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TDZE and runway elevation on IAP plates, why the difference?

Asked by: 8596 views Instrument Rating

Reference the subject title: If you go to SFO, all the approaches use runway elevation. But others obviously use TDZE. Sometimes, one runway will use TDZE, but the opposite direction rw will use runway elevation (reference KRBL and rw33 and rw15). Are they just changing the nomenclature here? I'm using Jepp plates. Thanks!

1 Answers

  1. John D Collins on Aug 01, 2014

    If you are referring to the fact that some runways specify THRE (Threshold Elevation) and others specify TDZE (Touchdown Zone Elevation), ICAO has adopted the use of THRE as its standard. TDZE is the highest elevation in the first 3000 feet of the approach end of the runway starting at the threshold. For runways that comply with standards, this may be up to 20 feet higher than the threshold elevation. In most cases, THRE and TDZE are the same, but not always.

    The US as part of its ICAO conformance policy decided to switch its approach charts from using TDZE to THRE and over 4000 approaches have been changed. I pointed out at the Aeronautical Charting Forum that 91.175 specifically specifies TDZE when authorizing descent below the DA/MDA using the approach lights as the only visual cue. It states: “The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable.”

    I asked, how could this be complied with unless the TDZE was on the approach Chart. The FAA checked into it and the attorneys agreed that it was not possible to comply with the regulation without TDZE. Big whoops. The FAA then stopped putting THRE on new or revised charts, has gone back to using TDZE and published a document that lists all of the TDZE values for airports who have approaches that are still using THRE as an interim fix until all those charts can be updated. This can be found at http://aeronav.faa.gov//d-tpp/1408/TDZE_rounded.pdf I don’t have Jeppesen service so I can’t tell you what they provide on their charts.

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