As occurrences of an aircraft running off the end of the runway and hitting things tend to leave both the aircraft and whatever it hits rather the worse for wear, much effort over the years has gone into both preventing runway overruns1 and mitigating the consequences of overruns when they do occur.
There are three main ways of mitigating the consequences of a runway overrun:
- Providing a long, clear, flat runway safety area (RSA) past the end of the runway, to allow an overrunning aircraft space in which to safely decelerate, free of the risk of striking anything else; this is highly effective, and the preferred solution for new runways and airports, but requires a considerable length of free area past the runway end, which is often not available at older airports (for instance, due to a runway ending just prior to the airport boundary, with dense residential or commercial development or major roads, rail lines, or waterways immediately adjacent, or due to terrain which drops off steeply past the end of the runway).
- Replacing solid objects past the end of the runway with frangible equivalents, which break apart harmlessly when struck by an aircraft instead of standing firm and potentially causing severe damage; this is nearly ubiquitous for airport equipment such as approach lights and radionavigation aids, which, by their very nature, must often be located within the RSA (and usually in the highest-risk portion of the RSA [along the centerline and extending quite close to the end of the runway], at that), but is not practical for structures beyond the airport boundary.
- Decelerating an aircraft very quickly, so that it stops before hitting anything solid; this is usually done using an engineered-materials arrestor system (EMAS), generally consisting of a bed of cellular-concrete or foamed-silica blocks which crush under the weight of an aircraft, causing the aircraft’s landing gear to sink into and drag through the EMAS bed and quickly bringing the aircraft to a stop. EMAS is the preferred solution in instances where terrain or land-acquisition factors make a full-size RSA infeasible.
There are at least 116 EMAS beds and counting at U.S. runway ends, but only 15 or so in the entire rest of the world combined. This despite the fact that airports which could benefit from EMAS are no less common elsewhere than in the United States; for example (sorted by number of dangerous runway ends):
- SBSP runways 17L, 17R, 35L, 35R: Heavy urbanisation and busy roads just past the ends of all of these runways; however, I am not, as of yet, aware of any accidents involving aircraft running off the ends of these runways.2
- VOML runways 06, 09, 24, 27: Very steep drop-off past the ends of these runways (the airport is perched on top of a hill); this has resulted in at least one fatal accident.
- CYYZ runways 23, 24L, 24R: Deep ravine and creek runs past the ends of the three runways; this has caused at least two accidents over the years.
- SCIP runways 10, 28: Steep drop-off past the end of the runway (the slope of the ground falls away below the runway near the end), most prominent for runway 10 but also present (albeit to a considerably lesser degree) for runway 28; fortunately, no accidents yet (of which I am aware).
To the best of my knowledge, none of these runways are equipped with EMAS.3
Why are EMAS installations apparently so rare outside the United States?
1: Methods for doing this include:
- Plowing, grooving, and sanding runways to improve traction (and, thus, aircraft braking capability) in adverse weather conditions.
- Requiring the use of landing-distance computations based on up-to-date braking reports to ensure that there exists sufficient runway for an aircraft to come to a stop on the paved surface.
- Training pilots to promptly apply all available deceleration devices during landing (especially in adverse weather conditions), such as wheelbrakes, spoilers, and reverse thrust.
- Preventing unnecessary high-speed rejected takeoffs through better pilot training, including:
- Not rejecting for tyre failures except at very low speed
- Not rejecting for any reason after V1 unless there is reason to think the aircraft incapable of controlled flight.
- Training pilots in the proper technique for rejecting a takeoff if one is necessary, including:
- Making a quick, certain go/no-go decision immediately upon experiencing a failure, rather than dithering on whether to reject or not as the aircraft continues to accelerate towards the end of the runway.
- Fully applying all available deceleration devices immediately upon making the decision to reject.
- Maintaining full application of all deceleration devices until the aircraft comes to a complete stop.
2: There were two accidents, on 16 and 17 July 2007, involving aircraft departing the paved surface of SBSP runways 17R and 35L, respectively, under conditions of heavy rain, the latter of which resulted in 199 fatalities; however, in both of these cases, the aircraft veered off the side of the runway before reaching the end, and, thus, an EMAS, even if installed, would not have prevented these accidents, as EMAS installations generally only cover the ends of the runway, not the sides.
3: The departure end of VOML runway 06 did have a sand-filled arrestor bed; however, it failed to stop the aircraft in the aforementioned fatal accident, while a full-fledged EMAS bed most likely would have prevented the plane from plunging over the precipice.