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:
"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.
This says there is no history of difficulties either with
equipment or crew.
Sounds like a healthy battery.
All "normal" except one. If the "one" was not normal, why no further investigation?
This argues with the foregoing statement about what has already been cited as abnormal operation of the ammeter.
They anticipated flight into IMC . . . not a place to be without a source of electrons.
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 equipment.
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.
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.
A question closer to the point . . .
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.
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.
Good move . . .
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.
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.
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.
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.
. . . weasel-words. "correct", "borderline", "misinterpreted" and "warning" do not assemble into a clear explanation of of why this flight launched in the first place.
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.
Exactly.
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 . . .