Dark-n-Stormy Night Stories Redux

May 8, 2004
Robert L. Nuckolls, III

On May 8, 2004 Mickey Coggins cited a "Never Again" story from pages of AOPA Pilot Magazine. He is properly concerned that while the story purports some clarity of thought and purpose to the operation of airplanes, the situation and conclusions cited in the story are unclear as to their usefulness.

Others on the AeroElectric-List have already picked up on flaws in this story's analysis and the supposed "lessons learned". Let us do a review of facts offered and see if logical conclusions differ from those of the story's author.

Dark-n-stormy-night stories can be sifted for words that add understanding to the physics of the problem. I think it helps if we toss out words that make it a better story and concentrate on the facts that offer understanding of the situation and physics. I've gleaned the article for data points and included them below with my comments formatted to stand apart from the story:

Never Again Online
Mission Critical (Excerpted)


  • Only an hour before, the three of us finished installation of the airborne photography equipment and exhaustive ground checks of the elaborate hardware.

       "Exhaustive" and "elaborate" are non-quantified. We
       can only assume that the narrator is telling us that there
       were battery-only operations of equipment on the ground
       before flight . . . the fact that battery only ops are
       mentioned suggests that he believes them to be significant
       in how the story unfolds later.
  • The system and crew had a history of working well together. The Piper Aztec was a bulletproof airplane highly respected by the crew.
        This says there is no history of difficulties either with
        equipment or crew.
  • Both engines started without undue hesitation . . .
       Sounds like a healthy battery.
  • . . . and all instruments and electrical checks proved normal, save one. The ammeter indicated a hairline discharge, probably a result of the loads placed on the electrical system during the LSP equipment checks prior to startup. No problem, we thought. The engine alternators would quickly bring back the charge.
       All "normal" except one. If the "one" was not normal,
       why no further investigation?
  • The runup indicated all systems go.
       This argues with the foregoing statement about what has
       already been cited as abnormal operation of the ammeter.
  • This would be a routine IFR flight with heater and hatch cover on.
       They anticipated flight into IMC . . . not a place to be
       without a source of electrons.
  • I settled in for a nap and the camera operator flicked on his reading lamp.
       Except for the "hairline discharge", stuff was working
       as expected including a reading lamp. The aircraft was
       not experiencing a bus voltage condition so serious as
       to produce noteworthy misbehavior of panel mounted
  • However, the ammeter persisted in reading a more pronounced electrical drain that troubled the pilot.
       We're not sure what kind of ammeter is being referred to
       here. It it a battery ammeter that reads minus-zero-plus
       like automobiles of yesteryear (and most light aircraft) or is
       it an alternator loadmeter that shows present system loads
       being carried by the alternator(s)? The the writer uses the
       word "discharge" which implies a left-of-zero reading on a
       battery ammeter . . . alternator loadmeters never produce
       readings that might be interpreted as a "discharge" condition
       so let us assume for the moment, he's talking about a minus-zero-
       plus reading battery ammeter.
  • It also seemed as if the instrument lights were not as bright as they should be. Was the ammeter malfunctioning or was there a short somewhere?
       Ammeters generally work or they don't. If the wires
       are hooked up and the instrument reads anything other
       than zero, then it's not malfunctioning. One can
       worry about "shorts" as root cause of the problem 
       but every significant short has a profound effect on
       the performance of some system or accessory. A short
       wouldn't be directly responsible for a less than
       comforting indication on the ammeter. 
  • He checked the circuit breakers. Were the alternators acting up?
       A question closer to the point . . .
  • One by one he tested the electrical loads to isolate the problem. Nothing remedied the situation but one thing was now clear: The aircraft was suffering an electrical failure.
       Not sure what "tested the electrical loads" means. It
       conjures up images of random switch-flipping hoping that
       at least one such action will cause the problem to clear.
       This is a trouble shooting procedure based on poor
       understanding of the system.
  • The loss of electronics meant flying the airplane with engine_driven, vacuum_powered primary flight instruments. The most precious, electrically dependent gauges were the radio navigation aids. With reference to these, the pilot could divert to the nearest instrument airport and let down through the scum.
       Yup, all those electro-whizzies on the panel do depend
       on a source of electrical energy. Most owners of light
       twins (even the heavy twins) have been sold a bill of goods
       based on the presumption that just 'cause we have TWO
       of some very useful items that we're far less likely
       to find ourselves in the situation being discussed.
  • All ancillary power drains were immediately shut down, including external navigation lights, anticollision beacons, internal post lamps, instrument illumination, heater, and autopilot.
       Good move . . .
  • The pilot could barely hear air traffic control. The system was failing fast.
       If the battery was so low that it affected apparent
       volume of received signals it wasn't just "failing fast".
       It would be less than 10.5/21.0 volts and already failed.
  • He turned down the volume to conserve precious power, dialed in 7600 on the transponder, and called in the emergency. The situation was becoming critical.

  • The pilot slowed the Aztec to its maximum landing gear extension speed and felt the wheels lock with confirmation from the three green indicator lights. It would be one less thing to worry about.
       If he got gear to extend, the battery was not yet failed.
       The low volume condition cited above was perhaps anxiety
       induced perception that differed from fact . . . or there
       was another problem working simultaneously with the low
       bus voltage.
  • The pilot decided to refrain from transmitting and maintained a listening watch for ATC's reassuring instructions as the controller painted the Aztec's position and cleared it for an emergency descent to Runway 26. If the radio failed completely, he was totally dependent on his navaids for the instrument approach. Were they trustworthy?
       This is unclear. It seems to differentiate "radios" from "navaids."
       If ATC is giving vectors then all he needs is a comm radio. If
       the comm radio fails due to low voltage, then it's a certainty
       that any other electronic devices useful for navigation will
       be similarly afflicted. The question of trustworthiness is
       rhetorical. Under these conditions no radio is trustworthy.
  • That night, a quick check of the aircraft's manual found the problem. Apparently the alternators would not kick in if battery power was depleted below a threshold amperage. This condition was created during static ground checks of the camera equipment, which resulted in alternator shutdown.
       Like most dark-n-stormy-night stories, it falls apart at
       the end. If they're talking about a check of the POH, then
       the condition cited does not speak of "threshold amperage" but
       a low voltage condition caused by a discharged, worn or
       damaged battery. These paragraphs speak to the need for
       some small amount of energy required to get alternators to
       come on line. 
       An alternator will come up with an assist of less than
       2 volts from a battery. This voltage will not close a
       battery contactor and bring the battery on line
       much less crank engines. If they got the battery contactor
       closed and cranked engines, the hypothesis of insufficient
       battery votlage for alternator excitation is not supported.
       The diagnosis of root cause suggests ground checks
       of the camera system as the primary cause of a 
       depleted battery such that alternators were
       incapable of operating. This cannot be so.
  • The ammeter reading was correct but so borderline that its warning was misinterpreted.
       . . . weasel-words. "correct", "borderline", "misinterpreted"
       and "warning" do not assemble into a clear explanation of
       of why this flight launched in the first place. 
  • The battery was charged overnight and the mission continued the next day without further incident. The lessons we learned were invaluable. No matter how insignificant a problem appears, it's still a problem that can lead to disaster. Know your systems and the significance of their impact on the mission.
       Like the wrap-up offered in the story I used to
       open chapter 17 in the book, this story only serves
       to spotlight a lack of understanding both on the part
       of pilots, manufacturers and those-who-know-more-about-
       airplanes-than-we-do in the design, certification and
       operation of aircraft electrical systems. These guys
       didn't have a clue as to what really happened.
       Brian Lloyd suggests this further interpretation based on his personal
       knowledge of the Aztec's systems:
    BL: I have an Aztec and I have studied its electrical system in detail. My immediate reaction is to quote some of our British bretheren and state, "what rubbish." (Actually I said something else but Matt says I am not to use those words here.)

    BL: If there is enough power in the battery to start the engines and run the lights and radios, there is more than enough to excite the alternators.

    BL: The Aztec has two alternators being driven by a single voltage regulator. The alternators are run with both fields and B-leads in parallel. I can find nothing in the system to ensure that they share the load equally but it seems to work just fine anyway.
       Don't know who "did it first" but when generators were replaced
       with alternators, there were no regulators available that would
       make two engine driven power sources work in precise concert with
       each other. I.e. share total system loads between them. Regulators
       for paralleling two generators were commonplace but not for alternators.
       While this didn't represent a big operational problem, it really
       BUGS the pilot of a twin system to see one alternator carrying
       most if not all of the loads.  At Cessna when we put alternators
       on the C-337, a number of schemes were tried. The most satisfactory
       from a perspective of bug-free pilots was to simply parallel
       two alternators and run them from a common regulator. This same
       system was used on Barons and now Brian tells us Piper did it too.
       To keep those-who-know-more-about-airplanes-than-we-do happy
       about loss of redundancy with a single-regulator approach to
       twin alternator systems, a second or "standby regulator" was
       added. The system included a panel mounted switch to select #1 or
       #2 regulator.  Problems with this scheme are obvious. There are
       single points of failure that can take out both alternators. It
       was about 15 years later that Cessna asked for a paralleling
       regulator design for the new C-303 Crusader. I designed one
       for Electromech to offer in competition with several other firms.
       I think ElectroDelta got the contract on that one. To my knowledge,
       the C-303 is the only certified ship to fly with truly paralleled
       but independent alternators. 
    BL: There is a second VR that may be switched into the circuit should the main VR fail. A toggle switch with a red flip-up cover on the lower center console, conveniently out of sight, selects the VR. (The Aztec isn't big on ergonomics and there are several controls that one must locate and operate by feel.)

    BL: So what could the problem have been? Well, my guess is that they had an overvoltage transient which tripped the overvoltage protection relay. This type of OVP latches and won't release until all power is removed from the buss. I bet dollars to donuts that if they had turned off the alternators (field switch) and then cycled the battery master, the alternators would have come back on-line.

       Interesting hypothesis. OV relays of the era are latching devices
       that will reset if the alternator field switch is simply turned OFF
       for a second or two and returned to ON. After "testing of
       electrical loads" cited in the story, the pilot may have
       missed the value in "testing electrical sources" as well. If
       you're reduced to random switch-flipping with hopes of fixing
       something, you would do well to flip ALL the switches.
    BL: It also points out how Bob's crowbar OVP system communicates the problem much more clearly to the pilot. When the field breaker pops you know what is wrong (or have a pretty good idea) and resetting the breaker puts everything to right unless there is really something wrong with the alternator system, at which time you resort to your essential bus (BN: endurance bus, ENDURANCE bus) and get on the ground.
       Close but no cigar. Instruments and breakers are POOR warning
       devices. This pilot noted abnormal behavior on the system's
       instrumentation during pre-flight and failed to react in any
       useful way. ACTIVE notification of low voltage is the true
       warning device. This is why that feature was built into the
       alternator controllers offered by B&C from day-one. If the
       only thing between you and the Dark Panel Syndrome is a battery,
       you need a really insistent light flashing in your face.
    BL: Getting back to the Aztec, it is possible that a weak battery would provide insufficient stabilization on the buss that the bus voltage could have risen too high when the alternators kicked on that the OVP relay was activated. Still, the battery would have to have been so dead as to not be able to power anything for that to be the case. Regardless, cycling the alternator field switch and the battery master should have cleared the problem.
      Given that the battery ran the airplane in a full-up IFR mode
      for so long after cranking two engines suggests that the battery
      was anything but weak. OV protection can be tripped by some
      kinds of transients. 
      Recall the writer told us, "The system and crew
      had a history of working well together. The Piper Aztec was
      a bulletproof airplane highly respected by the crew."
      If this aircraft had any past history of misbehavior in the
      OV protection system, one would have expected it to be a data
      point in the story.
      Brian further offers:
    BL: The Aztec has a combined volt/amp meter that has a three-position switch to display bus voltage or load on either alternator. Part of the run up check list is to cycle the switch through all three positions and abort if buss voltage is not correct or if either alternator shows no load (no output). So you can't miss this one. The meter readings are not subtle.
      Aha! Alternator loadmeters, not a battery ammeter. This data confirms
      the fact that pilot and storyteller for the incident under discussion
      had a poor working knowledge of this airplane's electrical system.
    BL: As I said in my previous message about this article, "what rubbish."
      I agree. When the writer said, "However, the ammeter persisted in reading
      a more pronounced electrical drain that troubled the pilot" it strongly
      suggests that the instrument was being interpreted as a battery
      ammeter. Alternator loadmeters don't display "drain". Our best interpretation
      of words in the story suggests the instrument was indicating zero output from
      the alternator(s). The pilot didn't have a clue as to the significance of
      what the instrument was telling him.
      Folks, this story was similar to story I used to introduce
      the topic of system reliability in chapter 17 of the 'Connection.
      These stories are worse than useless because (1) they offer little
      if any insight as to the simple-ideas . . . the physics of how
      the system works and (2) their conclusions based on ignorance
      become total fabrications . . . the garbage-in-garbage-out
      principal at its finest.
      It's good that these stories are cited and discussed here on
      the AeroElectric-List. UNDERSTANDING of the stone simple ideas
      upon which the system operates is only one antidote to the shining
      light of ignorance.
      Our spam-can flying brothers have no obvious mechanism with
      which to test the validity of the gospel according to AOPA
      Pilot, Flying, etc.  When folk who publish those magazines
      let stories like this go to press, they only serve to
      discredit themselves as a purveyor of useful knowledge.
      The OBAM aircraft community is constantly demonstrating
      both a willingness and ability to do much better. 
      Thanks to Mickey Coggins for bringing the article to the
      AeroElectric-List for considered critical review. Thanks also
      to Brian Lloyd for sharing his first hand knowledge of
      the Aztec's configuration.
      'Lectric Bob . . .