Logbook Research: It's not just an AD search any more

Logbook Research It's not just an AD search any more By Joe Hertzler The maintenance records of an aircraft are the single most important factor considered when evaluating an aircraft for purchase. The value of an aircraft can be affected...


To establish the current times and cycles for each engine and propeller, we recommend the following equation be used:

([Engine Time @ Install] - [Aircraft Time @ Install]) + [Current Aircraft Time] = [Current Engine Time]

To use the equation you must have the most recent installation of each engine and propeller within the component logbook as well as the airframe logbook. It's important to emphasize that the record of the most recent installation must be used. Sometimes it will be an overhaul entry or a hot section inspection or MPI, but just as often the engine or prop can be removed and replaced by a loaner for a repair and then re-installed following the repair.

For each component (engine and propeller) locate the last installation entry and write down the date, total time in service, and total cycles in service. Using those numbers, follow the equation above to derive the current time and cycles for that engine or propeller. Remember to use the same date (installation date) for aircraft and component times.

For example, if the engine time was 2,768.5 total hours and 3,467 total cycles when it was installed on the aircraft, and the aircraft time at installation was 3,567.2 hours and 3,617 landings, and the current aircraft time (today) is 4,724.5 and 4,978 landings, the current engine time and cycles would be derived as follows:

Hours = ([2,768.5] - [3,567.2]) + 4,724.5 = 3,925.8
Cycles = ([3,467] - [3617]) + 4978 = 4828

This reflects the accumulation of time and cycles equal to that of the aircraft since the last installation. In all cases, time in service accumulates the same as the aircraft. In most cases, the engine cycles accumulate the same as aircraft landings. However, in the rare instance that the operator counts the engine cycles differently, the operator's cycle count obviously should be used.

What we have learned through hundreds of records audits is that it's far more likely that you will find an error has been made in the tracking of the engine or propeller times and cycles and that the equation mentioned here will resolve any discrepancies. A large discrepancy in times and cycles, for the engines in particular, can have a substantial cost impact. A rule of thumb number, for example, for a PT6A-42 engine is $50 per hour or per cycle. This assumes that the average overhaul will cost $150,000, and that the average TBO is 3,000 hours (this does not include maintenance between overhauls). So if during research we find that the engine has 500 more hours or cycles than the owner is advertising, that translates into a $25,000 swing in value.

Once you have a good total time and cycles for each component, compare it to the times and cycles provided by the owner and note any discrepancies.

What is the inspection program?
The next step is to determine what inspection program the aircraft is utilizing. The program may be an annual/100-hour program for light piston aircraft, or the complex inspection program recommended by the manufacturer for multi-turbine powered aircraft (Ref 14 CFR Part 91.409). Another possibility is that the aircraft is currently operated under an Air Carrier certificate being maintained in accordance with an Approved Aircraft Inspection Program (AAIP). (For details on possible inspection programs refer to AMT Around the Hangar articles for July, September, and October 2001 also available online at www.amtonline.com).

Once you know the inspection program the aircraft is utilizing, you can compile a list of all of the requirements for that program. The idea is to get a good handle on the program and have it fresh in your mind when you begin reading through the logbooks. Compile the list in a way that will provide space to write down when each item was last accomplished and calculate when it is next due as you read through the logbooks.

Areas that can be more troublesome than others are major modifications. Major modifications generally require some sort of instructions for continued airworthiness (ICA). These instructions will mandate inspections that are required because the aircraft has been modified and the manufacturer's inspection program or other approved inspection program does not cover that area or component of the aircraft.

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