Wednesday, June 6, 2012

EMAS: Engineered Material Arresting System

G R Mohan | 1:07 AM | | | | | Best Blogger Tips

Aircraft overruns during landing and take-off are a frequent occurrence and statistically are the fourth largest cause of airline fatalities. . An overrun occurs when an aircraft passes beyond the end of a runway during an aborted take-off or while landing. To minimize the hazards of overruns, the Federal Aviation Administration (FAA) incorporated the concept of a safety area beyond the runway end into airport design standards.  However, there are many runways, where natural obstacles, local development and other constraints make the construction of the RESA impracticable. Recognising the difficulty associated with accomplishing RESA without compromising the performance capability of a runway, research programmes were initiated to find alternate and effective arresting solutions.

Engineered Material Arresting System is an evolution of a "soft ground arresting system" consisting of a bed of several thousand blocks of crushable cellular cement material installed on the runway overrun to decelerate an aircraft in an emergency. FAA Policy Order 5200.9 identifies EMAS as equivalent to a 1,000-foot long RSA and provides guidelines on comparing various runway safety area (RSA) improvement alternatives to the EMAS option.
The customised bed is designed to crush under the weight of an aircraft thus providing a controlled and predictable deceleration. The range of aircraft operating at each airport and available overrun distance on each runway are accommodated by varying bed dimensions and material properties. The depth of the EMAS gradually increases as the aircraft travels into the arrestor bed, providing increasing deceleration when required by heavier or faster aircraft. Aircraft run out distance will be determined by the aircraft size, weight, and speed and bed configuration.  Once stopped, the material allows the passengers to exit the aircraft safely and for the aircraft to be safely removed from the arresting system.
Typically, EMAS is laid full width of the runway, and is located slight distance away from the runway end to prevent damage due to undershoot landings. Although crushable under the load of an aircraft, the pavement strength is adequate to support movement of ARFF vehicles. The EMAS bed is painted with yellow chevrons to indicate that the surface is unusable.

With the standard EMAS solution, an EMAS with stopping capability of 70 knots and a safety area only 600 feet long would be required, provided vertical guidance is available for undershoot protection. The policy also introduces the need to evaluate a non-standard EMAS whenever a standard solution (1000 ft. RSA or standard EMAS) cannot be achieved within the maximum feasible cost. FAA believes that EMAS can still provide acceptable safety enhancement as long as 40 knots stopping capability is provided with the non-standard EMAS solution.
On the afternoon of January 24, 2005, the EMAS was put to its biggest challenge when a 600,000-pound Boeing 747 landed long and overran into the EMAS. As predicted, the aircraft was safely stopped by the EMAS with no injuries to the crew and damage to the aircraft was limited to replacing nine tires. The aircraft went back into service within seven days. Over the years, many aircraft overruns have taken place through EMAS. In all instances the aircraft have flown within a short span of maintenance activity, typically less than seven days.
One of the major advantages of the EMAS system is the minimal damage that the aircraft has undergone during such arrests. The aircraft were pulled out of the EMAS bed by a tow attached to the landing gear.
Since over 90% of all runway overruns have taken place below 70 Kts, EMAS caters to this energy absorption. The system requires minimal maintenance, apart from removal of snow to prevent accumulation and loss of friction. Repair and replacement, after engagement by an aircraft, simply involves removal of the damaged portion of the bed and relaying the bed with fresh cement blocks.
The EMAS system maximizes runway safety with the same level of superior overrun protection and yet offers easier and quicker installation, improved durability and greatly reduced maintenance. The system has the same proven performance and ability to safely arrest commercial aircraft. FAA Advisory circular AC 150 provides a performance specification for this system. The system is approved by FAA as well as by NTSB.

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