C172N & R series electrical malfunction

[kenguan]

I am comparing C172N and C172R POH Emergency checklist for electrical system in particular:

Electrical emergency procedure for C172N:
Excessive rate of charge:
1. Avionics power switch OFF
2. Master switch OFF both sides
3. Master switch ON
4. Overvoltage light check OFF
5. Avionics power switch ON
If overvoltage light illuminates again
6. Flight - terminate as soon as possible

Ammeter shows discharge:
1. Alternator OFF
2. Nonessential electrical equipment OFF
3. Flight terminate as soon as practical

Electrical emergency procedure for C172R:
Excessive rate of charge:
1. Alternator OFF
2. Non essential electrical equipment OFF
3. Flight - terminate as soon as practical

Ammeter shows discharge:
1. Avionics Master Switch OFF
2. Alternator CB - CHECK IN
3. Master switch OFF both sides
4. Master switch ON
5. VOLTS annunciator - Check OFF
6. Avionics Master Switch ON
If VOLTS annunciator illuminates again
7. Alternator OFF
8. Non essential electrical equipment OFF
9. Flight - terminate as soon as practical

Now notice the difference between the N & R for the same malfunction - they seem to be exact opposite actions for the same malfunction -
in the N, the procedure of 'ammeter shows discharge' is basically the same procedure for the R if 'excessive rate of charge'

Is there a different electrical system between the N & R?

[Pilot DAR]

It is my guess (and therefore not a certainty, until I check the circuit diagrams for each plane) that the systems are basically the same in both aircraft - other than the annunciation itself. The old "overvoltage" light was probably not entirely accurately named. It is more a light which tells you that the overvoltage sensor has taken your alternator off line to protect your avionics from over voltage. In accordance with the Flight Manual procedures, you might recycle everything to see if it comes back. I think that the newer Cessnas annunciate this a little differently, but really the affect is the same.

Consider it form that perspective, and see if it makes more sense....

[david viewing]

This seems to reflect varying opinions about the chance of fixing an alternator overcharge problem by cycling, something that's been discussed here before. Supposedly in the 'N' it's worth trying (maybe the mechanical voltage regulator would unstick) whereas in the 'R' it's not worth the effort or fire risk.

Contrarily, in the 'R' it's worth cycling to fix a trip out, but apparently not in the 'N'.

This conflicting advice, not for the first time, causes me to wonder if aeronautical engineers are truly comfortable with electrical things.

In reality and in any situation more than a few miles from the circuit, a certain amount of cycling is likely to be resorted to anyway. Many of these electrical problems (in my direct experience) are related to the crude and amateurish design of aircraft electrical systems, the obsession with screw terminals, thermal breakers and the poor protection of wiring in the engine bay. Amongst modern car manufacturers, only Jaguar could rival the carp electrical quality of the average light aircraft.(IHMO).

[Big Pistons Forever]

Pilot DAR is correct.

The electrical system in the N and the R are functionally the same. Both airplanes have a overvoltage sensor in the alternator control unit which takes the alternator off line if an overvoltage condition is detected.

The difference is the warning light. The N has an "overvoltage" light. This light illuminates if the overvoltage sensor (system voltage over 31.5 volts) is activated. The R (or S) model has no light indicating that the sensor has tripped, the only immediate indication of an overvoltage trip would be a discharge showing on the ammeter. If this is missed eventually the "low volt" light would illuminate as the battery ( which is now the sole provider of electrical power) starts to drop its voltage as it becomes discharged.

A few important points here

1) Over voltage and excessive rate of charge are not the same thing. It is possible to have an overvoltage without an excessive rate of charge and vice versa.

2) The ammeter is the instrument which will tell you what is going on and it should be regularly monitored in flight. If it is indicating a discharge (ie the needle is to the left of the center marker) then you do not have a functioning alternator and you had better do something as you will have no electrical power in as little as 30 mins. An inflight indication of a very high charging rate or discharging rate requires immediate action as it is an indication of something very wrong, with the worst case scenario the potential for an inflight electrical fire. If this indication is accompanied by a burning smell then the electrical fire emergency checklist should be actioned without delay.

3) The overvoltage sensor works by removing the current to the alternator field. If an overvoltage condition is suspected then the "ALT FLD" circuit breaker should be checked. If it is popped then you almost certainly have had an overvoltage condition. One reset is allowed but personally if I was close to an airport I would just land and sort out the issue on the ground.

[david viewing]

Totally agree with BPF except to say that the UK-spec 'low volt' light comes on pretty much immediately after the alternator fails in the PA-28. (I know - more than once!). I suspect the 172R is similar since it likes to flicker while taxying after landing, or even in the descent.

[Big Pistons Forever]

I suspect the 172R is similar since it likes to flicker while taxying after landing, or even in the descent.

If the low volt light is flickering like you said then something is wrong with the aircraft as the low volt light comes on at 24.5 volts and the alternator is more then capable of maintaining 28 + volts even at taxi RPM.

[Gertrude the Wombat]

Amongst modern car manufacturers, only Jaguar could rival the carp electrical quality of the average light aircraft.(IHMO).
The main problem with car electrics in my view (and I don't see why aircraft shouldn't be similar) is that when you start with the real basics at school the first thing they teach you is Ohm's Law ... and car electrics don't obey Ohm's Law.

[Well, they obey Ohm's Law as applied to the circuit as it actually exists, of course, which includes leaking insulation, resistive joints between wires, non-linearities in both, and so on. But this isn't, is it, the same as Ohm's Law applied to the circuit as it appears in the handbook, which, eg, shows pieces of wire as being passive components with zero resistance.]

[kenguan]

But the procedure for:
"Ammeter shows discharge" on an N series is the same as
"Ammeter shows excessive rate of charge" on an R series...

Two same procedures for two different malfunctions on two different series of aircraft...?
Electrical system is different betwwen an N and R or something is wrong somewhere...>

[India Four Two]

Gertrude,

The main problem with British car electrics in my view (in the old days) was:

Lucas, Prince of Darkness :E

Sorry, couldn't resist.

[cockney steve]

[QUOTE][Lucas, Prince of Darkness/QUOTE]For the younger reader, Lucas had a virtual market monopoly until the Japanese invasion finally took off in the late 70's.
the Lucas marketing slogan was "King of the Road"

I suspect IFT was one of us un the Trade who knew the truth! :O


Kenguan...Paradoxically, they're probably the same fault.
in an Alternator , the Rotor produces the Field magnetism and it's windings are fed by carbon brushes and slip-rings....the Field current can be fed by an archaic mechanical coil and armature reg, or an electronic Reg...the end result is the same......

Too much field current = high voltage Stator output (the main power terminal) = in extremis, boiled battery and fried electronics...NOTE in failure -mode, voltage normally rises/falls with engine-speed.

too little Field current...alty cannot reach battery-voltage and therefore battery will take the load until it's voltage drops to the Alty output -level
(too low to power the essentials)

Remember! the Donk. will carry on regardless- the outstanding advantage of Magneto Ignition....a generator, H.T.coil and Distributor all in one self-contained unit.

[India Four Two]

I suspect IFT was one of us un the Trade who knew the truth! http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/embarass.gif
No Steve, just an unhappy user of their products. Who remembers modifying a rubber glove to put over the distributor cap on early Minis, to keep the rain out?

The well-deserved epithet originated in the USA. I first heard it on the wonderful NPR Saturday morning show, Car Talk hosted by Click and Clack the Tappet Brothers!

Listen to their shows here:Car Talk (http://www.cartalk.com)