When it comes to an aircraft’s engines, how can a jet or turboprop operator protect themselves from nasty surprises? Dave Higdon highlights what to focus on when it comes to hourly engine maintenance programs and the maintenance schedules themselves…
Few things cause more concern among buyers of used business jets and turboprops than the thought of an unexpected maintenance event and the associated costs. Of particular concern is the health of the powerplant, and this is the main reason savvy operators enroll their aircraft's engines on an hourly maintenance program.
The security these programs offer holds sway in the marketplace, too, allowing operators to command higher prices when selling an existing aircraft that’s enrolled on a program. The financial stability and operational security these plans afford the seller typically transfers to an aircraft’s new owners, with all of the same benefits.
These programs come in varying forms, but the goal is essentially the same: Help owners and operators to avoid any expensive surprises from their aircraft. But what are the surprises that could await an unwary operator? Over the following paragraphs, we’ll examine some of the more common ones.
Missed Replacement of Rotables
Jet engine OEMs help ensure a long life for their engines through periodic inspections. As the engine advances in hours and cycles, a tear-down to assess the engine's condition becomes necessary, along with the replacement of any rotable components.
Rotable components are an inventory item that can be repeatedly and economically restored to a fully-serviceable condition. The servicing method is to swap out the component near the end of its life cycle or on failure. If that removed component can be restored to a serviceable condition it will be repaired and kept for another exchange.
Not all operators or maintenance shops always get it right when keeping up with the removal and replacementof rotable parts, however – particularly when their replacement cycle is on-condition. The same is true for periodic inspections performed via a borescope. It's rare, it’s a bad practice when it happens, but it does sometimes happen.
Incomplete Aircraft Logs
Some of the most expensive surprises may be avoided through a detailed inspection of an aircraft's airframe and powerplant logs. Depending on the age and life of the aircraft, such an inspection may involve multiple logbooks or electronic records, however. Ideally this logbook review occurs before a buyer signs a contract to purchase a used aircraft.
While it's not common, incomplete logs occur for different reasons. The easiest way to invite expensive surprises is failure to account for – and completely review – the aircraft logs, with a particular focus on service bulletins (SBs), airworthiness directives (ADs) and updates to the maintenance manuals for that particular engine.
Such an examination should be considered mandatory before the purchase of an aircraft, and as part of the preparation for enrolling an engine in an hourly maintenance program. No vendor is likely to accept an engine into its program without passing this baseline examination.
A seller's inability to provide complete logs should be considered a red flag warranting deeper examination of the aircraft itself, and its engines especially.
The Need for Routine Maintenance Adherence
Every engine OEM ships its product with specific instructions for the use and upkeep of the engine, along with an outline of periodic maintenance requirements and the time cycles during which these inspections and associated work should occur.
The manuals always cover the approved maintenance program and the cycles during which certain parts should be replaced (typically well ahead of any expected fatigue failures). Depending on the powerplant, those replacement cycles may be hours-based, cycle-based, calendar-based or even condition-based.
‘On-condition’ means the part can continue in service when a periodic inspection reveals no flaws, allowing the component to continue in service (sometimes almost indefinitely). Modern turbine engines for business aircraft boast remarkable inspection periods, and today’s modern powerplant can even exceed 4,000 hours.
In the life of a typical business jet – flying an average 350 hours annually – a 2,000-hour inspection cycle would arrive after approximately every 5.7 years of flying, and about 11 years between overhaul limits.
Even then more routine maintenance will still be needed. That means oil changes, filter checks and changes if needed, oil-level upkeep, heat exchangers cleaning.
A popular addition to the preventive maintenance available for jets is the internal engine wash. This process cleans the interior rotating components of debris, soot and other contaminants which can reduce engine performance and efficiency.
An engine operated within specifications, with no exceedances of temperatures or RPM could theoretically continue on the airframe for the expected life of the engine with only minimal maintenance costs or downtime.
It is, however, vital for anybody buying a business jet or turboprop on the used aircraft market to ensure the appropriate maintenance procedures have been adhered to.
So many components of a modern turbofan engine endure such extreme conditions that it's difficult to point out all of them. But engine OEMs mandate inspection cycles precisely because they help ensure the powerplant's full life.
Inlet Fan Blades: Subject to atmospheric temperatures and the impact of foreign objects (birds, dust, sand, volcanic ash, ice, snow and rainwater) these components come in for periodic inspection in fine detail to check for cracks which could lead to blade failure.
Compressor Stages:Unless something large gets past the inlet fan, the compressor stages should encounter nothing but air; air that increases in pressure and temperature with every succeeding stage. These are a routine object of detailed inspection and any damage detected will likely mean a complete tear-down.
Combustion Chamber: This is the metal can behind the compressor where the mixture of air and fuel feed a constantly burning blue flame producing the extreme pressures jet engines use to thrust the aircraft forward. Any crack or flaw in the combustion chamber can lead to a catastrophic failure, loss of the engine, or even the aircraft.
Power Turbine Wheels:Sitting in the flow of that blue flame, these turbine wheels spin at extreme speeds to drive the inlet fan, the compressor stages and in turn components such as starter/generators, oil pumps and more. A failed turbine blade can fly away at bullet speeds – fast enough to penetrate containment shields, the fuselage, the pressure vessel and wing components.
Maintenance Programs: What to Look For
Engine OEMs all stress the importance of adhering to the inspection cycles, the schedule for replacing time-limited components and rotables, as well as maintaining oil replenishment and replacement cycles. To ease the risks, vendors and other third-party providers offer hourly maintenance programs.
An operator considering enrolling an airplane (whether a powerplant or the whole aircraft) on one of these programs should thoroughly examine the coverage a program offers.
Just as the range of unhappy surprises is kept to a minimum through detailed examination of an aircraft’s logs, the key to a satisfactory experience with an hourly maintenance program begins with understanding what's covered, what's isn’t, and the operator's maintenance and record-keeping prior to signing the contact.
- Does the coverage include removal and replacement? Or only the work on the engine itself? The difference could be expensive.
- Does the coverage include a loaner engine in the event an engine must be removed for required work? Will the same coverage include the cost of shipping the engine to the overhaul facility?
- What accessories (if any) are covered - and to what extent?
- Does the coverage include overhauls, or only hot-section inspections?
- What happens after the engine returns to service on the airframe? Maintenance experts caution that particular care should be exercised with any engine reinstalled after a major overhaul or inspection tear down.
As durable as they may be, there is plenty of scope for a powerplant to become a source of problems. It's when such problems occur that the true value of an hourly engine maintenance program will become apparent to the operator.