I was thinking about this on my way into work this morning (maybe it was the dull grey weather...)

 

Last year a Singapore Airlines T7 ended up off the runway at Munich following what appeared to be a bodged autoland without the full protections in place. Basic explanations of ILS systems state that there are effectively 2 parts to the ILS array on the ground - the localiser beams, spread horizontally to line the aircraft up with the runway centreline, and the glideslope beams, spread vertically. It is implied (if not stated) that these are located are the near end of the runway you are approaching.

 

My question is - in the case of an autoland, what happens once you have passed over the near end of the runway? The Munich incident shows (I believe, and the fact that protections need to be in place to perform a full autoland) that the AP is still locked onto a localiser of some sort to guide it down the runway (rather than a pure heading, otherwise interference wouldn't be an issue, right?). So does the localiser at the near end have something that points down the runway as well or is there another one at the far end for an autoland capable runway? Same goes for the glideslope - my instinct would be that once the aircraft has passing over the G/S beam at the near end the AP will know the runway length and its own actual altitude AGL from the radio altimeter and can therefore calculate what it needs to do in terms of V/S from that? If so is that part of what makes an aircraft have an autoland capability above the AP just being able to follow the localiser and G/S?

 

Thanks

Localizers are placed beyond the far end of the runway and glideslopes are placed abeam the touchdown point. There is no need for any sort of 'secondary' transmitter.

Thanks.

 

So the localiser that you pass over before the threshold of (for example) runway 26, is in fact for runway 08 ILS? And the G/S signal is followed all the way to touchdown in an autoland?

Another quick question...

 

Is it/can it be a full hands-off operation right to the point of being stationary on the runway ie does it apply reverse thrust and brakes?

does it apply reverse thrust and brakes?

I take it "it" is the airplane? I don't think that any airplane engages reversers by itself, and I see two good reasons for it:

First, what if the airplane apllies reverse thrust before touch down by error? That might be fatal, though nowadays it's - at least for the pilots - to engage reversers in flight.

Second, thrust reversers are not allowed everywhere due to noise abatement.

Regarding the brakes: Every airliner has automatic brakes, which will slow down the aircraft after touchdown, and they will bring it to a complete stop, but this should not happen, since it will take more time for the aircraft to exit the runway and thus the following planes might be forced to go around,causing delays.

 

Regards,

Flo

Yes, by 'it' I mean the AP that is doing the autoland. I realise that even if it could stop the aircraft, it probably wouldn't for the reasons you cite. I guess if you look at it the other way round I am asking what does the crew do during an autoland and at what point do they take back control?

Good question on the autoland stuff. I guess it's a bit tricky (or just a long text) to answer since you are looking at different systems and levels of automation. For example, that rollout phase can offer AP guidance but can also be the part where the pilots take over completely. As far as I know, reverse thrust never is an automated step.

 

If you're up for a lengthy text about the technical and historical background, Wikipedia gives you something. http://en.wikipedia.org/wiki/Autoland

 

It's actually quite interesting to see which planes first enabled it and why. It has a lot to do with the need to fly to bad weather airports of course and then offered some tasks on the actual implementation. Also, the rw factors which lead to a deviation or unreliable signals (as mentioned by your example) are quite interesting. You will for example find different holding points at airports taking care of some influences.

 

In the sim, we actually have too good and too reliable signals when it comes to the ILS. Our problem may therefore be one of misplaced landing aids whereas the rw folks then have to deal with bad signals or 'just' deflected ones.

 

Another interesting part may be the implementation of new/future methods to precisely guide planes and allowing for an autoland. You surely saw that GPS approaches (or their evolution, so to speak) are taken over and later reach precision levels which would allow for a precise guidance. And that's without the need to install any navaids at airports, at least not in the sense of an expensive ILS array.

 

Also, the planes already use a multi-sensor setup. For example, the radar altimeter plays a vital role on the very last portion of the autoland, just before touching down. So that flare and touchdown phase rely on that system while the glideslope 'fades out'. Well, find the details in the above link.

 

You may also find different setups and legal boundaries on the very same plane. For instance, operators using the HUD instead of the autoland system. Or systems which are described as fail passive and fail operational. As said, the topic is pretty detailed.

 

The smaller the plane's category, the less autoland systems you may find. I think the regional jets are all on HUDs these days and therefore fly to low minima by guiding their pilots through the display setup in front of the windscreen. There's a cost factor involved of course and I think the HUD proves to be cheaper than keeping the autoland system legal and certified.

 

It's also nice to see how the 'cheap' general aviation sector handles the case. Not much autoland there, not even HUDs, but some enhancements on the displays and the representation of the flight path. 'Highway in the sky' and things. They also have that great synthetic vision which gives you a coloured representation of the surrounding terrain and it's hazards.

 

Business jets then sometimes feature enhanced HUDs which allow for night vision and some artificial colouring. So it's not like the airliner stuff is the most advanced one by design.

Yes, by 'it' I mean the AP that is doing the autoland. I realise that even if it could stop the aircraft, it probably wouldn't for the reasons you cite. I guess if you look at it the other way round I am asking what does the crew do during an autoland and at what point do they take back control?

 

Where they take back control depends a bit on the autoland system. Generally about 60 knots I think. Reverse thrust is never automated AFAIK, so the pilots would deploy reverse thrust on touchdown and then take over directional control at 60 knots. With a bit of luck one of our resident real world pilots will be along to confirm this.