Monday, March 22, 2010

Displaced Thresholds and Landing - Part II

JFK Has Five Displaced Thresholds 
What are the reasons a permanent displaced threshold will be established for a specific runway? There are only two reasons.  The first one is if an obstacle penetrates the obstacle clearance plane.  A displaced threshold will be established to assure clearance from the obstacle while making an approach in Instrument Meteorological Conditions (IMC). The second reason is for noise abatement. As far as I know, there is no way to tell them apart.    

I made an attempt in 1997 to get the FAA to consider re-locating the five different displaced thresholds in place at JFK in light of the newer, quieter stage II and stage III aircraft using JFK.  The FAA actually did do a study to see what the cost of moving the displaced thresholds would be.  The cost of the computer time to move the thresholds was estimated to be in excess of $100,000.

The reason for the high cost was the need to re-survey the heights and locations of all the man made obstacles that had been built in the obstacle clearance planes for the approaches.  As a result of the high cost of doing this, the displaced thresholds will most likely remain in their current locations. I doubt if we ever see them change.

Once again, I want to re-state the fact there is nothing that prevents you from touching down prior to the displaced threshold.  You will note upon inspection of the actual runway there are many tire marks prior to almost all of the painted threshold markings.

Consider the situation of a requirement for a no flap landing.  Without the use of wing flaps, the approach and touch down speeds are going to be much higher than normal.  Add to the situation a high gross weight condition along with a 9 knot tail wind and the actual touch down speed may exceed 230 knots. I recognize this set of conditions may be highly unlikely but it still remains a possibility.  If you are landing at JFK on runway 31L, I doubt very much if you would choose to land on or beyond the displaced threshold.

If there is an operational need for you to use all the runway, use it!


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