Pre-buy Inspection - Part 1 Logbook Entries

Friday, September 29, 2017, at the 42nd MAPA Homecoming, KNR gave a seminar titled “How to Buy a Used Mooney”. For those readers that were able to attend and for those of you that could not get through the weather, read on as ShopTalk covers how to do a logbook pre-buy on a used airplane and how to read logbook entries. At the Homecoming, we handed out a checklist, “How to Purchase a Used Mooney”. That checklist and a Servicing Schedule spreadsheet are available for download at These documents will greatly assist in organizing the data gleaned from the logbooks. Also at, a ShopTalk article from April 2004, Caveat emptor, caveat vendor looks at the pre-buy inspection from a different perspective.

When asked to do a pre-buy inspection, at first, I do not want to see the airplane. I want copies of all the logbooks back to when the airplane left the factory new, along with a current tach or Hobbs time whichever is used to determine total time. If we can get through the logbook review without too many Money Pit Items (MPIs – my term), then the actual aircraft should be inspected. This article, Part 1, will only discuss the logbook review.

Starting with a clean pad for notes (cross-reference with the checklist), read every entry in every logbook from the most recent to the first entries made. As we go through the logs, look to make sure there are no large gaps of time missing and that each logbook ends and the next log begins with no major gaps in tach or calendar time. For most major components, note both the tach time and date of installation, servicing, repair, etc. The date is just as important as the tach time. Tach time can be calculated to give us total time on each component but the date can tell us if the component was rebuilt or repaired when a possible airworthiness directive (AD) came out.

Going backward through the engine log, note when each magneto 500‑hour service was last done. Likewise, the last 500hr service was done on each alternator. When was the vacuum pump installed, new or rebuilt? When were the spark plugs newly installed? If the engine is turbocharged, note the time and date of the turbo and wastegate installation. A good example of the importance of the date is when Kelly Aerospace assembled a bunch of turbochargers improperly and the FAA issued an AD against them. Remember, the goal is to understand the condition of all these very expensive items. If you buy the wrong airplane with a bunch of worn out components, you own them.

Now understanding what a log entry says can be critical. For example, magneto entries: A mechanic claims to have done a 500‑hour service but only lists points and condenser replaced; this is not a complete 500‑hour service. If a magneto repair station records a magneto 500‑hour service and provides FAA Form 8130‑3 or yellow tag then a proper 500‑hour service was done. There is a lot more to a 500‑hour service than just replacing points and condenser.

Once we have figured out all the total times and what was or was not done to each engine item we will have a good understanding of the condition of the engine accessories. Of course, we will calculate the total time on the engine and its time since overhaul. Importantly, who did any major overhaul on the engine? The engine manufacturer will usually do the best overhaul possible as compared to some unknown mechanic in the field.

So does the engine overhaul on this particular airplane have used parts in it? Overhauled cylinders? Was the engine remanufactured to factory new limits? There are a handful of engine shops in the country that do a good job when it comes time to overhaul an aircraft engine, but the factory will not install overhauled cylinders on the engine they will warranty; only new parts will do and new parts are expensive. Might this engine still be in warranty? Some engine shops have warranty periods longer than the factory.

On turbocharged engines, we look for signs in the logbook of engines being operated at too high turbine inlet temperatures. These signs are premature cylinder, wastegate or turbo failures. Just because the airframe manufacture POH limits the TIT at 1650 degrees does not mean 1649 continuous is OK. Likewise, a CHT red line at 500 degrees does not mean 499 continuous is acceptable. A logbook recording multiple cylinder replacements provides a clue as to the future of an engine.

Most propeller logbooks are much easier to understand but there are some clues we don’t want to miss. Again we will calculate the total time on the prop and ask the question, “Is this the original prop that came with the new airplane?” If not, why was it changed out? A possible prop strike? Sometimes a prop change is an innocent event. An example of this is with the M20K two-blade McCauley prop. Over the years, mechanics will casually file off nicks at the ends of the blades. Then one day the prop is sent in for overhaul and, low and behold, it must be replaced. This particular prop has no diameter reduction allowed! Also, because new blade replacements are so expensive, many times a new prop is not much more money. These are ways a new prop can find its way onto the airplane you are looking to buy.

Another MPI is on certain older Hartzell two-blade props with the light hub installed, the HC C2Yk series. Per AD 2009-23-03, at every annual, this hub must be eddy current inspected. That inspection is about $300 at a properly equipped prop shop (transportation cost?). If the hub is found cracked, it is an expensive replacement.

Now that we have gone through the engine and prop logs and have our notes, it’s time to go through the airframe logbooks. Note the date and tach time for the following items: Any and all landing gear shock disc replacement, O2 bottle type and age in years along with the last hydro test date, the last IFR certification and ELT test and battery due date. And, of course, the date of the last annual.

The landing gear shock discs will last about 5-7 years on a long-body Mooney and about 12-15 years on a short-body Mooney depending on weather conditions and flight activity. If the plane sits outside and is flown twice a year these shock discs will not last as long as an airplane that is hangered and flies weekly.

The O2 bottle has a life limit if it is a 3HT or composite and must be removed from service once the limit is reached. The 3HT bottle has a 5-year hydro test cycle, life limit is 23 years. The composite has a 3-year hydro test cycle and a 15-year limit. Just to give the reader an idea of how big an MPI this one item is: a 3HT bottle in 2017 dollars will cost $1,600, a composite bottle over $2,000! You don’t want to miss this one on the pre-buy.

Another big ticket item is the airplane battery. A Mooney TLS/Bravo has two 24 volt batteries. Each one will set you back $600 at current (pun intended) prices, so knowing battery age is important. A typical AGM battery will last 5-8 years, a wet-cell battery may only last 2-3 years. Write down the battery type and the date it was newly installed.

As we go through the airframe logbooks, note any damage such as a gear up landing, wing skin replacement or replacing or re‑skinning control surfaces. All of these items, if not properly repaired, can lead to an airplane that won’t fly straight and level. Having to redo work to fix a crooked airplane can be another MPI.

With Mooney aircraft, determine if the fuel tanks have ever been resealed or if there are multiple entries of work on one tank, chasing a leak. MPI alert! To completely strip and reseal one troublesome tank will cost about $5,000. The leaky tank problem can be traced back to the landing gear shock discs needing to be replaced. When these discs get old they typically get hard as a rock and all the landing impact forces are transmitted directly to the tank structure. This loosens the sealant causing a leak. Wet wing leaks are OK on the SR-71 but not on your Mooney.

As these airplanes age, it is becoming difficult to find one that does not have some kind of damage history. The questions to ask about damage history are: How extensive was the damage and how many years and flight hours have been flown off since the repairs were completed? For example, it is not a good sign if the repairs were done many years ago but only 10 flight hours since. If, since the repair, the airplane has been regularly flown and has some significant hours, the new owner needn’t become a test pilot.

New paint and new interiors look good to a buyer but just like everything else with the FAA, there is a correct way to return the aircraft to service. If the airplane was repainted there must be a log entry stating the control surfaces were balanced following the pertinent service manual. A new or refurbished interior must have burn certifications that comply with FAR 23 appendix F (flame resistance).

If the burn certs are not mentioned and the interior is not covered under an STC then a burn test that meets FAR 23 appendix F must be accomplished before the plane can be returned to service during an annual. The same goes for the balance on the control surfaces, it must have been verified and documented.

Another item to look for concerns the landing gear motor and transmission. If there was any work done on this assembly, note the completion date and tach time. Most of us in the Mooney community remember the no-back-spring fiasco, but not many remember that Eaton made some improperly heat-treated no‑back‑springs. This is why it is important to have the date the work was done as it is the only way to determine if the Mooney you are looking to buy has a good or bad no-back-spring in the gear transmission.

Also, write down the make of the transmission if it is in the logbooks as some Mooneys came with an Eaton system and others came with a Plessy system (1986-1992 if I remember right). The Plessy system is interchangeable with the Eaton but you can no longer get any parts for the Plessy system. When it fails you will have to go to the Eaton system and Mooney gets over $20K for one of these motor/ transmission. If the system was overhauled, often it was only the transmission and the 30-year old electric motor was just reinstalled. The system will not work properly if one component is new and the other is at the end of its life. Both parts should be overhauled at the same time.

How many hours are too many on the landing gear motor/ transmission? Well, Mooney says a new no‑back‑spring every 1000 hours but this is extreme for most Mooneys. Think about how a Mooney operator runs his or her aircraft, normally it is one gear up cycle and then two hours later one gear down cycle and only once or twice a week. Not a lot of activity for the gear motor/ transmission as compared to a trainer with retractable gear that will see a dozen or more gear cycles per day. Most Mooneys are used for cross‑country flights not hops around the pattern. With that in mind, it is perfectly safe to overhaul the gear motor/transmission every 3-4 thousand hours except the early 1960s and 1970s Dukes gear systems.

Dukes electric landing gear systems in the early Mooneys have an AD against them as the transmissions are not very robust compared to the Eaton or Plessy systems. The main concern is the hours on the gears in the Dukes transmission. AD 75-23-04 is a multi‑part AD and each part must have tach time recorded when serviced. The Dukes gear system won’t set you back $20K but if abused and not properly serviced can cost close to $4,000 for repair.

Now that we have all the times and dates for all of the above items listed we can fill out our service schedule and determine what has been done and what items are overdue service. With that information, one can determine how ugly the new owner’s first annual will be. Now you have an idea of how much the seller should take off the sale price based on the data you have created, just from the logbook pre-buy inspection.

Part 2 of the pre-buy process will be on the actual airplane and what that inspection involves. Most used airplanes have some items wrong with them. You are just looking for that airplane with a low number of MPIs.

If you have questions about this or any other ShopTalk article, please e-mail me at or call me at my aircraft repair shop, 307-789-6866. All ShopTalk articles can be read here at www.knr‑ Until next time, enjoy flying your Mooney.