gradients - degrees v's percent

[desmotronic]

In regards to climb and descent gradients I've noticed they are usually expressed as percent i.e. number of feet vertical per 100 feet horizontal such as in SIDS, CAO 20.7.1B etc. However instrument approaches usually have the glide path expressed in degrees typically a 3 degree glide path.

By trigonometry the fixed relationship between the two is tan x deg = y percent.

Can anyone advise the rationale for the use of degrees v's percent?

I'm a new user of Jepps and noticed that they seem to give a hint in that percent gradients do not take the curvature of the earth into account whilst fixed glideslopes (which are exressed in degrees) do.

Is this the reason and if so why the different units? Or is there something i am missing?

[43Inches]

I think it may be the other way around in that the 3 degree profile is set from the touchdown zone and therefor is the angle from the earths surface at that point (no correction for curvature at all). The gradient (fraction or %) infers you must gain x feet for every nm covered so is relative to the earths surface (msl) at each mile. The later being referenced to altitude would infer that you would climb a gradient with refence to the earths surface.

[Arm out the window]

The curvature of the earth's only worth 100 ft or so over a distance of 20 km, so changes in elevation of land surrounding airfields is going to be far more significant, and we don't really think about that too much except in terms of high obstacles in the approach and circling areas.

I reckon it's just different ways of expressing the same thing, and my guess would be when instrument approaches were being designed in the early days, they hit on 3 degrees as being a reasonable slope, but we've found it more convenient to talk of percent gradient because it's easy to convert that to feet per minute for reference in the cockpit.

[Howard Hughes]

Curvature of the Earth?:eek:

I hope you are only talking from a theoretical perspective, cause when you have an engine failure at 50 feet it ain't really gonna matter, you will have more on your mind! ;)

PS: the curvature of the Earth actually aids you in an after take off situation!:ok:

[Arm out the window]

On that note, I've flown types that only climb on one engine because of the curvature of the earth!

[Wing Root]

The flat earth society (http://www.alaska.net/~clund/e_djublonskopf/Flatearthsociety.htm) would have fun with this...

[BombsGone]

Don't know the pucka answer but the practicalities are that % is easy to use when calculating rates of climb. Ground speed in knots times % works well. On an instrument approach Degrees is more useful especially if you have a HUD.

[JulieFlyGal]

they do it both ways so they can trick you in the ATPL exams :ugh:

[Icarus53]

Arm out has hit the nail - probably somewhere around the neck. Curvature is not really an issue here for starters.

A degree type gradient is usually used to describe a particular path through the air on which you intend to track. It is therefore extended from a particular point in space (a touchdown zone). Approach paths are a good example of this. % type gradients usually describe performance requirements (which primarily include climb gradients) which can be demonstrated from certain points and in various configurations. As it is a measure of aircraft performance at any given time, it is not applied to a specific point in space. As Arm Out suggests - it's easier to think of performance gradients in terms of % as you can quickly estimate your performance in the cockpit.

I'm sure someone must know better, but that's my story and I'm sticking to it!

Icarus

[A37575]

they hit on 3 degrees as being a reasonable slope

The original ICAO ILS recommended glide slope was set at between 2.5 and 2.75 degrees. Then around 1968 the Australian DCA were getting aircraft noise complaints from Sydney residents under the runway 07 ILS path. They stirred up the local member for Parliament and he pressured DCA to think of ways of ending the complaints (apart from beating up those who complained the loudest).

My boss in DCA suggested increasing the glide slope angle to three degrees - the theory being aircraft would be higher at the outer marker and thus less noise footprint. Of course it would make bugger-all difference but it satisfied the politicians who could go back to their bunch of bogans under the glideslope and assure them all would be sweet.

And thats what happened. The DCA Flying Unit DC3 and F27 did the calibration flights for the new three degree angle and presumably everyone was happy. Of course that meant DCA had a tiger by the tail and for standardisation all the ILS in Australia had to be jacked up to three degrees. In turn that cost a lot of tax payers' money in flying costs and trips to Perth and Darwin to fix up their glide slopes. DCA had officials on ICAO Visual Aids Panel and so ICAO thought this was a jolly good idea and three degrees became the standard. And all based on perceived noise reduction for a vocal minority swilling their beer under the 07 ILS.

But that wasn't the end of it. Someone in DCA said wot about the T-VASIS around Australia because they had all been set up for 2.5 degrees as well when first installed. So DCA Flying Unit pilots and technicians spent many weeks flogging old VH-CAN (DC3 Calibration ship) and/or the F27 VH-CAT around Australia re-calibrating all the T-VASIS to three degrees.

Much more than three degrees became a problem with bigger jets because of their inertia and I believe 3.75 degrees became the final upper limit for big jet transports.

Time dims the finer points but I think the story is reasonably accurate.