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Much to the chagrin of many of the World’s Gas Turbine Aero Engine (GTAE) Overhaul and Repair facilities- we are currently living in a period of aviation history that often deems it cheaper for an aircraft owner to purchase a used GTAE from a reputable source in-place of going through the greater expense of overhauling or repairing their own engine.

AvBuyer   |   1st March 2010
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Pre-owned Aircraft Engines:
The idiot’s guide to what you’re buying.

Much to the chagrin of many of the World’s Gas Turbine Aero Engine (GTAE) Overhaul and Repair facilities- we are currently living in a period of aviation history that often deems it cheaper for an aircraft owner to purchase a used GTAE from a reputable source in-place of going through the greater expense of overhauling or repairing their own engine.

Even though this is a relatively new philosophy to many of the owner/operators today- it still flies in the face of expected logic. The reason for this reversal in budgeting strategy is the significant drop in fair-market-value that has been experienced by the entire used aircraft fleet in the past fifteen months- or so. Aircraft values have dropped pretty evenly spread between forty to sixty percent across the board since the autumn of 2008. For many of the older jet aircraft this value hardship- coupled with the spike in crude oil prices of a year-or-so ago has resulted in them being withdrawn from service- withdrawn from sale- and subsequently culled for their engines- system components and other salvageable items.

An example of this is the Rolls-Royce Spey. The cost to perform a full overhaul is well over one million U.S. dollars per engine. Today it is easy to find a decent Gulfstream II with mid-time engines for less than one million U.S. Dollars for the entire aircraft.

Think of it as a convenient delivery service for your replacement engines - i.e. the donor aircraft that you have purchased flies to your hangar ready to be turned into what I learnt as an apprentice mechanic years ago to call a “Christmas Tree” (it bears presents (spares) for as long as it lasts until all that is left is a bare aluminium skeleton). The purpose of this article is to serve as an “idiot’s guide to what you’re buying” with the hopes that it might steer you away from making some potentially very expensive mistakes during the purchase of a pre-owned GTAE. First let’s cover the issue of the transfer of Maintenance Service Plans.

There are only six GTAE companies that manufacture 99.9% of all engines for all Business Jets. This will eventually change if Russian- Indian or Chinese Companies manage to find a place at this very exclusive dinner table. For now though- these six are (in reverse alphabetical order):

• Williams International
• Rolls-Royce
• Pratt & Whitney Canada (United Technologies) • Honeywell
• General Electric
• CFM (the joint venture between GE and Snecma).

Four of the six companies listed above- have their own series or individual maintenance service plans available for purchase by a GTAE owner/operator. Additionally there are independent service plan companies that will provide coverage on any of the covered engine types- and also coverage for the ones that are figuratively left out in the cold. Without analyzing any single program- the names of which are probably very recognizable to you:br> Powerplan/MSP/JSSI/ESP/TAP/Corp Care- it is very important to verify all of the following from the service plan administrator before you wire money for the purchase of the covered engine…

• Is the program completely paid up-to-date and current- with no issues that will prevent transfer of the entire coverage?
• Did the total number of hours’ flown/paid-for on this GTAE in the past twelve months- comply with the minimum hours contracted at the commencement of the coverage?
• If the GTAE has been stored either on-wing- or in a storage container- did the method of storage comply with the manufacturer’s and/or service plan provider’s required methods and procedures? Are you going to get 100% coverage for every part of the engine- or is there only partial or pro-rated coverage provided and paid-up on this particular GTAE; or if a modular engine- is only a specific module covered?
• Most aircraft salvage companies buy flyable aircraft for engine and parts harvesting. Some of these entities have an up-prepared or gravel strip for the aircraft to fly into for one last flight. One of the engine service plan providers has written language in their service agreement to disqualify any GTAE covered by their plan that is operated on a gravel runway. In the case of you purchasing this subject GTAE from a parts reseller- was the contract still legally in-effect and was it properly transferred to the existing owner who is now selling it to you?

Does it transfer: If you are expecting there to be any kind of coverage from a service plan to transfer after the purchase has been completed- do make certain that you have acceptable answers to the aforementioned list of questions. Why? Well- accrued or paid-in coverage amounts may come to $500-000 (US) or more- and the price that you have paid for the GTAE was probably based upon this transfer coverage amount going with the engine. Of course this is money that you will never see as real cash- but it will pay in-part or in-full for the cost of the next overhaul.

Service Bulletin Status: Another potentially very expensive issue is the Service Bulletin status of the candidate engine. As materials- manufacturing processes and finishing techniques are learned and improved as an engine fleet matures; the fatigue cycle lives of many of the engine components are increased under a Service Bulletin that requires an older component to be replaced with an up-dated one. In some cases the time between overhaul is also increased.

There can be a significant disparity between a GTAE with the latest Service Bulletin compliance status- and one that is low in-service time- but has not undergone an overhaul in a decade- or more. An example of this can be found with the impellors on the Pratt & Whitney JT15 and with some of the power turbine disks on PT6A engines. Different operating applications: Most- if not all of the GTAE models that are in-service within business aviation have also seen service with the military around the world. Unfortunately when like engine models are in circulation within two very different operating applications- i.e. civil and military- there is sometimes a chance that surplus engines and spares come available for purchase by the general public.

The FAA is so concerned with this issue that it believed it necessary to publish an Advisory Circular (No. AC20-142) on this subject- titled: ‘Eligibility and evaluation of U.S. Military Surplus Flight Safety Critical Aircraft Parts- Engines and Propellers’. This document is meant to provide guidance for an owner to determine if the individual surplus spares are eligible for use on a civilian aircraft. It is common to find that even though outwardly the component looks identical to an FAA certified component- it is ‘Military Unique’ and thus is not eligible for use; worst yet if it is allowed to be placed into service- using the words of the FAA it: “...could cause a catastrophic failure resulting in loss or serious damage to the aircraft or an un-commanded engine shutdown resulting in an unsafe condition.”

TBO- CSN matters: A significant slice of the value of a used GTAE is vested by its overhaul status- i.e. ’Time Before Overhaul’- and also ‘Cycles Since New.’ Obviously if an Overhaul on a typical engine costs $600-000 to perform- then an engine can be valued with this simple equation:

W+X+Y = Z
W = Core value; X = Overhaul value*; Y = Disk life remaining**; Z = Total value. (* Defined as Overhaul Cost divided by the Time Between Overhaul (TBO) in Hours- then multiplied by the Hours in Time Since Overhaul (TSO.); ** Defined as Each Individual Disk/Component Replacement Cost divided by the Fatigue Cycle Life in Cycles ‘Scrap Limit’ starting at Zero- then multiplied by the Cycles in Cycles Since New (CSN) for each part; then all added together.)

As you’ll see- there are a lot more issues to consider when determining the ‘used value’ of a GTAE than just using the simplified version of the formula provided above; however this approach is still fairly accurate if you need a ‘down-and-dirty’ figure.

When aviation was in its infancy- every part of an aircraft was ‘on-condition’ meaning that it was not replaced until it looked like it was ready to fail. It soon became evident that a life or service limit should be determined and imposed by the manufacturer of the specific part or component in question- hence maintenance schedules became both voluminous and problematic regarding the meshing of complex and irregular change cycles.

In accordance with cfr 14- FAR 91.409- the FAA requires that “...the registered owner or operator of each airplane or turbine-powered rotorcraft described in paragraph (e) of this section must select- identify in the aircraft maintenance records- and use one of the following programs for the inspection of the aircraft:

(1) A continuous airworthiness inspection program that is part of a continuous airworthiness maintenance program currently in use by a person holding an air carrier operating certificate or an operating certificate issued under part 121 or 135 of this chapter and operating that make and model aircraft under part 121 of this chapter or operating that make and model under part 135 of this chapter and maintaining it under $135.411(a)(2) of this chapter.

(2) An approved aircraft inspection program approved under $135.419 of this chapter and currently in use by a person holding an operating certificate issued under part 135 of this chapter.

(3) A current inspection program recommended by the manufacturer.

(4) Any other inspection program established by the registered owner or operator of that air plane or turbine-powered rotorcraft and approved by the Administrator under paragraph (g) of this section. However- the Administrator may require revision of this inspection program in accordance with the provisions of $91.415... It is common for many owner/operators that have a large fleet to adopt their own Approved Aircraft Inspection Program (AAIP) that places many of their components and parts- especially the Hot Section and Overhaul intervals of their applicable GTAE in their fleet- into an ‘On-Condition’ status.”

The ability to develop and monitor a AAIP is unique to a specific owner/operator- and is absolutely NOT transferable. You might be able to get a great price on a used GTAE that comes from a large operator- but it is useless to you unless you bring it back to a ‘normal-for-you’ compliance state- which might in your situation- mean that it must first be overhauled before you can release it for service in your fleet.

Fortunately for all of us- many of today’s aircraft and engine manufacturers are working hard to get their products eligible for operations under the oversight of the Maintenance Steering Group-3 (MSG-3); the aviation industries steering committee- which exists because many owner/operators want to see more of their maintenance schedule items to be classified as ‘On-Condition’ without having the ability to submit and have approved their own AAIP. This however is the subject for another article in the future.

Let’s now assume that you have determined that the genus of the GTAE that you are proposing to buy is impeccable. How about its internal condition? Is this a safe and dependable engine that is well worth the money that you are about to spend on it; or is it potentially a hand-grenade quietly waiting to let loose on-wing at the worst possible moment? How can you tell?

I strongly urge you that you shouldn’t buy unless the seller can provide you with an independent test-cell report for the engine- or at the very least a comprehensive speed- pressure- temperature and vibration report obtained after a full ‘five-point’ run and vibration survey has been performed. Don’t just stop there- though. You should also have the candidate engine internally inspected with the aid of a borescope. Unfortunately there are still areas of damage and fatigue that might be missed if the engine is not dismantled and inspected; but usually this in impractical in most situations.

The kind of problems that might be found during a borescopic inspection include- bent- burnt- twisted- nicked or missing compressor or turbine blades; leaking carbon or labyrinth seals; erosion- coking and sulphurdation (the rapid degradation of turbine blades through accelerated corrosion attack as the intake air combines with the sulphur from the fuel and reacts with the blade alloy at high temperature. Sulfidation corrosion is usually found on the blade root shanks and shrouds in an engines turbine section).

The problems that cannot be found without performing a tear-down of the engine may include: Lightning Strike damage- cracking and Excessive Bearing Wear. Except for Lightning Strikes- excessive wear will show up on the Spectral Oil Analysis report- if regular sampling has been carried out on the engine prior to your pre-purchase inspection and testing.

Just like a computer where it is often said that with “garbage entered as data- the computer shall output more garbage” meaning that the data is unusable- maintenance records are subject to the same problem. Apart from some entries either being entered in a foreign language- handwritten in an illegible hand- mis-spelled- or worse yet maintenance events not even being recorded in the logs- then I suggest that you walk away from the engine that has either incomplete or incomprehensible records.

It must be stated that much of what I have said about testing and evaluating a used GTAE before you buy- is rapidly becoming moot. The reason for this is that on-board engine monitoring systems are becoming much more prevalent- especially as the most modern business aircraft has a fully integrated digital avionics system that relies on constant data streams from a multitude of operating main and ancillary systems. An excellent example of this is the Dassault Falcon 7X with Full Authority Digital Engine Controls (FADEC) on all three of its Pratt & Whitney Canada PW307A engines. All three engines are fitted with vibration monitoring units and fan trim-balance system.

Additionally there are constant data parameters being sensed and used by the onboard master flight control computer- and then recorded and stored. If there is an ‘exceedence’ or a fault code is generated by any of the engines- or in-fact if a snapshot of the recorded monitored trends is requested- this data is automatically transmitted by Satcom to Pratt & Whitney in Canada which is watching its customer’s engines that are enrolled under the Eagle Service Plan.

We’ll conclude with a look at Meggitt Aerospace’s currently in-development “Turbine Tip Timing” System for today’s modern GTAE which will greatly improve engine reliability. Meggitt says “...Just as a glass can be made to vibrate and break by an opera singer- the blades in a gas turbine engine can vibrate under certain operating conditions- suffer fatigue and then break- often resulting in catastrophic engine damage…”

The company adds that its Turbine Tip Timing System employs “…microwave and eddy current technology to measure such vibrations in situ and via non-contact sensors. The measurement- known as tip timing- is made by accurately detecting the time it takes for each blade to pass the sensor. If the blades are vibrating- damage occurs from being struck by foreign objects- or they simply wear over time- blade passing times will begin to change randomly or drift.

A high speed data acquisition system with signal processing algorithms will collect this data so the engine operating state can be changed to avoid potentially dangerous situations.v If you have any questions regarding this article- or would like to receive some free advice- you are welcome to contact Jeremy at JetBrokers- Inc. at +1.636.449.2833- or email: jcox@jetbrokers.com

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