{
  "slug": "engine-reserve-fund",
  "title": "Engine Reserve Fund Calculator",
  "heading": "Engine Overhaul Reserve Fund Calculator",
  "category": "financial",
  "url": "https://www.revenuelab.fyi/toolbox/engine-reserve-fund",
  "summary": "How much to set aside per flight hour to fund the next overhaul.",
  "description": "An engine reaching TBO (time between overhauls) without a funded reserve forces owners into an unplanned $25,000-45,000 expense at the worst possible time, right when the airplane is grounded. This calculator works backward from overhaul cost and remaining hours to TBO to compute the per-hour reserve rate needed, and shows how the fund balance grows if you actually set the money aside rather than spending it. It also flags whether an engine flying below roughly 30-40 hours per year, which risks corrosion-driven early overhaul rather than a clean run to TBO.",
  "formula": "Reserve rate per hour = overhaul cost ÷ hours remaining to TBO. Fund balance after n hours = reserve rate × hours flown.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=engine-reserve-fund",
  "inputs": [
    {
      "id": "overhaulCost",
      "label": "Expected overhaul cost",
      "kind": "number",
      "hint": null,
      "default": 32000,
      "unit": "$",
      "min": 5000,
      "max": null
    },
    {
      "id": "currentHours",
      "label": "Current hours since last overhaul",
      "kind": "number",
      "hint": null,
      "default": 800,
      "unit": null,
      "min": 0,
      "max": null
    },
    {
      "id": "tbo",
      "label": "TBO (hours)",
      "kind": "number",
      "hint": null,
      "default": 2000,
      "unit": null,
      "min": 500,
      "max": null
    },
    {
      "id": "hoursYear",
      "label": "Hours flown per year",
      "kind": "number",
      "hint": null,
      "default": 100,
      "unit": null,
      "min": 5,
      "max": null
    }
  ],
  "outputs": [
    {
      "id": "reservePerHour",
      "label": "Reserve to set aside per hour",
      "format": "currency",
      "hint": null,
      "primary": true
    },
    {
      "id": "annualReserve",
      "label": "Annual reserve amount",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "remaining",
      "label": "Hours remaining to TBO",
      "format": "number",
      "hint": null,
      "primary": false
    },
    {
      "id": "yearsToTBO",
      "label": "Years to reach TBO at this pace",
      "format": "decimal",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Expected overhaul cost: 32000 $",
      "Current hours since last overhaul: 800",
      "TBO (hours): 2000",
      "Hours flown per year: 100"
    ],
    "outputs": [
      "Reserve to set aside per hour: $26.67",
      "Annual reserve amount: $2,667",
      "Hours remaining to TBO: 1,200",
      "Years to reach TBO at this pace: 12"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter expected overhaul cost ($).",
      "Enter current hours since last overhaul.",
      "Enter tbo (hours).",
      "Enter hours flown per year.",
      "Read your reserve to set aside per hour on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "overhaulCost": 19000,
        "currentHours": 475,
        "tbo": 1200,
        "hoursYear": 60
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "overhaulCost": 32000,
        "currentHours": 800,
        "tbo": 2000,
        "hoursYear": 100
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "overhaulCost": 51000,
        "currentHours": 1275,
        "tbo": 3200,
        "hoursYear": 160
      }
    }
  ],
  "limitations": [
    "Results are estimates before tax, fees, and inflation unless an input explicitly covers them.",
    "Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.",
    "This is educational maths, not financial advice. Check anything contractual with the lender or your accountant."
  ],
  "faq": [
    {
      "q": "Why does low annual usage matter for engine health?",
      "a": "Engines that sit idle for weeks between flights are more prone to internal corrosion on cam lobes, lifters, and cylinder walls than engines flown regularly, because moisture from combustion byproducts doesn't fully evaporate off internal surfaces without sustained heat. Many mechanics see engines flown under 40 hours/year require overhaul on corrosion grounds well before reaching TBO hours."
    },
    {
      "q": "Is TBO a hard limit I must meet?",
      "a": "For most Part 91 private operations, TBO is a manufacturer recommendation, not a mandatory replacement time — you can legally run past TBO if the engine passes inspections (compression checks, oil analysis, borescope) and remains airworthy. Commercial operations under certain Part 135/121 contracts, and some insurance policies, may require overhaul at or before TBO regardless of condition."
    },
    {
      "q": "Does this account for prop overhaul too?",
      "a": "No — this is engine-only. Propeller overhaul is a separate expense on its own TBO cycle (often shorter, like 2,000 hours or 6 years calendar limit, whichever comes first, for many fixed-pitch or simple constant-speed props), typically running $2,500-6,000, and should get its own reserve line alongside the engine reserve."
    },
    {
      "q": "What if I buy an aircraft mid-cycle on the engine?",
      "a": "Set current hours to the logbook time since major overhaul (SMOH), not total airframe time, since that's what determines remaining life to TBO. If the engine's maintenance and oil analysis history looks rough for its hours, consider a shorter effective remaining-hours estimate than the raw TBO math suggests to build reserve faster."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/aircraft-ownership-cost-per-hour",
    "https://www.revenuelab.fyi/toolbox/annual-inspection-cost-share",
    "https://www.revenuelab.fyi/toolbox/aircraft-leaseback-roi"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Engine Reserve Fund Calculator (https://www.revenuelab.fyi/toolbox/engine-reserve-fund)"
}