737 NG A/P hickup...
 

Intercepting an ILS, at LOC/GS capture I was surprised to see the autopilot struggling to follow F/D commands. We were clearly not following the cross and we were going up and down like a yo-yo and also banking back and forth. The signal on our screens seemed stable, so I just wonder what can have caused this? The plane was an old one in RYR terms.

Another situation I've had is LOC/GS interception with an unreliable GS, the A/P kept pithching up and down in a rather dramatic way, so we disconnected...at what point would it disconnect by itself? It was quite nasty.

Was there another aeroplane on approach in front of you maybe distorting the ILS signals?

Regards,
BH.

Ground movement of vehicles or aeroplanes in the vicinity of the aerials can cause this- I have seen it several times over the years. As to disconnecting itself, I think the autopilot would hang on for dear life. I do not think you would ever leave it in to the extent it would want to hold on. I have a wry smile to myself when people witter on about pilotless aeroplanes- examples like this and the BA 777 and many others, and faults 'not from the book' mean that although computers are a great help, for the unlimited future, human intervention is occasionally very necessary!

I like the one about 'why does it take 100 pilots to change a lightbulb?'......one to change the bulb and 99 to tell you how they did it on Tridents. If we're going to have a nostalgiafest, let's make it Tridents, not Tristars!

Well that's because thanks to the CRE, we are not allowed 'black' cats anymore in the UK. They are now 'cats with significant hair toning'. 'The man in the street' as an expression is now banned to prevent offence in the UK to the 'woman in the street' (which seems to be not banned)- Council staff banned from saying 'man on the street'... in case women are offended | Mail Online

What were we talking about again?

The obsolete design of the 737NG autopilot....:}

touché - one must keep a sense of humour throughout ....

The NG is actually a bit of a lash-up of an updating to the 300 -500 versions to try and bring the line up to near Airbus technology. Initial delight at a near 747-400 flight system becomes disappointment at what it can not do that its big brother can. You can tell the old technology behind the bright and shiny facade. The embarrasingly non-existent EICAS system, the total lack of autostart and auto-icing, the awful integration of systems that should really have been sorted out with the NG version, the ancient cockpit design- why ever couldn't they have replaced the front end with the lower drag quieter, 757 front? It really is still a pig dressed up in a DJ. They actually updated the 737 line to approximately 1985 technology....and that is what they are still selling today! And don't even bring up the 757 system! Noah had better equipment. How do Boeing get away with it? Not even a VORLOC switch! Can you believe it?

What was the question again? Oh yes- so that is probably why the ILS is prone to such external errors.

You can thank Southwest Airlines (the quite large American carrier) for this arrangement, as Boeing had every desire to truly update the design, however...it would have required a different (new) pilots type rating, and Southwest, having hundreds of the older aircraft, said...we won't buy any more if you do.

Case closed.

So I was told, anyway.

Well FR does fly to a fair few places where the ILS signal is pure crap, at times... not all are mentioned in the airfield brief like: LBC, TSF, and quite a few more…
:hmm::ouch:

But it does sound like the A/C managed to make a mess of it… even the FR A/C will at times need some TLC.;)

Gosh, ASFKAP, you are totally out of touch, Herb is gone...retired.
Actually, my info comes directly from a Boeing Sr Vice President, whom is very much in the know.

No Queen's or PM's here, I'm afraid.:bored:

Fortunately...OTOH, we have Dubya, just about as bad.:yuk:

Thanx for all inputs, and for the first case "vehicle/aircraft distorting the ILS signal"; this is not what I'm talking about. Steady signals, just weird actions by A/P.

ASFKAP: Excactly, which was my original question. How can the A/P commands be different to the F/D commands? I have noted that older aircraft tend to have more pronounced errors like this.
Another thing that springs to mind is the abrupt motions on the control column when initially engaging the A/P in such aircraft. You sort of notice straight away that something's not quite right.

Speaking of TSF Venice Treviso with the unreliable GS. First you're kept high due terrain then you have to dive for your profile and then cleared ILS. So your roughly at 15-20 miles doing 250kts with sp.brk. and capturing LOC/GS. With a bit of luck the G/S signal moves up making the A/P pitch up quite abruptly due to the speed. I swear I could actually feel the G's doing this. I've seen the A/P disconnect lots of times, but it seems it just won't in the APP mode. I sort of get the feeling that said mode has different criterias to the others...Dunno, maybe it's just me...