ILS Landings: Glide Slope and Middle Marker Beacon

[Aeger]

I've been trying to abide more closely to the ILS glide slope in low visibility landings, but one thing that seems to raise my eyebrow is how the glide slope bar skyrockets after passing the middle marker beacon (on a gradual descension).

 

My question is, is this a realistic characteristic of the ILS system or an improper modeling in DCS? In most of the ILS landing tutorials and flight (commercial) recordings I've seen so far, it seems the pilots can take a smooth gradual decent and ride the glide slope all the way to touch down. Of course if I try to do so in DCS, I'll greatly overshoot the runway due to glide slope instructions to pitch up.

 

Any input appreciated!

[Razor18]

I've been trying to abide more closely to the ILS glide slope in low visibility landings, but one thing that seems to raise my eyebrow is how the glide slope bar skyrockets after passing the middle marker beacon (on a gradual descension).

 

My question is, is this a realistic characteristic of the ILS system or an improper modeling in DCS? In most of the ILS landing tutorials and flight (commercial) recordings I've seen so far, it seems the pilots can take a smooth gradual decent and ride the glide slope all the way to touch down. Of course if I try to do so in DCS, I'll greatly overshoot the runway due to glide slope instructions to pitch up.

 

Any input appreciated!

 

Just asking to clarify: are you talking about the small white glide slope "arrow" on the left side, or are you talking about the yellow horizontal steering bar?

[Aeger]

Sorry, talking about the actual horizontal steering bar.

 

Thanks.

[Razor18]

Sorry, talking about the actual horizontal steering bar.

 

Thanks.

I tought so. Well it's a bit difficult to explain the effect of 3D geometry difference between your position (higher/lower) and the optimal slope measured in degrees changing with the distance to transmitter. Phew... when you are miles away, your deviation (or delta) up or down from the glideslope (and also localiser-wise BTW) can be relatively big, but this still results in a relatively small angular difference, cause you are far away. White GS arrow is just a little off, and the yellow bar doesn't needs to go very low to guide you with small correction back to the optimal slope. But as you come closer, the same altitude deviation from the optimal slope means now bigger and bigger angular deviation as you come closer to the source of the ILS beam. So the white GS arrow is more off, and the yellow bar needs to tell you to do even more correction to come back on the optimal glideslope.

 

Imagine flying inside a cone aiming for the tip of it. Far away you can be more off of it's center axis, and still far away from its "walls". As you get closer to the tip, the margin of error to stay away from the walls gets smaller and smaller.

 

Whichever explanation fits you better, stay away from chasing the yellow bar close in, because it only leads to an increasing oscillation you can't control at the end. Just rely rather on the white GS arrow, and try to maintain your vertical speed around the same value that kept you more or less on the glideslope early on.

 

Shout if still not clear... :smilewink:

[Scendore]

Razor gave you the clinical answer... The reality of flying a real ILS is that the closer you get to landing you have to use much smaller corrections. When you're in windy conditions it makes it more difficult. Use the heading that keeps you on course and power to keep you on glide slope. Try not to use pitch to capture the GS. Set the pitch for the desired decent rate then power to hold the GS. They key is to be stabilized in your approach which means you're configured for landing around the FAF or Final Approach Fix

[Mike77]

Razor gave you the clinical answer... The reality of flying a real ILS is that the closer you get to landing you have to use much smaller corrections. When you're in windy conditions it makes it more difficult. Use the heading that keeps you on course and power to keep you on glide slope. Try not to use pitch to capture the GS. Set the pitch for the desired decent rate then power to hold the GS. They key is to be stabilized in your approach which means you're configured for landing around the FAF or Final Approach Fix

 

 

Thank you for the explanation. I love this stuff!

 

Am I correct in thinking that this lines up with the statement from the book "Stick And Rudder": use Power for altitude... Pitch for speed.

 

I'm just not sure if staying on the glide slope requires a variation of this... embarrassing, but I just can't get my head wrapped around it.

 

Thanks again!

 

Mike

[MagnumHB]

Am I correct in thinking that this lines up with the statement from the book "Stick And Rudder": use Power for altitude... Pitch for speed.

 

I'm just not sure if staying on the glide slope requires a variation of this... embarrassing, but I just can't get my head wrapped around it.

 

That's the general idea, but this of course opens up a big can of worms discussion on jets vs. props, Air Force vs. Navy, personal preference, etc. Stick and Rudder pounds that into you so you understand it deep down, but it's also important to remember that pitch, power, speed, and descent rate are all interconnected. Adjusting one may require you to adjust the other, or adjusting one may fix more than one problem at once.

[Razor18]

Thank you for the explanation. I love this stuff!

 

Am I correct in thinking that this lines up with the statement from the book "Stick And Rudder": use Power for altitude... Pitch for speed.

 

I'm just not sure if staying on the glide slope requires a variation of this... embarrassing, but I just can't get my head wrapped around it.

 

Thanks again!

 

Mike

 

What you quoted is very true, but only at very low speed (approach speed it is), when you are flying so slow, that without adding power you probably can't even maintain altitude. In the "normal" speed zone of an aircraft, power will be speed and pitch will be altitude again!

Edited February 13, 2016 by Razor18

[Luzifer]

What you quoted is very true, but only at very low speed (approach speed it is), when you are flying so slow, that without adding power you probably can't even maintain altitude. In the "normal" speed zone of an aircraft, power will be speed and pitch will be altitude again!

 

That does not make sense, why would speed make a difference? Trim balance stabilizes the plane at a certain speed no matter the thrust*. Given the speed is the same, the rest follows from simple conservation of energy.

 

Of course that's the static situation for 1 g. To go from level flight to climb you have to have more than 1 g for a time (and less than 1 g to go to descent) so it will oscillate around the trimmed speed for quite a while until it stabilizes if you let it do it all on its own.

 

 

* That's a bit idealized assuming that thrust doesn't affect trim. For the A-10 it probably doesn't, much, but for airliners with engines under the wings more thrust creates a significant pitch-up moment.

[Razor18]

The originaly saying "pitch for speed, power for altitude" is not meant for the high speed flight, it's for the approach rather. As long you are not in the lower end of the speed envelope, you don't need to add (much) more thrust to climb, you just simply raise your nose. But when you are closer to stall speed, lifting your nose results in so much more drag due to the even higher AOA, that the deceleration is exponential, compared to the same raise of nose at high speed, even without considering G-s and trimm effect. Thats what the saying goes for, at least in my opinion.

[Luzifer]

Which is just saying that at higher speeds and smaller changes the effect isn't as obvious... But you would still have to retrim to keep the nose up and that means speed will drop. Maybe just slightly for a shallow climb, but the energy that goes into climbing still has to come from somewhere.

[Aeger]

Thanks for all the input guys. While I've done landings hundreds of times (apparently incorrectly), switching my focus on using speed to control my descent instead of pitch seems to be helping by a wide margin. I'm not feeling that disconnect/awkwardness I had previously with following the ILS glide slope bar. Much appreciated! :thumbup:

[djkling]

Thanks for all the input guys. While I've done landings hundreds of times (apparently incorrectly), switching my focus on using speed to control my descent instead of pitch seems to be helping by a wide margin. I'm not feeling that disconnect/awkwardness I had previously with following the ILS glide slope bar. Much appreciated! :thumbup:

 

Just remember that even your everyday "private pilots" have likely never done an ILS approach. It takes time, and most people do them in trainer aircraft for quite a while (C172's being the most common, or Cherokee's). They move slow and you really do learn to use power for altitude control and you use pitch for speed. The "cone" is also very much the proper analogy since that is how the ILS system works. When you get close small deviations deflect the needle much more, in small planes it's simpler since they move less quickly through the air; if you shoot an ILS approach you also sometimes get in low enough under IFR conditions where you can use visual rules at that point and it becomes less an issue. Anyhow, keep at it, it does get easier!