.  
FAA – AC 91-6A and 91-6B – Water, Slush and Snow on the Runway.

.  
JAR-OPS 1.515 – Landing – Dry Runways,

.  
JAR-OPS 1.520 – Landing – Wet and Contaminated Runways,

.  
UK CAA – AIC 11/98 – Landing Performance of Large Transport Aeroplanes,

.  
UK CAA – AIC 61/99 – Risks and factors Associated with Operations on Runways Affected by Snow, Slush or Water.

Getting to Grips withApproach-and-Landing Accidents Reduction
Factors Affecting Landing Distance
Page 7

Introduction
To ensure an optimum use of braking devices, the following aspects must be understood:
.  
Design and operation of each braking device (i.e., ground spoilers, brakes and thrust reversers);

.  
Distribution of stopping forces during landing roll;

.  
Type of braking required to achieve a desired stopping distance;

.  
Factors affecting the optimum use of braking devices; and,

.  
Applicable operational guidelines.

Statistical Data
Runway excursions and overruns account for 20 % of all approach-and-landing serious incidents and accidents.
Slowed or delayed braking action was a causal factor in 45 % of these events.
Landing overruns represent 80 % of all observed overrun events (i.e., including runway overruns following a rejected takeoff).
The use of braking devices is only one among several causal factors resulting in a runway excursion or overrun.
Braking Devices Overview
The following braking devices are used to decelerate the aircraft and bring it to a complete stop:
.  
Ground spoilers;

.  
Wheel brakes (including anti-skid and autobrake systems); and,

.  
Thrust reverser system.

Ground spoilers:
Ground spoilers deploy automatically upon main landing gear touchdown (if armed) or upon selection of the thrust reversers.
Ground spoilers provide two distinct aerodynamic effects, as illustrated by Figure 1 :
.  
Increased aerodynamic drag, which contributes to aircraft deceleration;

.  
Lift dumping, which increases the load on the wheels and increases the wheel-brakes efficiency.

Optimum Use of Braking Devices
Page 1

Negligible Weight 60 % Weight 85 % Weight on Main Wheels on Main Wheels on Main Wheels

130 % Drag
Increase from
Ground Spoilers

Touchdown Nose Wheel Down Ground Spoilers Extended
V APP ( V LS)

Figure 1
Maximizing Weight-on-Wheels and Aerodynamic Drag

Wheel brakes:
Braking action results from the friction force
Tire Braking Force

between the tires and the runway surface.
Antiskid activation 100
90
This friction force depends on several parameters:
.  Aircraft speed; 80
70

0 5 8 10 15 20 30 40 50 60 70 80 90 100
Slip Ratio ( % )

.
Wheel speed (i.e., free rolling, skidding or

locked wheel);

.
Runway condition (i.e., nature and depth of

Force ( % )
60 50 40
contaminant);

.  
Tire condition and pressure (i.e., friction surface);

.  
The load applied on the wheel (i.e., the friction force depends on the load applied on the wheel and on the runway braking coefficient);

.  
The number of operative brakes ( MEL status, as applicable).

Anti-skid systems are designed to maintain the wheel skidding factor (also called the slip ratio) close to the point providing the maximum friction force, approximately 10 % on a scale going from 0 % (free rolling) to 100 % (locked wheel), as illustrated by Figure 2.
　
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