An apprentice is working on the left engine and she shows me the broken tach cable. It has broken fairly near the engine end. "How do you get the rest of the cable out of the housing?" I ask.
"That's what I was wondering," she confesses.
The engineer explains, "Normally, you would push it out with the new cable as you installed it, but because we're converting to the digital system, it will just stay in there."
"Forever? Like a mammoth preserved in a tar pit?" I am enamoured of the image of this historical piece of airplane entombed in its housing for all time. "But how does the new one work without a cable?"
Apparently it determines up the propeller rpm from the magnetos. I think this is every bit as cool as the picture on an iPod shifting from horizontal to vertical when you tip the device. It's probably just as simple, more a breakthrough in someone thinking of doing it that way than in the technology itself, but I will be pleased to never see another broken tach cable again, even while I am carrying a fossilized one around.
So I guess the new digital tachometer takes readings from both magnetos at the same time, computes some sort of average and is smart enough to recognize the situation when one magneto has failed or is grounded for a mag check?
The digital aircraft tachometers I've seen are connected to both magnetos and will give an alert when one magneto is not firing. However, there's no need for an averaging routine, given that if functional, each magneto will give an identical number of impulses per crank revolution, the difference between the RPM derived from the left and right mags would be zero to a precision far beyond that needed. I'd expect it to be an either/or input.
A digital tachometer that responds to magneto pulses is susceptible to electrical noise if not designed carefully, and even then there could be unforseen failure modes. How do you distinguish a lightning strike from a magneto spark? Noise pulses can be interpreted as extra magneto pulses, leading to a false overspeed indication.
By pushing control buttons you can get a reading of the drop when you select one magneto off: no need to do the math to see if it is within the acceptable range.
I'll have to look at the POH supplement for the equipment to answer the question about false overspeeds. I imagine that it might ignore momentary discrepancies. Or maybe it goes whacky during a lightning strike, like every other instrument on the panel.
Free food really does make most anybody a much happier, more well-adjusted sort of a person.
Interesting... I could imagine the old cable, cased in carbonite, would bother me a little too.
Thank you Andrew - I second your comment. The question is, where can one get free food (not to mention beer) from a regular, reliable source?
My fish thinks I'm cool.
A digital tachometer that responds to magneto pulses is susceptible to electrical noise if not designed carefully
I beleive that the electronic tachometers sense from the primary side of magnetos and are connected to the magneto switch circuit under the instrument panel, (or wherever the switches are located) which is a much less noisy place than the engine nacelles. Besides, how often do you get hit by lightning? And assuming that the instrument hasn't been damaged (completely separate issue), does would momentary inaccuracy in your RPM be crucial, or even noticed?
A Squared said: "would momentary inaccuracy in your RPM be crucial, or even noticed?"
Well, if a computer is watching the RPMs, it might be. Increasingly, aircraft avionics are incorporating "safety" measures that interrupt the usual operation of the aircraft when an unexpected sensor reading is received. A false overspeed could start a unforseen chain of events whose original purpose might have been to protect the equipment from damage, but which ends up depriving the cockpit of information or control. I'm thinking about the electrical generator problems I commented on at http://airplanepilot.blogspot.com/2009/09/electrical-encore.html , where intermittent current sensor fluctations (caused by an intermittent connection on a circuit board) caused the avionics to shut down a generator, dropping power to the captain's console, the transponder, and all three radios.
Wyane Farmer wrote: Well, if a computer is watching the RPMs
Right, you do know that we're talking about a airplane with engines that are functionally identical to the ones made half a century ago, don't you? Ones with levers, and cables, and purely mechanical fuel controls.
Ah, thanks. No, I don't think I've gotten a clear idea of what technology "aviatrix" is flying.
It's the Flintstones up front and the Jetsons down the back.
Wayne wrote: No, I don't think I've gotten a clear idea of what technology "aviatrix" is flying.
OK, without getting too specific, and given that Aviatrix has recently mentioned magnetos, and that her airplane was built when wood paneling was popular for basement rec rooms, It's reasonably safe to assume that the engines are reciprocating, purely mechanical, and would not lose any functionality with a complete loss of electrical power.
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