BOB MACKNESS

ASSOCIATE TECHNICAL FELLOW

MECHANICAL/HYDRAULIC SYSTEMS

BOEING COMMERCIAL AIRPLANES

Operators typically purchase twin-aisle airplanes forlong-distance flights. However, when market conditionsdictate, operators may use some of these airplanes onshorter flights. In such instances, appropriate action by the flight crew can reduce the likelihood of brakeoverheating and concomitant departure delays.

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F L I G H T O P E R A T I O N S

MANAGING

UNEVENBRAKE TEMPERATURESON TWIN-AISLE AIRPLANES

DURING SHORT FLIGHTS

INTERPRETATION OF THE BRAKE TEMPERATURE INDICATION

The BRAKE TEMP light illuminates(or a BRAKE OVHT alert displays)when the temperature of a brake risesabove a predetermined level and turnsoff when all the brakes have cooled to a certain level (fig. 1). These levelsvary by airplane model and type of brake (table 1).

The BRAKE TEMP light andBRAKE OVHT alert indicate that theradiated and conducted heat from the brake may cause a wheel fuse plug to melt to release air pressure from thetire safely. Whether or not the fuse plugactually melts depends on how far thethreshold setting for the brake tempera-ture indication has been exceeded (i.e.,the higher the reading on the brake tem-perature display, the greater the likeli-hood of the fuse plug melting). Theaircraft maintenance manual describesthe procedures for checking for deflatedtires and hydraulic leaks. (Hydraulicleaks can result from overheated brake

FACTORS THAT LEAD TO UNEVEN BRAKE HEATING

Slowing an airplane requires the dissi-pation of kinetic energy. Kinetic energythat is not absorbed by the thrustreversers and speed brakes is convertedto heat energy by the wheel brakes. The weight and speed of the airplane during landing and taxiing determine how much energy the brake friction(i.e., heat stack) material absorbs.

Factors that lead to uneven brakeheating include variation in brake wear, inadvertent asymmetrical braking,and wind conditions.

Variation in brake wear.Brakes that are approaching wear limitscan become as much as 30 percent hotter than unworn brakes. This vari-ation in temperature simply reflects the difference in the amount of brakefriction material.

Inadvertent asymmetrical braking.Temperature differences between brakeson the left and right gears often arisefrom inadvertent asymmetrical braking.When the brakes and rudder pedals areused concurrently to maintain headingand keep speed under control, it is easy for flight crews to inadvertently applyasymmetrical braking inputs that —unknown to them — are being com-pensated for by the rudder. When thepedal brakes are applied lightly duringtaxi braking and landings, temperature differences among the brakes on eachlanding gear can increase quickly.

Wind conditions.Brakes cool by convection, and even a moderate breeze can result in tem-perature differences among brakes that are exposed to wind and those that aresheltered from it.

Given these factors, it is important toremember that temperature differencesbetween the two sides of an airplane do not necessarily mean that a problemexists with the braking system or thebrake temperature monitor.

BRAKE TEMPERATURE INDICATION

FIGURE

1When a twin-aisle airplane is used forshort-flight operations, brakes may heatunevenly on each landing, with the difference in temperatures becoming more pronouncedwith repeated landings. At the gate, the braketemperature display may indicate that somebrakes are hotter than others. Temperaturesmay continue to climb until the brake temperature light illuminates or an alert displays. Understanding the causes of unevenbrake heating and the meaning of the brake temperature indication can help flight crews minimize possible departure delays. This article explains the following:

1. Factors that lead to uneven brake heating.

2. Interpretation of the brake temperature indication.

3. Flight crew action that can minimize uneven brake heating.

1 2

Airplane model

767-200/300with steel brakes

767-200/-300with carbon brakes

767-400with carbon brakes

777-200/300with carbon brakes

MD-11with carbon brakes

Temperature thresholds (°C) Notes

■ The brake temperature monitoring system is optional.

■ Any brake reading that exceeds the threshold appears in white numerals in a white-bordered box on the engine indication and crew alerting system (EICAS).

■ The BRAKE TEMP light is white and is not an alert but an aid in the use of the brake cooling chart.

■ The alert is an amber BRAKE TEMP advisory message on EICAS.■ On the landing gear synoptic page, any brake reading that exceeds the threshold appears in white numerals in a white-bordered box.

■ The alert is an amber BRAKE TEMP advisory message on EICAS.■ On the landing gear synoptic page, any brake reading that exceeds the threshold appears in white numerals in a white-bordered box.

■ The BRAKE OVHT alert is a level 2 message on the electronic display.■ On the configuration page, any brake reading that exceeds the threshold appears in amber numerals inside an amber-bordered box.

Indication on Indication off

371 315

427 362

538 338

430 330

550 450

BRAKE TEMPERATURE INDICATION SYSTEMS BY AIRPLANE MODEL

TABLE

1

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767 carbon brake

Source: Honeywell Aircraft Landing Systems

differences between new and wornbrakes also will increase with repeatedlandings. Because of these factors,a BRAKE TEMP or BRAKE OVHT indication will more likely occur toward the end of the day and especiallyin hot weather.

If landing conditions for an airplaneexceed certain maximums listed in theairplane flight manual (AFM) quick-turnaround charts, departure may bedelayed for a prescribed period of timeto let the brakes cool. The airplane maybe allowed to depart before the end ofthat period if the true brake tempera-tures, as shown by the brake temperatureindication, are lower than those pre-dicted in the AFM.

FLIGHT CREW ACTION THAT CAN MINIMIZE UNEVENBRAKE HEATING

How the flight crew uses the brake system in conjunction with reverse thrustand the spoilers during landing signifi-cantly affects the amount of energy trans-ferred to the brake stacks. Flight crewscan use several techniques to arrive atthe airport gate with cooler brakes.

On long runways, aerodynamic dragand thrust reversers may provide plentyof deceleration at high speed, and braking can be delayed until taxi speedsare reached. If autobrakes are used, theflight crew should choose the lowestdeceleration setting commensurate withthe available field length; landing longand turning off short should be avoided.Brake energy balance is improved when the autobrakes are used all theway down to taxi speeds becausethe same hydraulic pressure is applied to every brake.

On the taxiway, moderate brake ap-plications followed by full pedal releasekeep the brakes cooler than would asteady application of light brake pres-sure (i.e., riding the brakes). Using thistechnique makes it less likely that theBRAKE TEMP or BRAKE OVHT indication will come on because of onedisproportionately hot brake. (It shouldbe noted that minimizing the number of brake applications can reduce wearon carbon brakes.)

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Editor’s note: For related information, refer to the Aug. 1, 1990, issue of the Boeing Flight Operations Review and the latest revision of the

Boeing Flight Crew Training Manual.

SUMMARYDuring short-flight operations, flight crews can take steps to pass on a cooler set of brakes to the next crew. Crews cancarefully manage brake energy during landing and taxiing by

■ Maximizing the use of thrust reversers and minimizing the use of brakes.

■ Using autobrakes or symmetrical pedal braking.

■ Avoiding riding the brakes during taxi.

If the BRAKE TEMP or BRAKE OVHT indication still occurs,published maintenance procedures should be performed.

Comparison of new (left) and worn carbon brakes Source: Messier-Bugatti

piston seals after a BRAKE TEMP or BRAKE OVHT indication hasoccurred.) If no anomalies are found,the maintenance crew may clear the airplane for dispatch.

Brake temperatures tend to increasefrom one landing to the next during short-flight operations. On the first landing of the day, the brakes are relatively coolbecause of the length of time since the

last significant energy input. When theairplane leaves the gate for the nextflight, however, the brakes will still bewarm from the previous landing, andmore energy will accumulate from braking during the taxi out. The brakeswill not cool completely on a shortflight, and during the next landing, thesame braking effort will produce higherpeak temperatures. The temperature