AIRCRAFT MAINTENANCE TECHNOLOGY
SEPTEMBER 2003
A discussion on PT6 engine
STCs
By Joe Escobar
The Pratt & Whitney
PT6 engine is in wide use today. Since its introduction in 1964, the PT6 family
of engines has achieved more than 247 million flying hours. As with any engine,
there are costs associated with scheduled and unscheduled maintenance of the
engine. The most expensive part of PT6 engine ownership can be the engine
overhaul. Because of this, some owners and operators are looking at ways to
extend their overhaul intervals. We will look at one option available to do just
that.
There are basically four choices when it comes to your PT6 engine overhaul:
In this article, we will
focus on the fourth option - using an STC to extend the engine TBO.
Using an FAA STC to extend TBO
For mechanics that aren't familiar with a TBO extension STC, it can seem like an
odd concept. Most STCs we are familiar with involve a change to the aircraft or
engine - installing a Raisbeck wing locker on a King Air for example.
But a TBO extension STC is a little different. Maintenance On Reliable Engines
(MORE) has developed an STC for an engine maintenance regimen designed for PT6
engines. It states that its STC is designed to extend life, reliability, safety,
and reduce the cost of operation and ownership. But what is unique is that there
are no actual changes to the engine. The program revolves around a change in
inspection intervals and maintenance requirements.
How it works
In the most basic sense, the MORE STC is an aggressive inspection system. The
STC increases the frequency and thoroughness of routine engine inspections. The
purpose is to find problems in their early stages. These problems then can be
corrected promptly. This early correction of engine problems leads to an
increase in the engine overhaul interval. The five main elements of the MORE STC
are:
Required inspections. An initial inspection is performed. This
establishes an initial baseline and shows the present condition of the engine.
An FAA Form 337 is required to incorporate the STC along with a notice to the
FAA informing them that the engine will be using the STC for the maintenance of
the engine per FAR 91.409. If the aircraft is transferred to another owner, a
notice to the FAA is required at the same time the new owner registers the
aircraft.
Engine performance information. Performed at regular intervals, engine
performance monitoring allows you to remain informed if an engine needs a
repair. It evaluates the efficiency of the engine on a scheduled basis and is
able to detect gradual changes over that time that will help pinpoint problems
while the corrective actions are small and less expensive.
Oil and oil filter analysis. These are performed at scheduled time
intervals. Oil and filter analysis can help users detect excessive wear of
components and can provide early warnings of impending failure of engine parts.
Sampling and filter analysis is performed using the MORE company's filter
analysis tool kit and oil sampling kit. Debris is removed from the oil filter
and analyzed. The filter is then cleaned in accordance with the P&WC manual.
Engine vibration analysis. Propellers are balanced on a regular basis.
Vibration analysis is performed and charted on cards. Specific speeds and RPMs
are then reviewed and identified to the corresponding speeds of critical parts
in the engine. Readings are compared from one inspection to the next and any
changes are noted and corrected. This can help reduce deterioration or rotating
parts that can be caused by excessive vibration.
Borescope inspections. Periodic borescope inspections of the internal
parts of the engine hot section and compressor are used to identify engine
problems in their early stages and to enable corrective action. Borescope
inspection of internal parts without the need to disassemble the engine allows
for the hot section to operate "on condition."
Modular techniques
PT6 engines that are maintained under the MORE Instructions for Continued
Airworthiness are separated into modules for record keeping purposes. This is
due to the fact that experience has shown that certain portions of the engine
need inspection/repair more often than others. Separating the engine into
modules allows each module to be repaired or overhauled when necessary. If the
remainder of the engine is operating normally, this allows the remainder of the
engine to remain in service until there is a reason for it to be repaired or
overhauled. The modules in the MORE Instructions for Continued Airworthiness
are:
The following example is
given on how the modular technique works:
An engine with 4,000 hours time since overhaul (TSO) experiences a lightning
strike. The power section module requires repair, but the owner decides to
overhaul it instead. The power section module will then have 8,000 hours before
overhaul is required again. In another 4,000 hours, the other three modules are
removed for overhaul, but the power section module will not need overhaul since
it has 4,000 hours to go before the next overhaul.
Taking an engine off of the MORE program
If for some reason, an owner wants to take his engine off of the MORE program,
what would it entail? A logbook entry? Well, it is not that simple. Since the
program is an STC, placing the engine on the program (or taking it off the
program) requires an FAA Form 337.
Twin engines
In a twin engine aircraft, it is possible to place only one engine on the MORE
program. However, it is not recommended. It is hard enough to keep up with
maintenance requirements and inspection due items when both engines are on the
same program. When engines are on different programs, this task becomes even
more difficult. This can lead to an inadvertent overlook of an inspection item
or maintenance requirement. If you are considering putting an engine on the MORE
program, it is beneficial to put both engines on it to avoid any potential
problems.
Become familiar with MORE
Under the MORE program, the inspections required and time between inspections
can vary significantly from P&WC requirements, so it is important to be
familiar with them to ensure that they are complied with.
Peter Boissonneault of Hyannis, MA-based Heritage Turbines explains the
importance. "If someone doesn't comply with a required inspection under the
MORE program, the aircraft is basically unairworthy. We run into problems
sometimes here at our shop. For example, an FBO may pull a set of nozzles off an
aircraft and send them in to us. They just send them in for a maintenance manual
flow check. But they don't tell us their engine is under the MORE program. Under
the MORE program, the nozzles are required to be overhauled at set intervals.
They are tracked. So we can send them a set of nozzles back that would be
airworthy on another PT6 engine, but can't be installed on their MORE engine.
Mechanics need to be aware of these things when sending components out to be
inspected."
Even if you don't have any aircraft on the MORE program, it is beneficial to
learn about how it works in case you are faced with working on a MORE PT6 engine
at your facility.
This has been a brief overview of the MORE program, one option for extending TBO
on your engine. For more information, you can contact MORE.