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Runway Arrestor Gear Systems

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Category: Runway Excursion Runway Excursion
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Content control: SKYbrary About SKYbrary
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Tag(s) Overrun on Landing
Overrun on Take Off

Definition

Aircraft arresting systems are designed to ensure that tactical military aircraft are able to stop on the runway during the landing roll or after a high speed rejected take off. This distinguishes them from an Engineered Materials Arresting System (EMAS) or net-type military aircraft barriers which are designed to mitigate the consequences of a runway overrun.

Description

Arrestor Gear installations using cables are integral to the routine operation of military aircraft from aircraft carrier decks and from runways. Whilst some of the references at the end of this article include detailed information on military use of such systems, the purpose here is to consider the operational safety implications for civil aircraft which take off or land on those runways used by both civil and military aircraft which have cable-type arrestor gear installed. These systems are found worldwide at both civil airports where tactical military aircraft are accepted and at military bases where civil operations are permitted.

A typical installation consists of a single cable spanning the runway towards one or both ends. When required, it is engaged by a hook fitted to many tactical military aircraft. These cables are typically 2.5 to 3.2 cm in diameter and when 'rigged and up' or in situ, are typically suspended at approximately 7.5 cm above the pavement surface by 15cm diameter rubber 'donuts' spaced along their length. However, some versions of the system can be retracted by remote control into an adjacent groove in the runway surface when not operationally required and thus the full length of the runway is restored to a normal condition.

A typical USAF arrestor wire installation (reproduced from Boeing AERO magazine issue 13)

Three main factors determine where the cables are located on runways:

  • the direction of engagement (unidirectional or bidirectional);
  • the 'runout' of the system - the distance from the cable to the point at which it will stop the aircraft which is typically in the range 300-350 metres
  • whether the system is typically used in both IMC and VMC or just in VMC

Typical locations for cables are shown in the diagram below:

A typical cable locations(reproduced from Boeing AERO magazine issue 13)

Operational Consequences for Civil Aircraft

The nose landing gear of some older transport aircraft types is fitted with gravel or spray/FOD deflectors. If these deflectors come into contact with the arrestor cable, they may shatter and create FOD hazardous to subsequent aircraft if not removed. Earlier model Boeing 737s which operated on gravel runways and all the MD 80/90 series/Boeing 717 are equipped with nose gear mounted deflectors. The 737 gravel deflectors have a ground clearance of as little as 2 cm but the MD 80/90 and Boeing 717 spray/FOD deflectors have a minimum clearance of 9 cm.

If a cable is not retractable, it can sometimes be disconnected and moved to the side of the runway when not required or alternatively, the 'donuts' supporting it in the functional or raised position can sometimes be moved along the cable to the sides of the runway, thereby allowing the cable (still under tension) to lie flush with the surface in a 'rigged and down' condition allowing landing gear to safely pass over it.

The option of a reduction in the declared length of a runway for civil aircraft is also possible by use of displaced landing threshold, but the landing performance must then be re-assessed. The risk of cable interference at the upwind end of the runway in the case of a rejected take off or the end of a landing roll is generally discounted due to the lower groundspeed at that point - typical aircraft manufacturer advice is to ensure that if crossing 'rigged and up' cables is unavoidable, it is done at a groundspeed of not more than 60 knots.

Arrestor Gear Position Marking

The only widely available requirements for indicating the presence of arrestor gear across runways used by civil aircraft are those published by the FAA. Their requirements for indicating the presence of a permanent aircraft arresting system cable crossing a runway used by civil aircraft are:

  • a series of reflective yellow circles each of diameter 10 feet /3.05 metres and 15 feet / 4.57 metres apart from edge to edge which extend across the full width of the runway such that the middle two circles straddle the runway centreline. Runway Designators must not be compromised and interference with normal runway markings should be avoided if possible.
  • Arrestor Gear Markers (AGMs) taking the form of an internally-lighted yellow translucent circle approximately 39 inches / 1 metre in diameter on a square black background should be present on both sides of the runway at or within 10 feet /3 metres of the cable position and equidistant from the runway edge.

These two requirements are depicted in the diagram below which was taken from the FAA AC 150/5220-9A 'Aircraft arresting systems on civil airports'.

FAA required arresting gear marker configuration (reproduced from AC 150/5220-9A)

If a temporary aircraft arresting system cable is installed across a runway used by civil aircraft, FAA requirements are that an appropriately displaced threshold must be established.

Although ICAO Annex 14 does not contain specific requirements for identification of the location of raised 'rigged and up' arrestor cables, at night they would constitute an obstruction and, as such, would require illumination. This is usually provided by an illuminated circle of the type required by the FAA.

Note that the U.S. term for arrestor cable installations is 'BAK' - derived from 'barrier arresting systems' but there are many other terms in use.

Operational Cautions

It is generally recognised that operations which involve regular crossing of 'rigged and up' cables are likely to increase both tyre wear and the risk of tyre damage, such that regular and careful inspection of the landing gear and tyres is warranted.

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Further Reading