Flight Safety
Declared Distances and Displaced Thresholds... When the runway is shorter than published.
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by Rick Braunig
here aren’t a lot of runways with displaced thresholds in Minnesota, but they are common enough that pilots should be familiar with them. When a runway has a displaced threshold, the landing distance available (LDA) is shorter than the runway length. The threshold to Runway 30 at Winona Municipal-Max Rick Braunig Conrad Field (KONA) is displaced by 480 feet, meaning that aircraft are not supposed to land before that point on the runway. When landing on Runway 30, the pilot does not have the full 5,679 feet to land on, but rather, only 5,199 feet. This information is shown on the top line of approach plates or in the Declared Distance information in the Chart Supplement (Green Book). Displaced thresholds are usually added to clear obstructions in the approach. There are requirements for
day and night operations. Of course, runways can be used in two directions, so it stands to reason that an approach obstruction to Runway 30 would be a departure obstruction to Runway 12. Departures are normally made with maximum thrust and the climb is normally at best angle of climb (Vx) until the objects are cleared. While an aircraft can legally use the entire runway for takeoff, that does not ensure that they will be able to clear the obstructions on climb out. If you notice on climb out that you are not going to clear the obstruction, the only choice left is to try to turn to avoid it. A good rule of thumb is to always plan to lift off by any displaced threshold on the reciprocal runway, giving you a better chance of clearing the obstructions. Unfortunately, the displaced threshold can be hard to see at night because the displaced threshold is not lighted for aircraft going the opposite direction. It is interesting to note that the LDA for Runway 12 at Winona is also 5,199 feet. There is no displaced threshold to Runway 12, so why can’t all 5,679 feet be used for landing? The answer is that the usable runway has been shortened by
approach slopes that are based upon the type of approach to the runway. In this case when we say type, we are talking about whether the approach has to be visual or if there is an instrument approach to the runway end. If an object penetrates the required slope, the FAA allows the use of a displaced threshold to clear the slope. Displaced thresholds are marked and lighted so the pilot can identify them during both
the distance required for the Runway Safety Area (RSA). The RSA is a defined surface surrounding the runway, prepared or suitable, for reducing the risk of damage to airplanes in the event of an undershoot, overshoot, or excursion from the runway. The RSA is also meant to support aircraft rescue and firefighting equipment if needed for an aircraft crash and to support snow removal equipment to help the airport keep
40 AUGUST/SEPTEMBER 2021 MIDWEST FLYER MAGAZINE
