Perhaps a very stupid question, but I run into something very strange from time to time.
I have set the ILS freq, and heading. Angling at a 30 degree towards the ILS.
My altitude is at the given alt of 1800 feet.
The ILS picks up and starts turning th plane. I can see that the altitude point is just above the 1800 feet.
Suddenly the ALT light of the mcp goes out, and in stead of starting the descent, the plane goes up. And that at a pretty fast rate. Within seconds it is at 2200 feet. Climbing at 3000ft/min.

 

I have asked fellow simmers, and most of them have that annoying issue. I mostly piick up by disabling the Autopilot and descend on hand, up to 2500ft/min.
Resulting in landing a bit to fast.

 

I wonder what causes this?

 

Thanks ahead,

 

RobB

Looks like you are trying to intercept ILS too low. Try intercepting the beam while flying at 2500-3000ft at about 8nm from runway. ALT light should go out in Aproach mode.

What aircraft are you using?

Are you sure it's not a localizer only approach or a backcourse? Do the charts match up to what you were trying to do?

I was today flying the Twinotter Extended. From EDDG/07 to EHBK/03.
But I also saw this on the Qualitywings RJ series.
Entry is normally 1800-2000 feet.
I was always to affraid to be to high.

 

Thanks for thinking with me.

 

RobB
 

At what distance are you intercepting ILS? What is your DME showing?

 

It still looks like you are below GP. Can you make a screenshot?

A screenshot afterwards is difficult. But I do my best, to reconstruct.

In short, I was inbound EH302. And turned 360, nearly at EH302 and the ILS picked up near D5.6BKZ.

Here is an Acars link to my PIREP: http://www.citylink-va.com/index.php/PIREPS/view/1355

At the bottom is the somewhat how I flew.
This exact the moment when the ILS picked up lateral: [21:47] - Flaps to position 1 at 1791ft and 103kts

Then a minute later, I had been at 2200 feet and corrected. Tried the Approach again around 1200 feet. Plane pulled up again. I was at that point a bit lower then the ILS shoot be.

 

Hope this is sufficient.

 

Is the ILS frequency the same in the sim? Sometimes the charts real world are different than the sim

In 2007 the ILS was different. 108.50. But that frequency does not work anymore, when I fly.
111.55 does work. As it picks up in the horizontal way. And I do not have the issue allways. 75% no issue.

 

RobB

Ok - I think I see where your problem is. You are flying RNAV transition in a non-RNAV plane and it simply lacks precision.

 

What happens is something like that - approach mode of your (quite primitive) AP had no time to intercept the GP. In this approach you need to get established on ILS having a very narrow vertical tolerance. Basically - you need to fly smoothly with only slight changes to speed and pitch and be on the correct altitude (correct - like just tens of feet from the assigned altitude).

 

You have not described what you had exactly done in your cockpit but what you should have done is to get established on localizer first (manually / in HDG mode / in NAV mode) and descent (manually or in V/S mode speed mode) to exactly 1240ft in 3.8nm distance from the DME. And then you should be able to activate the APPR mode provided that you are in descent and the AP does not have to do a big correction. Then it should guide you to the airport without any problems.

 

 

EDITED:

Checked in Sim.

 

If you follow the procedure EXACTLY (and I mean - just as it is drawn on your chart - with no mistakes) - the ILS and APPR mode will work like charm.

 

If you make a mistake - AP will behave badly.

 

You have two points where your altitude is mentioned - at D5.6 and at D3.8. Fly in direction of EH302 at 1800 and start to descent and turn to intercept the localizer. Keep your descent speed as described in the table on the chart. Keep your engine power the same or make just a slight changes (Twin Otter pitches when you make power changes - AP will not correct that on approach). Get to 1240ft at D3.8. In this point - engage APPR mode.

I switched from GPS to NAV around EH302. And used the HDG button. But maybe, I should try to go the point you say,
Stupidity is, that I think, that the ILS picks up at D5.8, but after closer inspection of the image I see D3.8 as the real entry point. You are correct.
Thanks, will watch this. Still learning, after many years. It is a great hobby, and you can extend your knowledge day after day.

;-)

 

Thanks.

 

RobB

D3.8 can be marked as FAF because it is RNAV transition.

 

In fact you can engage APPR mode earlier. It's not a problem. But you need to be in a location (position, altitude) and in attitude that will allow AP to work. Twin Otter does not have any fancy equipment so it can get you in trouble when you give him such a hard task as late ILS intercept. Give it a little help and the AP will do the rest

Will try it that way, and report my findings.

Thank you.

 

RobB

If you're in HDG mode the autopilot will only follow the heading bugged on your EHSI/PFD

 

You need to be in NAV mode (or LOC) to have the AP follow a GPS track or a VOR/localizer signal

I was until EH309 in NAV mode and switched to Loc at that point. Setting for 270, and slower turning northward.

Intercepting ILS at EH302, while having ALT+HDG+APR switched on.
Around EH302 the plane switched off HDG en started to turn to 033.

At D3.8 the ALT switched off.

 

RobB