G/S and VASI mismatch
 

Hi guys,
according to your knowledge,
in case of G/S and VASI (or PAPI) disagreement which one is theoretically to follow , especially, say, below 200'?:confused:

I would more than likely conduct a missed approach, however using my knowledge from an article i read in flight safety, it is possible for the PAPI to show an errounus reading due to refraction from particles in the air making an "on slope" light indicate fly up or fly down. The way in which to distinguish if this is happening is to see the intensity of the lights, the artile states that the erronus light may be dimmer then the correct lights.

John

Rely on the GS down to minimums, and then adjust according to the lights.
Several countries have mountainous terrain (except yours obviously) and you will find that the GS and PAPIs rarely are matching. This is because the GS has been adjusted so to provide the required OCA/H, 200' for CATI. However they don't have to adjust the PAPIs as they are normally standard 3,75 from minimums and down.

I go for the other way round! G/S for me, esp as the PAPI's at many airports I visit are calibrated for the longer aircraft and on my ickle 73 you frequently get 3 reds down final.

My reasoning: The PAPI's are calibrated to 200', the G/S (on a Cat 3 runway) is calibrated to 50'.

IMO, below 200', you should be flying to your aiming point on a visual approach, not 'following' anything. My requirements - just do not go above either PAPI or G/S (ok, you can flare:) ) and I'll be happy. The number of times I have to bend down and pick the ILS GP off the cockpit floor........................ :(

G/S vs papi or vasis
 

hello squak ident,

normally a papi gives correct glidepath information till 100ft AGL, where as a vasis can be followed till 200ft AGL.
when there is a small difference between electronic glide slope & papi or vasis,use common sense, have a good look at the rwy & if your aiming point is where it is supposed to be & remain steady in your windscreen & the rwy shape is ok(having taken into account the rwy geography, uphill, downhill, small, large widths etc. ...)your are on the right angle. keep in mind that differences that might exist between the electronic G/S & papi/vasis stems from the different limitations of both systems.
vasis/papis are correct somewhere from 4nm out(not a lot), daylight, haze, brume & pollution can discolor the lights, twr selected brightness has an influence, system can be inop or just not switched on, long grass can obstruct clear vision & finally the system can indeed be misaligned for a number of reasons.

electronic glide slopes are mostly certified from about 10nm, provided you are on the localizer(ok from 25nm out) & on the right altitude/height AGL, about 3000ftAGL/9-10nm out on a 3° glide path:3x3=9.
if the distance is limited, it should be mentioned on your approach chart, eg. catania,sicily, where glide slope is ok from about 5nm out if my memory is correct.

finally, i would say that if the difference is small, you can use either. if the difference is significant & in your view unacceptable, report it to the twr & use the one mark eyeball to determine which system is correct & make a safe landing.
hope this will be of any help.

Also worth checking the approach chart to make sure the PAPI and G/S are set at the same descent angle. It is not unheard of to get a glidepath that is different to the PAPI angle.

PP

Check the PAPI to ILS GS alignment from the chart, the standardised (ICAO) arrangement requires them to be accurately aligned for normal sized aircraft. If not then the difference and conditions of use should be stated on the runway/approach chart e.g. two sets of PAPI, one for long body aircraft, or terrain/obstacle clearance.
All ILS GS bend upwards (flatten out) the closer you get to the ground – parabolic effect due to the reflected beam. Thus do not use the GS below the approved minima; obviously Cat 2/3 installations are better than Cat 1.
A true PAPI system uses a projected light beam which is very accurate, significantly better than VASI. Again check the charts re PAPI or a hybrid PAPI/VASI installation. The old style VASI was most unreliable below 200ft – ‘all pink’.
When visual, follow the visual cues but use the PAPI for additional information i.e. sudden change in vertical speed or floating high.

Hey Folks:
All the answers here are quite correct -- but before I disregarded one or the other I'd make sure what the rules are -- and the rules do differ slightly from country to country. For example the Operating Rules in the US require that, "An airplane approaching to land on a runway served by an instrument landing system (ILS), if the airplane is ILS equipped, must fly that airplane at an altitude at or above the glide slope between the outer marker and the middle marker; and an airplane approaching to land on a runway served by a visual approach slope indicator must maintain an altitude at or above the glide slope until a lower altitude is necessary for a safe landing." Now, admittedly, this is for large airplanes -- but its a pretty good safety practice, and if both are available, its a pretty sure bet that anyone who sticks it in short of the threshold is going to have a lot of questions to answer.
_______
AirRabbit

Lota of factors involved here. For example, on a B747, once the gear is down the aircraft is using the GS aerials on the nose gear doors which are about 25' below the pilot's eyes - and that makes a difference! My company states the PAPIs should not be relied on below 200' and the GS is a better guide. On a 747, this will mean you are less likely to drag the wheels through the approach lights.

I do occassionaly fly into a non-towered airfield which has no glide slope, but does have PAPI - and at the relevant point on the approach they show (from left to right) White, Red, White, Red.

How do you follow those?

Don't. The indication you describe indicates that one or more is mis-aligned so they should not be used. Alternatively, disregard the outer two lights and just use the inner two as an APAPI system. You should certainly tell the airport operator as soon as possible though.
Oh for PLASI's!

Use the Glideslope. It's calibrated and electronically alarmed if it goes out of tolerance. The PAPI's, although checked on a Flight Inspection aren't alarmed or Calibrated to nearly the same accuracy.

In response to the comments by SixDelta, I was nearly going to say that there are some places around the world where ILS alarms are not always all that they should be. Of course, in such places, the calibration of a PAPI, VASI or VASIS would probably be even more suspect! :ooh:

However, I find myself in general agreement with the concept of using the ILS GS for guidance in preference to a PAPI/VASI/etc. Having been extensively involved in calibration (in a previous life), I can tell you that harmonisation of a PAPI (etc) with an ILS GS isn't always highly successful. :uhoh: