Adding Time to Your TBO: A discussion on PT6 engine STCs

Sept. 1, 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:

  • Overhaul the engines at the P&WC recommended hours
  • Request a recommendation from P&WC to extend TBO
  • Develop a maintenance program
  • Use an FAA STC to extend TBO

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:

  • Accessories
  • Compressor Module
  • Hot Section Module
  • Power Section Module

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 at its address at right.

More on the MORE program Heritage Turbines will be hosting a PT6A maintenance school in Hyannis, MA, on Nov. 5-6, 2003. The school is open to airlines, FBOs, mechanics, pilots, and government agencies. The school covers many aspects of PT6A engine maintenance including line maintenance, 100-hour checks, basic troubleshooting, and hot section inspections. Included in the school, TBO extensions, escalating time extensions, the MORE program and P&WC on condition program will be discussed. For more information you can call Heritage at (888) 621-7788 or visit its website at

Additional ReSource
Maintenance On Reliable Engines
1132B Airport Road
Minden, NV 89423
(775) 782-3346