No Ammeter Reading - Would You Scrub?

Discussion in 'Flight Following' started by sefeing, Oct 28, 2021.

  1. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    On the other hand, I know a guy who had a generator fail…turned out the broken gear put a hole in the case.

    Or when the generator failed because of a hydraulic leak that started a fire that the airplane didn’t tell me about. :(
     
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  2. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    You’ve got that backwards. ;)
     
  3. Clip4

    Clip4 Final Approach

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    I would have parked the plane and called it a day.
     
  4. Shepherd

    Shepherd En-Route

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    You have electricity in your plane?
    sigh.....
     
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  5. Brad W

    Brad W Cleared for Takeoff

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    Is that from an MEL? (serious question...I'm a rusty pilot and fuzzy on details now)

    I was in the habit of doing that most flights for a while...checking I mean. I wouldn't have cancelled like you said...but I'd be aware if for whatever reason I wanted to extend the flight and it was getting late.... and I could write it up for the next renter and the FBO to know about and fix
     
  6. bflynn

    bflynn Final Approach

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    Kinds of Equipment List. Some aircraft have them published in the POH
     
  7. Maxnr

    Maxnr Pre-takeoff checklist

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    Some have them as a separate bound document.
     
  8. unsafervguy

    unsafervguy Pattern Altitude

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    first regulatory issues, are you legal taking off with know inop equipment without it properly deactivated and logged? answer, no.

    how many single engine recips have an MEL? not many. if you have an MEL program, the proper sequence is, is it MELable? yes, get the A&P to MEL it unless you have a crew placardable program also.
     
  9. NoHeat

    NoHeat En-Route

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    In the 354 page POH for a SR22 (Gen 1/2) there is such a list, but it’s called “Kinds of operation equipment list.” It’s found in Section 2 Limitations.

    It has columns for VFR and IFR, for example ALT2 is required for IFR but not VFR., while both ALT1 and amp meter indications are required for both VFR and IFR.

    Older planes I’ve flown had a much shorter POH, without such a list.
     
  10. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    Don’t confuse Air Carrier MEL programs with Part 91 authorization/use of the MMEL.
     
  11. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    That’s not an MEL. It’s one of the documents listed in 91.213(d)(2)(I).
     
  12. Lindberg

    Lindberg En-Route

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    How would a functioning ammeter factor into any of these failures?
     
  13. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    Didn’t say it would.
     
  14. unsafervguy

    unsafervguy Pattern Altitude

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    I’m not, what sel Piston airplanes have an faa mmel? None that I know of.
     
  15. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    There is a generic single engine MMEL, but I don’t know of any Part 91 operators who use it.

    If they did use it, your post would still be incorrect.
     
  16. N747JB

    N747JB Final Approach

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    We heard a zing in the headsets and I figured out it was the belt. But, I was 23 year old pilot that was still semi-invincible! :D
     
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  17. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    Having the systems knowledge that you had a belt-driven rather than a gear-driven alternator makes a big difference in the decision to continue or land.
     
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  18. Robert Gee

    Robert Gee Pre-takeoff checklist

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    Induction style or shunt style ammeter? JK! Back to parking and ask for some free time next trip.
     
  19. Lndwarrior

    Lndwarrior Line Up and Wait

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    According to the OP, the gauge was the only information available on the electrical system in this plane. He/she had no idea what was going on under the cowl.

    Why is the decision even a question?
     
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  20. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    According to the OP, there was also an alternator light that worked properly.
     
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  21. Sluggo63

    Sluggo63 Pattern Altitude

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    This.

    Everybody glossed over the fact that the ammeter was zero, but the alternator light was out until he switched the ammeter off, and that's when the light illuminated.

    I'm not saying that he was wrong to scrub, but a little troubleshooting and system knowledge might tell you that all you had was a bag gauge but the alternator was still working.

    In the KC-135 we had an analog oil pressure gauge and also a "OIL PRESS LOW" light for each engine. If the gauge went to zero, the manual told you to do nothing unless the light was also illuminated.

    Again, I'll never second guess someone being conservative.
     
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  22. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    But did you depart that way?
     
  23. luvflyin

    luvflyin Touchdown! Greaser!

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    What about vice versa? Gauge says ya got pressure, but the warning light illuminates?
     
  24. Maxnr

    Maxnr Pre-takeoff checklist

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    Happened to me. L. base to final. CP flying. 1st indication: power fluctuation. 2nd indication: Warning light:"#1 fuel press low." Action: slammed #1 into X-feed. 3rd indication #1 fuel pressure gauge STARTS to drop. Landed OK. A&Ps found a bad check valve in #1 fuel system after 2 day tear down.
     
  25. Dan Thomas

    Dan Thomas Final Approach

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    The KC-135 won't have the same system so many small airplanes use. In most airplanes that don't have an ACU (Alternator Control Unit, the electronic doodad that unifies the regulator and overvolt circuitry), that alternator light simply tells you whether the regulator is switched on or not. The overvolt sensor is in the line between the ALT switch and the "S" terminal of the regulator, and if it senses an overvolt condition it shuts the regulator off (which shuts the alternator off) until you recycle the switch. The alternator belt could fail, or the gear drive, or the output cable or field wire could fall off the alternator, and that light will not illuminate. The alternator could fall right out of the airplane and it wouldn't light up. The light is fired by an internal source in the regulator that has nothing to do with alternator output. These older regulators were adapted from car systems, but the car was wired differently so that the light illuminated if the alternator quit. The airplane is wired so that the system can be shut off, and that function was sacrificed in the process.

    Switching the ALT switch off just tells you the light is working, and not much else. Pilots that aren't mechanics with some detailed knowledge of the system won't know that. It's easy to get complacent though ignorance.
     
    Last edited: Oct 29, 2021
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  26. Albany Tom

    Albany Tom Pattern Altitude

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    Yep! I'd have made the same decision. As others have pointed out, if it seems weird, bring it in.

    The risk here isn't that you don't have an ammeter, or that you aren't compliant with whatever rule. As I see it, there two risks. First, there's a good chance you don't have a functional charging system, so you're running battery. So you maybe lose the radio, and maybe kill the battery. Second risk is that you don't know WHY the ammeter isn't working.

    Maybe the ammeter is reading zero because the alternator/voltage regulator is dead. Not a big deal. Other possible reasons depend on the aircraft. In a PA-28, you could have an open wire anywhere from the alternator to the battery solenoid. If the alt warning lights depending on the alt switch, from my read of the schematic, the open would have to be between the alt light connection and the battery relay. If the wire is broken, is it flying around in the engine compartment somewhere? There's no fuse in that line shown in the POH. The service manual for some shows a breaker. A loose hot wire isn't good. Probably you could shut the alternator down and reduce that risk. That's a lot of stuff to sort out, in the air, though, with just the POH. All you know for sure is that there's a problem with the electrical system, and you don't know why. Bringing it in sounds like a good plan to me.
     
  27. Dan Thomas

    Dan Thomas Final Approach

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    The light is connected as shown below. I have added it into the circuit diagram as found in the Cessna Charging Systems manual.

    upload_2021-10-29_19-15-0.png

    The light is shown in red. It's connected to the regulator's "I" (indicator) terminal and the bus, usually at the alt field breaker. No breakers are shown in this diagram, for simplicity. When the ALT switch is turned on (shown as the "master" in this diagram, mistakenly except for the 150), the field relay closes and shorts the light and it goes out. When that relay opens, the light fires because it's connected to the bus and to ground through the regulator's 10 ohm and 50 ohm resistors. It's just that simple. Note that if the field wire fell off the alternator, or if the brushes failed, or if the output cable fell off the alternator, the light wouldn't illuminate. It's useless for telling us anything about the alternator's performance. All it tells us is that the regulator is shut off.

    upload_2021-10-29_19-30-21.png

    Here's the typical overvolt light setup. From that "I" terminal on the regulator to the bus. "13" is the light. "12" is the overvolt sensor.

    Now, some of those airplanes had the light labelled "Overvolt" and others "Low Voltage." There is no difference in the wiring or the function of the light. If the sensor detects an overvoltage condition, it interrupts the feed from breaker to the ALT switch ("3") and shuts off the regulator, and now we have a low-voltage situation because the alternator has stopped generating.
     
  28. Larry in TN

    Larry in TN Pattern Altitude

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    I've never flown the KC-135, but that situation often indicates that the oil filter is bypassing. The non-normal checklist will have you reduce thrust until the light goes out. If it doesn't go out, an engine shutdown is required.

    I had that exact situation in a DC8 over the Indian Ocean at around 3:00 AM. We shut it down and, since it was a four-engine jet, pressed on to Singapore where we landed on-time and under-burn. The filter had not bypassed. There is a pop-pin that indicates bypassing by the oil filter which the mechanic checked once we landed. It was a faulty oil pressure sending unit. The flight mechanic deferred it, in accordance with the MEL, and the next crew continued on to Tokyo.

    I guess my point is that it's often more complicated than you might first think. You don't necessarily know all the implications of the inoperative instrument or how it could affect your flight.
     
  29. Albany Tom

    Albany Tom Pattern Altitude

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    That's pretty cool. Quite a bit different from the Piper setup:

    pa-28-alt.jpg

    In this setup, the ALT light will illuminate either on the test voltage, or if the battery voltage is about 12V above the output of the alternator voltage. The piper setup is simpler, but I like it better.
     
  30. sarangan

    sarangan Pattern Altitude

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    For this reason, I start with just the battery side of the master switch, and then engage the alternator. This way you can verify both the discharge and charge indications. If the indicator is inop, you can catch it at this stage rather than wait until runup.
     
  31. Albany Tom

    Albany Tom Pattern Altitude

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    That's a great idea for the Cessna. For a Piper, the ammeter won't move until the alternator is on. It doesn't read battery discharge, only alternator output.

    Had no idea they were different until the above schematic post on the Cessna...I'd only read the Piper manuals.
     
  32. Huckster79

    Huckster79 Pattern Altitude

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    Not second guessing OP at all! Just answering above question.

    depends, I mighta but I fly out xtremely vfr bird… I don’t do controlled airspace, flaps are manual. If I plugged in my iPad n it charged… I may decide it’s just the meter. Solid daytime VFR… maybe. But there’s not many “other things” in my bird to be damaged - there’s just not a lot of equipment there nor anything electrical I truly need to fly safely.
     
  33. TrueCourse

    TrueCourse Pre-takeoff checklist

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    Another thing to watch for is when your POH says the alternator light will illuminate when the switch is off OR when there is low voltage. That may have been true when it rolled off the production line. Then, when a new version of an alternator control unit is installed the logic reverts to as described - illuminates only when the switch is selected off. Though it may have been documented in the maintenance logs, no one may have updated the POH and you’ll be reading (and depending on) incorrect information. This is particularly true if the plane changes ownership and the knowledge is not transferred with the plane.
     
  34. Dan Thomas

    Dan Thomas Final Approach

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    That Piper circuit will have the light come on if the alternator output fails. The light has positive at both ends when the alternator is operating, and if the alternator's positive fails, the bus's positive takes over and the light comes on. The grounded resistor can proved only so much current, and the alternator is stealing so much of it that the light can't illuminate unless the alternator fails. Much better setup.

    There are some other airplanes that have the voltage regulator connected like that. The overvolt line feeds both the S and A terminals on the regulator, so that a stuck regulator shutoff relay can still be shut off. Cessna has the A terminal fed off the alternator's output terminal, which is also connected to the bus via the ammeter. Can't shut that off unless you shut the master off. That A terminal tells the regulator what the voltage is doing, and also supplies the current for the field.
     
  35. Albany Tom

    Albany Tom Pattern Altitude

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    Yeah, I like the Piper setup a lot, too. I've wondered if it is a holdover from when pa-28's had generators rather than alternators. If I understand it correctly, in a generator system, there's the potential for the battery to try to drive the generator as a motor without protection, wasting both HP and battery current. With the alternator, there is a different risk, that without a battery connected to the alternator, you can very quickly fry the alternator windings on startup. So to fix both, they have that interlocking switch. However they landed on it, I like the configuration for the flexibility you describe.
     
  36. Dan Thomas

    Dan Thomas Final Approach

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    The generators had a different regulator that had some extra stuff in it. It had the usual voltage regulator, a current limiter (to avoid overloading the generator and cooking it) and a reverse-current cutout relay that prevented the battery from motoring the generator. Some ingenious stuff from a day before modern diode technology was available.

    upload_2021-10-30_20-10-45.jpeg


    The generator's output current all passed through the regulator.
     
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  37. Ghery

    Ghery Touchdown! Greaser!

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    If there is a doubt, there is no doubt. Scrub the flight and have it fixed.
     
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  38. woodchucker

    woodchucker Pattern Altitude

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    I witnessed a university flight school plane start up and try to taxi with two tie down straps and one wheel chock in place. They shut down, freed the plane and hopped back in to fly. I was thinking to myself if that was me I would pack it up and head to the local bar for lunch and a cold one.
     
  39. wsuffa

    wsuffa Touchdown! Greaser!

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    Would I have scrubbed it? I wasn't there, so I can't speak to the specific situation. I don't know the plane, the weather, or the equipment.

    Would I *always* scrub? It depends on the specifics of the plane, the condition, weather, alternates, distance to be flown, and availability of maintenance. And the aircraft required equipment for legal flight. For example, if there were an analog ammeter that was working fine and only the digital was out, and the analog could be considered primary, I'd probably go if the flight were short.

    Since my plane had ammeters (digital and analog) that measured across a shunt, I'd be more comfortable going if I could establish by a totality of systems that the battery was, in fact, charging (this might indicate that it's just the display portion of the digital meter and nothing involving the shunt). That comfort level would go up if it's clear-and-a-million VFR and I were operating from a non-tower airport (e.g. radios not required). I would not be comfortable under other circumstances. But some planes have the ammeter operate in a different manner, in which case I'd look at it totally differently.

    So I think the OP made a good choice with the information he had. Much better to scrub a flight than to discover other failures in flight.
     
  40. A Martin

    A Martin Pre-takeoff checklist

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    haaa .... good one ..... reminds me of the good old days where bush pilots opened up the vast Canadian northern wilderness.

    The only things electric were the two magnetos ..... and no airports .... you landed in clearings and on lakes .

    Navigate with magnetic compasses off by 30-40 degrees near the north pole

    .
     
    Last edited: Oct 31, 2021