Question on approach plates

[Lomcovack]

I am after some information on approach plates, in particulalr why is an approach generally given in degrees on a precision approach and in a percent on a non precision approach.
Views and further information much appreciated.

[reynoldsno1]

A precison approach has vertical guidance i.e. the aircraft is able to follow a electronically generated vertical path set at a predetermined angle.

A non-precision approach does not have such guidance. The vertical path must be judged by the pilot. As a guide, a descent gradient is given which the pilot can convert into a rate of descent depending on the groundspeed of the aircraft.

[extreme P]

Technically you could determine an angle on an NPA from the TCH+50' back to the FAF. Problem is there is no way to fly that angle with "raw data" that I am aware of. The Airbus bird and FPA can fly any angle but it is not raw data and still an NPA. Steam gauges (likely some exceptions but for demonstration purposes only please don't crucify me!) can't fly approach angles. Also the approach could be flown differently (dive and drive vs SCDA).

[blackmail]

gradients

both(degrees & %) can be used to calculate your rate of descent.
rod = speed(kts) x slope(in %) = speed(nm/min) x slope( in °). e.g. 120kts on a 5% or 3° glide path : rod = 120kts x 5 = 600ft/min = 2nm/min x 3° = 6(600ft/min)
kind regards,
bm

[FougaMagister]

... in other words, on a 3 degree slope, the RoD should be g/s x 5, or groundspeed / 2, x 10, i.e. 120 kts g/s = 600 fpm. So going into LCY, with its 5.5 degree glidepath, would (initially) require 1100 fpm - not that I have had the chance!

Cheers

[Old Smokey]

With the increasing availability of FPA/FPV in more modern aircraft, particularly SELECTABLE FPA/FPV, it would be highly appropriate to provide SID and APPROACH paths in Degrees, not % Gradient.

Conversion to a required V/S truly makes a Non-Precision Approach just that - Non Precision (and crude at that). The sensible use of FPA/FPV makes a Non-Precision Approach into a Semi Precision Approach, properly done, you can expect to break visual on the PAPI/VASIS every time.

Regards,

Old Smokey

[reynoldsno1]

So why can't that be done with an angle?

It can be, of course, but that wasn't the question being asked. There are still many aircraft flying IFR without recourse to FMS capabilities- a descent gradient of 300ft/NM (5%) is probably more useful to them than an angle of 2.94deg.

I was trying to keep it simple .....

[411A]

Simple is better, reynoldsno1, altho some might disagree.
I find that many with fancy FMS equipment are too busy pushing buttons and then wondering...what the heck is it doing now?

[Megaton]

In an Airboos, descent profile in degrees can be v useful when using the flight path angle - simply reach descent point (or bit before) and dial in -3 deg or whatever.

[Lemper]

all of the above seems well thought of, however, it might be also that the precision approach path is given in degrees and not in gradient because most aircraft AFM limitations publish the max. and min. glide path angle for autoland in degrees. e.g., B744 is 2.5 to 3.25 deg.

[bookworm]

I am after some information on approach plates, in particulalr why is an approach generally given in degrees on a precision approach and in a percent on a non precision approach.

I think the short and useless answer is "because Annex 4 requires it".

Annex 4 says:

11.10.8.5 Final approach descent gradient and, in parenthesis, descent angle to the nearest one tenth of a degree shall be shown for non-precision procedures with a final approach fix.

11.10.8.6 On charts depicting ILS/MLS and LNAV/VNAV approach procedures, the height of the ILS/MLS and LNAV/VNAV reference datum to the nearest half metre or foot and the glide path/elevation/vertical path angle shall be shown.

11.10.8.7 Final approach descent angle shall be shown to the nearest one tenth of a degree for instrument procedures with vertical guidance.

Why? I don't know.

[FE Hoppy]

Embraer 170/190 fly all NPA with constant angle if published as primary guidance is FPA and you can hand fly an angle just as easily as hand flying a rate of decent. Normally we would use FD in FPA mode with AP on. The only trick is starting the decent at the right time so your path ends above the threshold. VNAV GPATH does it all for you so the skill is management not handling.

[Lomcovack]

BOOKWORM

This is just what I was looking for, any body no the resoning behind it, as has been discussed an approach described in degrees may as well be described in percent.