A Complete Guide to Aircraft Engines

Aircraft Engine

Essential Knowledge for Buying Used Light Aircraft

If you have been perusing the airplane listings and casting a hungry eye at all of the gorgeous offerings, plenty of things might catch your eye. Paint and interiors that make a good impression are always nice. There are a few that stick out because of they seem to be priced much more reasonably than the other aircraft. But buried somewhere in the ad will be an indication of how much money you are really going to have to put into this airplane: The amount of time that the engine has been run or the number of hours since it was last overhauled.

Here is where it gets confusing: There are a variety of arcane terms used to describe engine condition to be found in many used airplane sales advertisements. SMOH, SNEW, SFOH, “fresh top”… The list of terms goes on and on. There is no other component on the used aircraft you buy that will give you more satisfaction than a good engine—and there is no component that can cost you as much cash and tears as a worn out engine. In this article, we’ll break down the engine speak so that you know what you are really getting for the money.

What is an aircraft engine overhaul, anyway?

Light aircraft reciprocating engines have dozens of moving parts that must work in perfect synchronization for the engine to function properly. Even though many of these parts are well lubricated by engine oil, they eventually become worn beyond tolerances. Clear signs that a used aircraft engine needs to be overhauled include excessive oil consumption, low cylinder compression, spark plugs that are wet with oil, and metallic shavings in the oil filter.

Each engine manufacturer stipulates an amount of time that the engine should run before an overhaul is necessary; this is called time before overhaul (TBO). 2000 hours is a pretty typical number for most light general aviation aircraft. There is a common misconception that an engine MUST be overhauled at or before the TBO; that isn’t necessarily true. For FAR Part 91 operations, it is perfectly legal and acceptable to fly beyond the TBO. So long as the engine health is monitored through rigorous record keeping of oil consumption, laboratory oil analysis, compression testing, and the occasional borescope inspection, there is nothing inherently unsafe about flying past the TBO. The inverse is also true: A poorly maintained or roughly used airplane (especially if it is turbocharged) may require an overhaul prior to the TBO. The moral of the story is this: engine overhauls are generally performed on condition rather than at a set time value.

A true engine overhaul involves taking the engine completely apart and inspecting each and every component for condition and wear in comparison to manufacturer specifications. Many owners opt to have this done by the engine manufacturer at their factory; often these overhauls include all new parts so that the engine is essentially brand new. These are referred to as zero time engines—used aircraft with zero time engines will command a premium price in the marketplace.

An overhaul can be performed by any A&P mechanic, although the quality varies; were parts replaced or was that worn crankshaft just ground down to barely within specs? A careful look at the maintenance logbook should reveal what was really done, but as with all things check diligently before you buy a used aircraft. A poorly done or just barely in spec overhaul is nearly as bad as an overhaul that wasn’t done at all.

In some used aircraft ads, you’ll see a reference to a top overhaul. This isn’t really an overhaul at all—it is only the overhaul or replacement of components external to the engine case, usually one or all of the cylinders. This is pretty common in turbocharged airplanes, since they tend to run at higher pressures and temperatures than their normally aspirated cousins. If you are interested in a used airplane that has had a top overhaul, caution is warranted; why was the top overhaul done? Was the airplane a hanger queen and the cylinders got corroded? Was there a cracked cylinder or a seriously malfunctioning valve? What inspections, if any, have been done to the cam and crank? Top overhauls can be responsibly done, but you should be very wary and ask good questions before you buy a used airplane that advertises that this sort of work was performed.

The advertisements—A sea of abbreviations

Here is a helpful guide to the engine abbreviations and terms you are likely to see in used aircraft advertisements:

  • TBO: Time before overhaul. As we stated above, this is a suggestion for Part 91 operations. Engines may require overhaul before or after this time, depending on condition.
  • SNEW: Since New. This was a new engine when it was put on the airplane.
  • SOH: Since overhaul. This is a pretty non-specific description, so you’ll want to ask good questions.
  • SMOH: Since major overhaul. The engine was completely dissembled, checked, had parts repaired, milled, or replaced as necessary. This is typically a good sign, but you’ll want to check out the logbooks and the shop that did the work.
  • SFOH: Since factory overhaul. The engine was overhauled by the manufacturer’s facility. Often, these are “zero time engines.” This will be a quality overhaul that you can have confidence in.
  • SPOH: Since propeller overhaul. Props need to be overhauled as well. While not part of the engine directly, a worn out prop hurts performance and can end up costing you money down the road. This typically applies to variable pitch (constant speed) propellers.
  • Zero time: The engine has been overhauled by the factory and is in a like-new state. The FAA considers this to be a new engine.
  • STOH or “top overhaul:” STOH stands for since top overhaul, so this is a measure of time on the cylinders and not a measure of time on the engine itself. There are no regulatory maintenance definitions that apply to top overhauls, so you’ll want to exercise caution here.

 

TBO and Used Aircraft Valuation:

As you scan the used aircraft listings, you’ll probably note a pretty wide range of prices between aircraft of the same price. Often, the answer as to why this disparity exists can be found in the engine descriptions. Aircraft with new or zero time engines can—and will—command a higher price on the used airplane market. Aircraft with engines near or beyond TBO will likely be less expensive—but is the used aircraft in question inexpensive enough to make up for the cost of replacing or overhauling a worn engine?

Overhauls are expensive. If you choose a factory overhaul option for your engine, expect to pay somewhere north of $26,000. A “field overhaul,” one done by a mechanic or facility other than the factory, can cost between $18,000 and $21,000. While these expenses are indeed eye-popping, they present something of an opportunity; you may be able to negotiate a lower sale price for your used aircraft by pricing in the overhaul it will eventually need. Keep in mind that if you choose to replace an engine, you can often get some money back on the engine case, although some manufactures stipulate that the engine must be below TBO to qualify for that rebate.

 

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