Glide Slope

I've been enjoying doing some "plane spotting" at various airports, and have found some nice places where I can sit and watch the planes coming in pretty low. It got me interested to look into how how they really get, since I'm not that great at judging distances.

I was reading on planespotter.com of a spot near LAX where planes are reported as being just 40 ft overhead of passers by. Is that realistic -- could they really be that low? How low do planes, like 737 size, get as they pass over the fence on the outside of an airport just before they get to the runway?

Looking at some specs on the internet, it looks like most planes use a 3 degree "glide slope". Does that mean that's the angle they're coming in to land, regardless of nose pitch? I kind of picture it like a right triangle laying on its side, with the sharp angle the touch down point. I was thinking that with Google Earth it might be possible to compute how high (or low) a plane coming in for a landing might be as it passes overhead from the view spot seconds before touchdown.

 

The decision height on a "glide-slope" is usually 200 feet agl. After that, the pilot controls the plane because he supposed to have the runway, or part of the landing system in sight. So the 3 degree part is accurate, until the field gets in sight of the pilot, then he'll adjust the altitude as necessary. That means where there is a roadway near the approach end of the runway, the distance would be less than what would normally be considered appropriate.

Art

 

A few ramblings......

If you are on a perimeter road a hundred yards from the end of the road the planes will only be a few dozen feet above you.

I really don't know alot but I do fly now and then....... you can probably find a lot of the information on approach plates.

Just to give you quick example from an approach I do alot..... the pattern altitude is 2000msl and we start descending at six miles out to the minimum descent altitude (MDA) which is 800msl to get down it takes about a 700fpm rate of descent or ~3 degree glideslope. We want to be at MDA 2 miles out and hold that altitude until we are a mile out (Missed Approach Point) at which time the field will hopefully be in sight and we will descend and land or if the field is not in sight we go around. The touchdown zone elevation is 300msl so between one and two miles out the plane is roughly 500 feet above the ground.

 

For commercial aircraft (121) you are 50 feet over the approach end of the runway, and touch down 1,000 feet from the threshold.

Bob

 

I just tried to find the minimum terrain clearance for glideslopes, but I can't find anything there. The closest I found was that 3 degrees is the usual glideslope angle, but it can be adjusted if local terrain dictates it. However... using a little bit of trigonometry you can figure out how high the airplane is going to be at any distance from the touchdown point. Here's the formula: H = tan(G) x D. G=Glideslope angle, and D is the horizontal distance from the touchdown point (keep in mind that the touchdown point is 1000' from the runway threshold on an ils.) Also, this would be the height above the ground ONLY if the ground were level. If you're standing on a hill that's 50 feet higher than the runway, you'd subtract 50 from the number you got from the formula.

Can anyone find in the regs or anywhere for that matter how much terrain clearance the glide slope guarantees you?