I am thinking of flying at a club that charges on tach time. I always notice that my tach time is lower than my hobbs time. How does tach time work? What are some ways to help minimize tach time??

 

The Hobbs meter is an actual electric clock that runs when the engine is running, usually by sensing oil pressure. Most rental planes have a hobbs.

Tach time is basically a mechanical counter that is drive by engine RPM. It is usually calibrated to approximate actual clock time at the typical cruise RPM for the engine, often 2300 rpm. Maintenance is based on tach time, so every airplane should have one. Some rentals save the expense of installing a hobbs by just using tach instead of hobbs for billing.

The good thing about this is that while taxiing or flying slowly, you get charged for less time. If a club has their rates adjusted for a typical renter's high-speed cross-country, a time builder might be able to log more time and get charged less by flying slowly at reduced power settings and engine RPM. Pattern work usually results in lower tach times, IIRC.

Many folks commonly multiply tach x 1.3 to get loggable time, but you can always legally use your watch and log the actual time from engine start to engine stop as flight time regardless of what the tach says.

 

Thank you rickair.
I have just a few specific follow up questions (since you know this stuff).
1. When will the hobbs start counting and when will it stop? (i.e, when the master is turned on/off, when the ignition is turned on/off, or when the engine is running (oil pressure in the green)).

2. You said an a/c tach can be calibrated to a normal operating rpm, say 2300. So if you go less than 2300, the tach time slows down. But, if your RPM's are above 2300, does the tach time count faster than actual time??

Thank you

 

Some operations like to rip you off, and will have the hobbs wired to the master...but some pilots use this to their advantage on cross-countries in non-radio / non-xponder airspace...

Normally, the hobbs should be running only when oil pressure is present, ie the engine is on. You can turn on the master and check atis, clearance, etc before start.

If the engine runs faster than the calibrated RPM, tach time would run faster than actual clock time...but since this normally only happens for a few minutes during takeoff and climb, tach time usually totals less than hobbs due to the VERY low rpm on taxi and descent.

 

I've had two BE-76 training planes with a Hobbs wired in a funky way. The first was at Sierra Academy (blah... talk about a ripoff!). Their hobbs was off the right engine. So we did almost all our single engine stuff with the right engine shut down. Another was N99UC, whose hobbs was wired into the Master Switch..... and would add another .1 or .2 to the flight time because my stupid instructor liked monkey-*ick around with the master on during preflight. What a jerk.


Anyway... just my little rant

 

Most twins have a hobbs that only senses one engine. Every light twin I've been in was configured this way from the factory.

The wiring to the master thing is BS. Especially when every 6 minutes that it's on is costing you upwards of $20

 

Some greedy FBOs do it though.

 

 

Hobbs time and Tach time usually both start moving at engine start. The difference is Hobbs is electrical and Tach runs off engine RPM. Tachs are set to work accurately at a certain power setting. If the Tach is accurate at 2400 RPM, then if u run the engine at 2200 RPM one hour of hobbs time would not equal one hour or tach time (it would be like .8 or .9. not a math wiz so dont know the exact). This becomes very evident in multi training when ur hobbs time is one hour and ur tach time is .5. I'd like to think i didnt spend 30 minutes with the master on but the engines not turning

 

Different Hobbs meters are wired differently. Some start when master is turned on, some start with oil pressure and master, some are wired to a strut switch for weight on wheels. Generally only a FBO trying to maximize revenue has it wired only to the master switch.

Joe