Skip to content

Corrective maintenance

Summary

Corrective maintenance repairs known damage or component failure. Unlike scheduled maintenance (interval-based), corrective maintenance is triggered by a finding from the post-flight check or post-crash inspection. The most common corrective tasks are arm shaft replacement (5 minutes), propeller replacement (2 minutes), and motor mount O-ring replacement (15 minutes). The arm shaft is designed to fracture on impact — finding a fractured shaft is a success of the failure hierarchy, not a build defect.

Concept

The failure hierarchy as maintenance guidance

libdrone's failure hierarchy deliberately routes crash energy toward the arm shaft first, protecting the electronics and PCCF layers. When a crash occurs, the arm shaft is expected to fracture. This is the design working correctly. Corrective maintenance after a crash therefore focuses on the arm shaft and motor before inspecting anything deeper.

The hierarchy: bumpers (absorb first impact) → arm shaft (fracture fuse) → motor (bend or strip threads before transmitting force to PCCF) → PCCF layer (last resort — structural damage here grounds the drone for full inspection).

A crash where only the arm shaft fractured is a best-case outcome. A crash where the PCCF layer cracked requires more thorough inspection and is a more serious airworthiness event.

Reference

Common corrective tasks and time estimates

Task Trigger Time
Prop replacement Any chip, crack, or deformation 2 min
Arm shaft replacement Lateral play at T-lock, visible fracture 5 min
Motor mount O-ring replacement Cracking, deformation, or post-crash 15 min
Motor replacement Non-spinning, hot, or damaged bearing 20 min
Capacitor replacement Visible damage, cracked body 30 min (resolder)
Conformal coating repair Chip or peeling after crash or repair 30 min + 24h cure

Arm shaft replacement procedure

  1. Remove battery. Disarm. Remove props on the affected arm.
  2. Disconnect MR30 motor connector.
  3. Remove 2× M2 screws at the arm tab (accessible from the arm root).
  4. Slide the arm shaft out of the T-slot along the arm axis.
  5. Inspect the T-slot walls for cracking. Run a finger along the inside of each wall. Any crack felt or visible → do not reuse this T-slot. Ground the drone and consult the repair procedure for PCCF layer replacement.
  6. Print a replacement arm shaft (PETG, standard profiles, ~20 minutes).
  7. Slide new shaft into T-slot. Verify tab seats fully with no rocking.
  8. Install 2× M2 screws finger-tight + 1/4 turn.
  9. Reconnect MR30. Install new props on this arm.
  10. Acoustic ping: tap the CF rods — all should ring. A rod that thuds has loosened — tighten the pinch bolt.

Motor mount O-ring replacement

→ See scheduled-maintenance for the complete O-ring replacement procedure. Corrective replacement after a crash follows the same procedure.

After corrective O-ring replacement, always re-torque the motor mount screws (0.4–0.5 N·m cross-pattern) and verify the passive cover does not contact the arm head except at the O-ring contact zones.

Prop replacement

  1. Remove the prop nut (CCW prop = CW nut thread; CW prop = CCW nut thread).
  2. Pull the prop straight off the motor shaft.
  3. Inspect the motor shaft for bending: roll it on a flat surface. Any wobble = bent shaft = motor replacement required.
  4. Fit new prop. Torque nut to 0.8–1.0 N·m. Verify prop seated fully.
  5. Run balance check on magnetic balancer before first post-replacement flight.

Procedure

Post-crash triage sequence

Run in this order — stop at the first hard finding:

  1. Props — visual inspect all 4. Replace any damaged prop before anything else.
  2. Arm shafts — press each arm laterally. Any play = fractured shaft. Replace before continuing.
  3. Motor mount O-rings — inspect for tearing or displacement on the affected arm. If torn: replace before continuing.
  4. CF rods — acoustic ping all 4. Dull sound = loosened rod. Tighten pinch bolt.
  5. T-slot walls — run finger inside the T-slot of any arm that experienced the crash impact. Any crack = ground the drone.
  6. Electronics — visual inspect all connectors, capacitor, and solder joints on the FC and ESC. Any loose joint or dislodged component — address before powering on.
  7. Conformal coating — if any chip or crack in the coating over the FC or ESC, repair before next flight in humid conditions.

Rationale

Why arm shaft replacement does not require special tools

The T-lock system was designed for field-replaceable arms with no tools beyond a 1.5mm hex key (for the M2 screws). A repair kit for field deployment is: two pre-printed spare arm shafts, an M2 hex key, four spare props, and four M5 prop nuts. Total mass: ~80g. A drone grounded by a fractured arm at a deployment site can be airborne again in under 10 minutes.

Connections

requires: - failure-hierarchy - arm-shaft - floating-motor-mounts related: - scheduled-maintenance - post-flight-check - coupon-validation leads_to: - scheduled-maintenance