said "The PAPIs may be disregarded below 200 ft AGL provided your aiming point is safe." Please don't selectively quote out of context.



A person complying with your statement could make a very shallow approach (4 reds on the PAPI) and think that their aiming point was safe because the can see it and see no obstacles between their eye and that point. Unfortunately, their eye is not at the wheels which could be dragging through the trees.



At airfields where the MEHT is less than 45 feet, then I have to fly 3 whites 1 red (or whatever it takes) to ensure my aiming point is safe.


You really need to look again at how PAPI work.

With a 45ft MEHT, flying with 1 white and 3 red gives you an extra 5.3ft.

Because unlike VASI based systems, all lights have a common origin.

So if the MEHT is 40ft, flying 1 white and 3 red only gives an extra 4.7ft. So you will not get your required MEHT by flying 3 white and 1 red.

For PAPIs, the method of increasing MEHT is to move the lights deeper into the runway.

These issues are not something that can be done on the fly. They require someone to sit down and wprk out the issues. Remember that a lower MEHT at the threshold can be acceptable in the case of a displaced threshold.


So if you are flying a B737, the ILS at LGW will put you in the 3 reds 1 white PAPI (which is set for B747) when you are below 200ft.


A totally separate issue. The PAPI are set for a B747 (A380?) but so too is the ILS. If that was not the case then how would the B747's wheels clear the threshold during a CAT 3 autoland?

I think here you are confusing eye to wheel height with eye to GS antenna height. Not only do these vary from type to type but they can also vary quite a bit in the same type.

Even forgetting PAPI and ILS for a minute, dont forget that the distance from the threshold to the aiming point varies with runway length. -

150m for less than 800m

800 to 1199 is 250m

1200 to 2399 is 300m

2400m plus is 400m

So even with no PAPI and no ILS if you fly a normal 3 degree approach to a 1100m runway you will cross the threshold lower than when you fly into a 3000m runway if you use the standard aiming point.

[rudderrudderrat]

The PAPI are set for a B747 (A380?) but so too is the ILS. If that was not the case then how would the B747's wheels clear the threshold during a CAT 3 autoland?



Because on a 747, the second glide slope aerial is on the landing gear door - some distance below the pilot's eye height.

[kijangnim]

Greetings,
We learn everyday, a second Glide Slope ANtenna on the landing gear door:confused:
The B777 is as long and I never heard of it, could you please bring some more details

[rudderrudderrat]

Hi kijangnim,

From memory, the first G/S antenna is near the nose of the aircraft and is used initially. When the gear is lowered, then a second antenna on one of the gear doors is used for the rest of the ILS.

Maybe it's because the 747 pilots are on the second floor higher up than 777. This old link has some more info: see AirRabbit's

archive/index.php/t-273878.html (http://www.pprune.org/archive/index.php/t-273878.html)

[Capn Bloggs]

The PAPI are set for a B747 (A380?) but so too is the ILS. If that was not the case then how would the B747's wheels clear the threshold during a CAT 3 autoland?
And furthermore, all the PAPIs at the major airports in Oz have been moved so they indicate on-slope for long-bodied aircraft flying the ILS. ILS TCH 50ft, PAPI MEHT 72ft.

That means for us "littlees", we get a "fly up" on the PAPI if dong an ILS, or alternatively get a "fly down" on the ILS if we're using the PAPI. Either is quite satisfactory as the runway is, well, only 3000m+ long. :ok:

[assymetricdrift]

*Edit* This is based purely on a smallish aircraft though, so it might not be reflective of the bigger picture.

Going into somewhere like Gatwick, if you follow the papi indication on the way down, you will find that your glide slope indication goes far too high. If you follow the ILS indication below minima you'll see that at 150 feet or so, you hit 3 red lights.

It's an interesting point, as obviously we are all taught to follow the papi indications on the way down, but there are many airports out there whereby if you follow the indication, by the time you have flared, you will be outside the TDZ.

Now, my question would be if you were doing a Cat II/Cat III landing, and with the autopilot disconnected at MUH, what the indication on the PAPIs would be? We did a practice Cat II into an airport recently, and when we came out of the simulated approach at 100 feet RH, the PAPIs were showing 3 red and 1 white.

I can't help but feel that maybe some papi bars out there are configured for the fuller figured aircraft...

I might of course be totally wrong though!

[Capn Bloggs]

Assymetricdrift,
Going into somewhere like Gatwick, if you follow the papi indication on the way down, you will find that your glide slope indication goes far too high. If you follow the ILS indication below minima you'll see that at 150 feet or so, you hit 3 red lights.
What is the TCH for the ILS and the MEHT for the PAPI? This will probably explain what you see.

Now, my question would be if you were doing a Cat II/Cat III landing, and with the autopilot disconnected at MUH, what the indication on the PAPIs would be? We did a practice Cat II into an airport recently, and when we came out of the simulated approach at 100 feet RH, the PAPIs were showing 3 red and 1 white.
From a previous thread (http://www.pprune.org/3485320-post17.html) on this issue, the Canadians switch off the PAPI during low vis ops so this phenomenon doesn't occur. If you're on-slope on the ILS at 100ft, doing 10ft/sec, you must be pointed at the correct aim point and unless you do something crazy with the flight path you're going to "hit" the runway at the right point. What the PAPI shows is irrelevant.

[planeboy_777]

I think this will sort out my doubt

I'm flying an A320 whose eye-to-wheel height is 24 feet.
the Airports TCH is 50Feet


Situation A:
if i'm approaching with the help of PAPI as my refrence
what will my Radio altimeter read?

Situation B:
if i'm approaching with the help of ILS GS indication as my refrence
what will my Radio altimeter read?



Please help :sad:

[Centaurus]

I am going back many years here but in another time I was working with the Australian Dept of Civil Aviation Navigation Aid Calibration Unit. This involved flight testing of ILS and also T-VASIS. Although the ILS was flown down to 50 feet to look for aberrations below the nominal Cat 1 of 200 feet, it was not intended that aircraft would follow the ILS glide slope below 200 feet since measurements below 200 ft were not always reliable.

I am sure measurements are more reliable now with the advent of lower DH. On the other hand the T-VASIS was only flight calibrated to 200 feet and it was never intended to be used below 200 feet because of lateral siting.

VASIS and PAPI are visual aids to give an accurate visual slope from 4 miles and again it was never intended to fly an ILS in IMC to 200 ft and then use the PAPI/VASIS below that. In any case if the aircraft is stabilised on the ILS at 200 feet there should be no need to further "fine tune" below 200 ft as the aircraft is within seconds of crossing the fence and heads should be up unless Cat 2 or 3 ILS.

Attempting to switch to a PAPI glide path indication from an ILS at 200 feet is asking for trouble as the siting of the PAPI may not be the same position as the ILS glide slope transmitter. In other words don't make things unnecessarily complicated when there is simply no need.

[compressor stall]

By my calculations, for every foot your eye height (published in your AFM) is less than that of the papi (in some countries published on the Jepp charts) you will land 7 meters long.