cessna 172 questions
wa...@esl.com
I rent a cessna 172 to do my flying. Recently some questions came up that I
would like 172 expert opinions on.
While doing the runup the alternator guage showed over charging. Having gone
through two alternators in my flying past, one going bizerk and getting
replaced and the other literally falling apart in flight, I decided to abort my
plans for flying. When I brought it back I was told that was because it had
been out the night before with lights on and that it just needed to be run.
First of all is this true? Is this a normal condition? If it is true, it says
to me that it doesn't supply enough power to run the lights and drains the
battery. Since it was only out 2.4 hours the night before what would happen if
it had been flying all night?
Second question. A couple days of before I had the same plane and was doing
pattern. After several landings, I noticed that when I gave it full throttle
to take off the oil pressure gauge jumped to high. When I quit climbing it
settled down to normal range. When on final and I was below 1000 rpm the oil
pressure gauge dropped out of the normal operating range. I brought it in and
was told that was normal when the engine was warm and that I just hadn't
noticed it before.
Is all of this true and I've missed things before or is it time to find a
different plane and/or place to rent from? Neither makes sense to me and I'm
leary.
If anyone could set me straight I would appreciate it. If possible, email
would be preferred
Thanks
Dr. Daniel R. Masys
>
>While doing the runup the alternator guage showed over charging. Having gone
>through two alternators in my flying past, one going bizerk and getting
>replaced and the other literally falling apart in flight, I decided to abort my
>plans for flying. When I brought it back I was told that was because it had
>been out the night before with lights on and that it just needed to be run.
>First of all is this true? Is this a normal condition? If it is true, it says
>to me that it doesn't supply enough power to run the lights and drains the
>battery. Since it was only out 2.4 hours the night before what would happen if
>it had been flying all night?
>
compensate for the drained battery. There is a genuine over-charging condition
in cessnas, caused by failure of the voltage limiting relay inside the
voltage regulator. When this happens, the charge needle goes all the way
up, pegs the needle, and the overvoltage protection relay kicks in and
shuts off the alternator circuit, which must then be manually reset.
In newer brand C models this occurence is announced by a red "High Voltage"
lamp on the panel. I had this condition develop on my 182 about a year
ago, necessitating a 350nm flight without radios; luckily it was vfr.
Point 2: Congratulations! You have just discovered that the total electrical
load on 172's flying at night often exceeds alternator output (especially
if rpms are kept low, as in night pattern work). In the 172 I used to own,
a night approach culminating in low rpms and deployment of electric flaps
used to cause a dim-out of the lights followed by a tripped main breaker.
It's a wonderful case of marginal engineering.
>Second question. A couple days of before I had the same plane and was doing
>pattern. After several landings, I noticed that when I gave it full throttle
>to take off the oil pressure gauge jumped to high. When I quit climbing it
>settled down to normal range. When on final and I was below 1000 rpm the oil
>pressure gauge dropped out of the normal operating range. I brought it in and
>was told that was normal when the engine was warm and that I just hadn't
>noticed it before.
>
>Is all of this true and I've missed things before or is it time to find a
>different plane and/or place to rent from? Neither makes sense to me and I'm
>leary.
>
High oil pressures are quite common when the oil is cold (most engine manu-
factures specify both hot and cold oil limits for acceptable oil pressure).
Both my 172 (Lycoming) and 182 (Continental) push into the high 90's when
taking off with engine temps on the cool side; the shop manuals say this
is ok. The phenomena is generally a function of oil viscosity, so if you
use a heavy weight oil, this will occur at higher temps also.
The low oil pressure is less common, since it generally doesn't take too
many hundreds of rpms to maintain pressure in the galleys (most of the
pump output just goes to the pressure-limiting bypass valve and back to
the sump); but engine manufacturers cite remarkably low pressures as
satisfactory--Continentals may run as low as 10 psi; Lycoming is justifiably
more conservative with their camshaft troubles, and has a higher number
in the book, but it is not a lot higher.
So, bottom line is, A+ for observation skills and everything you describe
can be "within normal limits." Rise up in thine 172 and go gently in the
pattern...
Dan Masys
ma...@lhc.nlm.nih.gov
Week 7 of air commuting; lost a day when the alternator went south...
Ian Kluft
>through two alternators in my flying past, one going bizerk and getting
>replaced and the other literally falling apart in flight, I decided to abort my
>plans for flying. When I brought it back I was told that was because it had
>been out the night before with lights on and that it just needed to be run.
It's normal. No problem. Typically, if the battery is recharging from any
amount of deficit, you'll see it. So, if the lights were being used the night
before or if it took a few tries to start it, you can expect to see the
ammeter show a charging indication.
What you can't hear, over the engine noise, is the bellowing moan from the
battery, "feeeeeeeeeeeeed meeeeeee!" :-)
---
Ian Kluft | #include <std-disclaimer.h>
ikl...@uts.amdahl.com | If I don't identify my aircraft...
UTS Systems Software Division | can I report a UFO sighting?
Amdahl Corporation, Santa Clara, CA | do I need a UFO rating?
Gregory TRAVIS
In <1991Oct16....@nlm.nih.gov> ma...@nlm.nih.gov (Dr. Daniel R. Masys) writes:
>In article <6...@esl.ESL.COM> wa...@esl.com writes:
>>
>>While doing the runup the alternator guage showed over charging. Having gone
>>through two alternators in my flying past, one going bizerk and getting
>>replaced and the other literally falling apart in flight, I decided to abort my
>>plans for flying. When I brought it back I was told that was because it had
>>been out the night before with lights on and that it just needed to be run.
Wanda - you should elaborate on what you mean by "over-charging." A couple
of needle's widths above 0 is NOT over-charging. Pegged at 60+ IS
over-charging. Dan talks about this, grave, situation so I won't
elaborate.
>Point 2: Congratulations! You have just discovered that the total electrical
>load on 172's flying at night often exceeds alternator output (especially
>if rpms are kept low, as in night pattern work). In the 172 I used to own,
>a night approach culminating in low rpms and deployment of electric flaps
>used to cause a dim-out of the lights followed by a tripped main breaker.
>It's a wonderful case of marginal engineering.
Careful, Dan! Wanda also didn't indicate what vintage 172 she was flying.
Unless I missed it. Anyway, there is a wealth of difference between a
14 volt 172 and a 28 volt 172.
[aside: Dan, tripping of the main breaker does NOT sound right! In
ANY case.]
From my 172 service manual for a typical 28 volt 172 I list the following
loads (typical for night flight):
Battery Contactor: .5 amps
Fuel indicators: .1
Beacon: 6.0
Instrument lights: 0.7
Position lights: 2.5
Turn Coordinator: 0.3
Clock: negligible
Map light: 0.2
------
10.3 amps for the "basic" load.
OK, let's also assume Wanda had the following:
Encoder: 0.1
Strobes: 3.0
ADF: 1.0
NAV/COM (385A): 2.25 (worst case - always transmitting)
NAV/COM (385A): 2.25 (the second one)
XPDR: 2.0
Glide Slope: 0.5 (just one)
Auto Pilot: 2.5
DME: 2.9
Pitot Heat: 2.9
Post lights: 0.6
-----
20.00 amps
She also had the landing light always on:
8.9 amps
And she had air conditioning:
6.7 amps
And she couldn't keep her hands off the flaps:
8.5 amps
-----
24.1 amps
For a total of:
54.4 amps
From a (standard) alternator that can generate 60 amps above 1400
engine RPM.
Now, mind you, the 14 volt systems, with 60-amp alternators, will require
double the amperage of the above 28-volt system. There, you might want
to leave the landing light off, turn off the air conditioning, and leave the
flaps alone until short final. It is conceivable that such an aircraft
would be taxing its electrical system at night. But just BARELY.
Dan is right about the oil pressure. High is nornal when the engine is
cold. Cessna's position about the "green arc" on the oil pressure
gauge is, from talking to them, that it is only valid when the RPM's are
within their green arc. In other words, as long as it's above 25 psi
(or whatever for your model) when the RPMs are OUT of the green
arc - everything's OK.
greg
--
Gregory Reed Travis C I C A
Center for Innovative Computing Applications|gr...@cica.indiana.edu (work)
Indiana University, Bloomington, IN 47405 |frognix!gr...@cica.indiana.edu (hm)
Dr. Daniel R. Masys
>
>Careful, Dan! Wanda also didn't indicate what vintage 172 she was flying.
>Unless I missed it. Anyway, there is a wealth of difference between a
>14 volt 172 and a 28 volt 172.
>
My 1970 172K was a 14v. machine with a 40 amp alternator, not a 60 amp.
>[aside: Dan, tripping of the main breaker does NOT sound right! In
>ANY case.]
If you add up all of the avionics, nav lights, strobes, 2 100 watt landing
lights, and toss in the flap motor on slow final...Plink! 40 amp breaker
pops some (but not all of the time). But hey, now that is someone else's
problem. BTW, has anybody seen an emerald green, gold and white C172
number N7193G? That was my bird, but its been sold a couple of times since
and flown the coop!
>
Dan Masys
ma...@lhc.nlm.nih.gov
Gregory TRAVIS
>In article <greg.687757183@weasle> Gregory TRAVIS analyzes the electrical load
>of a C172 night flight, then concludes:
>> ... It is conceivable that such an aircraft
>>would be taxing its electrical system at night. But just BARELY.
>It's a good analysis of the load, but I have to fault you on the conclusion.
>It's like assuming that a car that runs non-stop for 4 hours will take longer
>to cool down than one run for 2 hours.
Good point. And Dan's correction about his 14v/40amp bird is also
well-taken. Such an electrical system is truely marginal at best
for night flight.
However, my posting was centered about the "modern" (if you will) 172s
with 28v/60amp electrical systems. I think they have pretty robust
electrical systems.
Although I don't have the exact specifications on the alternators used,
one should bear in mind that alternator output does not fall off like
generator output. My alternator seems capable of keeping the battery
charged, at night, with anything over about 900 RPMs. I don't know about
the rest of you, but I don't get below 1500RPM until VERY short
final, especially at night when I fly the airplane all the way to touchdown.
[The alternator RPM is higher than engine RPM - a car alternator
usually turns twice as fast as the crankshaft; I can't remember what the ratio
is for my bird.]
All that aside, David and Dan are right - you're always going to see a
deficit on the battery's part right after startup. The length of
the deficit and its depth are dependent on many factors including
battery condition, temperature, cranking time, etc. And who flew the
plane the night before.
G. David Frye
of a C172 night flight, then concludes:
>would be taxing its electrical system at night. But just BARELY.
It's a good analysis of the load, but I have to fault you on the conclusion.
It's like assuming that a car that runs non-stop for 4 hours will take longer
to cool down than one run for 2 hours. Not true because the cooling system
keeps the engine in a certain temperature range. Likewise the alternator keeps
the battery at a "full charge", as long as it is able, but doesn't build up a
reserve of "extra charge". As soon as the engine is throttled back over the
numbers, the alternator stops charging at its full capacity. Figure 5 minutes
or so to taxi back to the FBO, landing light and radios on, flaps being
retracted, all with the engine at 1000 RPM or so, and you've got plenty of
time to drain _some_ of the charge. Next thing the battery is asked to do is
run the starter. It's not unusual for our C172M to show a +10A charge for
several minutes after starting, especially when the battery is old and won't
hold a full charge.
G. David Frye
-----------------------------------------------------------------------------
INTERNET: fr...@cerl.uiuc.edu PLATO: frye / s / cerl PHONE: (217) 333-7439
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Kevin P. Kleinfelter
That brings to mind my night training. My instructor had me do pattern
circuits as:
* Takeoff with no flaps, but landing light on
* Light off when turning crosswind
* Fly downwind with landing light off (of course position lights are on)
* Do my normal flaps stuff as I near and turn downwind.
By the time I'm downwind, I've gone to about 30 degrees by feel and
timing (did you ever try to find the doorpost mounted flap indicator
in an old C-150 at night?)
* Landing lights on.
* Turn final and land.
Afterward, I wondered about all that time in the pattern with only my position
lights to make me visible, but I never thought to ask. It makes good sense
when viewed in the context that some Cessnas may not be able to handle
concurrent landing lights and flap operation. Yet another reason I dislike
electric flaps!
--
Kevin Kleinfelter @ DBS, Inc (404) 239-2347 ...gatech!nanovx!msa3b!kevin
Dun&Bradstreet Software, 3445 Peachtree Rd, NE, Atlanta GA 30326-1276
WARNING: I have been advised that email to ke...@msa3b.UUCP may bounce.
It looks like email will have to go via 'gatech' because that is well-known.
Hal Kempthorne
after startup. Some discussion about load and charging capacity
followed in an effort to discover where all the electrons went.
What happens during starting is that the starter motor draws a
bunch of juice out of the battery. It may take 300 to 400 amps
for, let's just say, 10 seconds. After the engine starts, the
charging system goes to work replenishing the battery at a rate
of maybe 20 amps. So it takes 150 to 200 seconds to bring it back
to full charge. Even more if it took a lot of cranking to start
the engine. Of course, the Hobbs meter starts when the engine
starts so one wants to turn on all the radios right away too. :-)
Hal Kempthorne
Debonair N6134V