Max Glideslope Angle for Autoland

[JammedStab]

It says 3.25 degrees for the aircraft I fly. But there is one approach that we do where the Jeppesen chart says the GS angle is 3.2° but the FMC says it is 3.3 degrees. Which is the restricting number.

[Peter G-W]

Which angle do you think the ac flies?

[Denti]

Most probably a rounding problem. Guess in the usual way the FMC rounds 3.25 to 3.3.

[Skyjob]

Denti you would be correct had the chart been published at 3.25 degree.

FMC can only display one decimal and chart publishes two. It's then a rounding error and the FMC displays 3.3 degrees.

However it appeared to m the question is he other way around this case...

Jeppesen chart says the GS angle is 3.2° but the FMC says it is 3.3 degrees

Seems like a possible incorrect coding and should be reported to NavData provider for verification.

Next time you fly, temporarily select airfield again and runway with these details, verify that you posted this info correct (capture it on camera) then if need be send to Operations for verification.

Alternatively, post the image here so other's can assist in finding cause for your concerns (unofficially of course)

[Denti]

If you supply the airport and runway info i'll check my LIDO and can crosscheck next time i fly (which isn't all that often since my secondary job takes up too much time). There are sometimes differences between chart suppliers about stuff like that as well.

[JammedStab]

PANC ILS 15. Chart says 3.20, FMC actually says 3.31.

[aterpster]

JammedStab:

PANC ILS 15. Chart says 3.20, FMC actually says 3.31.

The FAA Datasheets System website, which is engineering source, says 3.20 degrees.

[Skyjob]

JammedStab I think an uploaded image would benefit those in the know, but it certainly seems to be an incorrect coding in the FMC which should be brought to the attention of Flight Operations if found to be the case.

An uploaded image should show the database coded angle and approach type selected

[Skyjob]

As the promulgated GP is 3.2 then this is the actual ILS Glidepath so it can be flown within the 3.25 maximum limit for autoland.
However when conducting a LOC approach the "calculated" Path will be as per FMC, thus 3.31 will be flown.

[underfire]

RWY 15 ILS Interesting to note:
ILS 15 3.20° TCH 58
VGSI 3.20° TCH 75 (?!?!)

[underfire]

Do they have the lights elevated for snow?

[Skyjob]

RWY 15 ILS Interesting to note:
ILS 15 3.20° TCH 58
VGSI 3.20° TCH 75 (?!?!)

VGSI and ILS are not be calibrated for same touchdown location.

VGSI and ILS glidepath not coincident

[PEI_3721]

The certification regulations might assume a maximum of 3.5 deg, but I cannot find a reference. Also 3.5 deg may the margin which a manufacturer has to show as being safe via development simulation, whereas the AFM value probably matches that demonstrated by flight tests.

An ‘ultimate’ limit depends on the aircraft type – aerodynamics, the flight guidance control laws, and the means of establishing a GS beam. RAE flight tests demonstrated 6 deg autoland with a BAC 111 and 5.5 deg with a HS748, flying MLS glidepaths.

[underfire]

Jammed, while you can use autoland on RWY 15, it is not really set up for that.

As OK noted, depending on your brand and number of sig digits avail, results may vary. The FMS will always round up, never down.

I cant look it up right now, but there are some pretty good geoidal ref differences up there, so this will contribute to differences as well, depending on your brands use of the ellipsoid.

[Capn Bloggs]

RWY 15 ILS Interesting to note:
ILS 15 3.20° TCH 58
VGSI 3.20° TCH 75 (?!?!)
By design, I suspect. We have similar here. The PAPI (aka VGSI?) is set for big/long aeroplanes where the eye height is well above the GS antenna height. I understand the idea is that when you pop out on the GS, the PAPI will also indicate "on slope". Obviously not as important for single isle machines on long runways (apart from the FDAP!).

[flyboyike]

I think it's odd that Jammed Stab has thus far refused to reveal what equipment he's on beyond "the aircraft I fly", despite being asked more than once (at least twice just by me). I certainly don't think he'll tell us what box he uses.

Why might someone be reticent about something so seemingly innocent?

[Citation2]

Your are flying an ILS approach not a FMC RNAV approach . So the glide slope displayed on your FMC is irrelevant , as you will be flying a glide . The autopilot / FD is coupled to the ILS frequency , not to the FMC database.

The limiting factor is the offical document like AIP, jeppesen ...

[Skyjob]

Back to the original question regarding what ILS angle can be flown, it is safe to say the one published is the one promulgated by the equipment, thus can be flown at the promulgated angle of 3.20.

The FMC coded angle is NOT used for the ILS as the G/S and LOC signal are followed, thus the check for angle would only be 'required' when conducting a VNAV approach. In case of a NPA, where the angle disagrees in FMC from published, DO NOT USE VNAV but use V/S.

[underfire]

OK,

Quite a bit appears to be in transition right now. The more that the ac use RNAV, LPV, and RNP approach procedures, the more these sorts of issues will come to the forefront.

The geoidal differences in the FMS, baro-vnav temp issues, and others will need to be addressed.

In working with several FMS systems, the associated coding, and the programming, there are significant differences, many of which, are now being realized.

With tailored procedures, the FMS rounding has always been a significant issue.

[JammedStab]

Jammed Stab,

Was able to check the PANC ILS 15 in two different current FMC Nav Data Bases....one shows 3.20 and the other, which only displays to tenths, shows 3.2....

Couldn't find a 3.31 for any approach, just curious, which box are you using?

Standard Honeywell Boeing FMC.

[latetonite]

And what if you fly a LOC approach, GS out? You will fly a VNAV path based on the GS in the FMS.

On a different note, the limitation of the autoland acceptable GS is valid with 10 knots tail- and 20 kts headwind. The apparent glide path decreases or increases quit a bit.

[aterpster]

latetonite:


On a different note, the limitation of the autoland acceptable GS is valid with 10 knots tail- and 20 kts headwind. The apparent glide path decreases or increases quit a bit.

Wind has no affect on the GS.

I believe you are thinking of rate of descent.

[latetonite]

Yes I am thinking of rate of descent. That is all the Autopilot has to cope with.

And that is where there are limits set in the angle.

The Angle itself, be it 3 degrees or 333 degrees, for the AP it is just a number.
To keep the aircraft under control is what matters.

Sorry if I expressed myself wrong.