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newSchoolWorks.com › VoTechWorks › Transportation › Aviation Maintenance › Airframe Systems and Structures › Lesson 6
TRN-AVIA22-02 · Lesson 6 · 48 clock hours · lab

Aircraft Landing Gear Systems

Objective. Students will inspect, service, and operationally test a retractable landing gear system—including hydraulic actuators, up/down locks, and position-indicating switches—and complete a logbook entry for the task.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Aircraft Landing Gear Systems — Inspection, Service, and Operational Test

The landing gear system is one of the most safety-critical assemblies on any aircraft. A failure during takeoff or landing can be catastrophic. As an aviation maintenance technician, you are responsible for ensuring every component—actuators, locks, indicators, and the strut itself—is within the manufacturer's limits before signing off a return-to-service statement.

How a Retractable Gear System Works

Most retractable gear systems use hydraulic pressure to raise and lower the gear. A hydraulic actuator pushes or pulls a drag brace or side brace to fold the gear into a wheel well. Up-locks hold the gear retracted in flight so it cannot fall free; down-locks (overcenter locks or mechanical hooks) hold the gear fully extended for landing. Door actuators open the wheel-well doors before the gear moves, and close them after retraction—this is called sequencing. The sequence is controlled by hydraulic sequencing valves or, on many modern aircraft, by electronic controllers.

Position-indicating switches (proximity switches or mechanical switches) trigger cockpit lights: green for each gear down-and-locked, red for gear in transit or unsafe, and no light (or a specific indication) for gear up-and-locked. If any light shows an unexpected indication during a retraction test, stop the test and investigate before proceeding.

Oleo-Pneumatic Struts

The main and nose gear struts on most aircraft are oleo-pneumatic (also called air-oil) struts. They contain hydraulic fluid in the lower chamber and nitrogen (never oxygen—oxygen is a fire hazard with petroleum fluid) in the upper chamber. When the aircraft rolls over a bump, fluid is forced through a small orifice, converting kinetic energy to heat and dampening the shock. The trapped nitrogen acts as a spring.

Strut extension is the amount the inner cylinder protrudes from the outer cylinder when the aircraft is on the ground. The aircraft maintenance manual (AMM) specifies an extension range—typically expressed in inches—that matches the correct fluid level and nitrogen pre-charge. A strut that is too low will bottom out on landing; one that is too high may indicate overservicing.

Performing a Retraction Test

Before any retraction test, verify the aircraft is on jacks rated for its gross weight and positioned on a level, stable surface. Confirm all personnel are clear of the gear swing path—moving gear can crush a limb instantly.

1. Jack the aircraft per the AMM jacking procedure; confirm safety locks are in place on the jacks.

2. Connect hydraulic ground power if required.

3. Follow the AMM functional test procedure step-by-step, calling out each step.

4. Observe door sequencing—doors must open fully before gear begins to move.

5. Check that all gear reach the up-lock position; confirm the cockpit shows the up indication.

6. Extend the gear; confirm down-and-locked indication and that all down-locks are engaged.

7. Measure and record door clearances if the AMM specifies limits.

8. Record all observations on the work order. Any out-of-tolerance finding requires a discrepancy write-up before return to service.

Inspecting and Servicing an Oleo Strut

When removing a strut or servicing it in place, always depressurize the strut before loosening the air valve core or any fitting. Nitrogen under pressure can violently expel a fitting. Wear safety glasses and nitrile gloves at all times. Place a drip pan under the strut before opening any hydraulic port.

Strut extension check: With the aircraft properly loaded or on jacks at the correct stance, measure from the top of the lower (inner) cylinder to the bottom edge of the outer cylinder (or per the specific AMM reference point). Compare to the AMM limit table.

Fluid level check: With the strut fully compressed (aircraft on ground, strut bottomed), remove the filler/bleed valve and check fluid level per the AMM. Add the approved hydraulic fluid type specified in the AMM if low; never mix fluid types.

Nitrogen servicing: Connect a nitrogen servicing cart with a calibrated gauge. Charge to the AMM pressure specification at the ambient temperature noted in the chart. Never use shop air or oxygen.

After reinstallation, torque all fittings and attachment hardware to AMM-specified values. Verify strut extension is within limits before removing from jacks.

Safety First

Completing the Logbook Entry

Under 14 CFR Part 43, every maintenance action must be documented. Your logbook entry must include: aircraft make, model, and serial number; date of work; description of the work performed; reference to the manual and revision used; part numbers of any replaced components; your name, certificate number, and type of certificate; and the return-to-service statement. The return-to-service statement certifies that the work was performed in accordance with the referenced data and that the aircraft is approved for return to service. An incomplete or missing entry is a regulatory violation—treat the logbook as seriously as the wrench.

Key terms9

Up-lockA mechanical latch that holds the landing gear in the fully retracted position during flight, preventing the gear from falling free.
Down-lockA mechanical device (often an overcenter brace or hook) that holds the landing gear in the fully extended position and absorbs side and drag loads on landing.
Oleo-pneumatic strutA landing gear shock absorber that uses hydraulic fluid and compressed nitrogen to absorb and dampen landing loads; sometimes called an air-oil strut.
Strut extensionThe measured amount the inner cylinder of an oleo strut protrudes from the outer cylinder when the aircraft is on the ground; used to verify correct fluid level and nitrogen charge.
Sequencing valveA hydraulic valve that controls the order of operation in a landing gear system, ensuring wheel-well doors open before the gear moves and close after retraction.
Position-indicating switchAn electrical switch (mechanical or proximity type) that senses gear position and drives cockpit indicator lights to show down-and-locked, in-transit, or up-and-locked status.
Return-to-service statementThe regulatory certification in a maintenance record, required by 14 CFR Part 43, in which a certificated mechanic approves an aircraft or component for return to service after maintenance.
Aircraft Maintenance Manual (AMM)The manufacturer's official document that specifies approved procedures, limits, torque values, and servicing data for a specific aircraft model.
Nitrogen servicingThe process of charging an oleo strut or hydraulic accumulator with dry nitrogen gas to the pressure specified in the AMM; nitrogen is used instead of air or oxygen to prevent moisture and fire hazard.

Worked examples4

1. The AMM states that the nose gear oleo strut extension should be between 3.0 inches and 3.5 inches. You measure the strut and find it extends 2.25 inches. By how much is the strut short of the minimum, and what action is required?
  1. Identify the AMM minimum extension: 3.0 inches.
  2. Record the measured extension: 2.25 inches.
  3. Calculate the shortfall: 3.0 − 2.25 = 0.75 inches below the minimum.
  4. Because the strut is below the AMM minimum, it is out of tolerance and must be serviced before the aircraft is returned to service.
  5. Service the strut by checking and correcting the hydraulic fluid level first (with strut fully compressed), then charge nitrogen to AMM-specified pressure. Re-measure extension to confirm it falls within 3.0–3.5 inches.
Answer: The strut is 0.75 inches below the AMM minimum of 3.0 inches. The strut must be serviced (fluid and nitrogen) and re-measured before return to service.
2. During a retraction test, you must verify that the main gear door clearance to the fuselage does not exceed 0.75 inches when the gear is fully retracted. You measure clearances at three points: 0.45 in, 0.68 in, and 0.80 in. Which reading is out of tolerance and by how much?
  1. List the three measurements: 0.45 in, 0.68 in, 0.80 in.
  2. Compare each to the AMM limit of 0.75 in maximum.
  3. 0.45 in — within limit (0.45 < 0.75).
  4. 0.68 in — within limit (0.68 < 0.75).
  5. 0.80 in — out of tolerance (0.80 > 0.75).
  6. Calculate the amount over limit: 0.80 − 0.75 = 0.05 inches over the maximum allowable clearance.
Answer: The third measurement (0.80 in) is out of tolerance by 0.05 inches. This discrepancy must be written up and corrected (gear rigging or door adjustment) before return to service.
3. A torque wrench reads in inch-pounds. The AMM specifies a fitting torque of 35 ft-lb. What is the equivalent value in in-lb that you must set on the inch-pound torque wrench?
  1. Recall the conversion: 1 ft-lb = 12 in-lb.
  2. Multiply: 35 ft-lb × 12 in-lb/ft-lb = 420 in-lb.
  3. Set the torque wrench to 420 in-lb and torque the fitting accordingly.
Answer: 420 in-lb.
4. After servicing a strut, you re-measure the extension and get 3.3 inches. The AMM limit is 3.0–3.5 inches. What percentage of the total allowable range does 3.3 inches represent, measured from the minimum?
  1. Calculate the total allowable range: 3.5 − 3.0 = 0.5 inches.
  2. Calculate how far the measured value sits above the minimum: 3.3 − 3.0 = 0.3 inches.
  3. Divide the above-minimum amount by the total range: 0.3 ÷ 0.5 = 0.60.
  4. Convert to percentage: 0.60 × 100 = 60%.
  5. The measured value is within limits and sits 60% of the way through the allowable range from the minimum.
Answer: 3.3 inches is within limits and is 60% of the way from the minimum to the maximum of the allowable range.

The math in this lesson1

Learners compare oleo strut extension and door clearance readings to AMM limits and compute how far out of tolerance they are.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Perform a landing gear retraction test on the training aircraft or full-scale trainer using the manufacturer's functional test procedure; observe sequencing, door clearances, and indicator light operation, and record all measurements and observations on the work order. · Remove, inspect, and reinstall an oleo-pneumatic strut; check strut extension, fluid level, and nitrogen servicing per the aircraft maintenance manual; compare measured values to AMM limits and annotate the service sheet. · Complete a simulated FAA-format maintenance logbook entry for the landing gear inspection task, including aircraft make/model/serial, date, description of work, reference to manual, part numbers, and the required regulatory return-to-service statement.
Materials: Training aircraft or landing gear functional trainer, aircraft maintenance manual, hydraulic service cart, nitrogen servicing cart, strut extension gauge, drip pan, torque wrench set, logbook entry form (FAA format), PPE (safety glasses, nitrile gloves)
Check: All AMM functional test steps completed in sequence with correct measurements recorded; strut extension and fluid level within AMM limits; logbook entry includes all required elements per 14 CFR Part 43—instructor rates entry complete/incomplete; minimum satisfactory on all three activities

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Airframe — Landing Gear
1. Why must the landing gear doors fully open before the gear begins to retract during a retraction test?
Airframe — Landing Gear
2. What gas must be used to service an oleo-pneumatic landing gear strut, and why is oxygen prohibited?
Airframe — Landing Gear
3. A cockpit landing gear indicator shows a red light after the gear handle is moved to the DOWN position and sufficient time has passed. What does this most likely indicate?
Airframe — Landing Gear / Safety
4. Before loosening any fitting or valve on a pressurized oleo-pneumatic strut, the technician must first:
Regulations — 14 CFR Part 43
5. Which of the following items is required in a maintenance logbook entry to meet 14 CFR Part 43 requirements?
Airframe — Landing Gear
6. An oleo strut measures 1.8 inches of extension. The AMM specifies a minimum of 2.5 inches and a maximum of 3.0 inches. What is the correct action?
Airframe — Landing Gear / Safety
7. During a retraction test, which safety precaution is most critical before hydraulic power is applied to cycle the gear?
Airframe — Landing Gear
8. An up-lock on a retractable landing gear system primarily serves what function?
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