Preventive and corrective maintenance

Why this matters

A car’s service book says to change the oil every 10,000 kilometres or every 6 months, whichever comes first. Drones need the same system. A propeller with a hairline crack or a bolt slowly loosening from vibration gives no warning until something happens. Good maintenance finds problems before they cause incidents, and records every job so it can be traced.

Two kinds of maintenance

• Preventive maintenance is done on a schedule to stop parts failing in service, such as inspecting propellers every 5 flight hours
• Corrective maintenance is done when a defect is found, from a pre-flight check, a pilot report or the diagnosis in module 3

Maintenance intervals can be set in three ways, used together:

Real intervals must come from the manufacturer’s manual for that model. ASTM F2909, the standard on continued airworthiness of lightweight unmanned aircraft systems, likewise relies on maintenance information from the manufacturer. The figures in this lesson are hypothetical values for practice.

from datetime import date

now = {"hours": 48.6, "cycles": 131, "date": date(2026, 9, 27)}
tasks = [  # task, last done (hours, cycles, date), interval (hours, cycles, days); None = unit not used
    ("propeller inspection", (45.0, 118, date(2026, 9, 10)), (5, 20, 30)),
    ("motor bearing check", (25.0, 70, date(2026, 7, 1)), (25, None, 90)),
    ("frame bolt torque check", (40.0, 100, date(2026, 6, 20)), (20, None, 90)),
    ("firmware and parameter review", (30.0, 90, date(2026, 8, 1)), (None, None, 180)),
]
for name, (h0, c0, d0), (dh, dc, dd) in tasks:
    left = []
    if dh:
        left.append(("h", round(h0 + dh - now["hours"], 1)))
    if dc:
        left.append(("cycles", c0 + dc - now["cycles"]))
    if dd:
        left.append(("days", (d0 - now["date"]).days + dd))
    status = "DUE" if any(v <= 0 for _, v in left) else "ok"
    print(f"{name:<30} {status:<4} " + ", ".join(f"{v:+g} {k}" for k, v in left))

propeller inspection           ok   +1.4 h, +7 cycles, +13 days
motor bearing check            ok   +1.4 h, +2 days
frame bolt torque check        DUE  +11.4 h, -9 days
firmware and parameter review  ok   +123 days

The job card

A job card links the assigned task, the steps from the manual, the inspection results and the person authorised to sign off, in one document.

A good job card states:

• Aircraft identity: serial number, firmware version and the manual revision used
• Steps and specified values: citing the manufacturer’s manual, such as torque values and test criteria. No generic torque value can replace the manual for every model
• Parts: part numbers and serial numbers (S/N) of both the removed and the installed parts
• Results and signatures: the person doing the work and the inspector should be different people, as far as resources allow
• Functional test after the repair, before return to service

The Thai CAAT guidance for developing an operations manual requires a maintenance log and a functional test after maintenance before the aircraft is used again. If key information is missing, such as a part number or test criteria, the job must be held. Never sign a job off in place of an actual inspection.

Reliability figures

With enough records, you can calculate fleet indicators. The observed event rate is the number of events divided by operating hours, and availability is the fraction of time the aircraft was ready for service.

events, operating_hours = 2, 100
up_hours, down_hours = 90, 10
print(f"observed event rate {events / operating_hours:.2f} per hour "
      f"({events / operating_hours * 100:.0f} per 100 h)")
print(f"observed availability {up_hours / (up_hours + down_hours):.0%}")

observed event rate 0.02 per hour (2 per 100 h)
observed availability 90%

The NIST engineering statistics handbook separates repairable systems from non-repairable parts. For a repairable system, the rate to track is the rate of occurrence of failures over time (ROCOF), which may rise as the aircraft ages, so do not simply invert the rate and call it “mean time between failures”. Finding no events in 100 hours does not prove the risk is zero. You must also define what counts as an “event” and how repeated alarms from one event are counted.

Module lab

Common mistakes

Summary

• Preventive maintenance follows a schedule; corrective maintenance follows a defect; both end with a functional test
• Intervals can use hours, cycles and days together, and are due when any unit is reached first
• A job card cites the manufacturer’s manual and records parts, results and signatures; hold the job if information is missing
• Event rate and availability are observed values that need definitions and limits stated

Check your understanding

1. A task is due every 10 hours or 60 days. It was last done at 20 flight hours, 50 days ago. The aircraft now has 29 hours. Is it due?
2. In question 1, if 12 more days pass with only 0.5 more flight hours, is it due?
3. 3 events occur in 150 operating hours. What is the observed event rate?
4. An aircraft is ready for 170 hours and not ready for 30 hours. What is its availability?
5. A job card has no post-repair test criteria. What should you do?