Increased navigational accuracy can place several aircraft on the same course in the same lateral position
Strategic lateral offset procedure (SLOP) is a solution to a byproduct of increased navigation accuracy in aircraft. Because most now use GPS, aircraft track flight routes with extremely high accuracy. As a result, if an error in height occurs, there is a much higher chance of collision. SLOP allows aircraft to offset the centreline of an airway or flight route by a small amount, normally to the right, so that collision with opposite direction aircraft becomes unlikely.
In the North Atlantic Region pilots are expected to fly along the oceanic track center-line or 1 or 2 nautical miles to its right, randomly choosing one of these three offsets on each entry to oceanic airspace. The aim is to not achieve an overall even distribution of one-third of all flights on each of the three possible tracks, as one might assume. When the procedure was originally developed, 4.9 percent of aircraft in most oceans could not offset automatically, so the centerline had to remain as an option. Because of the possibility of opposite direction traffic on the centerline, it is the least desirable option, with the highest risk. The procedure lowers the overall risk of collision should an aircraft move vertically away from its assigned level. This randomization has the advantage over a planned assignment of offsets to each individual aircraft in that it mitigates the collision hazard for same-direction flights should an aircraft be erroneously flown along a track that was not assigned by ATC.
SLOP is recommended for use in modern flight management system-based, RVSM (reduced vertical separation minima)-equipped aircraft operations to mitigate the midair collision hazard, which is amplified by the accuracy of modern aircraft navigational technology and onboard flight instruments.
Lateral navigation (left–right) based on global positioning system (GPS), and RVSM quality altimetry (up–down), are each so accurate in their own dimension that opposite-direction aircraft which are erroneously flying the same altitude on the same navigational path are very likely to collide.
In addition to mitigating en route midair collision hazard, SLOP is used to reduce the probability of high-altitude wake turbulence encounters. During periods of low wind velocity aloft, aircraft which are spaced 1000 feet vertically but pass directly overhead in opposite directions can generate wake turbulence which may cause either injury to passengers/crew or undue structural airframe stress. This hazard is an unintended consequence of RVSM vertical spacing reductions which are designed to increase allowable air traffic density. Rates of closure for typical jet aircraft at cruise speed routinely exceed 900 knots.
Wake turbulence is thought likely to be experienced by the lower of two aircraft when it arrives approximately 15–30 nm behind an opposite-direction aircraft which has crossed directly overhead on the same route. On November 13, 2015, ICAO published a revised version of Document 4444, Pans ATM Paragraph 16.5 that includes provisions for applying SLOP in a continental/domestic air space for aircraft that are capable of offsetting in tenths of a mile. Centerline is not an option as aircraft can offset up to one-half mile right of course, in tenths of a mile, providing 5 alternative offsets.
In January 2017, the ICAO SPG (Authority for the NAT region) published updated guidance indicating that SLOP is now a requirement on the North Atlantic, rather than a recommendation. The guidance was part of a number of changes that were contained in a revised 2017 edition of NAT Doc 007:North Atlantic Airspace and Operations Manual.
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From Wikipedia:
An engineered materials arrestor system, engineered materials arresting system (EMAS), or arrester bed is a bed of engineered materials built at the end of a runway to reduce the severity of the consequences of a runway excursion. Engineered materials are defined in FAA Advisory Circular No 150/5220-22B as "high energy absorbing materials of selected strength, which will reliably and predictably crush under the weight of an aircraft". While the current technology involves lightweight, crushable concrete blocks, any material that has been approved to meet the FAA Advisory Circular can be used for an EMAS. The purpose of an EMAS is to stop an aircraft overrun with no human injury and minimal aircraft damage. The aircraft is slowed by the loss of energy required to crush the EMAS material. An EMAS is similar in concept to the runaway truck ramp made of gravel or sand. It is intended to stop an aircraft that has overshot a runway when there is an insufficient free space for a standard runway safety area (RSA). Multiple patents have been issued on the construction and design on the materials and process.
FAA Advisory Circular 150/5220-22B explains that an EMAS may not be effective for incidents involving aircraft of less than 25,000 pounds weight. It also clarifies that an EMAS is not the same as a stopway, which is defined in FAA Advisory Circular 150/5300-13A, Section 312.
As of May 2017, the International Civil Aviation Organization (ICAO) has been working on developing a harmonized regulation regarding arresting systems.
Research projects completed in Europe have looked into the cost-effectiveness of EMAS. Although arrestor beds have initially been installed at airports where the runway safety areas are below standards, their ability to stop aircraft with minimal or no damage to the air frame and its occupants has proven to bring results far beyond the cost of installations. The latest report, "Estimated Cost-Benefit Analysis of Runway Severity Reduction Based on Actual Arrestments" shows how the money saved through the first 11 arrestments has reached a calculated total of 1.9 Billion USD, thus saving over $1 B over the estimated cost of development (R&D, all installations worldwide, maintenance and repairs reaching a total of USD 600 Million). The study suggests that mitigating the consequences of runway excursions worldwide may turn out to be much more cost-effective than the current focus on reducing the already very low probability of occurrence.
Higher EMAS bed with side steps to allow aircraft rescue and firefighting (ARFF) access and passenger egress.
The FAA's design criteria for new airports designate Runway Safety Areas (RSA's) to increase the margin of safety if an overrun occurs and to provide additional access room for response vehicles. A United States federal law required that the length of RSA's in airports was to be 1,000 feet (300 m) by the end of 2015, in a response to a runway overrun into a highway at Teterboro Airport in New Jersey.[ At airports built before these standards were put into effect, the FAA has funded the installation of EMAS at the ends of main runways. The minimum recommended overall length of an EMAS installation is 600 feet (180 m), of which at least 400 feet (120 m) is to consist of the frangible material.
As of July 2014, 47 United States airports had been so equipped; the plan was to have 62 airports so equipped by the end of 2015.[ As of May 2017, over 100 EMAS have been installed at over 60 US airports.
As of May 2017, there were two recognized EMAS manufacturers worldwide that meet the FAA requirements of Advisory Circular 150-5220-22B, “Engineered Materials Arresting Systems for Aircraft Overruns.” (The FAA must review and approve each EMAS installation.)
The first, original EMAS was developed in the mid-1990s by Zodiac Arresting Systems (then known as ESCO/Engineered Arresting Systems Corp.) as part of a collaboration and technical acceptance by the FAA. EMASMAX® (fourth generation EMAS) arrestor beds are composed of blocks of lightweight, crushable cellular cement material, encased in jet blast resistant protection, designed to safely stop airplanes that overshoot runways. Zodiac’s latest, most durable EMAS is installed on over 110 airport runways at over 65 airports on three continents. Zodiac's EMAS has undergone intense testing, including several live aircraft test runs at speeds up 55 knots and is the world’s first and only EMAS that has safely stopped aircraft in real emergency overrun situations at commercial airports.
In October 2016 EMAS saved Vice Presidential candidate Mike Pence's B737 from a runway overrun at La Guardia Airport, and in December 2018 EMAS saved a Southwest Airlines B737 at Burbank Airport.
Runway Safe EMAS (second generation EMAS) is a foamed silica bed made from recycled glass and is contained within a high-strength plastic mesh system anchored to the pavement at the end of the runway. The foamed silica is poured into lanes bounded by the mesh and covered with a poured cement layer and treated with a top coat of sealant.[
Runway Safe EMAS has been installed to replace older EMAS at Chicago Midway. Runway Safe has also installed an EMAS at Zurich airport 2016.
There is a third manufacturer, certified by the Chinese CAAC, with a product that is very similar to the original one of Zodiac ESCO.