How does an oleo-pneumatic shock strut work?

The strut is a cylinder and piston containing hydraulic fluid with a charge of compressed air or nitrogen above it. Landing impact drives the piston up and forces fluid through a metered orifice, often varied through the stroke by a tapered metering pin, and that restriction absorbs the energy as heat. The gas charge acts as a spring: it holds the aircraft at rest and extends the strut again.

Why is nitrogen used instead of shop air in a shock strut?

Dry nitrogen is inert and carries essentially no moisture. Shop air brings water vapor into the strut, which corrodes the chrome piston and the cylinder walls and can freeze in the metering orifice at altitude. Oxygen is worse than either, because under pressure and in contact with hydraulic fluid or oil it creates a serious fire and explosion hazard, so it is never used for servicing struts.

How do I tell whether a strut needs air or fluid?

Measure the exposed piston against the dimension in the maintenance manual, with the aircraft at normal weight on level ground. If the strut is low, add air and recheck. A strut that comes up to the correct dimension and then settles again over hours or days is losing fluid or gas past a seal, and it has to be deflated, serviced with fluid, and recharged.

What does the landing gear squat switch do?

It tells the aircraft whether it is on the ground. Mounted on a main gear strut or torque link, the switch changes state when the strut compresses under weight and it opens the landing gear retract circuit, so the gear cannot be raised while the aircraft rests on its wheels. On larger aircraft the same ground-air signal switches other systems between ground and flight modes.

When does the landing gear warning horn sound?

When the gear is not down and locked and the aircraft is configured for landing. Depending on the model, the trigger is the throttle retarded below a set position, the flaps extended to a landing setting, or both. It is a warning only. It does not prevent a gear-up landing, and it is the squat switch, not the horn, that blocks retraction on the ground.

Landing Gear Systems — FAA A&P Test Questions (ACS AM.II.E)

Landing gear is a systems area, and the airframe written test treats it as one: how an oleo-pneumatic shock strut absorbs a landing, what the structural members are called, and how a retraction system knows what to do and when. Every AMTprep question tagged to AM.II.E comes with a rationale and a handbook reference. The strut questions carry the most weight, and nearly all of them come back to one idea, which is that fluid absorbs the impact while air carries the static load. Get that straight and a whole cluster of questions falls into place.

What ACS AM.II.E covers

AM.II.E follows the landing gear material in FAA-H-8083-31, and the oleo-pneumatic strut sits at the center of it. On touchdown the piston drives up into the cylinder and forces hydraulic fluid through a metered orifice, and that restriction, not the air, absorbs the impact and turns it into heat. The compressed air or nitrogen above the fluid behaves as a spring: it supports the parked weight of the aircraft, sets how far the piston stays extended, and returns the strut after compression. The servicing questions follow from that. A strut sitting lower than the exposed-piston dimension in the maintenance manual is usually low on air, but a strut that settles again after charging has lost fluid, and dry nitrogen is the specified charging gas. Structure is mostly naming. Torque links keep the piston aligned with the cylinder so the wheel tracks straight, and they also limit how far the strut extends. The drag brace reacts fore-and-aft loads, the side brace reacts side loads, and a shimmy damper controls nosewheel shimmy. The retraction half is about sequence and safety. Sequence valves make doors and gear move in the correct order by holding pressure until the previous step finishes. The squat switch, also called the safety or ground-shift switch, senses that the strut is compressed and interrupts the retract circuit so the gear cannot be raised on the ground. A position warning horn sounds when the gear is not down and locked and the aircraft is configured to land, and every retractable system has downlocks, uplocks, and an emergency extension method independent of normal power.

Where this sits on the test

ACS AM.II.E is tested on the FAA Airframe written test, one of 1,748 ACS-tagged questions in the Airframe bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-31

Three traps candidates fall into

The most common wrong answer about struts is that the compressed air absorbs the landing shock. It does not. Fluid forced through the metering orifice absorbs and dissipates that energy as heat, while the air charge supports the static weight, sets strut extension, and pushes the strut back out.
A low strut gets more air and the technician moves on. If it settles again, the problem is fluid rather than air, and topping off with nitrogen only hides a leaking seal until the strut bottoms out on a hard landing. Service to the exposed-piston dimension the maintenance manual gives.
Shop air seems close enough for charging a strut, and it is not. Compressed air carries moisture that corrodes the strut internally and can freeze in the metering orifice at altitude, and oxygen under high pressure in contact with hydraulic fluid is a fire hazard. Dry nitrogen is specified.

7 free sample questions from ACS AM.II.E

AM.II.ETap an answer
What is the primary purpose of a torque link (torque arm) assembly on an oleo-pneumatic landing gear strut?
AM.II.ETap an answer
In an oleo-pneumatic (air-oil) shock strut, how is the energy of landing impact primarily absorbed?
AM.II.ETap an answer
An oleo shock strut on a parked aircraft is compressed more than the published exposed-piston dimension. The most likely cause is that the strut is
AM.II.ETap an answer
What is the primary function of the squat switch (landing gear safety switch) in a retractable gear system?
AM.II.ETap an answer
In a hydraulically operated retractable landing gear system, what is the purpose of the sequence valves?
AM.II.ETap an answer
On most retractable-gear aircraft, the landing gear position warning horn is designed to sound when the gear is not down and locked and
AM.II.ETap an answer
What gas is recommended for charging the air chamber of an oleo-pneumatic shock strut when a high-quality charge is required?

What is the primary purpose of a torque link (torque arm) assembly on an oleo-pneumatic landing gear strut?

ACS code: AM.II.E

Correct answer: To keep the piston and axle aligned and prevent the wheel from rotating about the strut

Rationale: The torque links connect the upper cylinder to the lower piston of an oleo strut. They allow the piston to move up and down (telescope) while preventing it from rotating, which keeps the axle and wheel properly aligned with the direction of travel. Fluid metering is done by the metering pin/orifice, not the torque links.

In an oleo-pneumatic (air-oil) shock strut, how is the energy of landing impact primarily absorbed?

ACS code: AM.II.E

Correct answer: By forcing hydraulic fluid through a metering orifice while compressing the air charge

Rationale: An oleo-pneumatic strut absorbs energy in two stages: hydraulic fluid is forced through a small metering orifice (dissipating energy as heat), and the trapped air/nitrogen charge is compressed, acting as a spring. Coil springs and rubber donuts are used in other shock-absorbing designs, not in oleo-pneumatic struts.

An oleo shock strut on a parked aircraft is compressed more than the published exposed-piston dimension. The most likely cause is that the strut is

ACS code: AM.II.E

Correct answer: low on air charge or hydraulic fluid

Rationale: Strut inflation is checked by measuring the exposed (shiny) portion of the piston against the manufacturer's specification. A strut compressed below the correct dimension indicates insufficient air charge and/or low fluid. Over-servicing with air would produce an over-extended strut, not a compressed one.

What is the primary function of the squat switch (landing gear safety switch) in a retractable gear system?

ACS code: AM.II.E

Correct answer: To prevent gear retraction while the aircraft weight is on the wheels

Rationale: The squat (safety) switch is actuated by the compression of a landing gear strut when the aircraft's weight is on the wheels. When compressed (on the ground), it opens the retraction circuit so the gear cannot be inadvertently retracted while on the ground. It does not meter hydraulic pressure or release up-locks.

In a hydraulically operated retractable landing gear system, what is the purpose of the sequence valves?

ACS code: AM.II.E

Correct answer: To ensure the gear doors and gear operate in the proper order during a cycle

Rationale: Sequence valves (timing valves) control the order of operation so that, for example, the gear doors open before the gear extends and close after it retracts. This prevents the gear from striking closed doors. Pressure relief and lock pressure maintenance are performed by relief valves and other components, not sequence valves.

On most retractable-gear aircraft, the landing gear position warning horn is designed to sound when the gear is not down and locked and

ACS code: AM.II.E

Correct answer: the throttle(s) are retarded toward the closed/idle position

Rationale: The gear warning horn typically activates when the throttle is reduced (as during an approach to land) while the gear is not down and locked, alerting the pilot to a possible gear-up landing. It is associated with low power/approach configuration, not with setting the parking brake or retracting flaps for cruise.

What gas is recommended for charging the air chamber of an oleo-pneumatic shock strut when a high-quality charge is required?

ACS code: AM.II.E

Correct answer: Dry nitrogen

Rationale: Shock struts are charged with compressed air or, preferably, dry nitrogen. Nitrogen is inert and dry, which prevents internal corrosion and condensation. Oxygen must never be used near oil because of fire/explosion hazard, and carbon dioxide is not used for strut charging.

Failed AM.II.E on your AKTR?

Enter the ACS codes from your FAA fail report and get a targeted study plan — the exact areas the examiner must re-test you on, with the handbook references for each.

Build my remediation plan

Frequently asked questions

How does an oleo-pneumatic shock strut work?: The strut is a cylinder and piston containing hydraulic fluid with a charge of compressed air or nitrogen above it. Landing impact drives the piston up and forces fluid through a metered orifice, often varied through the stroke by a tapered metering pin, and that restriction absorbs the energy as heat. The gas charge acts as a spring: it holds the aircraft at rest and extends the strut again.
Why is nitrogen used instead of shop air in a shock strut?: Dry nitrogen is inert and carries essentially no moisture. Shop air brings water vapor into the strut, which corrodes the chrome piston and the cylinder walls and can freeze in the metering orifice at altitude. Oxygen is worse than either, because under pressure and in contact with hydraulic fluid or oil it creates a serious fire and explosion hazard, so it is never used for servicing struts.
How do I tell whether a strut needs air or fluid?: Measure the exposed piston against the dimension in the maintenance manual, with the aircraft at normal weight on level ground. If the strut is low, add air and recheck. A strut that comes up to the correct dimension and then settles again over hours or days is losing fluid or gas past a seal, and it has to be deflated, serviced with fluid, and recharged.
What does the landing gear squat switch do?: It tells the aircraft whether it is on the ground. Mounted on a main gear strut or torque link, the switch changes state when the strut compresses under weight and it opens the landing gear retract circuit, so the gear cannot be raised while the aircraft rests on its wheels. On larger aircraft the same ground-air signal switches other systems between ground and flight modes.
When does the landing gear warning horn sound?: When the gear is not down and locked and the aircraft is configured for landing. Depending on the model, the trigger is the throttle retarded below a set position, the flaps extended to a landing setting, or both. It is a warning only. It does not prevent a gear-up landing, and it is the squat switch, not the horn, that blocks retraction on the ground.

All 40 ACS areas

General

Airframe

Powerplant

Primary sources

Drill ACS AM.II.E inside the full Airframe bank

Adaptive engine, timed exam simulator, ACS-code remediation, and oral prep — everything you need to pass.

Start free See plans