Definition
Hard-time is the oldest and most conservative maintenance control method. A component assigned to hard-time has a defined maximum permissible service life — expressed in flight hours, cycles, calendar time, or a combination — at which it is mandatorily removed from service for overhaul or replacement, without exception and regardless of apparent condition. The classic example is the piston engine Time Between Overhaul (TBO): a Lycoming IO-360 has a manufacturer-recommended TBO of 2,000 hours and 12 calendar years. At that interval, the engine is removed and sent to an overhaul facility whether it passed its last borescope inspection or not. Life-limited parts (LLPs) are always hard-time — FAA §39 and EASA airworthiness directives mandating LLP retirement at specific cycles are the regulatory instrument that makes the hard-time limit compulsory rather than merely recommended. Engine disc replacements at manufacturer-specified cycle limits (e.g., a GE CFM56-5B fan disk at 20,000 cycles), landing gear structural LLPs, certain helicopter rotor-head components — these have no on-condition option because the failure mode (uncontained disc failure, fatigue crack propagation to structural failure) does not provide adequate warning before a catastrophic event.
On-condition means the component remains in service as long as each periodic inspection demonstrates it meets specified serviceability standards — dimensional tolerances, surface condition, crack-free status, functional performance within limits — defined in the component's maintenance documentation. The decision to remove the component is triggered by a condition finding (a measured dimension outside tolerance, a crack visible in NDT inspection, a functional test failure), not by reaching a fixed interval. On-condition maintenance is appropriate for components whose deterioration is detectable before failure, whose failure is survivable or recoverable at any progression stage, and where inspection capability is cost-effective relative to hard-time overhaul. Structural components inspected by eddy-current NDT, turbine blades inspected by borescope, hydraulic pumps bench-tested at intervals — these are on-condition items under most Approved Maintenance Programmes.
Condition-Monitored Maintenance (CMM) is a third category defined in ICAO Doc 9859 Appendix to Chapter 6 and referenced in the MSG-3 methodology (Airline/Manufacturer Maintenance Program Development Document, Revision 2015.1, produced by the Air Transport Association): the component is allowed to operate to functional failure, with the organization monitoring fleet-wide failure rates and times-to-failure to determine if the failure rate is acceptably low and predictable, and without specific preventive maintenance. CMM is appropriate only for components where failure has no direct adverse effect on safety and where the economic cost of unexpected failure is low — consumable equipment, some cabin items, non-critical avionics accessories.
The Approved Maintenance Programme (AMP) under EASA Part-M M.A.302, or the Continuous Airworthiness Maintenance Program (CAMP) under FAA §121.367 or §135.411, defines the maintenance control method for every component in the operator's fleet. The method is determined through the Maintenance Steering Group (MSG-3) logic documented by the aircraft manufacturer or through individual component assessments. For flight training operators, the AMP or CAMP is typically based on the manufacturer's Maintenance Review Board (MRB) report — for EASA, the MRB becomes the basis for the Maintenance Planning Document; for FAA, it becomes the basis of the approved maintenance program through the Principal Maintenance Inspector's approval.
Reliability programs under FAA §121.373 (Continuing Analysis and Surveillance System, CASS) and EASA AMC M.A.302(d) allow operators with sufficient fleet size and data to propose adjustments to maintenance intervals — including shifting components between on-condition and hard-time, or extending on-condition inspection intervals — based on demonstrated in-service reliability data. An operator whose reliability data shows an on-condition component consistently passing inspections with significant margin until very late in an interval may have grounds to extend the interval. Conversely, consistently early on-condition removals may indicate the interval is too long for that operator's environment.
Why It Matters for Flight Schools
For flight schools and ATO operators, hard-time limits are the non-negotiable scheduling constraints that dominate maintenance planning. A Cessna 172 engine approaching TBO must be scheduled for removal at or before TBO, and that removal takes the aircraft out of service for the duration of the overhaul — typically two to four weeks for a typical GA piston engine. Failure to plan this in advance results in an aircraft grounded at TBO with no replacement, a revenue impact from the lost training capacity, and potential pressure to extend the TBO beyond the manufacturer-recommended limit. For FAA Part 91 operators, piston engine TBO is manufacturer-recommended rather than legally mandatory (the FAA does not mandate compliance with GA piston TBO under Part 91), but most flight schools' insurance policies and lender financing agreements do require compliance. For EASA Part-M operators, the AMP-defined interval is mandatory.
On-condition items create a different planning challenge: the timing of removal is unpredictable. A landing gear component that is on-condition may pass ten consecutive inspections and fail the eleventh — the removal is triggered by the inspection finding, not by the calendar. The operational risk is that an on-condition removal of a long-lead-time component can ground an aircraft for an extended period if the replacement part is not in stock or is not available from the supply chain at short notice.
How Aviatize Handles This
Aviatize's maintenance control module models each component in the aircraft's AMP with its control method — hard-time or on-condition — and its associated parameters. Hard-time components carry their current hours/cycles/calendar accumulated values against their limit, with projected-to-limit calculations based on current utilization rates. The platform generates lead-time alerts at configurable thresholds (e.g., 100 hours before TBO, 90 calendar days before calendar limit) so that maintenance scheduling and parts procurement can begin in advance of the compliance deadline. For fleet-wide campaigns — an AD requiring engine replacement within 500 hours on all aircraft of a given type — the hard-time tracking view shows every affected aircraft's current status and remaining interval in a single screen, enabling triage by urgency.
For on-condition components, Aviatize tracks the inspection schedule and the inspection outcome history. Each inspection result is logged against the component record — pass, pass with observation, marginal pass, fail — with the finding details and the next inspection due date set. When an on-condition inspection produces a marginal or fail result, the platform generates a work order for the corrective action and flags the component for elevated monitoring frequency. Over time, the accumulated inspection history for on-condition components feeds into the KPI reporting and dashboards module as a fleet reliability dataset — enabling operators who qualify for reliability programs to build the documented evidence base that CASS and EASA AMC M.A.302(d) require.