PAPIs Unusable below what height (if any)

[quarefellah]

Hi All,
I was always under the impression that PAPIs were not to be used close to the ground as they are based on a particular "eye to wheel height" but I can't find anything written down about this. I have an idea that they are not to be used below 200'.
Anyone flying a less than big aircraft into a big aircraft airport (e.g. CDG) will know what I mean when I say that the PAPIs and ILS G/S do not correspond below approx 150' and invariably show 3 reds for 'smaller' aircraft.

Safe Flying

QF

[The African Dude]

At my place we use PAPIs and G/S until minimums, after which we use constant rate of descent. So the profile is effectively standardised even for different PAPI configurations.

AD

[compressor stall]

Well they are useable, but if you are in a smaller aircraft, you will land long.

Look up the chart and see the eye height for the papi. Look up your flight manual and see your eye height. Work out the difference, then use some basic trig to work out by how much would you land long if you followed it down...

In practice it's not a huge issue, as if the eye height is set for 747 or a380 it will be a long runway. I follow it to minimums then ignore the papi and aim for the markers.

[Tee Emm]

I have an idea that they are not to be used below 200'.

That was certainly true when I was involved in DCA flight testing of radio navigation and visual aids many years ago. Below 200 feet was the cut-off height for T-VASIS certainly but I am not sure about PAPI because it was not operational then in Australia. In any case, if the aircraft is using either of the two visual slope aids it is assumed that the approach is already stabilised by 500 feet and by 200 feet the angle should not have changed measurably. However if the ILS glide path is available it is far better to stay on that glide slope until close to the flare because of the limitations of visual approach slope systems and their siting to the side of the runway.

[capt. solipsist]

assumption: you are flying a light jet (a320, b737)

it depends on where the papi is located, whether in the 1000' point or 1350' point. if it is in the first, then it is ok (but doesn't really make sense because you should be concentrating on the aiming point by this time).

obviously you are gonna land long if you follow a papi located at 1350' .

[fireflybob]

PAPIS are a "point source" aid so should become more accurate as you get closer in, as opposed to the old VASIs which were not and I certainly recall advice not to use the latter below 200 ft for that reason. Remember there were 3 bar VASIS so wide bodied aircraft with larger MEHT followed the "top" 2 lights and other aircraft followed the "bottom" two lights.

Then there is the issue of mutual alignment (or not) between PAPI on glideslope indications and ILS.

[ft]

However if the ILS glide path is available it is far better to stay on that glide slope until close to the flare because of the limitations of visual approach slope systems and their siting to the side of the runway.

As the GP can (and frequently will) bend up and/or have significant structure past minima, that's not something I'd recommend. Besides, there's quite a bit of damping on the receivers, three second time constant. You're acting on old information. Doesn't matter much with a mile to go, but in close, well...

The PAPI shall provide correct information up to eight degrees from the unit in azimuth, meaning you should pretty much be in the roundout by the time you lose PAPI guidance due to the siting of the units.

Cheers,
Fred

[Tee Emm]

owever if the ILS glide path is available it is far better to stay on that glide slope until close to the flare

Fred. Agreed. Thanks for pointing that out. I should have amplified the expression `close to the flare`.

What I should have said was stay on the ILS glide slope within reasonable limits rather than completely disregard it. In another life with a government radio navigation aids testing unit, the ILS flight test required (among other things) flying the glide slope to below the standard Cat 1 DH to check for unusual indications of the type you mentioned. If something did not look right, technical investigation followed and if the problem could not be fixed, a note would be included on the relevant approach chart.

[ft]

And due to the flaring characteristic inherent in the design of the GPs, the part past point B (1050 m prior to the threshold, or slightly above 200' on a 3 degree GP) is excluded when measuring the angle of the GP.

Old news to Tee Emm, but probably an interesting tidbit for many others who think the GP goes straight to the TDZ!

Time allowing, I think I may be able to find a diagram of the GP past point B somewhere in my archives.

Cheers,
Fred