Avoiding overrun accidents on contamined runways
Landing on slippery surfaces requires precise touchdown in addition to judicious use of brakes or thrust reverse.
By Nihad Daidzic
Overrun accident on snow and ice-covered Runway 27 at CLE (Hopkins, Cleveland OH) on Feb 18, 2007. An Embraer 170 operated by Shuttle America (dba Delta Connection) ended up only feet from the road leading to NASA Glenn Research Center.
On average, every month, somewhere in the world, a large transport-category airplane exits the runway end in an overrun accident. An overrun simply means that the airplane was not able to stop in the available runway.
In most cases runway contamination is the main cause, followed by inappropriate touchdown control, excessive speed, poor braking management, etc. Corporate airplane statistics are even worse, as they often have to land on lower-quality runways and do not use the protections built into Title 14 CFR Parts 121 and 135—not that these offer absolute safety either.
Problems with contaminated runways are not restricted to landings. Serious danger exists in high-speed rejected takeoffs (RTOs). Often the performance takeoff decision speed is not adjusted properly for the level of contamination and the attempt to abort takeoff near V1 can result in runway overrun.
Runway contamination is defined per JAR-OPS as when more than 25% of the surface area of the runway is covered with a water film in excess of 3.0 mm (about 0.12 in) or its equivalent in slush, compacted or wet snow, or ice. For example, slush 5.0 mm deep, with a specific density of 0.7 g/cc (700 kg/m3), has a water equivalent depth of 3.5 mm.
The basic problem with runway contamination is that it significantly reduces tire/surface friction, which is a principal tool in stopping an airplane. For example, a Boeing 767-300ER landing at 300,000 lbs, with a touchdown groundspeed of 140 kts, will have to dissipate 260,774,348 ft-lb of energy (353,673,704 J or Nm) or about 100 kWh.
This is about 23,707 hp (17,684 KW) of kinetic energy dissipated in about 20 sec of deceleration. One such landing could provide daily energy needs for several households if we could somehow capture and regenerate it.
The 4 principal deceleration forces acting on an airplane in deceleration run are braking friction, reverse thrust, aerodynamic drag and runway surface gradient (slope). Let’s start by considering the less significant ones.
Runway gradient effect is very small overall compared with other decelerating forces. The maximum allowable average slope on runways operated by commercial operators is 2%. A 25,000-lb corporate jet would experience an average gravitational-component retarding force on landing of only 500 lbs (1/50 of the weight or 0.02 G) on a continuous 2% uphill runway.
For all practical purposes, then, runway gradient can be neglected, even for downhill landings, except for a very few exotic runways around the world.
Aerodynamic drag contributes only a few percentage points to the overall retarding force, and as the airplane slows down that contribution diminishes rapidly, since the aerodynamic drag is a function of the square of the true airspeed.
Initially, at high airspeeds, the aerodynamic drag can cause deceleration of up to 0.07 G. Although raising spoilers after touchdown increases airplane drag, the principal use of the spoilers is to reduce lift coefficient and put as much weight on the wheels/tires as possible.
In modern civilian airplanes aerodynamic drag becomes only marginally significant when landing on contaminated runways and during high-speed ground-run. Aerodynamic drag caused by crosswind on a slippery runway can actually push the airplane sideways, causing veer-off. (See Pro Pilot, Sep 2009, pp 54–58).
Reverse thrust is an important decelerating device which becomes the dominant stopping tool when landing on contaminated and slippery runways. Reverse thrust can be generated either by deflecting the cold-bypass air via cascades and blocking doors in high-bypass turbofan engines, or with clamshells or buckets in low-bypass turbofan and turbojet airplanes.
Braking friction coefficient and cornering (wet runway) coefficient as a function of tire slip for given ground speed. When the tires are locked the braking and cornering coefficients become sliding. Modern ABS systems work on the low-slip side of the friction curve. (Not to scale!)
In this way, an average 40–50% of maximum forward thrust can be achieved for reverse-thrust operations. Reverse thrust in turboprop airplanes, using negative blade pitch, is more effective than reversed jet stream.
The problem with reverse thrust is that it can blanket certain flight control surfaces—such as the rudder in aircraft with rear-fuselage-mounted jet engines—and cause steering problems. Also, the possibility of reingesting the hot exhaust gases and the boundary-layer separation in the jet engine intake lip could cause compressor stall and flow reversal with damaging consequences.
But this will mostly depend on the airframe/ engine integration design and varies from airplane to airplane. For example, the Boeing 747 can use thrust reversers until full stop in an emergency—and has.
Some airplanes can even back up using reverse thrust, but in most airframes the reverse thrust has to be at idle reverse by about 60 kts. In modern turbofan engines, with their high bypass ratio, the maximum reverse thrust reaches 40% of the maximum forward thrust.
Considering that the thrust-to-weight ratio in such airplanes (at SL standard ISA) is 0.30–0.35, it follows that the retarding force is between 0.12 and 0.15 G (12–15% of the landing weight).
Braking friction coefficient as a function of ground speed and level of contamination. ABS performance is illustrated in red. (Not to scale!)
Braking friction is the dominant retarding force when landing on dry paved runways. Although the wheel was “invented” more than 10,000 years ago, a modern tire is a hard-working and sophisticated high-tech product.
Today’s tire is a complex composite built mostly of vulcanized rubber, and is an “elastomer” whose complicated nonlinear viscoelastic properties are still not understood fully. The tire has to sustain and transmit various normal and shear (longitudinal and lateral) loads in the small contact patch (“footprint”) with the runway.