Twin locator approaches

[sprucegoose]

Can anyone provide some input regarding the number locator beacons required to conduct a twin locator approach. Where does it say both beacons must be servicable? I was taught to fly the approach with one beacon tuned in front of the aircraft and one behind. Fly towards the head of the needles to correct track and aim to keep both needles diametrically opposite to each other.

However where does it say that that you cannot use just the beacon in front of you for tracking once you have passed the first beacon? Can you do a twin locator/DME approach with one of the beacons unservicable? I need text references if anyone can help here.

[Reimers]

At my company, the only time we ever fly twin Locator (NDB+NDB) approaches is when going to Russia. As some aircraft are only equipped with one ADF, there is a higher minimum published for these aircraft. If one ADF was inop you can still do the approach.
Sorry, no text references, though...

[Tinstaafl]

I understand that you can't conduct a Twin Locator approach if one of the NDB's is u/s.

The design criteria for the minima on the approach are predicated on the additional tracking accuracy provided by the pair of NDB's used in conjunction with each other. The 1st NDB allows the a/c to be positioned more accurately for the commencement of final.

Of course there may be an alternative single NDB approach published using one of the locators, but that depends on the what the relevent authority decides. All other factors being equal, a single NDB approach will have a higher minima due to the lack of positioning accuracy when commencing final.

Rather a while since I studied this so any errors are purely my own!

[sprucegoose]

Yes well... the situation we had was thus in the simulator. We were tasked with flying a RWY16 twin locator/DME at Melbourne. My partner during his crack at it elected to put both ADF's to the second beacon along track after passing the first beacon at the IAF. I raised the question in the debrief if that was acceptable for a twin loc. approach. After all it is a TWIN locator approach. I pontificated at length about how I was taught to fly that type of approach and the theory involved. The instructor agreed that he had also been taught that way but there was no reference in any manual, regulation or other wise that listed minimun nav aid requirements for said procedure. Indeed the theory book I refered to was written by an ex Ansett pilot and my method of execution ( based on this theory text ) was simply one persons technique and not necessarily the only way. I just want some reference to this type of approach that says or implies that both beacons must be servicable and or used for tracking.

Thanks for the feedback thus far. Incidently its a prick of a way to get a big jet down to earth!

[This message has been edited by sprucegoose (edited 24 September 2000).]

[ironbutt57]

legalities aside..it enhances horizontal situational awareness to have the two locators tuned as opposed to using one..it seems to me to privide a clearer picture of the aircraft's position along the final approach path..fly the same approach at MEL in the sim during recurrent as well...works fine..lasts a long time!
:) :) :)

[mustafagander]

It is axiomatic that unless all nav aids named in the proceedure nameplate are serviceable, that approach is not able to be flown. Think about a VOR/DME proceedure - take away the VOR, what's left??? or the DME, where are we distance-wise??? Likewise with a twin locator. If both aids weren't necessary, they wouldn't be named, it would be a locator approach.
If one of the aids is not displayed, I would expect Oz AIP ENR 1.5-7 para 1.10.1 (b) Missed Approach would apply .....radio aid fails....its failed alright - you turned it off! Also try ENR 1.5-13 para 1.12.1 descend when "established" - not easy if the aid is not displayed.
Yeah, I know - you're established on the other one, BUT both are named in the proceedure and are necessary as far as the designer is concerned.

[Capt Claret]

Mr Goose sir,

I'm not aware of any requirements for the mumber of ADFs or which aid they're tuned to. My approach would depend upon the # of ADFs.

If 2 ADFs, one tuned to each locator. After station passage overthe first locator, I'd track to the second, as the needle wouldn't be as sensitive given the distance to the 2nd locator.

If only 1 ADF, after station passage over 1st locator, tune & ident second locator, and again track to it.

As far as the locators are concerned, I agree with mustafa, that if both locators aren't serviceable, then it really can't be a twin locator approach, can it?

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bottums up !

[reynoldsno1]

Tin is absolutely correct - the use of the twin locators reduces the obstacle clearance area in the final approach segment. Using only one beacon will increase the area, and, depending on the obstacle environment, may increas the OCA/H. You may also lose a final approach fix.

The cause of the US Navy 737 that crashed in Yugoslavia was partly attributed to an aircraft trying to fly this type of approach and getting it wrong (only one ADF installed).

[sprucegoose]

I agree with what I'm getting here and I appreciate the responses.

Assuming all radio beacons were working can one fly a twin locator approach if the aircraft is only fitted with one ADF? I can't find my Trevor Thom book http://www.pprune.org/ubb/NonCGI/frown.gif

[Tinstaafl]

My understanding is that you must have both locators transmitting but need only one ADF receiving to do the approach.

Once established over the 1st aid then it has done its job ie position the a/c within a defined area prior to commencing the next segment of the approach. Thereafter the 2nd aid is acceptable to be used for tracking to stay within the required tolerances.

Playing devils advocate here, but if both receivers are required at what point is the changeover specified for tracking from the locator behind, to tracking to the locator ahead?

It's easy to say 'at the mid-point' but for a TWIN LOC only approach ie no DME, this doesn't seem to provide a means to determine this. Timing guidance isn't given for the segment between the aids (haven't got an approach plate with me so must rely on memory).

Given the different tolerances different receivers provide there is also a possible tracking discrepancy on the changeover between the two.

I realise this is somewhat pedantic given that it's a non-precision approach, but just my thoughts. Using two ADFs then I have one tuned in front & one behind as this saves twiddling the buttons during the approach. After the 1st LOC I track using the ADF tuned to the beacon ahead.

I find the one behind is useful once overhead that beacon for determining if I'm L or R of the track by the way the needle falls (slow, fast, to the L or R etc). During inbound to beacon #2 then the combination is great for determining L or R of the track quickly. Even so I still track using the navaid ahead.

[Lurk R]

Is there a maximum distance between the 2 NDB's to maintain effectiveness of the approach? Just thinking of instances where only 1 ADF is being used and the effect of a stiff crosswind which pushes you slightly off track between the 2 beacons?

[Slasher]

Jepp charts specify the PRIMARY aid to be used for navigation in the top right corner of the approachp plate. Its my understanding if that aid is unservicable, your not legal to do that particular approach.
Say ABC is an NDB at 10nm and DEF is one at 4nm. If ABC is the Primary Designated Aid then ABC must be servicable in order to do that approach. If ABC is out you cant legaly use DEF on its own.

[gaunty]

sprucegoose
Creeeeak groooaan, click, sproing, sounds of memory booting up.

I suspect the answer may well be in the question.
Locator beacons MF L(lower power than) are subtly different to MF NDB and one can be easily confused with the other. The Locator ranks after the NDB in order of precision.
See Jepp Radio Aids AU21-22.

As I recall it, Locator beacons usually radiate at less than 50 watts and are used primarily to locate the aircraft near an aiport runway centreline prior to or as part of an approach, or as an LOM or Locator at a OM site as an IAF on a Localiser or ILS.

I suspect that those Locators that now appear to be bereft of any apparent proximity to a localiser/ILS (such as ARC and MEA at AMML)are the remnants of an ancient VAR system, but are still useful.

It is their lower power and usually low frequency which limits their usefulness as tracking aids unless 2 are colocated and can be used concurrently/contiguously or whatever.
AMML Twin Locator Rwy 17, ARC and MEA are only 5.6 miles apart
I also suspect that although the AIP appears silent on the matter directly, or I can't find it, it is implied in Track Keeping "only those aids which specifically define the relevant track must be used for tracking keeping", provides the requirement for them BOTH to be operating.
Further I seem to recall that under the IFR Minimum Required Nav Systems for RPT; and Charter/Airwork when using NDB's (Locators) as the primary means of navigation that two ADFs are required.
To satisfy the monitoring requirements each Locator being used during the approach must tuned and both monitored for operation.
Seems to me that therefore you need both Locators serviceable and 2 serviceable ADFs to legally conduct a Twin Locator.

BTW thanks it was great company the other night :)

[Tinstaafl]

I think the RPT requirement for 2 ADFs isn't applicable to Twin LOC approaches.

Under Oz rules a private flight is allowed to do a Twin LOC approach. Also Aerial Work & Charter operations require two independent navaid systems. Two ADFs are not specified so a single ADF & a single VOR meet the navaid equipment requirement.

Good point about the difference between a LOC & an NDB. I was being a bit slack in my wording.

[bookworm]

While it doesn't directly address the question, it's worth mentioning that the accident that took the life of the then US Commerce Secretary, Ron Brown, took place on a twin locator approach into Dubrovnik.

The accident aircraft, a USAF CT-43 (B737), had only one ADF, but flew the approach anyway, apparently in violation of at least Air Force regs. A charting error was probably the most important factor in the accident, but the lack of navigational precision close to the MAP locator was also key in this CFIT tragedy.

[Streamline]

In my opinion this is a problem that is situated at the bureaucratic level.

There are more examples of this kind of bull****.

It often boils down to the fact that (and you all confirm my reasoning because you are actually asking the question) the guy who makes the plates doesn’t have a clue himself because he is not a pilot, hence doesn’t think about it.

So if you want to sort this out for every particular case you get to your local pilot union and get it sorted out with the local CAA, they will update the AIP and they the guy’s at Jeppesen or whatever will follow.
Every instructor trying to see it otherwise is compromising his authenticity.

I know I am pushing my opinion a bit hard here but every professional pilot has the right to clear cut information definitely as far as approach plates is concerned.

Details matter if lives are involved.



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Smooth Trimmer

[reynoldsno1]

Streamline - do you acrtually know what goes into the design of an instrument procedure? I think not otherwise you would realise that "the guy" who "makes up" the plate isn't a civil servant sitting in a back office somewhere with a pencil and paper & a bad idea.
The design process is exacting, exhaustive and exceedingly protracted. Users (drivers and controllers) are encouraged to participate, but often don't. Most pilots I work with are totally ignorant of the requirements and implications of PANS OPS and/or TERPS. Find out who these guys (or gals) are in your neck of the woods and talk to them - you will find out you can both learn a lot from each other

[sprucegoose]

Good answers all round. Thanks gaunty, I had fun as well. We must do it again sometime. I agree with Streamline to a degree on this one as well. So little is published in relation to the particular procedure that as we can see from the many answer here there is certaintly some room for error. I will put the question to a contact at CASA and see what I get for a response. I'll let you know.

[Streamline]

reynoldsno1

Yes I do know about Pans Ops and Terps, I have the copies at home and studied them.

We at our company we even X-check the calculations of some other countries and sometimes we are surprised and learn from it.

There are third world countries everywhere that have signed up with ICAO to apply Pans ops etc but...

Therefore some countries and companies have elected to make their own maps and plates and are no longer dependent on the competence of third parities.

It’s their decision and presumably not a cheap one but they think it’s worth it.

Fact is that someone has raised a question, because apparently there is a problem with some approach plates.

So if the plate is ambiguous then you have the right to ask for clarification about the plate to whoever made that plate.

No offence meant to whoever made the plate however, but details matter when lives are involved.

I know companies that forbit their crews to go into certain airports while others do not.

Civil pilots try to stay out of trouble while the military go and look it up. (it's the nature of their mission of course)
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Smooth Trimmer

[This message has been edited by Streamline (edited 02 October 2000).]

[gaunty]

For those with the time and interest got to
www.dogpile.com (http://www.dogpile.com)

my fave metasearch engine, type PANS OPS or TERPS with the usual parsing in the search window and press "fetch"

Settle back and be prepared to spend an interesting cuppla hours.

You may not find EVERYTHING you are looking for but will leave more edumacated :)

Wish they'd had this internet thingy when I was a lad.



[This message has been edited by gaunty (edited 03 October 2000).]

[gaunty]

Oh and BTW Ozzers who complain about ASSY airspace planning have a decko at this as the tip of one of the icebergs at LAX.
http://www.terps.com/

[Outside Loop]

I could be wrong but I was under the impression that the use of multiple NDB's,was to provide a further descent/descent limitation fix (no DME),rather than to aid tracking accuracy.Hence, both NDB's must be servicable but you can fly the proceedure with a single ADF, or with both ADF's tuned to the same aid

In the interest of orientation though I would have thought it more practable to allocate an ADF to each NDB as you described.

Remember the Westwind in Alice.

[reynoldsno1]

The use of twin locators serves both purposes. Having a FAF reduces the minimum obstacle clearance by 50 ft in the final approach area, but the additional tracking guidance reduces the size of the final approach obstacle clearance area - the areas are trapezoids that get larger the farther you are away from the facility providing (angular) track guidance.

The 737 that crashed in Yugoslavia hit high ground that was right on the edge of the the obstacle clearance area associated with the 1st locator beacon. It would appear that the aircraft used ONLY that beacon for tracking guidance and consequently never got to the MAPt, and was on a continuous divergent path from the FAT and MA track.

[Centaurus]

The USAF B737 accident exposed an interesting example of Murphy's Law. If I recall correctly the USAF had a published general SOP which directed that once established on final instrument approach, you were not permitted to change a navaid frequency. When the B737 departed the first NDB (I understand that for some reason there was only one ADF on the aircraft - maybe a TACAN RMI was fitted) adherence to this SOP meant that the crew were not permitted to change frequency to the next NDB in the procedure. Obviously someone in USAF HQ figured that there was a risk of mis- selection, so they neatly removed that risk and caused a worse risk to surface.

The aircraft was never stabilised - it was high and fast from the beginning and the workload built up. That second beacon near the airport was vital as it defined the missed approach point as well as a tracking aid I think.
But the crew never selected it. They either forgot due to hurried unstable approach or they stuck to a dangerous SOP and did not select the next NDB.

On any normal twin locator approach (assuming two ADF in the aircraft), once the aircraft is comfortably tracking on final and having passed the first beacon, I prefer to now bring both ADF's up to the beacon ahead as a precaution against mis-selection and a general back up to what may be the missed approach point. Purely personal opinion though.