Fog Question ?

[J4CKO99]

Ok, might be speaking out of turn here and in the wrong forum (so mods, feel free to move) as a long time lurking SLF but sat here in Knutsford, Cheshire, right under the flightpath of Manchester Airport and was chatting to a colleague who flew in from Aberdeen this morning, he had some delays, MAN is down to single runway operations and they ended up circling for a bit due to the fog, which has since descended again, I can hear planes overhead but cant see them.

So, we were talking, all being slight plane geek/enthusiast and fairly frequent fliers about what it must be like to be a pilot in these low visibility conditions so were wondering what pilots feel like when its like that as despite the technology, training, professionalism, procedures and dilligent ATC we reckopn it must still be a little bit scary, driving in was bad enough so pilotting a massive airliner must be disconcerting as itis a bit of a leap of faith doing 200 odd mph on approach when you cant see a bloody thing, I always get uneasy when we descend through thick cloud.

So, hats off to those sat at the front and we are interested in how you feel in these situations, is it business as usual or do you sit there, like us, feeling a bit twitchy ?

[Checkboard]

It's business as usual. Concentration is a bit higher in adverse weather, and planning for a missed approach a bit further forward in the thoughts, but that's about it.

Remember that professional pilot's spend their working lives in aeroplanes - I have far more time in an aircraft seat that a car seat, for instance.

[28L]

In a slightly bizarre way I quite enjoy putting into practice what we do in the simulator on a regular basis. Nowadays (in my experience operating ex-LHR) this sort of fog happens very infrequently.
It's the operational side which is VERY frustrating....delays...de-icing (if it's freezing fog, which it is at the moment). All the hassle is on the ground. Once I'm in the air I reeeeeelax! For the most part, the only scary bit about making a low-visibility approach is that I'm about to meet the chaos on the ground once again.
Remember that although the autopilot is flying the approach and landing we are monitoring everything as it happens - height, speed, position etc - and are able to discontinue the approach/landing at any stage up to (and including) touchdown.

[J4CKO99]

Very reasurring, not that I thought you sat there sweating and gibbering I hasten to add :ok:

I think with me its the fact I cant see anything and start wondering whats in the fog, the Tenerife accident(s) always spring to mind but I do appreciate that was a long time ago, I think it goes back to being a kid and hearing that my dads boss had been killed on the Dan Air one.

[Ex Cargo Clown]

I can't actually think of an accident due to thick Cat III type fog, maybe the Aeroflot Perm one, but that had plenty of other factors, and the KLM one had even more reasons.

It appears to actually be a concentrating factor, rather than a distraction.

[Lord Spandex Masher]

J4CKO99,

Don't forget that there are some airlines out there who do not use auto land.

Some aircraft are fitted with a head up display and the pilots then manually fly a CATIII approach and landing!

Hope that reasures you a bit more:E

[JEP]

You may add SAS in Linate - several factors of course, but it might not had happened in CAVOK.

[M.Mouse]

Flying in 'fog' i.e. cloud is something that routinely happens and is no big deal because large airliners are flown accurately by reference to instruments rather than by looking out of the window so flying without being able to see out is routine. Unlike a car you are generally being controlled and with good situational awareness it is not stressful because you know that you are unlikely to hit anything!

Making an approach in low visibility is practised in my company every six months to maintain competency and recency. I have made few really low visibility approaches and landings. The one that sticks in my mind is landing a B757 at LHR in 75 metres. We caught a glimpse of the odd light at around 200' then nothing more until the nose wheel touched down. It requires a great deal of concentration for a relatively short period of time in order that you react instinctively in the event of a technical failure. Other than that I would not call it stressful.

[andrasz]

I can't actually think of an accident due to thick Cat III type fog...

I can think of several, some in quite recent memory (PAF 101, anyone... ?), a common cause seems to be doing NP approach in Cat III weather...

Probably what you meant was no accidents while performing a Cat III approach in a suitably equipped aircraft, I cannot think of one either.

[Arfur Dent]

Taxying to the stand in 50 metres is far more tricky than the approach itself - especially when there is no ground control radar. Airports are notoriously badly marked and you really need to be careful that you are exactly where you think you are. There are some big airports out there.

[MungoP]

In fact accidents due to fog are pretty rare and have been for the past 30+ years. Company SOPs plus the legalities of making an approach give the pilot a clear way out if the vis is below minima. This wasn't always the case and in the bad old days companies might have been guilty of encouraging the crews to push down a little too far in an attempt to get in. Also it's generally accepted inthe industry that no more than 2 approaches should be attempted before diverting. A lot of blood was spilt before these rules were adopted and being able to divert without worrying about losing your job is a luxury we enjoy at the cost of those who went before us. The real concern is less to do with making the approach and more to do with fuel... in these days of accountants sitting comfortably (and safely) at their desks dictating terms on fuel uplifts, choosing just when to go for the alternate (knowing that a whole bunch of others might be about to head there ahead of you) can be problematic.

[INKJET]

I think that guys that are only Cat1 and no autopilot have a far more demanding job in these conditions, in general terms in a Cat3 aircraft if you can legally make the approach you will get in, in fact i can't think that i have ever gone around from a cat3 auto-land other than in the sim, my only scary approach was a cat2 into BRU (aircraft only cat2) when someone cut the power supply to the glide-path antenna with a JCB when we were about 400ft by the time i'd worked out what was wrong we were at 250ft and i was just about to throw it away when i got the lights (not the motorway!!)

[Nicholas49]

For the most part, the only scary bit about making a low-visibility approach is that I'm about to meet the chaos on the ground once again.

28L: your comment made me laugh out loud!

Can I ask a technical question here? This thread seems most relevant.

I understand every approach has a decision height. I understand that when you reach the decision height, you must go-around if you cannot see the runway lights. Hope that's correct.

But I do not understand how that rule applies on a CAT3 approach where, I imagine, you cannot see the runway even at decision height? Is there a different decision height for each type of approach (i.e. CAT1/2/33?) Or have I misunderstood something?

Many thanks for having the patience to explain to a non-professional.

Nick

[Denti]

There are CAT III approaches with a decision height and then there are those without. A usual decision height is 50ft, that is usually low enough to see the required visual segment usually approach lights only. If the crew does not see enough it has to execute a go-around. Without a decision height you do not take a decision or take them at an alert height which can be quite a bit higher than the CAT IIIa DH (200ft for us, below that landing is assured with any single failure, be it ILS or engine failure). For approaches with no decision height there is no visual requirement before touchdown, however you need a minimum RVR to be able to leave the runway, usually 75m.

[GlueBall]

CAT-III and CAT-IIIb alert height is typically 50 and 20 feet respectively [measured from the main gear]. It's a height based on the auto land fail-operational status from which a missed approach would be initiated. During a go around as such, the mains would likely touch the pavement.

[west lakes]

Having recently been given a video of a CAT III landing from the captains seat by a friend, respect to all. I actually drove past the threshold about 40mins before it was filmed and visibility as atrocious from a car

[Exaviator]

Whilst an instrument approach in marginal conditions is fairly routine to a trained and experienced crew, for the long haul pilot it usually happens at the end of a long and tiring duty period and you are at the back end of the fatigue curve.

Extra attention is required under these conditions or it can lead to poor decision making and falling into the trap of "Get home-itus", which is often a cause of landing accidents.

But to the pilot who maintains his professionalism, fly's a accurate approach and landing in fog the hard part after vacating the runway is often finding the way to the parking gate.:ok:

[Doors to Automatic]

I can understand that low viz landings are routine, with the help of Autoland, but what about take-offs? As they are always manual, blasting off in zero visibility must be very disconcerting?

[Denti]

Glueball, the higher an alert height the better, as below alert height a landing is assured with any single failure. We do use the 200ft alert height provided and certified by boeing, as of course after 200ft only multiple failures affect us now, whereas above the alert height even a single failure leads to a lower autoland status and most probably into a go-around. Alert height is in "my" operation only used when approaches with no DH can be flown, otherwise DH will be used.

Decision height on the other hand is based on go-around capability where a visual segment and a man made decision is still required, mainly fail passive and HUD based systems. The usual lowest DH for medium type aircraft is 50ft.

[HEATHROW DIRECTOR]

<<however you need a minimum RVR to be able to leave the runway, usually 75m.>>

Even though Heathrow has an excellent ground lighting system so pilots just follow the green lights from the runway to the parking gate, I have seen fog with patches so thick that pilots could not see the "greens". Then we used the ground radar to talk them off the runway. Great fun!!

[Nicholas49]

Denti, thanks for the explanation.

There are a few things I still don't understand:

- is the decision height specific to the type of approach being flown (CAT I/II/III) or specific to the airport runway? In other words, can it vary for the same runway in different weather conditions?

- how do you know the difference between alert height and decision height? Does the aircraft have a separate aural call-out / indicator for them?

CAT IIIa DH (200ft for us, below that landing is assured with any single failure, be it ILS or engine failure)

I really don't understand this and have wondered about this before. Surely if there is an engine failure at 200 ft, the approach becomes destabilised and you lose the localiser alignment? Are you saying the aircraft's autopilot is able to re-stabilise the approach with the one remaining engine (I'm talking twins) at 200ft and you can continue the landing without a go-around? If so, I say "wow", I never knew that!

Nick

[fireflybob]

The original specification for autoland on the DH Trident (which had a triplex system) was a failure rate of 1 in 10 to the six - ie in one in 10 million landings!

[Denti]

- is the decision height specific to the type of approach being flown (CAT I/II/III) or specific to the airport runway? In other words, can it vary for the same runway in different weather conditions?

The decision height is specific for the type of approach (CAT I/II/III), the aircraft equipment (fail operational and fail passive) and local geographic situation, ie. the runway.

- how do you know the difference between alert height and decision height? Does the aircraft have a separate aural call-out / indicator for them?

It is a specific difference between fail operational equipment and fail passive equipment only applicable to CAT III approaches. For example we do fly one type of aircraft of which some have fail operational autoflight systems and others don't. On those planes with fail operational autoflight systems we can fly CAT III approaches with no DH which means no requirement to see anything before nosewheel touchdown, in that case we have an alert height below which only multiple failures will lead to a go-around. On those planes with fail passive only autoflight system (which can be a downgraded fail operation system as well) we do have to use a DH because we have to see something to decide if a safe landing is assured or not.

So in short, we know the difference by equipment, not by any different aural alert or callout, as we do not get any automatic callout during fail operational CAT III approaches.

I really don't understand this and have wondered about this before. Surely if there is an engine failure at 200 ft, the approach becomes destabilised and you lose the localiser alignment? Are you saying the aircraft's autopilot is able to re-stabilise the approach with the one remaining engine (I'm talking twins) at 200ft and you can continue the landing without a go-around?

Well, exactly at 200ft is bad, one foot below or lower is safe ;) Yes, below the alert height any single failure won't affect a safe automatic landing. That can be an engine failure, the complete loss of the ILS, loss of a hydraulic system etc. It is actually quite interesting during initial CAT IIIb training to see the autoflight system coping with those failures, especially when you are used to CAT IIIa (fail passive) operation and the resulting go-around for each of those failures.

[Nicholas49]

Thanks Denti.

Well, exactly at 200ft is bad, one foot below or lower is safe

This is the part that seems completely counter-intuitive if ones follows (erroneously, it seems!) the logic that the closer you are to the ground, the less time you, or the autopilot, have to make corrections to glide-slope/localiser in the event of an engine failure. But you are saying it is unsafe to continue the approach at and above, not below, 200ft? This sounds strange to the non-expert!

If I understand correctly: on a CATIII approach using the autoland function, if an engine fails below 200ft, will the autopilot apply the correct amount of rudder to stop you veering off the runway on touchdown? How on earth does it know how much input is needed?!

[wiggy]

Surely if there is an engine failure at 200 ft, the approach becomes destabilised and you lose the localiser alignment? Are you saying the aircraft's autopilot is able to re-stabilise the approach with the one remaining engine (I'm talking twins) at 200ft and you can continue the landing without a go-around

Why would you lose the localiser alignment? In simple terms the autopilot ( assuming it's a three axis version) will just apply whatever correction is needed to keep the localiser centred.

[Nicholas49]

Why would you lose the localiser alignment?

I thought that would be an inevitable consequence of an engine failure when you are established on the ILS. It appears I was wrong. ;)

I'd be interested to hear an answer to my second question. How does the autopilot know how much rudder to apply after a single-engine autoland, given that the ILS provides no alignment assistance after touch-down? I thought you would have to fly a single-engine landing manually so the pilot can judge how much rudder input is needed. Is that incorrect?

[reak6ion]

As far as my knowledge goes,on the ground the aircraft will keep on tracking the yaw bar which is nonetheless the localizer beam and thus the rwy centerline.
Correct me if im wrong:)

[Sky Wave]

Loss of GP or Localiser signal even below ALERT height will mean a Go Around for us. A320 family.

It's one of the functions that is linked to the RED - AUTOLAND light. It basically means the aircraft cannot land and you must go around.

That said the red light will not come on if you lose the glidepath signal below 100ft as the aircraft is no longer using GP it uses RADALT. It will also not come on if you lose LOC below 15ft, however I guess that there is no way for it monitor the roll out so the Captain better be ready to take over!

If I understand correctly: on a CATIII approach using the autoland function, if an engine fails below 200ft, will the autopilot apply the correct amount of rudder to stop you veering off the runway on touchdown? How on earth does it know how much input is needed?!

Firstly at the point you touch down both engines should be at idle so there is very little rudder required to stop you veering off the runway. We can autoland with up to 20kts crosswind which is much more challenging for the autopilot.

Prior to touchdown the Autopilot just reacts to the situation it's presented with. A strong gust of wind at 200ft will cause more disruption to the flight path than an engine rundown. If the engine fails the auto thrust will increase power on the good engine to maintain the speed and the AP will just fly the ILS. No dramas!

[Denti]

Interesting. I was kinda surprised that the 737 can continue its autoland after alert height even if it completely looses the ILS signal. But at least in the simulator it works as advertised, never tried it in real life.

Below Alert Height any degradation in autoland status except NO AUTOLAND is suppressed until below 40kts GS and on ground, NO AUTOLAND is of course a go around and autopilot disconnect.

[Nicholas49]

Thanks Sky Wave - that clears it up.

Denti- if you completely lose the ILS signal in the 737, where is the autopilot/autoland taking its back-up signal from?

I must say the prospect of an aircraft landing itself, on one engine, in zero visibility, with a crosswind, is very impressive!

[Sir George Cayley]

Interestingly, some aircraft that are certified to CAT lllB will touch down after a go around is initiated from the DH. Must confuse the pax getting airborne again:ok:

Sir George Cayley