A Tale of Two Instruments

Every pilot that has ever rented an airplane will instantly recognize this little instrument. The Hobbs meter is typically tucked away in an obscure corner of the panel, overshadowed by the other instruments, which are bigger and much cooler looking. No passenger has ever looked at a Hobbs meter with awe and asked the pilot “Oooh….what’s that one for?” There are no graphics. No sexy colors. No dials, buttons or switches. Just a boring, black and white timer.

But study after study confirms that the tiny Hobbs meter reflects one of the leading reasons general aviation pilots stop flying. At almost every airport in the country where you can rent an airplane, the Hobbs meter is what is used to clock how long the plane was used. And “clock” is an accurate description of what the Hobbs meter is. Typically, they are started when sensing oil pressure. Thus, the moment you start the airplane, the Hobbs meter springs into action, counting time and slowly draining your checkbook.

Imagine you are a student pilot, and your trusty instructor is seated beside you for a lesson. You start the plane and get ready to taxi. As often happens, a question arises. It could be about the weather, traffic on the ramp or in the pattern, or any one of the many issues student pilots must master. A conversation ensues, and after a few minutes you take your feet off of the brakes and move forward with the lesson.

Or imagine you are an experienced pilot. It’s a busy day, and there is a lot of landing and departing traffic. The friendly ATC Ground person tells you to taxi and “hold short for incoming traffic.”

In each of these instances, the Hobbs meter is running silently in the background, counting the minutes in cold, impersonal precision. As long as your airplane engine is running, the Hobbs meter is doing its job and counting…..counting…..counting…..

Conversely, there are few instruments on an airplane that get more focus than the tachometer. The tach tells the pilot how fast the engine is running. Embedded within each tachometer is a timer that looks very similar to our nemesis the Hobbs meter. But don’t be fooled by their similar appearances. The tach has a quirk that every pilot paying for their flying based on how long they use the airplane needs to understand. The tach timer isn’t like a clock that mindlessly ticks away time. Tach timers count based on the engine RPMs. This is really important and is the core point of this blog. Pay attention!

When you are idling on the ramp, the tach timer counts much more slowly due to the lower engine RPMs. Remember that the Hobbs meter doesn’t care what the engine RPM is. As long as the engine is running it marks time at the exact same rate. The tach timer is different. For all of those flight activities where the engine isn’t running at full power (taxi, landing, power-off manuevers, slow flight, etc) the reduced engine RPM will cause the tach timer to run slower.

How much slower you ask? That all depends on the type of flying you are doing. If you start the engine, take off quickly and climb to cruise altitude for a two-hour cross country, the two timers will be almost identical. But much of the time general aviation involves more landings, takeoffs, and taxi time. (lower RPM) The general rule of thumb is the tach timer runs about 20% slower than the Hobbs meter. For student pilots that spend lots of time on the ramp discussing a landing or takeoff, that difference can be 30 – 40%. It’s a lot. Get your calculator and do some math. It’s a big number!

Remember earlier where I wrote that almost every place that rents airplanes uses the Hobbs meter? Almost every place. The Back 40 Flying Club is an exception. For our members, the rental time is calculated from the slower-counting tach timer. And that dear reader, translates into extra money in your pocket. Flying is an expensive hobby – we get that. But, the Back 40 does everything possible to keep general aviation pilots in the air. Lowering the cost for our members is a key piece of that strategy. Check us out. The Back 40 has been saving pilots money since the last century.