100 hour Inspections and Tach.

[Amish RakeFight]

I know that some flight schools (or any other type of operator for that matter) use the tach numbers to determine when an aircraft is due for its 100 hr. I'm talking about the straight numbers read off the tach, not the conversion of hours (1.3 or was it 1.2? x tach). Obviously, the tach method will yield a longer service interval than the Hobbs would as the tach numbers turn slower than the Hobbs.

Aren't the 100 hr inspections predicated upon flight hours, and normally the Hobbs is used to account for this, as when recording flight time in a logbook.

 

[erj-145mech]

Either method is acceptable to the administrator. Since the tach only runs one hour for one hour is at full power setting, and you don't usually fly at full power, it won't run 1 for 1. Many aiplanes don't have recording tachometers, so another method will be used instead. One my plane, I have the hour meter switched from the landing gear safety switch. It a flight hour meter, when the gear is on th ground, it not flying.

 

[Learsforsale]

Since the tach only runs one hour for one hour is at full power setting, and you don't usually fly at full power, it won't run 1 for 1.

I am glad you brought that up, because I have always wondered what determines tach time. Just to make sure I understand, lets say the max RPM of an engine is 2700 Rev. per minute x 60 minutes = 162,000 revolutions = 1 hour on the tach. time.

 

[Donsa320]

Tach time

I just replaced the recording tach on my Champ and the new one is advertised to record 1 hour when the engine is run for 1 hour at 2150 rpm, the nominal cruise rpm. More time if over 2150 and less time below 2150. Everyone seems happy with that arrangement.

DC

 

[a&p cfiguy]

Just to clarify, 100 hour inspections are due when the airplane reaches 100 hours "time in service", which is defined as time when the wheels leave the ground to time when wheels touch down (or something like that). Since the tach doesn't start fully counting until the rpms reach cruise power setting, this is a rough way of measuring time in service and is acceptable. Many hobbs start counting when the oil pressure comes alive, some start counting when master is turned on, some start counting when the squat switch on the gear is engaged (which is the most accurate measure of time in service).

A lot of people log their flight time straight off the hobbs meter, which is wrong. Logging flight time starts the second the aircraft moves under its own power and stops when the aircraft comes to a complete final stop. No hobbs meter measures this. The only way to measure this is by stopwatch. Example, you start the engine(s), hobbs start counting, tach is counting but very slowly. You get the ATIS, then your clearance, run pre taxi check, then 6 minutes later, power up to start your taxi. Hobbs already counted .1, but legally you cannot log this flight time because the aircraft hasn't moved yet. It takes you 12 minutes to taxi and hold short for takeoff before you're airborne (.2 hobbs, maybe .1 tach so far). You fly for 30 minutes then land (.5 hobbs, .5 tach). Taxi back in for another 6 minutes (.1 hobbs, 0 tach). Total times .9 hobbs, .6 tach. Loggable flight time .8. Aircraft time in service .5. The tach is close but not totally accurate of time in service, but since it's more on the conservative side, it's acceptable to measure time in service off the tach.

The ONLY way the measure loggable flight time is to use a stopwatch or mark the time when the wheels move. There are however a few ways to measure time in service of the aircraft, a tach, a hobbs with oil pressure switch, hobbs with squat switch, or simply keeping track with a written log.

 

[Amish RakeFight]

The above is true, however I believe the tradition of logging flight time according to the HOBBS meter probably originates from keeping track of rental charges. OF course it depends on how it's rigged, (most I knew of ran offthe master switch - one of the first "checklist" items to be turned on).

Aside from that, although the plane isn't moving under its own power throughout the Hobbs ticking, there is still a lot of learning and tasks being accomplished which fairly represent piloting experience.

 

[NoahWerka]

I thought "tach time"as when the engine was turning over: i.e: the faster the crankshaft turned,the faster the "tach time". The hobbs meter was only electrically activated by either the ignition switch, or the oil pressure switch. Have I been mis-informed for the past 36 years?

 

[Gatorman]

I just have a chat with my chief inspector about this a few minutes ago...wierd. I was telling him about the knuckle head in the other thread that wanted to 'disconnect' his Hobbs.

Anyway, we came to the conclusion that there are several different ways to calculate the times as required by the FARS to be recorded.whether it is by way of Hobbs, Stop Watch, Tach, or WOW.
FARs are not specific in this mattet except that they are to be recorded.

The I twisted his mellon a little and asked about maintenance time, does is count?
If I did a compressor wash on a jet aircraft, obiousily the time that the engine is in rotation does not count (althought it does build up oil pressure during this time), but when I have to crank it up and do the drying runs and then preformance runs, does this time go towards the aircraft airframe/engine times?
The reason I ask this...are the times on the engines while on in the test cell counted?


Hehehhe, this is fun....like stirring the pot.

 

[avbug]

If I did a compressor wash on a jet aircraft, obiousily the time that the engine is in rotation does not count (althought it does build up oil pressure during this time), but when I have to crank it up and do the drying runs and then preformance runs, does this time go towards the aircraft airframe/engine times?

The answer is yes, and no...depends on the manufacturer. Some count a start and a stop, others a start, power up to a particular power setting or value, and a stop, others a cycle for each power up beyond a certain value. In my experience, few operators abide by this, however, and count a cycle only as engine start to engine stop. Very few count the maintenance cycles or log them, and as the hour meter seldom runs in high performance aircraft without either adequate airspeed or the gear being up, there's really no record of the run time or engine values (save for downloadable systems with trend monitoring).

Garrett/Honeywell, for example, is changing the process of logging cycles to do as P&W did some time ago, assigning values as percentages of a cycle, depending on what was done.

Most operations save dollars and cycles by not counting whatever they can get away with.