ILS Approach Failing

[Henry3324]

I was flying from Gibraltar-London Heathrow on a Boeing 737-800 default plane.

I tuned to London Centre and begin my descent as accepted by ATC. I was directed to land at RWY 9 and after switching Approach frequencies, I followed the standard steps. NAV mode, NAV1: 109.50 Enabled from Standby, COURSE: 93 HEADING: 60 as instructed by ATC. I was then told to 'Fly Straight In Runway 9' After I was down to 2500 ft the pink diamond began to fall and as it touched the line by the Altitude Info, I clicked APP (Approach) and it disabled Altitude Hold and Heading. The plane lost altitude, getting further and further down, but there was no sign of the runway.
I didn't understand why the I failed the ILS Approach and what I did incorrectly, as I followed procedures as shown on guides and YouTube videos which all seem to work correctly..
I know you can't trust Auto-Pilot by itself but confused with where I messed up.

[rjlfry]

Try tuning both nav1 and nav2 to 109.50 rwy 9R .

[Morag]

Sounds like you were just a little late pressing the APP button. You need to click it before the final descent, the autopilot should hold your altitude until you reach the glide slope then it'll follow the path down to the runway.

[SteveFSX/P3D]

My understanding of it is that the APP should be selected before the glideslope gets level with the aircraft - to ensure you are intercepting from below to ensure capture. The aircraft in effect flying in to the glideslope whilst armed for capture. According to a lesson I read, If you press APP when it's level you might not capture or get a false capture. You can also fail to capture descending down to fly through the GS from above. So you should be below the GS upon capture.

[De Kaashaas]

What Steve said: in FSX the glideslope must be intercepted from below. This is not the case in reallife but poorly simulated. Also, make sure you're not too fast and you're not on a backcourse. The latter would steer you away from the runway instead of towards it. Also make sure you don't accidentally push or bump your controller/joystick as this disengages the autopilot. I don't know how this is simulated in the default 737 but it might catch you by surprise as well.

[Dan-TXHills]

Originally posted by De Kaashaas:

What Steve said: in FSX the glideslope must be intercepted from below. This is not the case in reallife but poorly simulated.

This may have been a mis-typing. It IS the case in real life. (In real life) the glide slope signal has echos at every multiple of the glide slop angle - so, if you have a three degree glide slope signal there will be an echo at 6, 9, 12 ... degrees (in reality it is unlikely that you would have more than the 9 degree false echo since the signal power degrades with each echo and probably would be too weak to receive). To make matters worse, if you intercept the first echo (6 degree, in this example) the glide slope sensing is reversed so if you appear to be below the glideslope you are actually above it (or, more correctly - actually above the first false echo of the glide slope).

I do not know if FSX has false glideslope signals - and maybe this is what you meant - that this is the case in real life but poorly simulated (in FSX).

Looking at the approach plate for the Cat 1 ILS Rwy 09R approach to EGLL you should have been at the correct altitude to intercept the glide slope (this happens about 7.5 NM before the runway threshold).

Morag is correct - you may have not engaged the approach function soon enough. You can do this anytime once you assure you are receiving the correct glide slope (not a false echo - but again, not sure if that happens in FSX) and the aircraft will fly the glide slope once it captures. Or maybe something to do with another setting - VLOC vs. GPS possibly? A bit out of my realm here - I disconnect the autopilot once I have confirmed glide slope reception and hand-fly the approach.

Dan

[SteveFSX/P3D]

Originally posted by Dan-TXHills:

Originally posted by De Kaashaas:

What Steve said: in FSX the glideslope must be intercepted from below. This is not the case in reallife but poorly simulated.

This may have been a mis-typing. It IS the case in real life. (In real life) the glide slope signal has echos at every multiple of the glide slop angle - so, if you have a three degree glide slope signal there will be an echo at 6, 9, 12 ... degrees (in reality it is unlikely that you would have more than the 9 degree false echo since the signal power degrades with each echo and probably would be too weak to receive). To make matters worse, if you intercept the first echo (6 degree, in this example) the glide slope sensing is reversed so if you appear to be below the glideslope you are actually above it (or, more correctly - actually above the first false echo of the glide slope).

I do not know if FSX has false glideslope signals - and maybe this is what you meant - that this is the case in real life but poorly simulated (in FSX).

Looking at the approach plate for the Cat 1 ILS Rwy 09R approach to EGLL you should have been at the correct altitude to intercept the glide slope (this happens about 7.5 NM before the runway threshold).

Morag is correct - you may have not engaged the approach function soon enough. You can do this anytime once you assure you are receiving the correct glide slope (not a false echo - but again, not sure if that happens in FSX) and the aircraft will fly the glide slope once it captures. Or maybe something to do with another setting - VLOC vs. GPS possibly? A bit out of my realm here - I disconnect the autopilot once I have confirmed glide slope reception and hand-fly the approach.

Dan

Thanks Dan, that's very interesting. (I know you meant the same De Kaashas, thanks)

I suppose those phantom signals have caused a few tragedies in real life :-(

I wonder if the likes of PMDG or A2A would simulate the capture of those false higher signals despite FSX not generating them?

[Dan-TXHills]

Yep - I believe there have been several dozen incidents involving intercepting a glideslope false echo. This one did not turn out well -

https://www.flightglobal.com/news/articles/spectre-of-false-glideslope-emerges-in-bishkek-747-c-433991/

Dan

[rjlfry]

I did a test yesterday default 737 glideslope intersept at 3000ft no problem 9L EGLL,
bear in mind no pc simmulator will ever be 100% true to real life if it could think of all the money airlines spend on millon pound simmulators when all they need is pc they can get from a shop.

[De Kaashaas]

Indeed it was a typo, thnx for the more extensive info! :-) I indeed intended to say that the 'squared echoes' are poorly simulated in FSX, or in the big corporate sims for that matter. Crew are actually all undertrained in this aspect, allthough some companies have it embedded in their line-training and fly their cadets intendedly above the glideslope for awareness purposes. Having seen it once makes it more top of mind. Problem here is that not every ILS-transmitter necessarily gives this echo.

And rjlfry even the multi-million sims will never exactly behave as a real aircraft does. A real aircraft is subject to the 'analog' laws of nature. The behaviour of an aircraft in any simulator will always be a result of digital algorythms and predicting the most likely scenario and therefor will never be the same thing as in real life.

Offcourse a million dollar corporate sim will be more acurate than our Intell based home computer, simply because they can handle less algorythms at the same time. ;-)

[Henry3324]

Thanks everyone for answering- I'm very impressed with the large details of answers and how FSX:SE is very strong. I've tried different approach methods as you all suggested and I'm back up and flying fine now. :)