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Twin locator approaches

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Twin locator approaches

Old 3rd Oct 2000, 06:25
  #21 (permalink)  
gaunty
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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/
 
Old 3rd Oct 2000, 18:44
  #22 (permalink)  
Outside Loop
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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.
 
Old 5th Oct 2000, 19:14
  #23 (permalink)  
reynoldsno1
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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.
 
Old 7th Oct 2000, 17:08
  #24 (permalink)  
Centaurus
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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.
 

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