Aeronautica

MANAGING

EVEN UN
ON TWIN-AISLE AIRPLANES DURING SHORT FLIGHTS

BRAKE TEMPERATURES

Operators typically purchase twin-aisle airplanes for long-distance flights. However, when market conditions dictate, operators may use some of these airplanes on shorter flights. In such instances, appropriate action by the flight crew can reduce the likelihood of brake overheating and concomitantdeparture delays.
BOB MACKNESS ASSOCIATE TECHNICAL FELLOW MECHANICAL/HYDRAULIC SYSTEMS BOEING COMMERCIAL AIRPLANES

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

AERO

11

hen a twin-aisle airplane is used for short-flight operations, brakes may heat unevenly on each landing, with the difference in temperatures becoming more pronounced with repeated landings. At the gate, the brake temperature display mayindicate that some brakes are hotter than others. Temperatures may continue to climb until the brake temperature light illuminates or an alert displays. Understanding the causes of uneven brake 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.

W

1

FACTORS THAT LEAD TO UNEVEN BRAKE HEATING

2

INTERPRETATION OF THE BRAKE TEMPERATURE INDICATION

1
FIGURE

BRAKE TEMPERATURE INDICATION

Slowing an airplane requires the dissipation of kinetic energy. Kinetic energy that is not absorbed by the thrust reversers andspeed brakes is converted to 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 brake heating include variation in brake wear, inadvertent asymmetrical braking, and wind conditions.

Variation in brake wear. Brakes that are approaching wearlimits can become as much as 30 percent hotter than unworn brakes. This variation in temperature simply reflects the difference in the amount of brake friction material. Inadvertent asymmetrical braking. Temperature differences between brakes on the left and right gears often arise from inadvertent asymmetrical braking. When the brakes and rudder pedals are used concurrently to maintain heading andkeep speed under control, it is easy for flight crews to inadvertently apply asymmetrical braking inputs that — unknown to them — are being compensated for by the rudder. When the pedal brakes are applied lightly during taxi braking and landings, temperature differences among the brakes on each landing gear can increase quickly. Wind conditions. Brakes cool by convection, and even a moderatebreeze can result in temperature differences among brakes that are exposed to wind and those that are sheltered from it. Given these factors, it is important to remember that temperature differences between the two sides of an airplane do not necessarily mean that a problem exists with the braking system or the brake temperature monitor.

The BRAKE TEMP light illuminates (or a BRAKE OVHT alertdisplays) when the temperature of a brake rises above a predetermined level and turns off when all the brakes have cooled to a certain level (fig. 1). These levels vary by airplane model and type of brake (table 1). The BRAKE TEMP light and BRAKE OVHT alert indicate that the radiated and conducted heat from the brake may cause a wheel fuse plug to melt to release air pressure from the tire safely.Whether or not the fuse plug actually melts depends on how far the threshold setting for the brake temperature indication has been exceeded (i.e., the higher the reading on the brake temperature display, the greater the likelihood of the fuse plug melting). The aircraft maintenance manual describes the procedures for checking for deflated tires and hydraulic leaks. (Hydraulic leaks can result from...