PAPI and ILS glide slope not in agreement

[Evros 310]

Hello,

Lately I have found that PAPIs for many sceneries do not coincide with ILS glide slope. It all looks good until about 2nm out where I start seeing three red, indicating that I am too low. Should I continue on ILS path, I touch down directly at TDZ while PAPI might show four red by half mile final. Should I follow PAPI, I would hit the runway too early or crash before it altogether. It's not for all airports, but I think it's for the majority of them. I only use addon airports. P3D v4.5H2.

Thanks

[Chock 17,769]

It will depend on which aeroplane you are flying as to whether the ILS agrees with the PAPI/VASI/markings. It is a known issue in real life, which is why the markings on a runway are designed to compensate for it. To explain... 

If you are in an average-sized aeroplane, the visual approach aids (either the painted aim point marker, or the PAPI/VASI lights) will work just fine to put your main wheels down perfectly on the touchdown zone. But, imagine the same approach in a Boeing 747, where you are thirty feet higher up than you were in that average-sized aeroplane. In that 747, you are going to be seeing more white lights on the PAPI when coming in for the exactly same approach path which that other aeroplane flew simply because you are sat higher up. If on the other hand you dropped your approach path in that 747, to visually agree with the PAPI lights, you would be undershooting the touchdown zone slightly because the 747 is massive and very long with the main wheels a very long way back from the cockpit.

Because of this phenomenon whereby the approach aids will differ for different cockpit heights, the markings on a runway are positioned to give you some leeway in where you touch down, because the lights alone will not work perfectly for every size of aeroplane. So a runway's marking are designed to compensate for that, like this:

The start of the runway where the concrete is fully capable of taking the weight of a touchdown is the piano keys, any bit before that (sometimes marked with chevrons) is merely a blast area. Then 500 feet past the piano keys, is the touchdown marker (so if you undershot by 500 feet, you'd still be on the runway), then 500 feet past that, is the visual aim point which is typically along side the PAPI lights (so if you overshoot the touchdown marker by 500 feet, you are still okay). Then every 500 feet after that there is another marker right up to the middle of the runway, where of course the markings reverse to work for the other direction.

So the margin of error can be quite large between the visual cues and the ILS, but the runway is designed to mean this is still okay if you are anywhere well within that 1,000 foot range which straddles the ideal touchdown point. This is why you will see a big black streak from tire marks which is usually well over 500 feet long across the touchdown point on most runways. It's all the different types, big and small, either landing a bit short or a bit long of the ideal marker point.

I used to know a Boeing 747 pilot who also flew gliders at my airfield, and you could always tell when he was at the controls of a glider on the approach, even if you were miles away; he'd be the one flaring his glider when it was still fifty feet off the deck because he'd been flying 747s all week and had got used to the visual picture from up high in the flare. 🤣

I suspect he might have been undershooting visual approaches a bit and greasing it down near the piano keys in his 747 as well after a weekend of getting used to flaring a glider.

 

 

Edited June 4, 2020 by Chock

[jon b 2,047]

Nice explanation that Alan, I could do with showing it to  a number of 747 FOs who I’ve flown with who just don’t understand this concept of PAPIs obviously working on eye height not wheel height.

A lot of 747 operators stipulate a height from  where the  PAPIs should be ignored , typically below 500ft or 200ft. I was starting to get a little annoyed with FOs calling “3 whites” at me on very short finals below 200ft. Firstly it’s a non standard call , secondly the aircraft, if they’d looked they’d see the aircraft was spot on the electronic glidepath.

The 747-400 actually has 2 glide slope receivers due to its sheer size. The first known as the capture antenna sits in the nose cone, then there’s the tracking antenna that sits on the nose wheel door which works once the wheels are down and is used for the actual landing as it is more in line with main gear height.

Often approach charts are annotated with remarks that the PAPIs and glide slope are not coincident 

 

[Alpha Floor 237]

In real life it also happens that PAPIs are sometimes set to a different angle that the ILS glideslope (although I doubt this is the case in P3D, I may be wrong.)

The correct procedure is: follow glideslope down to minimums if you are flying an ILS approach. Ignore them past minimums. Follow PAPIs if you are flying a visual approach.

[Vineguy 26]

I fly for a virtual airline that for them to grade your landing as optimal you have to touch down between 100 and 300 FPM and within a certain distance from the threshold. I fly the 737-800 and on approach the localizer it will show I am too low and so will the VASI or PAPI  lights. Should try to land just past the piano keys to be around 1200 feet at touchdown?

[Alpha Floor 237]

1 hour ago, Vineguy said:

I fly for a virtual airline that for them to grade your landing as optimal you have to touch down between 100 and 300 FPM and within a certain distance from the threshold. I fly the 737-800 and on approach the localizer it will show I am too low and so will the VASI or PAPI  lights. Should try to land just past the piano keys to be around 1200 feet at touchdown?

You should aim for the "aiming point" which are 1000 ft down the runway from the threshold. After flare, usually you will touch down slightly past the aiming point but within the touchdown zone. Don't obsess with vertical speed; it's more important to touch down with the correct attitude, within the touchdown zone, aligned with the centre line and without floating too much. 

Have the aircraft perform an autoland to see how it does it. 

[KenG 492]

Well first and foremost look at the IAP.

 

Many of them will tell you right in the profile view that VGSI (Visual Glide Slope Indicator) and ILS glidepath not coincident. That means when you are on the ILS GS you will not be on the PAPI.

The approach chart will even go so far to tell you by how much you will be off. VGSI Threshold crossing height is 71' while the ILS crossing height is 53'. So if you are on the GS you will be below the PAPI on this approach. 

This has to do with the placement of the GS antenna and the placement of the PAPI lighting system. In this case the GS antenna or the area where the signal is reflected is closer to the threshold then the PAPI. 

Procedures produced by PANOPS are not as stringent as TERPS so PANOPS procedures may not have the VGSI - GS message even though the difference exists. 

*****

As for rate of touchdown. Maximum touchdown is a limitation of the aircraft, usually based on landing gear strength. It can usually be found in the limitations section of the AFM and may vary based on landing weight.  The Touchdown Zone is defined in the U.S. AIM as the first 3,000' of the runway. IAW U.S. ATP standards pilots are expected to place the airplane at the aiming point markings, -250/+500 feet, or where there are no runway aiming point markings, 750 to 1,500 feet, from the approach threshold of the runway. You can see putting the airplane in the right portion of the runways is far more critical then trying to grease it on. The problem is VAs have no method of accurately and automatically judging touchdown zone so they use the landing rate collected by tools that may or may not be accurate. 

Now depending on aircraft geometry trying to place the trucks into the ATP touchdown standards will cause the airplane to be above or below either the GS or the VGSI.  So the pilot is trying to guess truck height which in Airplanes like the 747 or A380 can be difficult. Even in much smaller airplanes like the G650 the viewpoint of the pilot versus the gear height is significant. When you put the FPA on the Captain's Bars the gear will actually contact the runway 303' prior to the aim point without flare. So in the Gulfstream the flare distance has to be ~300' to bring the gear onto the aimpoint. 

With the old VASI many airports with 747 service had three sets of VASI lights. Smaller airplanes would fly the closest pair of lights and Jumbos would fly the further set of lights. Sort of the same thing can be done with PAPI instead of flying 2 white over 2 red you fly 3 white over 1 red. In the real world this is easier to do as the lights turn pink as they  transition from red to white. In P3D the lights are binary i.e. either red or white and have no transition. 

All of that being said the passengers only notice how firm the landing is and how long it takes to taxi back to the terminal or parking. I have operated into 15,000' runways where the parking was on the far side of the airport from where I was landing and there was a line of jetliners waiting. If it put the airplane down on the markers I could stop the airplane and taxi off into a line of airplanes waiting for take off. The have to taxi a couple of miles to parking.  In some occasions I would brief that we would touchdown midfield. The airplane I was flying could easily stop in 1,500'  so I was well within the limitation of the airplane and I helped Tower out by not having to roll out over a mile before departing the runway. I am sure there are 121 types having kittens just reading that. Well if you are a complete slave to your airline SOPs then so be it. Do what the company hired you to do. Fortunately, I was allowed to adjust my approach and landing based on airport and environmental conditions. 🙂

[dbw1 445]

9 hours ago, Evros said:

Hello,

Lately I have found that PAPIs for many sceneries do not coincide with ILS glide slope. It all looks good until about 2nm out where I start seeing three red, indicating that I am too low. Should I continue on ILS path, I touch down directly at TDZ while PAPI might show four red by half mile final. Should I follow PAPI, I would hit the runway too early or crash before it altogether. It's not for all airports, but I think it's for the majority of them. I only use addon airports. P3D v4.5H2.

Thanks

In the real world this is not uncommon and is acceptable. In fact sometimes in low approaches the papi/vasi may be shut off.

[Evros 310]

Thank you all for well detailed explanations, I appreciate that.

I thought something was out of order because for as long as I can remember, ILS and PAPIs have always coincided for me. Just to clarify, are you all experiencing similar behavior?

[Chock 17,769]

3 minutes ago, Evros said:

Just to clarify, are you all experiencing similar behavior?

Yup. see it all the time in the sim because I fly all kinds of different sized aeroplanes from the same airfield a lot. As noted, it depends on the aeroplane and the airport a lot of the time, but certainly the PAPIs don't always provide the perfect guide to getting the thing down and should really be something which assists you in positioning yourself into a stable approach where the visual aim point and touchdown markers look like they are stationary on the windscreen.

Normally, in decent weather in the sim, I use the same method I use when landing an aeroplane in real life, which is a technique pilots sometimes call the 'TLAR method' (That Looks About Right): If there is a PAPI/VASI, I'll use that as a guide to when to start the descent once I've turned onto finals, but once I'm on the way down and configured for landing, I'll use the aim point on the runway and concentrate on keeping that immobile on a spot on the windscreen and not worry too much about whether I'm coming in at 2.5 degrees, 5 degrees or whatever. In other words, if the descent angle is not excessive to the point where a flare would not arrest it comfortably, and the aim point is staying in the middle of the windscreen, showing I'm angling off enough to counter any crosswind, then 'that looks about right' and it will be a good landing.

You can't always do this of course, at some airfields, you will not so much fly the four corners of a circuit of get a vector to capture a localiser, as come in curving from the base leg to the final. An airfield where I have done this a lot is the glider airfield Camphill in Derbyshire which, being situated on top of a hill and with you coming into the wind, is of course prone to severe downdrafts on final/short final. So in that case you come in from quite a high up and make a steep fast turn on finals to ensure there isn't a severe variance in the AoA of both wings, then continue descending at a really very steep angle, flying at twice the stall speed of the aircraft (which for most gliders would be about 70-80 knots). The speed gives you plenty of penetration and gets you through the wind gradient quickly. Then, once you know you're going to make it for sure, you sort out any excess speed or height by playing the spoiler lever backwards and forwards like a mad trombonist! The forthcoming 'Deadstick' flight simulator will be providing plenty of opportunities for this kind of 'seat of the pants' flying, so look out for that one.

If you want to try a technique which is worth knowing for if you ever have to make a forced landing, here's another good one to practice with. Go in a large empty room such as a sports hall, get a book and put it in the middle of the floor. Now, imagine that is the field you are going to land in. Pick a spot on the book where you'd want to touch down, and walk some distance from it. Now mentally draw a line up from that spot on the book out to what would be 400 feet AGL on the approach, that's your turn point for finals, from that point, mentally draw a line 90 degrees off from that up to where 800 feet would be, that's your base leg turn, then draw a line 90 degrees off from that and that's your downwind starting at 1,000 feet AGL. Now move to that point and walk the circuit crouching down as you progress, and that will show you what your airfield should look like when you make your turns. In your sim, get used to what cars, houses, trees and stuff look like from various altitudes, so that if you cannot rely on your altimeter to give you the exact height above your chosen field, you will be able to guess altitude fairly well. I used to have an instructor who would cover the altimeter and make me do that so I'd be able to judge circuits into emergency forced landing fields and it is very good practice. I was cr@p at it at first, but after a while I got pretty good at judging heights visually.

If you want to try that in a sim and you have some kind of light aircraft or glider in your sim (especially one which has spoilers), then it is good practice for getting used to a maneuvering for decent sight picture and what you can and can't get away with to do that. The beauty of doing it in a sim of course, is that it doesn't matter if you mess things up. The hard part is that unlike in real life, you don't get the disturbed airflow battering the elevators which you can usually feel through the stick and hear in the airframe, which tells you you are getting near the stall, unless of course you have a force feedback joystick.

[ryanbatc 12,856]

Informative thread!!

[Samaritano 304]

Great thread. I really appreciate Ken and Jon sharing their pilot knowledge. I wish every thread in Avsim was like this one. 

 

[C2615 319]

As an scenery maker, I noticed even if we just plugin the exact number of GP and PAPI pozition provide by AIP, it not always align. Probably because ADE doesn't factory in the height of GP antenna and possible systmatic disturbance of real radiio wave.

Also I guess the deflaction show in game is based on the "ref points" of the aircraft sim, rather than the antenna position of the real aircraft.

[dbw1 445]

13 hours ago, C2615 said:

As an scenery maker, I noticed even if we just plugin the exact number of GP and PAPI pozition provide by AIP, it not always align. Probably because ADE doesn't factory in the height of GP antenna and possible systmatic disturbance of real radiio wave.

Also I guess the deflaction show in game is based on the "ref points" of the aircraft sim, rather than the antenna position of the real aircraft.

In the real world......The PAPI GS and the ILS GS are not always the same and this is acceptable as long as the PAPI is checked to be within tolerances - with frost in the ground it can shift.  Sometimes in IFR weather the PAPI may be shut off.

[DEHowie 269]

Over the last 15 years ICAO has mandated numerous changes particularly at larger airports.

Runway markings have changed doing away with traditional 500,1000 and 1500’ markers bringing in runway markings based on runway length ie less than or greater than 2400m runways.

This also changed papi crossing heights at most larger airports to 70-74’ eye height. 
Hence papi and ILS slope indications frequently do not coincide.