Definition

Threshold. The beginning of that portion of the runway usable for landing.

Source: ICAO Annex 14: Aerodromes

Decription

The threshold is normally located at the extremity of a runway, if there are no obstacles penetrating above the approach surface. In some cases, however, due to local conditions it may be desirable to displace the threshold temporarily or permanently. Factors to be considered include:

• obstacles near the approach path (including mobile objects such as vehicles or trains).
• the reference datum of the ILS/MLS.
• pavement characteristics.

The threshold is often used for a reference when establishing the location of various lighting and marking elements, such as:

• Elements of the approach lighting and the visual glideslope indicators
• TDZ and aiming point marking.

The obstacle clearance height is referenced to the threshold elevation (and not the aerodrome elevation):

• for precision approach runways.
• for non-precision approach runways when the threshold elevation is more than 2 m (7 ft) below the aerodrome elevation.

Information about the threshold location and elevation can be found in the AIP, part aerodromes (AD).

Threshold marking is provided on paved instrument runways (and also on some paved non-instrument runways and some unpaved runways). It consists of several stripes along the runway, placed symmetrically about the centreline. The number of stripes is between 4 and 16 depends on the runway width. The length, width and the spacing between the stripes are defined in Annex 14.

Threshold lighting is provided for runways equipped with runway edge lights (with some exceptions). It consists of a row of green lights placed at right angle to the runway axis and as close as possible to the threshold. The number of lights depends on the type of approach (non-precision, CAT I or CAT II/III). Normally, wing bars are provided in addition to threshold lighting for precision approach runways for additional conspiquity.

Example of marking and lighting of the threshold (at the runway extremity) is shown in the picture below. Marking and lighting for displaced thresholds are discussed in a dedicated article.

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Further Reading

• ICAO Annex 14: Aerodromes, Volume 1
• ICAO Doc 9157: Aerodrome Design Manual, Part 4