Upper side lobe of glideslope

[Send Clowns]

I've been reading one of the threads in "Reportiong Points" dealing with the complaints against RyanAir. On two occasions capturing the glideslope from above is mentioned. In both cases it is implied that this is not desirable for safety and is common and becoming more so, but the comments though critical are fairly blasé.

Now I am not yet a commercial pilot, but I recently went through a JAA fATPL course, and now teach groundschool including some Radio Nav, so the theory is still fairly fresh, and this is not allowed. There seemed to be no conceern that it is not legal on a procedure to capture the glideslope from above.

1) Is this happening often, and is it getting more common?

2) are over-hurried procedures to blame?

3) Is capture from above, with the risk of capturing the first upper sidelobe, dangerous? I know that if the pilot is visual or is checking off height/distance it should be obvious, but it has been shown before that the latter is not always done. Secondary indications such as low power/high speed may be ignored - we have all been taught how the mind rejects information not consistent with the model we have developed of our current situation.

4) If so what can/should be done about this, both by a pilot brought in too tight and expected to capture from above, and in the long term about the apparent increase in this practice?

[Captain Stable]

None of this is authoritative, please feel free to correct me anyone who knows more than I do...

1) Capturing a G/S from above is not illegal. It is highly unsatisfactory from a safety point of view, however, for various reasons You are in a low-energy situation, with low power. Your rate of descent is high and therefore, on capturing, needs to be reduced hard just as you are needing to pile power on to maintain the G/S You're heading towards the lumpy bits faster than was ever intended As you say, there is a possibility of capturing a side-lobe2) Yes, it is happening more often.

3) Various factors to blame Excess speed (cf debates on a certain LCA elsewhere) Increasing load on controllers and airspace Less familiarity nowadays than hitherto among controllers with aircrafts' requirements4) Most FD/AP systems require the aircraft to have locked on to the LZ a certain time (anything up to 30 secs depending on type) before they can capture the G/S. If an aircraft is vectored onto the LZ too close in, they have less chance of establishing in time. The G/S comes and goes, and you then have to chase it down. Altenatively, at too high a speed or too tight an intercept angle, you go straight through the LZ, have to turn and reintercept from the other side. But you get the picture.

So, all things considered, more pressure, more crowded skies, and aircraft not helping themselves by maintaining too much speed.

For safety's sake, what should really be done if above the G/S and too close in is ask for vectors back onto the LZ slightly further out. But TMA's are busy nowadays and companies won't like the added delay.

[john_tullamarine]

Following on from Capt Stable's comments,

(a) the ILS is a very old technology system and has its share of problems. One Australasian airline very nearly put a 767 into the water a year or so back when the automatics "captured" a non-existent glidepath and the crew were slow to pick up the error. Fortunately the safety culture in the particular airline was sensible and proactive (rather than punitive) and the resulting very extensive investigation produced a number of surprises ... indeed there was a suggestion that some seemingly inexplicable crew action accidents in the past may well have been caught out by a similar event ...

(b) consider also the complementary problem of false course captures .. some of the older automatics will quite cheerfully capture whichever signal occurs first .... GP or LLZ ... false or correct .. pick whichever you like. The same can happen to the unthinking pilot who is handflying the approach with brain in neutral. I had an instance with a 732 sim session not so long ago ... command upgrade chap (working more than quite hard under my tender and gentle care, I might add .. and doing pretty well) .... he selected the automatics for capture with some distance to go and then allowed himself to become distracted by other distractions (would I put in distractions ?) ... whereupon the machine captured a false LLZ path quite some considerable angle away from that which was desired. The subsequent "approach" proved to be quite puzzling to the crew as the picture wasn't right ... but, never having seen it before, and apparently not familiar with the possibility, they were unable to work it out under the remaining workload pressures ... I had allowed it to progress to see if the prioritisation penny dropped .... after we eventually froze the sim ... it ended up being a useful flight management discussion point.



The main point of my commentary is that both of the above problems ... and a good many others, can be avoided if we use the ILS in the manner intended.... it should be considered as an electronic funnel .... it is up to THE PILOT to find the entrance to the funnel and then it works just fine.

Far too much these days, we see the automation passive pilot flicking switches and pressing buttons with gay abandon but without any regard to the fact that these things are computers .. and computers screw up with regular monotony. If you trust it with your life without a healthy degree of suspicion and scepticism .. then there is a not insignificant risk that eventually you will pay for your casual attitude with your life and those of your passengers.

It is most important that the pilot take active responsibility for feeding the aircraft into the ILS at the appropriate point ... and actively check that both the location (by whatever means the approach plate specifies) and height are correct .. and likewise again at the marker ... it all works well most of the time .. but when it screws up .. it can do so big time ....

[Send Clowns]

Yes, John, that pretty much squares with my concerns. In fact is was knowing about your 767 case in (a), and the reports from it that highlighted some accidents that may have resulted from similar problems, that lead to my comments abut missing the cues that the wrong glideslope was captured.

Captain Stable

That is interesting, as I know that more than one FTO is teaching that capture of the glideslope from abovde is illegal. In fact I was told so in both groundschool and IR training. It seems however that even if legal it is not at all safe!

[john_tullamarine]

Indeed .. I was working in that neck of the woods at the time and got a fair bit of feedback on the 76 incident ..... talk about a close call .... of very great importance was the particular airline's safety philosophy and culture ... the crew made a detailed report and 'fessed up to their own shortcomings .. and the company's response was to get to the core of the causative problems rather than take the easy way out and blame the crew ... a lot of other operations could well follow their lead ... As a result the investigation was extremely in-depth, wide-ranging, and, in the end analysis, very valuable.

I think that we just have to accept that ATC workload and traffic fit in busy places will always leave us with having to accept the occasional high approach ... not a major problem if the pilots have their thinking bits switched on and, if not pushed to silly extremes, is not unduly hazardous. I would suggest that the main aim is to have a bottom line in one's operation ... and don't go outside it ... to decline under circumstances which are not reasonable might cause a little ATC discomfort ... but that's the way it goes from time to time ...