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False glide slopes

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False glide slopes

Old 26th Jun 2013, 23:01
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False glide slopes

Hi all,

I have been doing some research into false glide slopes. Most of the research and documents point towards false lobes which are at odd multiples of the actual path. So for a 3 degree approach there are catchable glides at 9 and 15 degrees. At 6 and 12 degrees there are null zones giving a failure flag.

Now this seems contradictory to what I have read about reports of people catching 6 degree slopes.

Does anyone know what the automatics of a transport jet will capture, is it the 6 or 9 degree slope and what are the indications?

Thanks
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Old 27th Jun 2013, 01:48
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catchable at 9 or 15 degrees?!?!?!

WTF are U talking about?
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Old 27th Jun 2013, 03:16
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WTF are U talking about?
Probably something like this:

Report: Air France A343 at Paris on Mar 13th 2012, intercepted mirror glide slope, large pitch oscillations and approach to stall

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Old 27th Jun 2013, 14:47
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My experience for what it is worth. Approaching to land at Manila in a 737-200 many years ago. Due ATC we were kept high (4000 I think) for noise abatement reasons and intercepted the localiser around 8 miles. That means the glide slope should have been on slope of three degrees at roughly 8 times 320 = 2560 ft. Sea level airport approx.

We were above 8/8th stratus cloud which was tops around 2000 ft and base about 1000 ft (from memory). We were cleared to descend when established on the localiser and had already dirtied up and reduced airspeed in preparation for a steep descent from 4000 where the glide slope indicator should have displayed full scale fly down. Wind was light and variable in the air and on the deck.

But it didn't and in fact displayed a slightly fly down indication of one quarter a dot. This obviously did not compute with the DME v height calculation. By the time we were around 2000 ft and going into cloud layer the glide slope indicator still showed very slightly fly down while the speed was accurate for the configuration of land flap, but the rate of descent was well over 1300 fpm and stable.

I guessed we had a false glide slope indication since in another life I did navaid flight calibration work and knew a bit about the symptoms although had never actually seen one that I knew of. I was about to go-around when we cleared cloud and had runway visual and obviously very high on slope. The ILS glide slope indicator still showed one quarter dot fly down....The T-VASIS was no help as it was all over the place and making no sense.

Manila was 11,000 ft length so we adjusted the visual approach to reduce rate of descent to land further along the runway. Landing safe with plenty of runway to spare.

Contacted ATC and said the ILS glide slope had a flyable false glide slope of around 5-6 degrees and the T-VASIS was dangerously useless and should be de-commissioned until fixed.

ATC replied they had a permanent instruction to have the T-VASIS on for all landings and that is why it was on. They added that they had serious earth tremors that day which may have knocked the T-VASIS boxes around but they would investigate it anyway and thanks for the info. But they still left the T-VASIS on!

Next day they arranged an ILS calibration test and found a flyable false glide slope around six degrees. Maintenance of the Manila navaids was a well known problem but it seemed we certainly had flown the six degree slope.

Several months earlier a China Airlines Boeing 707 from Taipei had crashed well short of the same runway on a visual approach. The 707 was a write-off. The captain swore he was on glide slope all the way, but that he needed idle thrust and speed brakes to keep the speed back to a manageable figure. The rate of descent must have been horrendous in a 707 in that configuration and idle thrust but he said he was on slope.

See this link: AirDisaster.Com Accident Photo: China Airlines Boeing 707 B-1826

Asked why he didn't go around when it was obvious close in something was wrong, he told the investigation he was worried the noise of the engines spooling up would frighten the passengers

It would seem he had experienced a similar false ILS glide slope that we had a few months later but for some reason he pressed on regardless. An ethnic culture of real men don't go around, perhaps?

Although as far as I recall, the subject of the possibility of a flyable false glide slope was not mentioned in the accident report, I wouldn't be at all surprised that was what he was unknowingly following while ignoring other information that would have proved a glide slope problem. Eg DME v Height.

Last edited by Centaurus; 2nd Jul 2013 at 13:42.
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Old 28th Jun 2013, 17:45
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NZ60 False glide slope incident. Top notch pilots.




Last edited by WhyByFlier; 28th Jun 2013 at 17:46.
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Old 2nd Jul 2013, 14:01
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It would seem he had experienced a similar false ILS glide slope that we had a few months later but for some reason he pressed on regardless. An ethnic culture of real men don't go around, perhaps?
Slight thread drift but still related. The picture of the burning China Airlines 707 at Manila after hitting short of the runway, is truly horrendous. False glide slope or not and remember that this crash happened over 33 years ago, it seems that, as so often happens in that part of the world, ethnic culture still plays a role in why some pilots refuse to accept they should go-around. The Lion Air 737 crash into the water at Bali is a case in point.

Last edited by sheppey; 2nd Jul 2013 at 14:03.
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Old 2nd Jul 2013, 14:16
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There are 2 false G/S's. One at 1 1/2 degs and 6 degs.
(Half and double) There are no flags either if you capture the wrong G/S.

This is why the final approach fix check, is critically import.

False localiser is 45 degs each side of centre.

Last edited by doubleu-anker; 2nd Jul 2013 at 14:23.
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Old 2nd Jul 2013, 14:30
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NZ 60 technically wasn’t a false GS; it was a ground failure which radiated a test signal but without identification of the error condition. The industry has taken action to prevent this.
False glideslopes occur at alternate multiples of the main beam, e.g. 6 and 15. AFAIR these are not null signals but indicate reversed sense and may not be flagged as ‘off’ or unusable.
There should always be check of altitude against range during an approach – altitude as the first priority as it is the killer parameter.
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Old 2nd Jul 2013, 14:57
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There are 2 false G/S's. One at 1 1/2 degs and 6 degs.
I don't know about these days, but certainly during my time on flight navigation aid calibration in the 1970's we checked for false glide slopes as part of the commissioning process and scheduled flight checks. There were false glide slopes which were about 60 degrees, and noticed by flying in level flight along the localiser until past the glide slope transmitter. Of course these were un-flyable unless you were a Stuka dive-bomber - but they did exist.
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Old 4th Jul 2013, 02:30
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NZ60 False glide slope incident

[Background - Centaurus and I were both doing some contract work for ANZ in Auckland at the time. One (or two ?) of their accident investigators had done their initial training in Oz on a course for which I was one of the instructors .. so we had struck up a renewal of the acquaintance and were privileged to get some insider insight into the company approach to the investigation ..]

Credit where credit is due. My observations at the time were that ANZ had a very adult and non-punitive approach to the investigation and resolution. Most creditable.

Frightening circumstances and very lucky to have had a happy ending.
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Old 4th Jul 2013, 04:24
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Burbank, Ca, BUR had a glide slope that showed on glide slope no matter where you were. We did the normal crossing altitudes and realized the GS was inop but no flags and centered. It was VFR so no problem but the airliner behind us got a low altitude alert from tower so the airliner said we are right on GS and tower said the GS is notamed inoperative. I guess it was due to maintenance but the ATIS for BUR is unreadable until low altitude because of a same frequency up north blocking it.
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Old 4th Jul 2013, 04:39
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As far as the false glide slopes at 6, 9 and 12 degrees unless you are reentering from outer space a bit of situational awareness should handle that.
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Old 4th Jul 2013, 08:22
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Quite right Bubbers....but as has been ably demonstrated by AF at CDG and Tunis in the last 18 months there is still significant room for improvement w.r.t. SA. Automation dependency? Fatigue? Culture? Or just "gash"?

Just back up your indications with a bit of basic mental arithmetic and you could save yourself from a world of hurt.....an if it's going to , just go-around...please.
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Old 4th Jul 2013, 10:32
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The company I fly for had a false GS capture, which according to the flightsafety departement and the manufacturer of the airplane was a 9 degree GS in reverse sensing.

The pilot flying totally lost SA and started descending fully configured and at final speed to pick up the GS from above (was a little high, but not disastrous). Due to being overloaded (still in line training) he descended with less than 3 degrees, so not enough to follow the GS, let alone pick it up from above.
The instructor saw it, gave hints, but decided to let the trainee go on for a while to show his error. In the instructors mind a go around was already decided upon and just before he wanted to call it, the aircraft intercepted the 9 degree GS, which resulted in the plane trying to follow it upwards, so it started to climb with 9 degrees.
Needless to say the autothrust had problems keeping up and a certain shaking sound was heard for a split second before they could take over.

What we have been told after investigation is that the 6 degree path would produce a flag in the cockpit but the 3 degrees (obviously) and 9 degrees are there, with the 9 degree in reverse sensing.
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Old 8th Jul 2013, 13:05
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I suspect this thread may have become remarkably topical?
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Old 8th Jul 2013, 16:54
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Something that is unclear - does the 6 degree slope exist or not? My understanding was that it existed, but some comments here are making it sound that in the 6 degree region, fail flags are displayed?
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Old 8th Jul 2013, 17:08
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I suspect this thread may have become remarkably topical? Why?
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Old 10th Jul 2013, 01:54
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I was taught that the false slopes alternate by reversing the direction. In other words above the normal slope the first false slope will tell you to fly up, and the next one above (presumably quite a bit above) will tell you to fly down etc etc.

Seems opinions differ, but in any case a cross check of the profile is surely a critical SOP.
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Old 10th Jul 2013, 02:02
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As far as the false glide slopes at 6, 9 and 12 degrees unless you are reentering from outer space a bit of situational awareness should handle that.
Yes, ROD is a good clue!
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