ILS Glideslope Indication Disagrees with PAPIs

[Guest]

Can anyone explain this?Flying down the ILS Rwy 23 at Glasgow EGPF - fully established on the Localiser and Glideslope. Become visual at 250 ft RADIO and the PAPIs are showing FOUR WHITES, yet the ILS says I'm on the slope????The plane is actually above the glideslope so why is the ILS indicating correctly??Any help much appreciatedRobbie

[marcom]

Hi Robbie,this is a known problem. Most of the PAPIs are located falsely. They should be 1000ft from the Rwy threshold, but most are much nearer to the beginning of the runway. Thats why they indicate that you are too high. Its annoying, especially when you fly the last part visually and follow the PAPI, you get to hear the glideslope warning.

[Guest]

Thanks for your reply. It was starting to do my head in! :-8 :-lol Is there any way around it? Do add-on sceneries have PAPIs in the correct position?CheersRobbie(-: :-scatter

[marcom]

I havent seen any :-(The best was 3 white/1 red when completely established on the G/S. But a little high on the G/S (half a dot) and you get 4 whites.Mark

[gosta]

Hi there,I believe the problem is caused by the way FS displays the lights. To determine the cockpit's position, FS uses the pilot's viewpoint as a reference. In many add-on planes (including PIC), this is set outside the actual cockpit in order to give a more realistic view. The aircraft's position in relation to the glideslope, however, is determined by the actual aircraft geometry. So, when in doubt, follow the G/S rather than the PAPI lights. You can test this by flying the approach with the default Cessna, see if you'll get a discrepancy between G/S and PAPI lights in that aircraft - most likely not.Cheers,Gosta.

[Guest]

HiWho remembers how to fly a Cessna ??Happy flightsPS. Thanks for clearing things though

[Guest Ian_Riddell]

On the 747 (in the real world), I believe this phenomenon is due to the fact that the aircraft's Glideslope receive antennae are located on the nosewheel doors, perhaps 15~20 feet below the cockpit. When the aircraft reaches the threshold, the PAPI (as seen from the cockpit) will show that you are somewhat high on the PAPI glideslope.The 767 should suffer from the same effect, but it will not be so pronounced: the Glideslope antennae are relatively close to the cockpit (near the bottom of the nosecone).http://members.ozemail.com.au/~b744er/767/ILSAntennae.jpghttp://members.ozemail.com.au/~b744er/767/ILSAntennae2.jpgHope this helps.Cheers.Ian.

[Guest]

Incredible!You are the MAN Ian!!!!!!! ...better than the Sky-God's because without guys like you, there would be no Sky-God's!Rob.

[marcom]

Hi Ian!Good point! I once talked to a 757 F/O who said they would see 2 reds/2 whites while flying an ILS, but when shooting a visual approach they would aim for 3 whites/1 red, just to be on the safe side. A 767 being bigger, could very well show 3 whites/1red on a standard approach, but then you would need more than half a dot above G/S indication to get 4 whites. So it could be a combination of both (or all three issues discussed)- glideslope antennae being lower than pilots line of sight,- PIC767s perspective being more forward and- misplaced PAPI in standard scenery.Regards,Mark

[eurowing]

hi all,someone having an FAR/AIM handy?? i currently don

[Guest Ian_Riddell]

>Incredible! >>You are the MAN Ian!!!!!!! ...better than the Sky-God's >because without guys like you, there would be no Sky-God's! LOL... not quite, Rob, but thanks for the vote of confidence. To be honest, I'm just repeating what I'd heard on websites like this... all I did was add pics ;-)Cheers.Ian (Ego currently recovering from a nasty incident somewhere near Vancouver on Worldflight 2002) :-hahhttp://www.worldflight.ozflight.com.au/boo.../fullroster.php

[Guest anthonyj]

Every ILS chart will have an annotation on it regarding TCH, or threshold crossing height. This is the height of the ILS beam as it crosses the runway threshold. In addition the runway directory, often on the back of the aerodrome chart, will have details about each runway, including the visual approach slope indicating system used and a TCH or MEHT (mean eye height threshold). This is included in most airlines' documentation. This is the height that the pilot's eyes will be as the aircraft crosses the threshold with an on slope indication on the VASIS/PAPI. It also gives the angle to which the VASIS/PAPI is aligned. Using Runway 05R at Auckland as an example, the ILS has a TCH to a displaced threshold of 50 feet. The 3 degree PAPI TCH is 73 feet. This is a difference of 23 feet!Now for the application. In a B767-300 with the ILS antenna at 50 feet the main gear is 30 feet off the ground. The pilot's eyes are 58 feet above the ground. As you see the pilot's eyes are 15 feet below the on slope indication of the PAPI, which is enough to get a fly up indication on this particular runway. Our company policy is to transition to a visual aimpoint on the runway, 1130 feet in, and to fly to that. In this case the glideslope would be more use than the PAPI. In a 747-400 you would have on slope indications both visually and electronically.Depending on the local application of the glideslope and VASIS/PAPI there are times in the real aircraft that you brief 'I will see x many lights fly up/fly down'. At 1000 feet you will be looking on slope, but at 50 feet it has transitioned to what you included in your briefing. It is not uncommon to have 'off-slope' indications, but as long as the aircraft approaches at close to 3 degrees, the wheels pass more than 25 feet over the threshold and the aircraft touches down in the touchdown zone all is well!

[de Lattre WGA]

Hello,All this is very interesting and all of us could spend hours to talk about ILS antenas, eigh of the aircraft and so on, but the problem still remain.FS2002 is not powerfull enough to make the differences between the eight of a 747 and a cessna. But FS2002 as a huge limitation conducting to have a difference between the ILS deviation bars and the Papi lights.Normaly, in the real world, the ILS antena on the ground are located at the end of the RWY, and some times far behind the threshold. LSGG for example, but it is not the only one. the problem with 2k2 is the fact that the Glide slope is positioned erraticaly from one airport to another. Thus if you land at KORD, for example or at LFPG RWY 26R, you will descent gently on the glide with the LOC and GLIDE centered, as you get to the narrower of the signal, thus closer to the ground, you will see that, assuming your speed and vertical speed on the glide are stabled, the papi lights will change and give the impression that you are not well positioned. It simmply because in the FS2K2 world, ILS signal come from the landing theshold, and will give you on very short final erratic infos. When I say threshold, I mean the very begining of the RWY, not the touch down zone as it should be.One way to verify this statment, take an aircraft and fly an ILS, and you will se the AP chasing the glide at or below 600 to 500 ft AGL.The only work around I have found is commercial addon having their own auto pilot box, unlike the DF737, in which I usualy disconect the AP around 1000ft AGL to cure the problem.That is also why if you shoot a CatIII ils with an aircraft in FS2k2, you will have to disconect rapidaly the AP after touch down caus the AP will not be abble to track on ground the LOC of the ILS.In the real world, the AP, provided the aircraft and crew have the certification, would track the runway centerline almost to a complete stop before vacating.Hope in fs2004 they will cure that problem.Cpt Cyrille de Lattre