bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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 Posted: Tue Dec 08, 2009 5:11 am Post subject: Who Has Got The Time? |
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12/6/2009 (and again on 12/8/2009)
Hello Fellow Pilots and Builders, The aeroelectric list has been filled
lately with discussions of hardware (meters) that keep track of time. Just
exactly what time is being kept, and for what purpose, is unclear. To shed
some light on the subject let's start by taking a look at what 14 CFR
(FAR's) say about time.
We'll take flight time first, then TIS (Time In Service):
1) Here is how pilot flight time is defined in section 1.1 of the FAR's :
"Flight time means: Pilot time that commences when an aircraft moves under
its own power for the purpose of flight and ends when the aircraft comes to
rest after landing;"
I am not aware of any meter that could keep accurate track of such time.
You'll find many pilots, and airlines, that do not keep track of flight /
pilot time in conformance with this definition. Standard compliance by all
pilots with this definition is unlikely because there is room for
interpretation.
Does "moves under its own power for the purpose of flight" mean the instant
the wheels start to roll as you leave the parking space in order to go
flying? Then that XXX minutes plus that you spend taxiing, doing engine run
up, and waiting for takeoff clearance at the end of the runway, would all be
flight time. Considering the delays involved in operating at some airports
one could become a multi thousand hour flight time pilot very quickly using
that interpretation.
On the other hand one is certainly exercising some very important PIC duties
from the time he leaves the parking space until starting takeoff roll.
Should all of that time be ignored and not recognized in some fashion?
2) Here is how section 1.1 of FAR's defines TIS (Time In Service):
"Time in service, with respect to maintenance time records, means the time
from the moment an aircraft leaves the surface of the earth until it touches
it at the next point of landing."
I suppose that there is hardware that could record this exact time, but it
certainly is not in common use in our category of airplanes.
So let's take a look at three common timing devices: the Hobbs meter, the
mechanical engine RPM based tachometer, and the software associated with
electronic flight instrumentation or engine instrumentation:
A) The Hobbs meter (you can look here for a quick review):
http://en.wikipedia.org/wiki/Hobbs_meter
It doesn't take very long to conclude that "Hobbs meter" has become a very
generic term (like Kleenex or Scotch tape) and that Hobbs meters come in
wide varieties and can be connected to record a wide variety of time. I
guess it is possible for a Hobbs meter to be connected up so that it records
one of the time definitions in the FAR's, but it doesn't seem easy.
B) FAR 91.205 (a) and (b) require an engine tachometer to be installed in
any powered standard category civil aircraft even for day VFR operations. By
far the most common type of tachometer found is the mechanical RPM based
tachometer. Presumably the purpose of the required tachometer is to inform
the pilot of his engine's RPM at any given instant, but somewhere along the
line these tachometers began including the total time of engine operation.
This required someone to decide how to convert instantaneous RPM into total
elapsed time of engine operation.
If, say 2,000 RPM were chosen as the standard mechanical ratio to convert
one minute at this RPM into one minute of elapsed engine operating time then
any engine operation at less than 2,000 RPM generates less than one minute
of elapsed engine operating time and any RPM greater than 2,000 generates
more than one minute of elapsed engine operating time. See some of the
tachometer conversion ratios between RPM and time available here:
http://tghaviation.rtrk.com/?scid=387399&kw=3649251
In any case it does not appear that any mechanical engine tachometer can
generate either of the elapsed times defined by the FAR's.
C) There is such a huge variety of electronic flight and engine
instrumentation systems and their associated software (and the ability of
the operator to modify the software in some cases) that any accurate
comparison of one airplane's / engine's / pilot's time to another airplane's
/ engine's / pilot's time would require some detailed examination of the
processes used to generate that time.
Again the probability that an electronic system would automatically generate
elapsed time in exact compliance with either of the FAR time definitions is
not likely.
So what is the builder / pilot to do? My suggestions:
a) Don't get all wrapped around the axle about generating time. Many people
are not recording flight time or time in service the same way that you are.
b) Pick some hardware and a system of documentation that seems to fit your
needs and go with it.
c) Be consistent in how you do things so that you can view and show the
results with some confidence.
d) Be wary of someone else's time claims, but don't make a big deal of time
unless you are billing by the hour.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
PS: My choice for my airplane was just to record "Tach time" from my engine
electronic data system for both flight time and TIS. My engine data system
generates zero
tach time anytime the engine RPM is less than 1,500 RPM and constant equal
running time in hours and tenths any time the engine RPM is above 1,500 RPM.
Recording this time for both flight time and TIS short changes me on pilot
flight time since a fair amount of time while practicing landings in the
landing pattern the engine is below 1,500 RPM. On
the other hand it will take much longer elapsed time for my engine to reach
its 2,000 hour TIS to be due for overhaul. This approach greatly simplifies
my bookkeeping.
Back when I was flying rental aircraft I just went with the flow and used
the FBO's Hobbs meter time for flight time -- I paid for it, I should be
able to log it.
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