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Old 23rd May 2007, 10:35
  #14 (permalink)  
Prof. Airport Engineer
Join Date: Oct 2000
Location: Australia (mostly)
Posts: 726
These aircraft speeds seem to be pretty close to the RESA designer assumption. The FAA (in their AC150/5220-22A on EMAS systems) call for the design entry speed to be 70 knots.
To the maximum extent possible, the EMAS must be designed to decelerate the design aircraft expected to use the runway at exit speeds of 70 knots (approach category C and D aircraft) without imposing loads that exceed the aircraft’s design limits, causing major structural damage to the aircraft or imposing excessive forces on its occupants . . . . Standard design conditions are no reverse thrust and poor braking (0.25 braking friction coefficient).
Turning to the pavement discussion, the stopway and RESA can be pretty light/thin pavements. Mainly because they are only trafficked occasionally, and also because they can be weak such that aircraft sink into them so long as they do not rip off the landing gear. The RESA however is required to take the weight of fire engines, which are in the F50/F100 aircraft weight class, and that means some sort of pavement is usually needed.

For the unpaved or dirt stopway case, the designer (hopefully) has considered the wet and the dry case. It is likely that the stopway has been surfaced with enough gravel to give it some bearing capacity (and to make sure it doesn’t erode with the wind and rain), and typically the gravel has strength even when wet.

For the paved stopway case, the designer will quite properly use a thinner pavement than the runway, although it is usually a designed structure in its own right. I can illustrate this with the concrete design open on my desk at the moment (and the principles also hold true for flexible pavements). The design principle for concrete (rigid) pavements is to control fatigue from bending. The busier the runway, the lower the allowable working stress for the concrete in the slab. Analogous to aircraft structural design and fatigue, I suspect. Stopways can be thinner than runways because they have (hopefully) much less traffic.

For this runway designed for the MD-11 as the critical aircraft (which is more demanding than a B744), the allowable working stress is 348 psi (sorry about the American units) and the slab thickness required for 150,000 coverages (about 36,000 annual departures) over a B subgrade is 400mm (15.75 inches). For the stopway, assuming only a few excursions per year, the allowable working stress is 515 psi and the slab thickness required is 290mm (11.5 inches). So the stopway would be designed as a thinner pavement quite properly and would work well, yet it could easily be said that it doesn't have the same bearing or wearing qualities as the runway.

As old, not bold indicated, there aren't too many airport staff in any country that would know this level of detail. They typically refer such questions to their consulting engineers who usually have some pavements experience; and they in turn might refer the harder questions to a specialist pavement engineer. In Australia, there are less than half a dozen such specialists, and in Africa much the same.
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