Obtaining Aircraft Usage Data - A Hypothetical Scenario

Jaybird180

Jaybird180

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Jaybird180
Owner Bob has maintained meticulous records and insists that his high time engine and airframe was well taken care of with nary a trouble. For example, in addition to the standard maintenance records, he logs separately how often he washes, waxes and vacuums the interior of his normally aspirated single engine airplane. He proudly displays this level of detail to the prospective purchaser, John who after several extensive telephone conversations travelled across the border from Canada to see Bob's American registered airplane. John is a numbers-minded purchaser and interested in specifics of the engine, which showed low oil and fuel consumption, high-compressions, low noise, sewing-machine smoothness and rocket-ship power delivery during the test flights despite its time significantly over TBO.

A year ago, Bob had a serious auto-accident where the car caught fire. No one was injured, but after a few weeks rest, Bob went to visit his AME and after an extensive physical the AME offered to renew his 1st Class early. The AME noticed that Bob was unusually withdrawn and after asking, Bob explained how his personal flight logbook got lost at the scene of the auto-accident containing every adventure he had with this airplane. Bob's AME couldn't repair his broken heart nor could he talk him out of voluntarily surrendering his medical.

Considering that Bob has records above and beyond what's normal he asks on a hunch to see Bob's Journey Log to get an idea of Bob's usage and treatment of the engine and airframe over the past several years. John believes that he can crunch the numbers of distance travelled, ground hours, air hours, tach hours and hobbs hours to draw inferences about Bob's habits with engine care.

Bob, being a good American, has no such thing; he separately logged each and every flight by recording date of flight, Hobbs and tach time and fuel used, always 100LL. The lost flight log perhaps could have filled in some of the missing data points John wants, but it is unavailable. With a significant non-refundable deposit of American greenbacks, John secures the logs to study over the weekend in his hotel room.

Here's the question:
What statistical inferences can John make about the usage history from the numbers, given he only has date of flight, Hobbs, tach time and fuel used?
 
The dates will tell you how regulary it flew. The hobbs and tach times will tell you how hard he ran the engine. That is about it.
The flight log has fuel used - what about oil?

What about the maintenance logs? Those are what I'd be most interested in reviewing - history of compressions, oil changes, oil analysis, parts replaced, etc.
If the usage log shows regular use, and an inspection of the plane shows it in good shape both mechanically and aesthetically, what is the problem? Do you really care how often it was washed and vacuumed? If it ever was, that is more than most planes get :rolleyes:

I'm flying a C172 with over 2700 hrs tach time. It flies very reguarly (training) and is very well maintained. Static RPM, overall performance, Oil consumption, compressions, oil analysis are all normal.

IMHO, regular use (like multiple times per week) is more important than anything for long engine life. Lack of abuse (mostly high temps) is a close second.
 
You can tell how much time is spent in cruise (as opposed to approach, slow flight, pattern, etc.) with hobbs/tach.

You can tell how hard the engine has been run with mean GPH (fuel used / hobbs).

Most importantly, you're looking for trends. Is mean GPH climbing with time? Maybe there might be some mild compression loss (which can be hard to find in a cranking compression test).

Frankly, you will find more about engine condition with a used oil analysis than you will in any analysis of logs, no matter how complete. Insist on one as part of a pre-buy inspection. That tells you what is wearing into the oil, and by how much.
 
it's an awful lot of thought to put into rolling the dice on an engine.

if you want to buy an airplane then get the cleanest, most corrosion free airframe you can find of the model you want. Everything else, radios, engines, props, are just attachments that come and go.
 
Jaybird180 said:
Owner Bob has maintained meticulous records and insists that his high time engine and airframe was well taken care of with nary a trouble. For example, in addition to the standard maintenance records, he logs separately how often he washes, waxes and vacuums the interior of his normally aspirated single engine airplane. He proudly displays this level of detail to the prospective purchaser, John who after several extensive telephone conversations travelled across the border from Canada to see Bob's American registered airplane. John is a numbers-minded purchaser and interested in specifics of the engine, which showed low oil and fuel consumption, high-compressions, low noise, sewing-machine smoothness and rocket-ship power delivery during the test flights despite its time significantly over TBO.

A year ago, Bob had a serious auto-accident where the car caught fire. No one was injured, but after a few weeks rest, Bob went to visit his AME and after an extensive physical the AME offered to renew his 1st Class early. The AME noticed that Bob was unusually withdrawn and after asking, Bob explained how his personal flight logbook got lost at the scene of the auto-accident containing every adventure he had with this airplane. Bob's AME couldn't repair his broken heart nor could he talk him out of voluntarily surrendering his medical.

Considering that Bob has records above and beyond what's normal he asks on a hunch to see Bob's Journey Log to get an idea of Bob's usage and treatment of the engine and airframe over the past several years. John believes that he can crunch the numbers of distance travelled, ground hours, air hours, tach hours and hobbs hours to draw inferences about Bob's habits with engine care.

Bob, being a good American, has no such thing; he separately logged each and every flight by recording date of flight, Hobbs and tach time and fuel used, always 100LL. The lost flight log perhaps could have filled in some of the missing data points John wants, but it is unavailable. With a significant non-refundable deposit of American greenbacks, John secures the logs to study over the weekend in his hotel room.

Here's the question:
What statistical inferences can John make about the usage history from the numbers, given he only has date of flight, Hobbs, tach time and fuel used?
Click to expand...


If you know the Hobbs to tach time and the and the reference RPM for the tach, you can figure a bit on how hard he ran the engine, but that's about it, outside of that any inferences would be stupid to try to consider.
 
Henning said:
If you know the Hobbs to tach time and the and the reference RPM for the tach, you can figure a bit on how hard he ran the engine, but that's about it, outside of that any inferences would be stupid to try to consider.
Click to expand...

If Bob's airplane has the stock hobbs and tach, where would the reference RPM be found?
 
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