Chapter 2 - Aircraft Undercarriage System

Question 1

What are the basic undercarriage leg configurations?

Answer 1

Telescopic

Lever suspension

Forward lever

Sideways lever

Quadrilateral deformable

Semi-articulated

Question 2

What is the purpose of the shock absorbers in the undercarriage?

Answer 2

Dampen shock of:

• Landing
• Taxying
• Movement over uneven surfaces

Question 3

What two basic types of undercarriage shock absorber are used?

Answer 3

One utilised the incompressibility of oil

One utilises combinations and oil and nitrogen

Question 4

What are the basic features of a retracting undercarriage?

Answer 4

Doors and Fairings

Sequencing

Jacks and Linkages

Question 5

What role does Sequencing serve in retractable undercarriage systems?

Answer 5

It is a series of hydraulic valves or electrical switches that ensure everything happens in the correct order by not permitting one part of the sequence to commence until the preceding part has been completed.

Question 6

What is responsible for holding the undercarriage in raised position?

Answer 6

Up locks

Question 7

What is responsible for holding the undercarriage in its lowered position?

Answer 7

Down locks

Question 8

What are the two common types of Down Lock in use?

Answer 8

Geometrically locked hinged lever

Integral with the extension jack

Question 9

What does no light or flag on the undercarriage control represent?

Answer 9

Unit is locked UP

Question 10

What do red lights or flags on the undercarriage control represent?

Answer 10

Gear is travelling

only between up and down?

Question 11

What do green lights or flags on the undercarriage control represent?

Answer 11

Gear is locked DOWN

Question 12

What are WOW switches and what is their purpose?

Answer 12

Weight On Wheels

Activated by the deflection of the undercarriage on the ground, used to prevent operation of the retraction mechanism and to unlock the ability for nosewheel steering.

Signals from these switches are used in other aircraft systems to either initiate their deployment upon take off or landing or to prevent their operation in the air/on the ground.

Question 13

What are the design criteria for aircraft wheels?

Answer 13

Light weight

Minimum size

Good fatigue resistance

Accommodation for the brake unit and dissipation of the heat generated during braking

Easy tyre replacement

Question 14

Why are aircraft wheels made in two halves?

Answer 14

The two halves unbolt which allows tyres to be fitted without stretching over the rims, and the wheels house the wheel bearings.

Question 15

What type of tyres are aircraft usually fitted with?

Answer 15

Radial ply tubeless tyres

Note - Be able to label the parts in the diagram in the course manual

Question 16

What are the advantages of radial ply tubeless tyres over the earlier cross ply tyres fitted with inner tubes?

Answer 16

Higher strength

Lower Weight

Cooler Running

Better overload capabilities

Question 17

What is the purpose of ‘creep marks’ on aircraft tyres?

Answer 17

Allows creep to be monitored in tubed tyres because the valve could break if the inner tube is forced too far round the wheel and puts extra stress on it.

Could cause a blowout on landing.

Question 18

What is the name of an emergency gear extension system that uses pneumatics (nitrogen)?

Answer 18

Blow-Down System

Question 19

What is a Multi Disc braking assembly?

Answer 19

High performance disc brakes constructed as multiple stacks of discs made from carbon composites which are able to operate at the necessary temperatures.

Consists of rotors and stators.

Pressure of braking presses the rotors and stators together, resulting friction provides the retarding force.

Question 20

When braking, what does the point of skidding depend upon?

Answer 20

Condition of the runway surface

Vertical load which the aircraft tyres exert on the ground

Retarding force applied by the brakes

Question 21

Does changing the weight on the tyres affect the speed at which full aquaplaning occurs?

Answer 21

No, as the contact area also changes and the ratio of weight to area remains constant.

Question 22

What causes aquaplaning?

Answer 22

Tyre displaces the stationary water, resulting in a change in momentum of the water, creating hydrodynamic pressure which reacts on the runway and tyre surfaces.

Question 23

What is brake modulation?

Answer 23

Early anti-skid systems used the inertia of a flywheel to sense rapid changes in wheel rotational speed during a skid.

Reduced hydraulic pressure during a skid to stop it, reapplying it once the skidding reduced/stopped.

Created an on/off pulsing of the brakes, known as brake modulation.

Question 24

How do modern anti-skid systems work?

Answer 24

Control technology varies the frequency (and amplitude (pressure)) to maintain the braking forces at a level immediately below that which would cause a skid for all speeds and surface conditions.

System also holds off the brakes until after touchdown, either for a given time or until the wheels have spun up to a set speed.

(Also applies the brakes after takeoff to spin down the wheels after undercarriage retraction has taken place)