diff --git a/Nasal/glide_slope_tunnel.nas b/Nasal/glide_slope_tunnel.nas
new file mode 100644
index 000000000..8d60eb944
--- /dev/null
+++ b/Nasal/glide_slope_tunnel.nas
@@ -0,0 +1,132 @@
+# Draw 3 degree glide slope tunnel for the nearest airport's most suitable runway
+# considering wind direction and runway size.
+# Activate with  --prop:sim/rendering/glide-slope-tunnel=1
+
+var MARKER = "Models/Geometry/square.xml";	# tunnel marker
+var DIST = 1000;				# distance between markers
+var NUM = 30;					# number of tunnel markers
+var ANGLE = 3 * math.pi / 180;			# glide slope angle in radian
+var HOFFSET = 274;				# distance between begin of runway and touchdown area (900 ft)
+var INTERVAL = 5;				# check for nearest airport
+
+var voffset = DIST * math.sin(ANGLE) / math.cos(ANGLE);
+var apt = nil;
+var tunnel = [];
+setsize(tunnel, NUM);
+
+
+var normdeg = func(a) {
+	while (a >= 180)
+		a -= 360;
+	while (a < -180)
+		a += 360;
+	return a;
+}
+
+
+# For each runway complement the opposite end, as airportinfo() returns only one end.
+#
+var complement_runways = func(apt) {
+	foreach (var rwy; keys(apt.runways)) {
+		if (!string.isdigit(rwy[0]))
+			continue;
+
+		var number = math.mod(num(substr(rwy, 0, 2)) + 18, 36);
+		var side = substr(rwy, 2, 1);
+		var comp = sprintf("%02d%s", number, side == "R" ? "L" : side == "L" ? "R" : side);
+		var r = apt.runways[rwy];
+		apt.runways[comp] = { lat: r.lat, lon: r.lon, length: r.length,
+				width: r.width, heading: math.mod(r.heading + 180, 360),
+				threshold1: r.threshold2,
+				#threshold2: r.threshold1
+				#stopway1: r.stopway2, stopway2: r.stopway1,
+		};
+	}
+}
+
+
+# Find best runway for current wind direction (or 270), also considering length and width.
+#
+var best_runway = func(apt) {
+	var wind_speed = getprop("/environment/wind-speed-kt");
+	var wind_from = wind_speed ? getprop("/environment/wind-from-heading-deg") : 270;
+	var max = -1;
+	var rwy = nil;
+
+	foreach (var r; keys(apt.runways)) {
+		var curr = apt.runways[r];
+		var deviation = math.abs(normdeg(wind_from - curr.heading)) + 1e-20;
+		var v = (0.01 * curr.length + 0.01 * curr.width) / deviation;
+
+		if (v > max) {
+			max = v;
+			rwy = curr;
+		}
+	}
+	return rwy;
+}
+
+
+# Draw 3 degree glide slope tunnel.
+#
+var draw_tunnel = func(rwy) {
+	var m = geo.Coord.new().set_latlon(rwy.lat, rwy.lon);
+	m.apply_course_distance(rwy.heading + 180, rwy.length / 2 - rwy.threshold1 - HOFFSET);
+
+	var g = geodinfo(m.lat(), m.lon());
+	var elev = g != nil ? g[0] : apt.elevation;
+	forindex (var i; tunnel) {
+		if (tunnel[i] != nil)
+			tunnel[i].remove();
+
+		m.set_alt(elev);
+		tunnel[i] = geo.put_model(MARKER, m, rwy.heading);
+		m.apply_course_distance(rwy.heading + 180, DIST);
+		elev += voffset;
+	}
+}
+
+
+var loop = func(id) {
+	id == loopid or return;
+	var a = airportinfo();
+	if (apt == nil or apt.id != a.id) {
+		apt = a;
+		var is_heliport = 1;
+		foreach (var rwy; keys(apt.runways))
+			if (rwy[0] != `H`)
+				is_heliport = 0;
+
+		if (is_heliport) {
+			#print(apt.id, " -- \"", apt.name, "\"  --> heliport; ignored");
+		} else {
+			complement_runways(apt);
+			draw_tunnel(best_runway(apt));
+			#print(apt.id, " -- \"", apt.name, "\"");
+		}
+	}
+	settimer(func { loop(id) }, INTERVAL);
+}
+
+
+var loopid = 0;
+
+settimer(func {
+	props.globals.getNode("/sim/model/geometry/square/top", 1).setBoolValue(1);
+	setlistener("/sim/rendering/glide-slope-tunnel", func(n) {
+		loopid += 1;
+		if (n.getValue()) {
+			loop(loopid);
+		} else {
+			forindex (var i; tunnel) {
+				if (tunnel[i] != nil) {
+					tunnel[i].remove();
+					tunnel[i] = nil;
+				}
+			}
+		}
+
+	}, 1);
+}, 0);
+
+