Braking systems in modern commercial aircraft are amazing feats of technology. The most extreme capability required from any commercial aircraft's brakes is to stop the aircraft in the remaining runway distance following a rejected take-off (RTO) called just before decision speed, with the aircraft operating at its maximum take-off weight. As Messier-Bugatti-Dowty notes on its website, bringing a large jet such as an A340 or a 777 to a halt in several hundred metres - travelling at 300 kph (186 mph) at maximum take-off weight - means the aircraft's brakes have to absorb more than one billion joules of energy in a few tens of seconds.
This is more than 125 megajoules per individual wheel-and-brake assembly in a widebody with eight main landing gear wheels. (Since less than 10 per cent of a commercial aircraft's weight falls on its two nosewheels, airliners don't have brakes in their nosewheels: all their brakes are fitted to wheels in main landing gear bogies.) Most commercial aircraft have a set of brakes for every main gear wheel, but Airbus' A380 has been designed with brakes on 16 of its 20 main gear wheels, with no brakes installed on the two rear wheels on each six-wheel outer main gear bogie.
Much of the energy generated by brake action in a rejected take-off is absorbed by the rotors and stators of the brake discs in the form of heat. So it is not surprising that the temperature reached by the carbon-carbon composite material which forms the brake discs and heat sink in most modern commercial jet brakes can go far above 2,000°C on the surface of the discs. The bulk of the heat sink itself reaches 1,700°C, according to Frank Crampton, SVP of sales and marketing for Meggitt Aircraft Braking Systems.
However, a metal alloy aircraft wheel starts going soft at about 200°C. "Trying to manage the heat sink inside the wheel at over 1,700°C is a big thermal management problem - you need a lot of shielding and a lot of ventilation," says Crampton.
During a rejected take-off, an associated problem is that the tyres on the aircraft's main gear wheels also become very hot - potentially so hot that once the aircraft has stopped (and cleared the runway if possible), the tyres could explode and thus create a shrapnel and fire hazard for anyone evacuating or otherwise near the aircraft. Accordingly, main landing gear wheels must be fused to allow tyre deflation before they reach a temperature at which they would explode.
Requirements for aircraft brakes
A rejected take-off represents an extreme requirement for brake capability, as does an overweight and overspeed landing. But since the pilots of any commercial aircraft potentially need to carry out either manoeuvre in an emergency, every brake manufacturer must design its products to be capable of handling safely the most extreme performance requirements that pilots might ever ask of them.
Not only that, says Nathan Drevna, a spokesman for Honeywell Aerospace Landing Systems, but the brakes must also remain functional after an emergency stop or an emergency landing to ensure that the aircraft will not roll away during evacuation of its passengers and crew. Other capabilities required before an airliner type can be certificated are that wheels must be able to absorb normal operating loads without failure over wheel rolling life (which is up to 50,000 miles) and that any wheel failure must first appear in a benign area such as a spoke, not a critical area such as a wheel hub or tubewell.
Additionally, says Drevna, any wheel without a tyre (the tyre having burst on landing or braking) must be able to roll on the runway for up to three miles without failure. It is also vital that brake running clearance must exist in any operating conditions, in order to prevent brake drag.
There are many other requirements which manufacturers of brakes and brake control systems for airliners - whether mainline or regional aircraft - must take into account. Emmanuel Lafaye, chief project engineer of Messier-Bugatti-Dowty's Wheels and Brakes Division, points out that aircraft brakes and their controls must be designed to cool off as fast as possible so that the aircraft's turnaround time can be as rapid. This is because brake effectiveness reduces with brake temperature, and no aircraft of a given type with its brake temperature registering above a specified limit would be given permission to take off, since the brakes would not be capable of handling the extreme RTO requirement.
Franck Ybert, systems engineering director of Messier-Bugatti-Dowty's Systems Equipment Division, says that since loss of wheel braking is a highly critical failure condition, brake control system architectures must be designed to incorporate extensive redundancy. In this way the systems can maintain appropriate braking capability in the event of a hydraulic or an electrical failure.
For hydraulically powered braking systems, this leads manufacturers to use two segregated hydraulic circuits and a set of redundant electronic systems with separated power supplies. These are generally associated with automatic system reconfiguration functions, so that pilots can quickly recover the capability to decelerate the aircraft should a hydraulic or electrical system failure occur while the aircraft is braking.
Additional brake performance requirements
Manufacturers of wheels, brakes (and their control systems) and tyres face yet more performance requirements. To meet stopping distance requirements, brake control systems must have efficient anti-skid functions which can deal with variable runway states, dry or wet. In systems terms, says Ybert, this drives a need for fast software control loops hosted in dedicated digital control units, which (in hydraulically actuated brake systems) control high-response servovalves. This ensures that brake pressure can be regulated sufficiently to maintain the wheel speeds at the skid ratio which provides the maximum of friction between the tyre and the runway - and so the maximum of aircraft deceleration.
Crampton says another performance objective is to ensure that, by means of the brakes and the design of the wheel and the tyre, the tyre generates the greatest possible tractive braking efficiency in its contact with the runway. If 100 per cent represents the best possible theoretical available braking force the tyre can produce, he says, typical standards achieved today for modern brake/tyre combinations are in the 90 to 95 per cent range. The figure is generally at the higher end of the range for braking on dry runways and at the lower end for braking on wet runways.
High braking efficiency is necessary because it allows the brake control system's anti-skid function to maintain wheel speed very close to the 'skid ratio' which provides maximum friction between tyre and runway, according to Ybert. This maximises aircraft deceleration for any given runway condition.
Ybert says another requirement for a brake control system is that when pilots make manual braking inputs via their brake pedals, the need for controllability demands a fast initial response from the brakes, without inducing brake overshoot. High static and dynamic accuracy is also needed between brake pressure/torque demand and the actual pressure and torque produced. For autobraking, there must be very high accuracy between the actual aircraft deceleration and the deceleration commanded, irrespective of any external factors, such as spoiler activation, that may affect aircraft deceleration.
Despite all these exacting control requirements, Drevna says airliner brake manufacturers must design their brakes to be as light, as reliable and as long-lived as possible. One reason for this is that "aircraft brake linings wear and need to be replaced frequently compared to other aircraft systems that may require little and infrequent maintenance", he says. "The infrastructure required to support this regular overhaul is a significant driver in the lifecycle cost of the brakes."
The carbon brake revolution
A key improvement here has been the development within the past two decades of carbon-carbon composite materials to replace steel as the braking friction and heat sink material used in commercial aircraft brakes. For about 60 years, from the 1930s through until the 1990s, steel was used as the brake-disc material in nearly all commercial aircraft brakes. Due to their design age and/or their overhaul status, many commercial aircraft still operate today with brakes which feature steel rotor and stator discs as their heat sinks - and do so perfectly normally.
But in a rejected take-off, a steel heat sink melts at around 1,100°C, according to Crampton. However, the carbon-composite heat sink material in a carbon brake sublimates - it doesn't melt - at about 2,700°C. "So the heat sink is a lot more capable than in the past," he says. That said, the manufacturer still has to be able to solve the problem of keeping the heat sink from heating up the wheel and tyre surrounding it to the point where they fail.
Although carbon-composite brake discs are still more expensive to produce than steel discs, they have other highly attractive properties which now make them more compelling than steel as a brake material. For a start, a carbon disc stack can weigh as little as one-quarter of the weight of a comparable steel heat sink, reducing aircraft weight substantially. Modern carbon-composite brake discs also wear much more slowly than do steel discs.
This has allowed brake manufacturers to extend average hard times between overhauls for wheels and brake discs from 1,000 landings to 2,000 landings. In some cases, such as the E-Jet family (for which Meggitt makes the brakes), brake overhaul intervals are now 3,000 landings - which means that even in regional airline operation, an aircraft with carbon brakes may still operate for about a year before its brakes need to be overhauled.
However, steel brake discs are not necessarily replaced entirely at the first overhaul interval, notes Kurt Burkhalter, a technical expert in Meggitt's Aircraft Braking Engineering Department. As long as a thinning rotor is not out of specification and doesn't reduce the brake disc stack's overall heat sink weight to below that required to deal with a rejected take-off, it can be moved from a high-wear part of the disc stack to a lower-wear location.
The use of carbon-composite heat sinks has become so important in the commercial aircraft brakes business that each of the four main OEMs - Messier-Bugatti-Dowty, Honeywell Landing Systems, Meggitt Aircraft Braking Systems and UTC Aerospace Systems - develops its own, trademarked and strictly proprietary carbon materials. "It's a black art," says Crampton.
Part of this black art is in developing anti-oxidation coatings which help protect carbon-composite brake discs from the aggressive de-icing solvents which airports now routinely use on runways. As a result of their location within aircraft wheels, a foot at most from the ground when the aircraft is not in the air, aircraft brakes are very exposed to any chemicals with which airports treat their movement areas.
Ceramic matrix composites represent one possible future area for brake disc materials development, but Burkhalter says carbon-carbon composites have lots of advantages which will keep them "state-of-the-art for a while" in aircraft brakes. Manufacturers continue to invest heavily in reducing the cost of manufacturing carbon-carbon materials and increasing further their time between overhauls.
Other factors such as low operating noise, an extensive operating temperature range and an ability to operate without causing undue vibration in the airframe also make carbon-composite brakes attractive. "To date we haven't found anything better than carbon-carbon composites," says Burkhalter. "I think a lot of mining is yet to be done with carbon-carbon itself in terms of [improving] cost, smoothness and overall braking system optimisation."
Another key technological development which continues to alter the aircraft brakes landscape is the advent in the 1970s of commercial brake-by-wire control, first used on the A300B4 by Messier-Bugatti-Dowty. Today the phrase still generally refers to systems which are controlled by means of electrical commands passing down wires from the cockpit brake pedal inputs to hydromechanical metering valves, which then deliver metered hydraulic pressure to the brakes.
Until recently, aircraft with brake-by-wire control systems were also designed with traditional brake-by-cable systems as emergency back-ups, for use in the event of a failure of the main braking control. However, says Ybert, more recent aircraft designs have used full brake-by-wire control systems, in which all brake inputs from the hydromechanical actuators are based on electrical signals only and no brake-by-cable back-up system is present.
In brake-by-wire systems, braking control functions and/or antiskid algorithms are hosted in digital control units. Ybert says these are connected to the hydraulic equipment for the braking system, to the brake control inputs - brake pedal electrical positions and the autobrake switch - and to wheel speed sensors.
The next logical step in the evolution of brake control and actuation is now upon the industry. While Airbus has adopted a relatively conservative design approach for the A350 XWB's brake systems, by retaining hydromechanical actuation, Boeing has gone all-electric with the 787. (Messier-Bugatti-Dowty makes brakes for both the 787 and the A350 XWB.) This means that its brake actuators use electric motors instead of hydraulic power.
"Electric actuation provides a better overall availability for the operator, as the electric motors or actuators can be replaced on-wing," says Lafaye. "Electrical actuation provides, first, improved availability for the brake and second, reduced maintenance time. When renewing the friction material on the brake, there are no seals or other brake parts to be replaced, but just the actuator to be placed in the new brake position." Additionally, he notes: "Electrical actuation shall benefit from our research and technology programmes, allowing actuation downsizing. This will permit not only weight savings, but also possibilities to design electrical actuation for small brakes."
According to both Lafaye and Crampton, an important advantage electric brakes offer over hydromechanical brakes is that even if an actuator becomes inoperative, the other actuators can adjust and take over its load, so the net braking effect will be the same or nearly the same. If one hydromechanical brake becomes inoperative because of a hydraulic leak, the aircraft's braking capacity suffers. Importantly for operators, an aircraft cannot be dispatched with a hydromechanical actuator not functioning, but an aircraft with electrically actuated brakes can be dispatched with an actuator inoperative.
That said, electric brakes have their downside too, according to Crampton. Electric brake systems are more expensive to produce than their hydromechanical counterparts - at least, they are today. And while hydromechanical brakes use power from hydraulic lines which serve the landing gear and so which are already located close to the brake actuators, in aircraft which use electric brake actuators new power sources have to be installed specifically to power the actuators.
In other respects, says Lafaye, there is little difference between hydromechanical and electrical actuation. "Braking performance is linked to the brake sizing - the diameter and number of discs - and the same performance can be achieved [using] a hydraulically activated or an electrically activated brake."
Brake OEMs' horizons expand
Other trends are also changing the brake manufacturing industry in basic ways. One is a welcome expansion of the OEMs' business activities as a logical result of advances in electronics. These have seen early anti-skid brake control systems develop into brake-by-wire control systems and from there into fully digital control of brakes, offering functions such as autobrake features. Crampton says brake OEMs are now seeing yet another functional evolution.
As the cost of aircraft manufacturers' new programmes continually rise and they seek greater risk-sharing participation from their top suppliers, they are now asking brake OEMs to do much more than just design brakes and the associated brake-control systems. "We're being encouraged to expand brake control into larger areas," says Crampton. These include nosewheel steering control; control of landing gear extension and gear retraction; monitoring the positions of landing gears and doors; and other functions.
As a result of Meggitt's experience in designing high-performance sensors, an OEM asked the company to design a tyre pressure monitoring system, so Meggitt designed a very short-range wireless system which transmits data from the hubcap of the wheel to the axle of the landing gear.
"This is a brave new world for us," says Crampton. "The industry is pushing its supply base to take on more and more." For instance, since both brake controls and nosewheel steering systems are covered by ATA Chapter 32 maintenance standards and procedures, an aircraft manufacturer might find it convenient to ask a brake control supplier to produce an aircraft's nosewheel steering system too. "We've been expanding. We used to have ten LRUs (line replaceable units) - now we have 20 to 30," in any given aircraft programme.
Other brake manufacturers are seeing the same trend. With fuel costs remaining high and the concept of non-engine taxiing continuing to gain momentum, Honeywell and Messier-Bugatti-Dowty are co-operating to develop a 'electric green taxiing system' ('EGTS'). This system uses the aircraft's auxiliary power unit (APU) to drive electric motors attached to wheels in aircraft main gear bogies. These electric motors are powerful enough to taxi the aircraft to and from the runway hold area, where the aircraft can start its engines before taking off or shut them down after landing.
In the fast-developing business area, other companies such as Wheeltug (with its nosewheel-drive electric-motor system, which has already attracted advance orders for more than 600 units) and Lufthansa and partners (with their 'eTaxi' system which, like 'EGTS', is designed to drive main landing gear wheels) will provide stiff competition for Honeywell and Messier-Bugatti-Dowty. But it is clear that in parallel with the evolution of traditional aircraft brakes and controls into new and more sophisticated systems, the companies which make aircraft brakes are evolving just as rapidly and in similarly exciting ways.