Hobbs vs Tach Billing: Which Meter Should You Charge On?

Bill on whichever meter you want, as long as the hourly rate was built for that meter. That's the short version of Hobbs vs tach billing. Flight schools and FBOs mostly charge Hobbs time, flying clubs often charge tach, and both can recover the same cost per flight. It goes wrong when a group picks a rate on one meter and bills it on the other, because on a trainer flown at conservative power AOPA's Flight Training magazine puts the gap at as much as 20%, with tach the smaller number.

1. Why the two meters disagree

A Hobbs meter is a clock. Once it's running, an hour on the ground costs the same as an hour at cruise. A tach timer counts engine revolutions and is geared to keep true time at one particular RPM, usually somewhere near cruise. Below that RPM it records less than an hour per hour. Taxiing, the runup, pattern work, and long power-off descents all go onto the tach at a discount, which is why the gap changes from one flight to the next and from one airplane to another.

2. Hobbs vs tach billing: who uses which, and why

AOPA notes that FBOs typically bill by Hobbs and flying clubs often bill by tach. The reasons are practical. A school wants a number the student can see tick over on the panel, and Hobbs is that number. A club that only tries to cover its costs tends to bill on the meter its costs follow. TSS Flying Club, for example, bills on tach because fuel and maintenance make up most of the airplane's usage cost and both track tach time. The club tells prospective members they'll likely pay for 10% to 25% fewer hours than they would on a Hobbs-billed rental.

That 10% to 25% doesn't make tach billing cheaper on its own. If the club sets its tach rate correctly, the member pays the same for the same flight and only the number of billed hours goes down. If it doesn't, the club runs a deficit.

3. How to convert a Hobbs rate to a tach rate

Say a club 172 needs $150 per Hobbs hour to cover fuel, maintenance, reserves, and its share of fixed costs, and the club's own logs show tach running 15% behind Hobbs on average. A tach hour is therefore 0.85 Hobbs hours of cost recovery, and the equivalent tach rate is $150 / 0.85 = $176.47. Call it $176.50.

• Average flight, 1.4 Hobbs: logs about 1.19 tach. Hobbs billing charges 1.4 x $150 = $210.00. Tach billing charges 1.19 x $176.50 = $210.04. Same flight, same money.
• Pattern session, 1.0 Hobbs: lots of low-power time, so suppose the tach logs 0.75. Hobbs billing charges $150.00, tach billing $132.38.
• Cross-country at cruise, 2.0 Hobbs: suppose the tach logs 1.85. Hobbs billing charges $300.00, tach billing $326.53.

Across the fleet the money comes out even, but tach billing moves cost away from pattern and taxi-heavy flying and toward cruise. Members doing touch-and-goes pay less and members flying long cross-countries pay more. That can be a reasonable policy, but the membership should choose it on purpose rather than discover it.

The expensive mistake is different. A club that switches from Hobbs to tach and keeps the $150 rate is underwater on day one. At 400 Hobbs hours a year, it bills 340 tach hours and collects $51,000 against the $60,000 it needed. Nobody raised dues and nobody cut a rate on purpose, yet the club is $9,000 short at year end.

4. What the meter choice does to your engine reserve

An engine reserve is the overhaul cost divided by the hours left to TBO. Engine makers state TBO in hours of operation, and in practice many shops and owners log engine time off the tach. Check which meter your engine logbook uses, and build the reserve per hour on that meter.

If the logbook runs on tach and the reserve works out to $18 per tach hour, a club that bills on Hobbs should collect $18 x 0.85 = $15.30 per Hobbs hour at a 15% gap. Charging $18 per Hobbs hour over-collects by about 18%. That's money in the right fund, but the extra comes out of members' pockets. The reverse is worse. A reserve calculated per Hobbs hour and charged per tach hour comes up short every flight, and the shortfall only shows when the overhaul invoice arrives.

5. Should you record both meters?

Yes, even if you bill on one. Recording both at the start and end of every flight costs a few seconds and buys you several things:

• A check on the meters themselves. If the Hobbs advanced 1.3 and the tach didn't move, one of them has failed, and you'll catch it on the first flight instead of at the annual.
• A check on the people writing the numbers down. A tach-to-Hobbs ratio that is wildly off for one flight is usually a typo.
• Your real ratio, per aircraft. The 15% in the example above should come from your own logs, and it will differ between a trainer and a cruise-heavy club airplane.
• The maintenance clock. AOPA mentions schools using the Hobbs to keep tabs on when the next 100-hour inspection is due, while engine time often lives on the tach. Whatever your maintenance program counts, record it.

No FAA rule tells a club or a school which meter to bill on. It's a business decision, and it belongs on your rate sheet, printed next to whether the rate is wet or dry.

6. Pick the meter, then set the rate

1. Pull the last six months of flights and compute the tach-to-Hobbs ratio for each aircraft.
2. Build the cost per hour on the meter you intend to bill, including the engine reserve on the meter the logbook uses.
3. Publish the meter, the rate, and the wet or dry basis together on one rate sheet.
4. Recheck the ratio at every rate review. A new instructor who does more pattern work, or a member who flies mostly cross-country, will move it.