Flight Training - May 2013 - 46

As your flight instructor probably pointed out during your first
cockpit familiarization, along
with all the other instruments
in your cockpit 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,
usually whenever the engine is
running. The old Hobbs meter
is what the FBO probably uses
to bill you, as well as keep tabs
on when the next “hundred hour”
inspection is due.
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
rented a Piper Cub at the grass field near
my home, we didn’t have a Hobbs meter;
we just used our watches and the honor
system. But each time the big hand went
another six ticks around it meant that I
was out another four bucks. (Ah, those
were the days!) When I rent a Robinson
helicopter, you can bet there’s a Hobbs
in there, and that six minutes becomes
almost $30. Get a checkout in a King Air
C90 twin turboprop, and each six-minute
digit might run you roughly $100! That
certainly does confer a new perspective
on going once around the pattern.
Almost everywhere I’ve rented aircraft
in the Mid-Atlantic area, flight time is
usually reckoned by the reading on the
Hobbs meter. But there’s another, more
merciful variant; if you’re in a flying club,
your flight time may be tallied by the
tachometer. These two types of meters
actually keep time quite differently.
Being aware of how each functions won’t
ease all the pain, but it might mitigate
some puzzlement.
The oil pressure gauge is important
to the operation of the Hobbs meter.
46 /

FLIGHTTRAINING.AOPA.ORG

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.

Actually, for most airplane
Hobbs meter installations, oil pressure is
what makes it start
“ticking.” There’s
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 pounds
per square inch absolute—the switch
contacts close, the circuit is completed,
and at that point, your wallet becomes
eligible for a new weight-and-balance.
The meter reckons 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 nervous....)
Not all Hobbs meters are connected to an oil pressure switch. Some
airplanes have an “airspeed” switch,
which basically only counts flight
time. (Like the oil pressure switch, it
activates when reaching a particular
airspeed, usually takeoff speed.) Some
complex airplanes use the squat switch
or gear retraction, basically accomplishing the same thing, which would
imply you could taxi until the cows
came home and the owner wouldn’t
care—but don’t bet on it. Some are
wired to the ignition switch. And
another, less common method is when
the Hobbs meter is connected to the
battery master switch, which means, of


http://FLIGHTTRAINING.AOPA.ORG

Flight Training - May 2013

Table of Contents for the Digital Edition of Flight Training - May 2013

Flight Training - April 2013
Contents
Right Seat
Centerline
Letters
Soaring in New Zealand
News Success Story
How It Works
Since You Asked
After the Checkride
ASI News
Final Exam
Flight Training Excellence Award Winner AOPA Action
News
Member Products
News
Flying Carpet
Flight Lesson
Checkride
North to Alaska
Make Order Out of Chaos
Time Is Money
Technique
Weather
Fortune Teller
Career Advisor
Tech Talk
A Real Workout
Great Influence, Great Responsibility
Time for a Change
Advertiser Index
Debrief
Flight Training - May 2013 - Flight Training - April 2013
Flight Training - May 2013 - Cover2
Flight Training - May 2013 - Contents
Flight Training - May 2013 - 2
Flight Training - May 2013 - 3
Flight Training - May 2013 - Right Seat
Flight Training - May 2013 - 5
Flight Training - May 2013 - Centerline
Flight Training - May 2013 - 7
Flight Training - May 2013 - Letters
Flight Training - May 2013 - 9
Flight Training - May 2013 - Soaring in New Zealand
Flight Training - May 2013 - 11
Flight Training - May 2013 - News Success Story
Flight Training - May 2013 - How It Works
Flight Training - May 2013 - Since You Asked
Flight Training - May 2013 - After the Checkride
Flight Training - May 2013 - ASI News
Flight Training - May 2013 - Flight Training Excellence Award Winner AOPA Action
Flight Training - May 2013 - Final Exam
Flight Training - May 2013 - 19
Flight Training - May 2013 - News
Flight Training - May 2013 - 21
Flight Training - May 2013 - 22
Flight Training - May 2013 - 23
Flight Training - May 2013 - Member Products
Flight Training - May 2013 - News
Flight Training - May 2013 - 26
Flight Training - May 2013 - 27
Flight Training - May 2013 - Flying Carpet
Flight Training - May 2013 - 29
Flight Training - May 2013 - Flight Lesson
Flight Training - May 2013 - 31
Flight Training - May 2013 - Checkride
Flight Training - May 2013 - 33
Flight Training - May 2013 - North to Alaska
Flight Training - May 2013 - 35
Flight Training - May 2013 - 36
Flight Training - May 2013 - 37
Flight Training - May 2013 - 38
Flight Training - May 2013 - 39
Flight Training - May 2013 - Make Order Out of Chaos
Flight Training - May 2013 - 41
Flight Training - May 2013 - 42
Flight Training - May 2013 - 43
Flight Training - May 2013 - Time Is Money
Flight Training - May 2013 - 45
Flight Training - May 2013 - 46
Flight Training - May 2013 - 47
Flight Training - May 2013 - Technique
Flight Training - May 2013 - 49
Flight Training - May 2013 - Weather
Flight Training - May 2013 - 51
Flight Training - May 2013 - 52
Flight Training - May 2013 - Fortune Teller
Flight Training - May 2013 - Career Advisor
Flight Training - May 2013 - 55
Flight Training - May 2013 - Tech Talk
Flight Training - May 2013 - A Real Workout
Flight Training - May 2013 - Great Influence, Great Responsibility
Flight Training - May 2013 - Time for a Change
Flight Training - May 2013 - Advertiser Index
Flight Training - May 2013 - 61
Flight Training - May 2013 - 62
Flight Training - May 2013 - 63
Flight Training - May 2013 - 64
Flight Training - May 2013 - 65
Flight Training - May 2013 - 66
Flight Training - May 2013 - 67
Flight Training - May 2013 - 68
Flight Training - May 2013 - 69
Flight Training - May 2013 - 70
Flight Training - May 2013 - 71
Flight Training - May 2013 - Debrief
Flight Training - May 2013 - Cover3
Flight Training - May 2013 - Cover4
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