Turbine Pilot

Tips for putting a stop to long landings

Passengers and pilots alike are too often guilty of judging an entire flight by the landing. That brief instant when the wheels gently caress (or rudely slap) the runway is the defining moment by which the rest of the flight is remembered. Considering this harsh reality, the garden-variety landing rollout usually plays a distant second fiddle to the touchdown itself. When, after all, was the last time that anyone wrote a poem about stopping an airplane? But the metamorphosis from sleek flying machine to docile ground vehicle should be more than an afterthought. It requires both reliable systems and good pilot technique.

Jets commonly depend upon three related systems for ground deceleration: tires and brakes, ground spoilers, and thrust reversers. How well they work together determines stopping distance. How skillfully the pilot uses them in concert can determine whether passengers disembark feeling warm and fuzzy, or like one of Agent 007's favorite martinis (shaken, not stirred).

During landing, it is the tires that first make contact with earth's surly bonds. Jet aircraft tires are built to withstand the extreme stresses associated with relatively high-speed touchdowns (or aborted takeoffs). Landing speeds for most jets are higher than typical cruise speeds of some light aircraft — usually in the range of 110 to 150 knots for airliners and business jets. Throw in the possibility of malfunctioning flaps and slats or overweight landings, which can necessitate even higher touchdown speeds, and it is obvious that jet aircraft tires must be built to take a pounding. Tire manufacturers designate maximum speed ratings for aircraft tires. A rating of 195 knots is common for many commercial jet aircraft. Although this sounds high enough for most contingencies (and usually is), it can be exceeded if one of the above-mentioned situations occurs at a high-altitude airport like Denver International or Mexico City. Although indicated airspeed at touchdown will be the same as at a sea-level field, true airspeed-and thus ground speed-will be higher.

Like any other tires, those found on jet aircraft are optimized to work best when properly inflated. Heat buildup caused by excessive flexing of the tire sidewalls is minimized when pressures are maintained correctly. But long periods of taxiing, or several closely spaced takeoff and landing cycles, can cause even properly inflated tires to absorb heat faster than it can be dissipated. When enough heat builds gradually in this manner, temperature-sensitive fusible tire plugs melt, releasing tire pressure and preventing a catastrophic tire explosion. Fusible plugs don't help, however, if a tire becomes weakened or damaged without an accompanying gradual temperature increase. This could occur during the stresses of a hard landing, for example, especially if the tire has a preexisting cut or other damaged area.

The ability to release tire pressure gradually is a critical safety feature on large aircraft. Tire explosions have been known to kill or maim persons unfortunate enough to be standing nearby at the time. Exploding tires have also caused serious systems damage to aircraft in flight, and there have been instances where in-flight tire explosions have resulted in the loss of aircraft. To reduce the likelihood of a fire stemming from overheated tires, dry nitrogen, not ordinary air, is used to inflate them.

Each tire is designed to support a certain proportion of the aircraft's weight. When one or more tires bursts or deflates, the aircraft's weight-bearing footprint changes, in the same way that a person's changes if his or her weight is shifted mostly to one foot. When this occurs, the weight-bearing limits of the remaining good tires can be exceeded, which can weaken those tires. Depending upon the aircraft's weight and the number of deflated tires, it may be necessary to replace all of the remaining tires, even though to the naked eye they appear undamaged.

With the advent of large jet aircraft having considerably higher landing speeds than their piston predecessors, the deficiencies of traditional manual aircraft brake systems were quickly revealed. Without any sure way of knowing how much brake application was too much for the tires to handle, even pilots with good braking technique were blowing tires regularly. In 1946, a company called Hydro-Aire Inc. installed the first-ever antiskid system on a U.S. Air Force Boeing B-47 bomber. The so-called Mark I system was crude by modern antiskid standards, but it did the job. Besides reducing the frequency of tire blowouts, the system also provided a slight improvement in overall braking efficiency.

An antiskid system works by monitoring the speed of each wheel through the use of rotary transducers mounted on the aircraft wheel axles. These in turn send electrical signals to a control unit. If the control unit detects a difference in wheel speed, such as might occur if a tire hydroplanes, it signals an antiskid valve that cycles rapidly, modulating hydraulic pressure to that wheel's brake. This allows the tire to again begin applying friction force to the runway surface. By precisely modulating brake pressure so that braking force is maintained very close to the optimum levels for the conditions, an antiskid system can slow an aircraft more effectively than can a pilot using manual braking.

Braking efficiency is a factor of both the runway surface condition and the particular tire/brake combination in use on an aircraft. It is a measure of how much of the available runway friction is used to stop the aircraft in the shortest possible distance. In the case of the Mark I, efficiencies of about 60 percent were achieved.

Hydro-Aire went on to develop more sophisticated antiskid systems, and today its designs fly on everything from the Boeing 777 to the Bombardier Global Express and even the space shuttle. The company's current state-of-the-art antiskid design is designated the Mark V digital brake-by-wire system. It is a microprocessor-based design in which the conventional cable-operated brake-metering system is replaced with an electrical transducer located at the brake pedal. The transducer senses brake pedal input and transmits a corresponding control signal to the brake-metering valve. Braking efficiencies of as high as 98 percent have been achieved with the Mark V, which is used on such aircraft as the Learjet 45, the Global Express, and the Boeing Business Jet.

Modern antiskid systems have advanced beyond simply protecting against blown tires. They now optimize braking efficiency under all runway conditions, enhance tire cornering capability, provide protection against hydroplaning, and protect against inadvertent brake application at touchdown. In some aircraft, they are also used to apply brakes to wheels during landing-gear retraction. This helps to prevent undue stress on the landing gear itself, caused by the considerable gyroscopic inertia of the spinning wheels as the gear moves through its retraction cycle. Antiskid systems generally extend tire life considerably, since damage such as flat spots caused by prolonged skidding is practically eliminated.

For a given runway condition, each aircraft's braked tires are capable of producing a given amount of friction force. In a normal landing, this force is shared for two main purposes: to slow the aircraft, and to allow the pilot to react to lateral forces such as a crosswind, in order to maintain directional control. Friction force in excess of what is needed to slow the aircraft provides additional cornering capability, so the higher the braking efficiency available, the more lateral control the pilot will have. On a dry runway, there is generally plenty of friction force available to comfortably do both jobs. On a runway reported as having only fair to poor braking action, considerably less friction force is available. The antiskid system is designed to milk as much of this friction force as possible, more efficiently than a pilot could do manually. How much more efficiently? Compared with average manual braking, antiskid shortens stopping distance on dry, hard runways by 30 to 40 percent. On wet or icy surfaces, it shortens ground roll by as much as 60 percent.

Autobrake systems go hand in hand with antiskid systems on many jet aircraft. They contribute an additional degree of braking efficiency compared to what a pilot can supply by manually applying brakes, even with an antiskid system to help. Generally, autobrake systems have two main modes: rejected takeoff (RTO), which applies maximum possible braking in the event of an aborted takeoff, and landing mode. Landing mode allows for a range of desired landing deceleration settings, often labeled as MIN, MED, and MAX. Some aircraft have up to five brake settings, labeled 1, 2, 3, 4, and MAX.

Modern autobrake systems maintain the desired deceleration level by taking into account not just the runway braking efficiency as determined by the antiskid, but also whether or not reverse thrust and spoilers are being used to decelerate the aircraft. Thus, if a pilot selects MED for landing, the autobrakes will supply more brake pressure if brakes alone are used, and less if other aids such as spoilers or reverse thrust are also applied. In either case the net effect will be the same: a medium level of deceleration as felt by pilots and passengers.

During a rejected takeoff with the RTO mode armed, maximum autobraking is applied immediately if the thrust levers are returned to the fully retracted position or if thrust reversers are activated. Simulator studies have shown that the chances of an aircraft's exiting the runway during a rejected takeoff are reduced if autobrakes are employed until the aircraft comes to a complete stop. Pilots braking manually in an RTO situation might have a tendency to fall back on their more relaxed braking experiences with hundreds or thousands of previous normal landing rollouts. They are lulled into relaxing braking efforts if the crisis appears under control, when in fact it is not.

Autobrakes can always be overridden by the pilot, simply by applying brake pressure or by selecting the Off position on the autobrake control panel.

The job of ground spoilers in helping to decelerate an aircraft is a simple one. They destroy lift, allowing the weight of the aircraft to more quickly settle upon the wheels. This in turn allows the brakes to do their job more effectively.

Thrust reversers can contribute a little or a lot to the deceleration effort, depending upon many variables, including the efficiency of the particular reverser design and the runway condition. Some thrust reverser systems are able to produce upwards of 50 percent of the forward thrust capability of the engine. Thus, such a system on a twin-engine jet with 12,000-pound-thrust engines could supply as much as 12,000 pounds of total reverse thrust. Typically, considerably less than full reverse thrust is used during a normal landing rollout. On a dry runway, reverse thrust systems shorten average landing rolls by 10 to 15 percent. On contaminated runways with poor braking action, they may contribute far more than that.

Of the three aids to stopping a jet on the runway, the brakes themselves are by far the most important element. Learning to use all three systems in concert for consistently smooth and safe arrivals is where art meets science in the cockpit.