Of the nearly 36,000 airports around the world that are classified as civil, military, or joint-use, approximately 3,800 are used for scheduled commercial service. Worldwide, an estimated 2,000 aircraft arresting systems are installed at facilities in 64 countries. The highest concentration of these systems is on military runways or where the military is a major tenant of a commercial airport. Commercial operators mainly encounter these systems at joint commercial-military airports or when operating charters to military airfields. If the nosegear spray and gravel deflectors used on some commercial airplanes come in contact with the arresting systems, the deflectors could shatter, creating foreign object debris (FOD). In extreme cases, the FOD could damage a critical airplane system.
Aircraft arresting barriers. These devices, which do not depend on arresting hooks on aircraft, stop an aircraft by absorbing its forward momentum in a landing or aborted takeoff overrun. These systems are most commonly net devices (fig. 1), but they also include older devices that catch the main gear struts. The barriers typically are located in the overrun of the runway, are unidirectional, and can have collocated or interconnected arresting cables as part of their configuration. They sometimes are used for special purposes, such as stopping the space shuttle.
Aircraft arresting cables. Arresting cables span the runway surface and are engaged by the aircraft arresting gear hook (fig. 2). Cables are typically 1 to 1 1/4 in (2.5 to 3.2 cm) in diameter and suspended 1 1/2 to 3 in (3.8 to 7.6 cm) above the pavement surface by rubber donuts 6 in (15.2 cm) in diameter. Used primarily by aircraft built in the United States and Europe, arresting cables have been used by the military since the late 1920s on aircraft carriers and for land-based runways.
Three main factors determine where cables are located on runways: the direction of engagement (unidirectional or bidirectional); the runout of the system, which is the distance from the cable to the point at which the aircraft stops (typically 950 to 1,200 ft, or 290 to 360 m); and whether the system is typically used under visual meteorological conditions or instrument meteorological conditions. Figure 3 shows the typical locations for arresting-cable installations. An aircraft operating on runway 10R would use the cable at the far end for both landing and aborted takeoff unless the aircraft had an emergency, at which point the arresting gear nearest the approach end of the runway would be used.
Engineered materials arresting systems (EMAS). EMAS, which are constructed of high-energy-absorbing materials of selected strength, are located in the safety area, or overrun, of the runway. They are designed to crush under the weight of commercial airplanes as they exert deceleration forces on the landing gear. These systems do not affect the normal landing and takeoff of airplanes. More information concerning EMAS is in FAA Advisory Circular AC 150/5220-22, Engineered Materials Arresting Systems (EMAS) for Aircraft Overruns.
Airlines have numerous concerns about operating commercial airplanes on runways with aircraft arresting systems. These include airplane nosegear interference, trampling of the arresting cable, adjustments to declared distances, dealing with arresting barriers, runway availability, airplane maintenance, and unintentionally engaging arresting systems.
Normal procedure is for the rubber donuts to be approximately 6 ft (1.8 m) apart, starting 3 ft (0.91 m) from the runway centerline on runways 200 ft (61 m) or less in width. For runways wider than 200 ft (61 m), the donuts are placed 8 ft (2.4 m) apart. To minimize potential damage to the nosegear deflectors, airplanes with such attachments should slow-taxi over the cable, avoiding the donuts. If the nosegear spray deflector is damaged and removed, in accordance with the minimum equipment list, the airplane is limited to operating on dry runways until the deflector is replaced.
It is important to note that the cable must be kept under tension, whether lying on the pavement or elevated by the donuts. Otherwise, the cable could be lifted by the airplane landing gear and contact the bottom of the fuselage or antennae located on the lower fuselage. (See rigged and down, rigged and up, and out of battery in \"Common Terms\")
Dealing with arresting barriers. Nets are typically located in the overrun area near the runway threshold. If the net is in the raised position at the departure end, it should be treated as an obstruction that has to be cleared by 35 ft in accordance with the FAA federal aviation regulations and an adjustment made to the TORA. There are rare situations in which a net has been located in the actual runway. In these cases, provided the net lies flat on the runway and is under tension, it can be rolled over. If the net is bunched and lying on top of the runway, the airplane should not cross it.
Runway availability. A commercial airplane following a military aircraft could experience a delay landing if the military aircraft engages the arresting gear. The flight crew of the commercial airplane should expect to execute a missed approach while the military aircraft is removed and the arresting gear reset. Typical cycle times for arresting gear can vary from 3 to 10 minutes depending on the type of system. (See cycle time and reset time in \"Common Terms\".)
Airplane maintenance. If the flight crew believes the airplane nosegear has contacted one of the hard rubber donuts supporting an arresting-gear cable, a visual inspection of the nosegear spray deflector should be conducted to verify whether it has been damaged. A similar visual inspection would apply if the flight crew thought that the cable had made contact with the belly of the airplane. For airlines that routinely operate on runways with arresting-gear cables, additional visual inspections may be conducted depending on the type of arresting systems installed and to what extent the airplane interacts with the system.
Unintentionally engaging arresting systems. Occurrences of commercial airplanes being engaged or tangled in arresting cables are rare. In the 1970s, a DC-10 had a rejected takeoff during a flight test. In this case, the fuse plugs on the leading tires on one main landing gear had deflated, and during the takeoff roll the arresting cable was snagged, causing the airplane to stop.
In 1995, an MD-88 was starting its takeoff roll when the nose landing gear snagged the arresting gear, bringing the airplane to a halt. It was inspected with no damage found and dispatched shortly thereafter.
Carrying any gear behind the seatpost of a full suspension bike is challenging. With regular fabric seat packs there are three issues. Firstly they impede dropper post use. Secondly, they must be kept small (sub 7 litres) in order to avoid the rear wheel driving into the bottom of the pack. And thirdly, they are often in the way of your backside when making steep descents.
The AeroPack does a fair job to resolve those issues, however, it is still not without its compromises. Due to the weight of your gear sitting directly on the wheel, it changes the unsprung mass of the wheel (similar to URT suspension system in the 90s), which will therefore change the suspension dynamics and create more chatter on rowdy trails.
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This article provides guidance for tower/approach controllers on what to expect from an aircraft experiencing the effects of landing gear problems and some of the considerations which will enable the controller, not only to provide as much support as possible to the aircraft concerned, but also maintain the safety of other aircraft at or in the vicinity of an aerodrome and of the ATC service provision in general.
Fixed, non-retractable landing gear was common in the early days of aviation but is now only seen on light aircraft. Commercial airliners use complex retractable undercarriages with multi-step automated retraction and extension sequences, and various systems to provide status information, redundancy and control. One such system provides easily interpretable indicator lights to provide the positional status of the landing gear. The principle is simple - a green light when the landing gear is down and locked and a red light when there is a discrepancy between the gear lever and landing gear positions. The unsafe indication might be the first sign of a problem related to the proper preparation of the landing gear for landing. Depending on the aircraft type and landing gear retraction system, the exact nature of the problem may vary significantly.
Fuel consumption, with High Lift Devices (flaps and slats) deployed and landing gear either fully or partially extended, is significantly higher than is the case when the aircraft is in a \"clean\" configuration. Flight Management System (FMS) fuel predictions are usually not accurate when the aircraft is not in the \"normal\" configuration for the phase of flight.
Due to the variety of modern aircraft landing gear design, it could be quite difficult for a non-professional to distinguish between normal and abnormal gear operation. In the case of a partial extension, any visual inspection should be done only by a qualified professional.
In case of a gear problem, the crew bears significant stress. They will need time to fully assess the nature of the problem. Further steps could include crew visual inspection (depending on aircraft design), alternate extension procedures which may include manual emergency landing gear extension, or flight manoeuvres designed to force the drop of the landin