Landing gear employing a rear-mounted wheel is called conventional or a tail wheel/dragger [Figure 1:]
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:
Allows adequate ground clearance for a larger propeller
More desirable for operations on unimproved fields
Disadvantages:
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
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
Tail-wheel (Conventional)
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:
It allows more forceful application of the brakes during landings at high speeds without causing the aircraft to nose over
It permits better forward visibility for the pilot during takeoff, landing, and taxiing
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
Tricycle
Pontoons:
One or more pontoons, or floats, are mounted under the fuselage to provide buoyancy
Fixed gear is designed to simplify design and operation
Advantages:
Always deployed
Low cost
Disadvantages:
Creates constant drag, mitigated by the use of a cover called a fairing
Pilot Handbook of Aeronautical Knowledge, Fixed Landing Gear
Pilot Handbook of Aeronautical Knowledge, Fixed Landing Gear
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
Pilot Handbook of Aeronautical Knowledge, Retractable Landing Gear
Operating Retractable Landing Gear:
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
Landing Gear Switches and Indicators:
Airplane Flying Handbook, Landing Gear Handle
Airplane Flying Handbook, Landing Gear Position Lights
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, such as the flaps [Figure 7]
Landing gear position indicators vary with different make and model airplanes but 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, Inspection Points
Tigerchocks
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
Retractable 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
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 pre-planned, 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
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 of some common errors that lead to accidents:
Forgetting (neglecting) to extend landing gear
Inadvertently retraction of the landing gear
Activating gear, but failed to check gear position
Misuse of emergency gear system
Retracting gear prematurely on takeoff
Extending gear too late
In order to minimize the chances of a landing gear related mishap, pilots should:
Use an appropriate checklist
A condensed checklist placard 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:
Brakes are necessary to slow the aircraft down after touchdown on the runway to a speed at which they can turn onto a taxiway
In the case of most modern airplanes, airplane brakes are generally disc brakes
They consist of multiple pads (called caliper pads) that are hydraulically squeezed toward each other with a rotating disk (called a rotor) between them
The pads place pressure on the rotor which is turning with the wheels
As a result of the increased friction on the rotor, the wheels inherently slow down and stop turning
The disks and brake pads are made either from steel, like those in a car, or from a carbon material that weighs less and can absorb more energy
Because airplane brakes are used principally during landings and must absorb enormous amounts of energy, their life is measured in landings rather than miles
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
Tow Bars:
Aircraft Tow Bar
Tow bars attach to the aircraft in order to better steer the aircraft during ground tows
It is critical to remember to remove the tow bar prior to takeoff! here is why
Common Training Aircraft Landing Gear Characteristics:
Cessna-172:
Tricycle type w/ steerable nose wheel
Shock absorption provided by tubular spring steel main landing gear struts and the air/oil nose gear sock strut
Each main gear is equipped with a hydraulically actuated disc type brake on the inboard side of each wheel
Each brake is connected, by a hydraulic line, to a master cylinder which is attached to each of the pilot's rudder pedals
Effective steering is accomplished through nose wheel steering by using the rudder pedals (ground)
Nose wheel turns about 10° each side of center
Applying left or right brake, results in differential braking
Minimum turning radius using differential brakes is about 27 feet
Piper Arrow:
Retractable tricycle landing gear
Hydraulically actuated by an electrically powered reversible pump
Operation takes about 7 seconds
Has emergency gear extension lever to equalize system pressure
Nosewheel is spring assisted during freefall
Nose gear steerable 30° arc each side of center
WARNING GEAR UNSAFE displays when:
Gear up and power reduced below approximately 14" manifold pressure,
Gear selector UP on the ground with the throttle in retarded position, or
Flaps extended beyond 10° without gear down and locked
Warning horn sounds at 90 Hz
Main gear uses Cleveland single disc hydraulic breaks
Toe breaks and parking brake use separate cylinders but a common reservoir
Landing Gear Malfunctions:
All Landing Gear Malfunctions:
Treat any indication of a malfunction as actual
The pilot should not recycle the landing gear if an irregular indication is experienced
Consider landing at an airfield with Crash Fire Rescue
Before Landing:
Exhaust all avenues for correcting the problem (maintenance, ATC, any other instructors)
Consider exhausting all fuel possible to reduce the risk of fire
After Landing:
Remain on the runway and contact the maintenance department to inspect the gear before it is taxied or towed back to the ramp
Landing Gear and Tire Servicing:
Tires may be serviced with an air compressor or nitrogen for most every general aviation aircraft
Commercial aircraft require nitrogen to compensate for lower temperatures at altitude, increasing tire pressure stability and overall physical integrity
Conclusion:
Follow established procedures and practice to avoid distractions which can lead to landing with the gear up
