Tach time or Anytime

[okpipepatrol]

What is the standard method in tracking time for an aircraft engine relating to maintenance and TBO?

[PappyJ]

I think Engines go on Tach time. Rentals are charged on HOBB's time.

Finally, Pilots are paid by in accordance with the following formula.

HOBBS TIME - TACH TIME = PILOTS PAY (Hours) :ugh:

[SNS3Guppy]

What is the standard method in tracking time for an aircraft engine relating to maintenance and TBO?

Okpipepatrol,

The short answer is that there isn't a standard method. When you say tach time, you're referring to the "time" obtained off the tachometer. This isn't actually a measure of time, except when the engine is operating at cruise RPM's. The hours are recorded much more slowly at lower RPM's, which include anything less than the maximum rated RPM for your aircraft, in most cases. If you're cruising at a lower RPM, you will see more time on your watch than on the tach at the end of an hour of cruise flight. If you check "tach time" at the end of a flight, you'll find less time recorded than you'll show on your watch or on an electronic hour meter (hobbs, etc). The tachometer is recording time at a slower rate when the engine is operating at reduced RPM's.

Because of this function, many operators elect to use tach time to measure maintenance intervals; it means more operating time between intervals, which equates to less money spent (more use for less dollars).

Some operators go off "hobbs" time, which is to say, off an electronic hour meter (hobbs is just a brand, and there are several). This usually doesn't provide an accurate measurement either, however, as there are a number of ways that the meter can be "plumbed" into the system, or installed. Some hour meters work off an oil pressure switch; electricity is only supplied to the electrical hour meter when the battery master switch is on and the engine (at least one of them) has oil pressure available.

In other installations, the hour meter runs any time the airplane is receiving electrical power, either from the battery or the aircraft generator/alternator. In this case, if there's power available, the hour meter is running. This installation is often used for billing purposes on rental aircraft. The renter ends up paying considerably more (several tenths of an hour, at least) for the rental, than the owner actually sees for maintenance. If the renter puts on 1.3 hours, the owner might only show it as 1.0 hours for maintenance purposes, for example. Thus he can make 130 hours of revenue between 100 hour inspections.

Some aircraft use other means of determining the hours for maintenance purposes. Some airplanes don't abide 100 hour and annual inspections, of course, and use progressive or phase inspections. This isn't very common on small, light aircraft. Generally, you'll find that the owner or operator will use whatever recording system available that will give him or her the most advantageous use of the aircraft, or maximum operating time for the minimum recorded time toward a maintenance interval.

Some maintenance is done based on cycles or landings. This typically applies to large airplanes and turbine powered airplanes, though not always. The operators specific maintenance program will spell out the details that are used in each particular case. Some operators and owners aren't aware that the FAA authorizes one to use nearly any maintenance program, so long as it's approved first and meets all the necessary criteria. If you're flying your Cessna 170 on pipeline patrol a thousand hours a year, for example, and don't like to have it down for a week or two at a time for the annual and for sporadic 100 hour inspections, then putting it on a progressive inspection program makes sense. This involves doing individual pre-programmed inspections throughout the year, such that at the end of the year, everything has been done on the airplane that needs to be done. Some prefer this method as being less intrusive on aircraft availability, and it means that the aircraft gets looked over more often; less time between inspections and less to go wrong. Things get caught quicker, and the costs go down.

TBO, as you brought it up, is also a misnomer that's often misunderstood. It really applies to first-run engines and to remanufactured engines. Most pilots don't understand what "overhauled" really means, and think that it involves rebuilding the engine to new standards. This isn't the case at all, and an overhaul can mean an inspection with nearly nothing (or nothing at all) replaced: "overhaul" literally means to inspect and find within tolerances. If your engine has been inspected and found to be within tolerances, it could actually be considered "overhauled." This doesn't mean that the engine should be expected to last any particular period of time after that "overhaul."

14 CFR 43.2 provides the breakdown for what's required for an item to be called "overhauled:"

§ 43.2 Records of overhaul and rebuilding.

(a) No person may describe in any required maintenance entry or form an aircraft, airframe, aircraft engine, propeller, appliance, or component part as being overhauled unless—

(1) Using methods, techniques, and practices acceptable to the Administrator, it has been disassembled, cleaned, inspected, repaired as necessary, and reassembled; and

(2) It has been tested in accordance with approved standards and technical data, or in accordance with current standards and technical data acceptable to the Administrator, which have been developed and documented by the holder of the type certificate, supplemental type certificate, or a material, part, process, or appliance approval under §21.305 of this chapter.

(b) No person may describe in any required maintenance entry or form an aircraft, airframe, aircraft engine, propeller, appliance, or component part as being rebuilt unless it has been disassembled, cleaned, inspected, repaired as necessary, reassembled, and tested to the same tolerances and limits as a new item, using either new parts or used parts that either conform to new part tolerances and limits or to approved oversized or undersized dimensions.

As you can see, what constitutes an "overhaul" is highly subjective, and one can't make assumptions when an overhaul is performed, beyond that it's been cleaned, tested, and found to be within limits. Accordingly, once an engine is past it's first TBO period, one shouldn't give much credence to the TBO numbers for planning or condition. Some operators have meticulous programs which actually increase the TBO of engines in 25 hour and 50 hour increments, by proving repeatedly at each TBO interval that their program of inspection and maintenance is so good that the engine can go longer than originally forecast. I've worked for several such operations operating and rebuilding the engines.

You can also see that the hours applied toward TBO are variable. The tach use of time for record keeping makes sense on the one hand, because it does suggest varying the time applied to the engine's history based on how hard it was working at any given time, and it only covers the time that the engine was operating. On the other hand, if one has a lot of taxi time, the tach is still running, albeit slowly, and doesn't represent that ground time very well. The same goes for reduced RPM operations in flight, when the tach records more slowly.

[Big Pistons Forever]

What is the standard method in tracking time for an aircraft engine relating to maintenance and TBO?

As SN3 noted there is a variety of ways used to measure time. The most correct method is to log actual wheels up to wheels down time and use that to track maintainance. The most favoured methods usually involve schemes that under state actual airtime in order to reduce maintainance costs and over state flight time in order to increase revenue.