What are Taxiway Lead-on Lights?

Taxiway Lead-on Lights

Definition

A taxiway lead-on light is a taxiway centre line light that guides a pilot who enters a runway.

The FAA describes taxiway centerline lead-on lights as lights that give visual guidance to persons entering the runway (AIM 2-1-5). They use green and yellow, like the lead-off lights that guide traffic out of a runway. The colours warn pilots and vehicle drivers that they are in the runway environment or in the ILS critical area, whichever is more conservative.

The taxiway light page covers all taxiway light types. It mentions lead-on lights in one paragraph. This page covers the lead-on lights alone.

Lead-on lights and lead-off lights

The two groups use the same pattern and often the same fixtures.

The FAA lead-on fixtures are bidirectional. One side gives the lead-on function and the other side gives the lead-off function. A fixture that shows yellow for the lead-off function also shows yellow for the lead-on function (AIM 2-1-5).

Colour pattern

AC 150/5340-30J, 4.3.2.2, sets the pattern:

• The colour coding begins with a green light at the runway centre line.
• The lights alternate green and yellow to one light position beyond the runway holding position or the ILS critical area holding position.
• The fixture before the holding position is always bidirectional. It is green when a pilot approaches from the taxiway and yellow when a pilot approaches from the runway.

If the layout gives an odd number of colour-coded lights, the first two lights on the runway are green. The AC gives this rule for lead-off lights (4.3.2.1.3).

Where the FAA installs them

The FAA ties lead-on lights to low visibility operations (AC 150/5340-30J, 4.3.5):

• Lead-off and lead-on lights are recommended where runway visual range (RVR) is below 1,200 ft (365 m).
• Lead-on lights should extend to the point of curvature on the runway if the runway has departure minimums below 600 ft (183 m) RVR.
• Where nobody operates below 1,200 ft (365 m) RVR, lead-on and lead-off lights may not stand within the runway confines.
• Extra lead-off and lead-on lights should not stand before the point of curvature on the runway. They would weaken the visual difference between an acute-angled exit and other exits.

How lead-on lights work with a stop bar

A controlled stop bar operates in conjunction with the lead-on lights that run from the stop bar toward the runway (AIM 2-1-10). The stop bar is red and the lead-on lights are off. After the ATC clearance, ATC turns the stop bar off and the lead-on lights on. The stop bar page covers the stop bar itself.

AC 150/5340-30J, 4.8.7.3.2, divides the lead-on lights into two segments:

1. Segment 1 begins at the stop bar and is 155 to 165 ft (47 to 50 m) long.
2. Segment 2 is the rest of the lead-on lights, to the point of curvature at the runway centre line. This applies if the distance from the stop bar is under 300 ft (90 m).
3. If the distance is more than 300 ft (90 m), segment 2 may be all the remaining lights. It may also be shorter, if segments 1 and 2 together are at least 300 ft (90 m) long.

Two sensors reset the lights. Sensor 1 stands near the end of segment 1 and sensor 2 near the end of segment 2.

1. When ATC presses the stop bar button, two backup timers begin to run. The stop bar goes off and both segments come on. The first timer runs about 45 seconds and backs up sensor 1. The second timer runs about 2 minutes and backs up sensor 2.
2. When the aircraft or vehicle reaches sensor 1, the stop bar comes on again and segment 1 goes off. This protects the runway against a trailing aircraft or vehicle.
3. When it reaches sensor 2, segment 2 goes off.

Sensor 1 may fail while its timer still runs. If sensor 2 trips then, both segments go off and the stop bar comes on (4.8.7.3.3).

Within 2 seconds of the button press, the stop bar goes off and the lead-on lights come on (4.8.7.4.2). The tower panel shows the real state within 5 seconds (4.8.7.5).

Controlled stop bars and their lead-on lights must be monitored electronically (4.8.7.5). A standard L-827 monitor can show a fault when about two stop bar or lead-on lights do not work. An L-829 CCR with integral monitor can do the same. It cannot say whether the failed lights are next to each other. A visual inspection must find that out (4.8.7.5, items 5 and 6).

Rules for pilots

The FAA gives two rules to pilots in AIM 2-1-10:

• A pilot should never cross a red stop bar, even if ATC has given a clearance.
• If the lead-on lights go out after the pilot crosses a stop bar, the pilot holds position and contacts ATC.

The EASA equivalent

CS-ADR-DSN does not use the name “lead-on lights”. Two specifications cover the same ground:

• CS ADR-DSN.M.710(c)(3) calls for fixed taxiway centre line lights that alternate green and yellow where the lights must show how close a runway is. The pattern begins at the edge of the ILS/MLS critical or sensitive area. It can begin instead at the lower edge of the inner transitional surface, whichever is farther from the runway. It continues to the runway.
• CS ADR-DSN.M.730(c)(6)(iii) and (iv) says that centre line lights beyond a lit stop bar should be off for at least 90 m. It also says that the stop bar and the lights beyond it are interlocked. One is on when the other is off.

What this means for an inspection

An inspection of lead-on lights checks these items:

• Each light is lit and shows the right colour in the right position.
• The first light at the runway centre line is green, and the colours alternate to one light beyond the holding position.
• The bidirectional fixture before the holding position shows green to the taxiway and yellow to the runway.
• The lights are not hidden by dirt, snow or damage.

A drone flight can record whether each light is lit, its colour and its position on the layout. The sensors, the timers and the tower panel belong to the control system, and a fly-over does not test them. TarmacView’s taxiway lights inspection covers the lights on the taxiways.