When Should You Do It?
Traditionally, an HSI is required when the engine reaches the halfway point of its time between overhauls (TBO). If
the TBO is 4,000 hours, an HSI should be scheduled at 2,000 hours of operation. This is known as a
‘hard-time’ HSI. The TBO of an engine is specified in its maintenance manual.
However, thanks to the increasing sophistication of engine health
monitoring systems in recent years, including technologies like P&WC’s FAST™ solution,
it’s become common for HSIs to be done ‘on condition.’ This means that, rather than being
performed after a fixed number of hours, an inspection only happens once the data indicates that it’s
“We monitor parameters like the inter-turbine temperature [ITT], the speed of the compressor and fuel
consumption,” explains Valdemar Porto, Customer Manager, Regional Turboprops at P&WC. “When we see a
trend of deterioration in performance, that tells us exactly when to do the HSI.”
While an HSI is usually done as a preventive, planned maintenance activity, unscheduled inspections are sometimes
needed. For instance, if the battery is not in good condition when the engine starts up, it can result in an
‘overtemp’ (high ITT) situation, warranting an unscheduled HSI.
How is It Done?
For a small aircraft engine like a PT6A, an HSI is a relatively quick and simple procedure that can be done on-wing.
It simply requires opening up the hot section and examining the components using standard tools, plus any
specialized tools indicated in the technical manual.
But, this approach is not possible for all engines. For instance, the way the larger PW100 turboprop engine is
installed in the nacelle means it has to be removed and sent to the shop for an HSI.
Any certified technician can do an HSI. That means somebody who has taken a course from P&WC and
received approval from the local aviation authority, such as Transport Canada or the FAA in the United
- VALDEMAR PORTO, CUSTOMER MANAGER, REGIONAL TURBOPROPS, P&WC
Why is It Required?
There are many excellent reasons to perform an HSI: It helps make sure that the aircraft engine will continue
delivering the expected power and performance. It will increase the engine’s durability. And it can reduce
maintenance costs by providing operators with more control over their maintenance activities.
A recent HSI is also an asset when selling a used aircraft. An engine that is ‘fresh since HSI' or
‘fresh since overhaul’ is more valuable than one which has not been inspected recently.