Just like the weather, about which everybody jokes but against which no one ever takes action, there is another equally uncaring adversary. It is a mere mechanical foe, a simple instrument, and one that we don’t really even need to keep in our scan to keep the shiny side up. But while we proudly total the growing hours in each succeeding page of our logbooks, we must first reconcile this sepulchral tally at the end of every flight. That opponent, gentle reader — the Hobbs meter.
THE METER MAID
In the bold and dashing days of pilots from generations past, pilots paid for their time, and got paid for it, to be sure, but it wasn’t metered out in such precise six-minute intervals, as it is now. Actually, it all started with the earliest electrically wound automobile clocks, and a man named John Weston Hobbs. In 1938, he founded the company which parlayed this interest in accounting into a timekeeping empire that now makes meters for lamps, switches, and many other devices not just in aviation, but also in the agricultural, automotive, construction, marine, medical, as well as recreational industries. As your flight instructor probably pointed out during your first cockpit familiarization, along with all the other instruments is this little set of numbered wheels behind a small window (circular or rectangular) that reads off tenths of hours, hours, tens, then hundreds, then thousands. The old Hobbs meter is what the FBO probably uses to bill you — as well as keep tabs on when the next “hundred-hour” is due.
EVEN DICK CAN’T BEAT THIS ONE
I guess it’s just another morbid curiosity of mine, or perhaps I’m moved to write about it because I’ve always mused over the coincidence that the owner of the primary FBO at my home drome happens to be a gentleman by the name of Jonnie Hobbs! (Yes, he gets asked that question from time to time, and no, there’s no relation … though he has said that he wishes there were.) But seriously, this is one somewhat sinister but sacrosanct realm wherein even our own George Wilhelmsen’s reckless and compulsively non law-abiding acquaintance Dick would not dare to trespass. Nor would any A&P working on the right side of the tracks … or so I’d like to think.
DOING A 360 (IN SECONDS)
Of course, oil pressure is important. So are other instruments, depending upon your particular brand of flying. Our fuel gauges, though notoriously inaccurate, do get our attention when they register near empty. And speaking of empty, the inevitable implication is that this is exactly what the Hobbs meter does to your bank account. If you’re renting your aircraft from an FBO, you can know with the certainty of death and taxes that every tenth-hour digit on the far right of its display is setting you back a few dollars. When I was a Cub renter at the grass field near my home, we didn’t even have a Hobbs meter; we just used our watches and the honor system. But each time the big hand went another six ticks around meant that I was out another four bucks. (Ah, those were the days!) When I rent a Robinson helicopter there’s a Hobbs in there, too, but suddenly those same six minutes burn about $18. Get a checkout in a King Air C90 let’s say, and each six-minute digit will run you somewhere around 90 or 100 bucks! It certainly does confer a new perspective on going once around the pattern, doesn’t it?
TWO KINDS OF SNITCHES
Almost everywhere I’ve rented aircraft here in the mid-Atlantic area, flight time is usually reckoned by the reading on the Hobbs meter. But there’s another, more merciful variant, and if you’re in a flying club, you may get to benefit from its softer-hearted cousin, the tachometer. I’ll cover that one too, in a minute. Of course, one of the luxuries for those lucky few of us who own our own aircraft is that even the Hobbs meter recedes from its status as an ever-present annoyance to that of only a perfunctory factotum for a far-off TBO. But for the rest of us … it matters. These two types of meters actually keep time quite differently. Being aware of how they each function won’t ease all the pain, but it might mitigate some puzzlement.
BACK TO OIL
First, the Hobbs meter: A moment ago, I mentioned that the oil pressure gauge is important. Actually, for the vast majority of airplane Hobbs meter installations, oil pressure is what makes it start “ticking“. There is usually a wire from “ground” to one terminal connector of the Hobbs meter, and a wire from its other terminal then goes to a pressure sensor or switch, and another wire completes the circuit to the power supply. When oil pressure rises beyond some predetermined minimum value, usually about 10 psi, the switch contacts close, the circuit is completed, and at that point, your wallet becomes eligible for a new weight-and-balance. The meter measures off time at the same speed as your wristwatch, whether you’re climbing out at full power, or sitting with the engine idling at 950 rpm and number three for takeoff. You can’t save money by shutting off the master switch, because it’s almost never connected through it. (You could shut down the engine, but that might make the folks behind you a little nervous…)
Not all Hobbs meters are connected up to an oil pressure switch. Some airplanes have the mechanism activated by an “airspeed” switch, which basically only counts flight time. (Like the oil pressure switch, it activates when reaching a certain airspeed, usually takeoff speed.) Some complex airplanes have it hooked up to the “squat switch” or gear switch, basically accomplishing the same thing. That would also mean you could taxi until the cows came home and the owner (apparently) wouldn’t care (but don’t bet on it). Some are wired to the ignition switch. And another more uncommon method is when the Hobbs meter is connected to the battery master switch (which means of course, that you could be paying even if the engine isn’t running … but once it is … well, I won’t go there).
THE KINDER, GENTLER REAPER
Then comes the tachometer. In addition to an analog dial showing the engine rpm, there’s often a meter within its face, having a presentation much like that of the Hobbs. The traditional ones are “mechanical” like this recording tachometer, although many are digital, and some do not record hours at all. But for those that do, they usually don’t mark the passage of time like the Hobbs meter. With the tach-based hourmeter, there is a geared drive from the registered rpm, connected to the engine via cabling, but it is made to keep “true time” only at one particular engine rpm, which in a typical trainer at high cruise might be, say 2500. If you’re rolling down a two-mile taxiway at Big City International at 1250 rpm, the tach time will only register six minutes for every 12 minutes of actual clock time, at that slower RPM. And when you’re siting there behind three other airplanes, idling at 800, stewing about the long wait, well, if you’re paying by tach time, things could be worse.
If you’re flying along on your cross-country flight at 2100 rpm, or doing pattern work at 2000, well then it’s kind of like relativity: it’s as though time is going by more slowly for you … and your wallet. Flying clubs often bill their members by tach time, where you ring up less time than you actually flew, because all they really need to do is make ends meet. An FBO bills by Hobbs time because not only do they want to make ends meet, but they want to make a profit as well. (And again, if you own your own plane, well then the only concern you might have is perhaps flying at that unpublished “best miles per gallon” airspeed.) For an airplane that you rented from the FBO, your natural inclination will probably be to go ahead and fly at a higher power setting that makes the tach hour nearly equal a clock hour, because that’s what you’re going to get billed for, anyway! For most trainers, flown at conservative power settings, the difference between Hobbs and tach time might be as much as 20% (with tach time being the smaller of the two of course). For those that pay by the tach hour, figuring out what the most economical “best tach speed” is for cross-country flight might be a useful exercise.
TRUST, BUT VERIFY
Some operators ask their renter pilots to write down the start and end times for both Hobbs and tach hours. This helps alert their maintenance folks, so that any malfunction in either instrument will be more easily noticed, and keeps the “accountability trail” as current as possible. For example, it is theoretically possible for congealed oil to hold the Hobbs switch open after an unsuccessful engine start on a cold morning. If your rental log can prove it was at such-and-such when you first got there, no one can point an accusing finger at you. (Obviously, before you fly any airplane that isn’t yours, cover yourself by making sure that what the previous person wrote down for an ending time is what you see when you first look inside the cockpit! It is a rare pilot who has been flying for awhile who hasn’t caught a bookkeeping error, like an undocumented maintenance flight, for example.) Also some operations request that if, when you shut down, the tenth-hour is anywhere between one digit and the next (perhaps even if it’s just starting to move) that you round up to the next higher tenth. In a way, if you fly much at all, it all comes out even in the end. The advantage to a particular rule like this is that at least it removes the uncertainty about what to write down when you check back in with the dispatcher.