Top

Landing Gear

Introduction:

  • Landing gear is designed to support the load of the aircraft for surface operations
  • The landing gear typically consists of three wheels:
    • Two main wheels (one located on each side of the fuselage)
    • A third wheel positioned either at the front or rear of the airplane
  • When the third wheel is located on the tail, it is called a tail-wheel, and the design is referred to as conventional gear
  • When the third wheel is located on the nose, it is called a nose-wheel, and the design is referred to as a tricycle gear
  • Aircraft can also be equipped with floats for water operations or skis for landing on snow
  • Types of landing gear:
Tail-wheel Pilot Handbook of Aeronautical Knowledge, Landing Gear
Figure 1: Tail-wheel (Conventional)
Figure 2: Tricycle

Seaplane

Ski-plane
Figure 3: Seaplane
Figure 4: Ski-plane

Conventional Gear:

  • Landing gear employing a rear-mounted wheel is called conventional landing gear [Figure 1: Tail-wheel (Conventional)]
  • Tail-wheel landing gear aircraft have two main wheels attached to the airframe ahead of its Center of Gravity (CG) that support most of the weight of the structure
  • Advantages:
    1. Allows adequate ground clearance for a larger propeller
    2. More desirable for operations on unimproved fields
  • Disadvantages:
    1. With the CG located behind the main gear, directional control of this type aircraft becomes more difficult while on the ground
      • If the pilot allows the aircraft to swerve while rolling on the ground at a low speed, he or she may not have sufficient rudder control and the CG will attempt to get ahead of the main gear which may cause the airplane to ground loop
    2. Lack of good forward visibility when the tail-wheel is on or near the ground
  • These inherent problems mean specific training (FAR 61.31) is required in tail-wheel aircraft

Tricycle Gear:

  • Landing gear employing a front-mounted wheel is called tricycle landing gear
  • Tricycle landing gear aircraft have two main wheels attached to the airframe behind its CG that support most of the weight of the structure
  • Additionally, a nose wheel will typically provide some sort of nose wheel steering control
  • Advantages:
    1. It allows more forceful application of the brakes during landings at high speeds without causing the aircraft to nose over
    2. It permits better forward visibility for the pilot during takeoff, landing, and taxiing
    3. It tends to prevent ground looping (swerving) by providing more directional stability during ground operation since the aircraft's CG is forward of the main wheels
      • The forward CG keeps the airplane moving forward in a straight line rather than ground looping

Pontoons:

  • One or more pontoons, or floats, are mounted under the fuselage to provide buoyancy
  • By contrast, a flying boat such as the Consolidated PBY Catalina, uses its fuselage for buoyancy
  • Either type of seaplane may also have landing gear suitable for land, making the vehicle an amphibious aircraft

Pilot Handbook of Aeronautical Knowledge, Fixed Landing Gear
Figure 5: Pilot Handbook of Aeronautical Knowledge,
Fixed Landing Gear

Fixed Gear:

  • Fixed gear is designed to simplify design and operation
  • Advantages:
    • Always deployed
    • Low cost
  • Disadvantages:
    • Creates constant drag

Retractable Landing Gear:

  • A retractable gear is designed to streamline the airplane by allowing the landing gear to be stowed inside the structure during cruising flight
  • The primary benefits of being able to retract the landing gear are increased climb performance and higher cruise airspeeds due to the resulting decrease in drag
  • Retractable landing gear systems may be operated either hydraulically or electrically, or may employ a combination of the two systems
  • Warning indicators are provided in the cockpit to show the pilot when the wheels are down and locked and when they are up and locked or if they are in intermediate positions
    • Systems for emergency operation are also provided
  • Disadvantages:
    • Increased weight
    • Increased cost
    • Limited to high performance aircraft

Pilot Handbook of Aeronautical Knowledge, Retractable Landing Gear
Figure 6: Pilot Handbook of Aeronautical Knowledge, Retractable Landing Gear

Function:

  • The landing gear, if retractable, may function with either electrical or hydraulic power
  • Electrical:
    • An electrical landing gear retraction system utilizes an electrically driven motor for gear operation
    • When a switch in the cockpit is moved to the UP position, the electric motor operates
      • Through a system of shafts, gears, adapters, an actuator screw, and a torque tube, a force is transmitted to the drag strut linkages
      • The gear retracts and locks
      • The struts that open and close the gear doors are also activated
    • If the switch is moved to the DOWN position, the motor reverses and the gear moves down and locks
    • Once activated the gear motor will continue to operate until an up or down limit switch on the motor's gearbox is tripped
  • Hydraulic:
    • A hydraulic landing gear retraction system utilizes pressurized hydraulic fluid to actuate linkages to raise and lower the gear
    • When a switch in the cockpit is moved to the UP position, hydraulic fluid is directed into the gear up line
    • The fluid flows through sequenced valves and down-locks to the gear actuating cylinders
    • A similar process occurs during gear extension
    • The pump which pressurizes the fluid in the system can be either engine driven or electrically powered
    • If an electrically powered pump is used to pressurize the fluid, the system is referred to as an electro-hydraulic system
    • The system also incorporates a hydraulic reservoir to contain excess fluid, and to provide a means of determining system fluid level
    • Regardless of its power source, the hydraulic pump is designed to operate within a specific range
    • When a sensor detects excessive pressure, a relief valve within the pump opens, and hydraulic pressure is routed back to the reservoir
    • Another type of relief valve prevents excessive pressure that may result from thermal expansion
    • Hydraulic pressure is also regulated by limit switches
    • Each gear has two limit switches-one dedicated to extension and one dedicated to retraction
    • These switches de-energize the hydraulic pump after the landing gear has completed its gear cycle
    • In the event of limit switch failure, a backup pressure relief valve activates to relieve excess system pressure

 Airplane Flying Handbook, Landing Gear Handle
Figure 7: Airplane Flying Handbook,
Landing Gear Handle

Landing Gear Switches and Indicators:

  • Landing gear position is controlled by a switch in the cockpit
  • In most airplanes, the gear switch is shaped like a wheel in order to facilitate positive identification and to differentiate it from other cockpit controls [Figure 7]
  • Landing gear position indicators vary with different make and model airplanes
  • The most common types of landing gear position indicators utilize a group of lights
  • One type consists of a group of three green lights, which illuminate when the landing gear is down and locked [Figure 8]
  • Another type consists of one green light to indicate when the landing gear is down and an amber light to indicate when the gear is up
  • Still other systems incorporate a red or amber light to indicate when the gear is in transit or unsafe for landing
  • The lights are usually of the "press to test" type, and the bulbs are interchangeable
  • Other types of landing gear position indicators consist of tab-type indicators with markings "UP" to indicate the gear is up and locked, a display of red and white diagonal stripes to show when the gear is unlocked, or a silhouette of each gear to indicate when it locks in the DOWN position

Operational Preflight:

 Airplane Flying Handbook, Landing Gear Position Lights
Figure 8: Airplane Flying Handbook, Landing Gear Position Lights
  • Because of their complexity, retractable landing gears demand a close inspection prior to every flight
  • The inspection should begin inside the cockpit
  • The pilot should first make certain that the landing gear selector switch is in the GEAR DOWN position
  • The pilot should then turn on the battery master switch and ensure that the landing gear position indicators show that the gear is down and locked
  • External inspection of the landing gear should consist of checking individual system components [Figure 11-10]
    • The landing gear, wheel well, and adjacent areas should be clean and free of mud and debris
    • Dirty switches and valves may cause false safe light indications or interrupt the extension cycle before the landing gear is completely down and locked
    • The wheel wells should be clear of any obstructions, as foreign objects may damage the gear or interfere with its operation
    • Bent gear doors may be an indication of possible problems with normal gear operation
    • Shock struts should be properly inflated and the pistons clean
    • Main gear and nose gear up-lock and down-lock mechanisms should be checked for general condition
    • Power sources and retracting mechanisms should be checked for general condition, obvious defects, and security of attachment
    • Hydraulic lines should be checked for signs of chafing, and leakage at attach points
    • Warning system micro switches (squat switches) should be checked for cleanliness and security of attachment
    • Actuating cylinders, sprockets, universals, drive gears, linkages and any other accessible components should be checked for condition and obvious defects
    • The airplane structure to which the landing gear is attached should be checked for distortion, cracks, and general condition
    • All bolts and rivets should be intact and secure

 Airplane Flying Handbook, Landing Gear Safety Switches
Figure 9: Airplane Flying Handbook,
Landing Gear Safety Switches

Landing Gear Safety Devices:

  • Most airplanes with a retractable landing gear have a gear warning horn that will sound when the airplane is configured for landing and the landing gear is not down and locked
  • Normally, the horn is linked to the throttle or flap position, and/or the airspeed indicator so that when the airplane is below a certain airspeed,configuration, or power setting with the gear retracted, the warning horn will sound
  • Accidental retraction of a landing gear may be prevented by such devices as mechanical down-locks, safety switches, and ground locks
  • Mechanical down-locks are built-in components of a gear retraction system and are operated automatically by the gear retraction system
  • To prevent accidental operation of the down-locks, and inadvertent landing gear retraction while the airplane is on the ground, electrically operated safety switches are installed
  • A landing gear safety switch, sometimes referred to as a squat switch, is usually mounted in a bracket on one of the main gear shock struts [Figure 11-8]
  • When the strut is compressed by the weight of the airplane, the switch opens the electrical circuit to the motor or mechanism that powers retraction
  • In this way, if the landing gear switch in the cockpit is placed in the RETRACT position when weight is on the gear, the gear will remain extended, and the warning horn may sound as an alert to the unsafe condition
  • Once the weight is off the gear, however, such as on takeoff, the safety switch will release and the gear will retract
  • Many airplanes are equipped with additional safety devices to prevent collapse of the gear when the airplane is on the ground
  • These devices are called ground locks
  • One common type is a pin installed in aligned holes drilled in two or more units of the landing gear support structure
  • Another type is a spring-loaded clip designed to fit around and hold two or more units of the support structure together
  • All types of ground locks usually have red streamers permanently attached to them to readily indicate whether or not they are installed

Emergency Gear Extension Systems:

  • The emergency extension system lowers the landing gear if the main power system fails
  • Some airplanes have an emergency release handle in the cockpit, which is connected through a mechanical linkage to the gear up-locks
  • When the handle is operated, it releases the up-locks and allows the gears to free fall, or extend under their own weight [Figure 10]
    • Due to the wind stream, limitations may apply to the speed at which the mechanism may be employed to ensure the gear lock when extended
  • On other airplanes, release of the up-lock is accomplished using compressed gas, which is directed to up-lock release cylinders [Figure 11]
  • In some airplanes, design configurations make emergency extension of the landing gear by gravity and air loads alone impossible or impractical and so provisions are included for forceful gear extension in an emergency
  • Some installations are designed so that either hydraulic fluid or compressed gas provides the necessary pressure, while others use a manual system such as a hand crank for emergency gear extension [Figure 12]
  • Hydraulic pressure for emergency operation of the landing gear may be provided by an auxiliary hand pump, an accumulator, or an electrically powered hydraulic pump depending on the design of the airplane

 Airplane Flying Handbook, Landing Gear Release Handle  Airplane Flying Handbook, Compressed Gas  Airplane Flying Handbook, Landing Gear Crank
Figure 10: Airplane Flying Handbook,
Landing Gear Release Handle
Figure 11: Airplane Flying Handbook,
Compressed Gas
Figure 12: Airplane Flying Handbook,
Landing Gear Crank

 Airplane Flying Handbook, Gear Placards
Figure 13: Airplane Flying Handbook,
Gear Placards

Takeoff and Climb:

  • Normally, the landing gear should be retracted after lift-off when the airplane has reached an altitude where, in the event of an engine failure or other emergency requiring an aborted takeoff, the airplane could no longer be landed on the runway
    • Landing gear retraction should be preplanned, taking into account the length of the runway, climb gradient, obstacle clearance requirements, the characteristics of the terrain beyond the departure end of the runway, and the climb characteristics of the particular airplane
    • The landing gear should not be retracted until a positive rate of climb is indicated on the flight instruments
  • If the airplane has not attained a positive rate of climb, there is always the chance it may settle back onto the runway with the gear retracted
    • This is especially so in cases of premature lift-off
  • The pilot should also remember that leaning forward to reach the landing gear selector may result in inadvertent forward pressure on the yoke, which will cause the airplane to descend
  • As the landing gear retracts, airspeed will increase and the airplane's pitch attitude may change
    • The gear will take several seconds to retract and becoming familiar with the sounds and feel of normal gear retraction so that any abnormal gear operation can be readily discernible
  • Gear retraction and locking (and gear extension and locking) is accompanied by sound and feel that are unique to the specific make and model airplane
    • Abnormal landing gear retraction is most often a clear sign that the gear extension cycle will also be abnormal

 Airplane Flying Handbook, Inspection Points
Figure 14: Airplane Flying Handbook, Inspection Points

Approach and Landing:

  • The operating loads placed on the landing gear at higher airspeeds may cause structural damage due to the forces of the airstream
  • Limiting speeds (not found on the airspeed indicator), therefore, are established for gear operation to protect the gear components from becoming overstressed during flight
  • They are published in the AFM/POH for the particular airplane and are usually listed on placards in the cockpit [Figure 13]
    • The maximum landing extended speed (VLE) is the maximum speed at which the airplane can be flown with the landing gear extended
    • The maximum landing gear operating speed (VLO) is the maximum speed at which the landing gear may be operated through its cycle
  • The landing gear is extended by placing the gear selector switch in the GEAR DOWN position
  • As the landing gear extends, the airspeed will decrease and the pitch attitude may increase
  • During the several seconds it takes for the gear to extend, the pilot should be attentive to any abnormal sounds or feel
  • The pilot should confirm that the landing gear has extended and locked by the normal sound and feel of the system operation as well as by the gear position indicators in the cockpit
  • Unless the landing gear has been previously extended to aid in a descent to traffic pattern altitude, the landing gear should be extended by the time the airplane reaches a point on the downwind leg that is opposite the point of intended landing
  • The pilot should establish a standard procedure consisting of a specific position on the downwind leg at which to lower the landing gear
    • Operation of an airplane equipped with a retractable landing gear requires the deliberate, careful, and continued use of an appropriate checklist
    • When on the downwind leg, the pilot should make it a habit to complete the landing gear checklist for that airplane
  • Standardization ensures:
    • It ensures that action has been taken to lower the gear
    • It increases the pilot's awareness so that the gear down indicators can be rechecked prior to landing
  • Unless good operating practices dictate otherwise, the landing roll should be completed and the airplane clear of the runway before any levers or switches are operated - especially the flaps - as it allows the pilot to focus attention on the after landing checklist and to identify the proper controls
  • This will accomplish the following: The landing gear strut safety switches will be actuated, deactivating the landing gear retract system

Transition:

  • Pilots transitioning to retractable gear airplanes should be aware that the most common pilot operational factors involved in retractable gear airplane accidents are:
    • Neglected to extend landing gear
    • Inadvertently retracted landing gear
    • Activated gear, but failed to check gear position
    • Misused emergency gear system
    • Retracted gear prematurely on takeoff
    • Extended gear too late
  • In order to minimize the chances of a landing gear related mishap, the pilot should:
    • Use an appropriate checklist (A condensed checklist mounted in view of the pilot as a reminder for its use and easy reference can be especially helpful)
    • Be familiar with, and periodically review, the landing gear emergency extension procedures for the particular airplane. Be familiar with the landing gear warning horn and warning light systems for the particular airplane. Use the horn system to cross-check the warning light system when an unsafe condition is noted
    • Review the procedure for replacing light bulbs in the landing gear warning light displays for the particular airplane, so that you can properly replace a bulb to determine if the bulb(s) in the display is good. Check to see if spare bulbs are available in the airplane spare bulb supply as part of the preflight inspection
    • Be familiar with and aware of the sounds and feel of a properly operating landing gear system

Struts:

  • Struts transmit shock loads of landing, takeoff, and taxi to the airplane structure
  • 3 types of landing gear struts:
    • Bungee
    • Spring
    • Oleo
  • Oleo struts are composed of oil and air (typically Nitrogen)
  • A piston absorbs the shock during operations
  • Oil absorbs landing shocks
  • Air absorbs taxi shocks
  • Bungee and spring struts slowly distribute forces to the airframe at acceptable rates to reduce bouncing tendency

Steering:

  • Steering is typically controlled through the rudder pedals but in larger aircraft, separate controls are used
  • A steerable nose-wheel or tail-wheel permits the airplane to be controlled throughout all operations while on the ground
  • Steerable wheels are linked to the rudders by cables or rods, while castering wheels are free to swivel
    • In both cases, the aircraft is steered using the rudder pedals
  • Aircraft with a castering wheels may require the pilot to combine the use of the rudder pedals with independent use of the brakes

Brakes:

  • Used to slow the aircraft
  • Typically located on the main gear only
  • Applied by either a hand control or by foot pedals (toe or heel)
  • Foot pedals operate independently and allow for differential braking and can supplement nose-wheel/tail-wheel steering for ground operations
  • Disc brakes most common on trainers
  • Most brake systems are hydraulically actuated
  • Air brakes may be used to slow the aircraft for landing and while in flight
  • Breaks controlled by top of the rudder pedal to apply pressure
  • A parking break helps keep the break applied during ramp operations

Emergencies

  • Landing Gear Fail to Retract
  • Landing Gear Fail to Extend
  • Blown Tire
  • Brake Failure
  • Brake Fire

References: