Sherlock Ohms

Turn on the UHF and the Plane Flips Over

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lessons for life
analog_Steve   9/9/2015 9:31:36 AM
Nice to hear from another former avionics tech!   I worked on avionics on A-4 Skyhawks and while my electronics knowledge was limited, I learned a lot about troubleshooting.   As you show, the ability to listen to the description of the problem, look for the symptoms, try to isolate the problem, etc, is important whether you are fixing a bad coax or trying to understand why your newest circuit isn't working the way you thought it should. 


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Re:UHF radio causes brake failure
Cleocat96   9/9/2015 9:42:48 AM
Years ago I was called into a consultation with a radio shop who serviced school buses. New UHF 100W radios had been installed and soon complaints of brake failure when transmitting were heard. We found the coax was good but routed in with the same wire budle as the antilock brake. Re-routing the coax solved the issue.

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Coax fraying
armorris   9/9/2015 10:06:55 AM
I'd ask why the antenna coax is getting frayed in the first place and fix that as well. I'd have all of the aircraft modified to eliminate the potential problem.

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Re: Coax fraying
patb2009   9/9/2015 10:34:24 AM
I have no idea if there was a system for sending "Information" around to the

other squadrons but, that's also something to do as well as add to the 

Maintenance manuals, or trouble-shooting issues as well as the pilots checklist.


If the manuals had a notation under Radio: Other Issues. Fraying of the UHF lead coax in the aft body may cause significant interference to the Stabaility Augmentation system

and cause uncommanded actuation of the Flaperons leading to Departure from controlled flight.


And add that to the StabAug System or Controls system.  Note: Even when powered off, an induced current from the UHF radio may cause uncommanded actuation of the flaperons threatening control and departure.


finally i'd add to the pilots checklist..  Key UHF Radio and observe all flight surfaces.


It's not just knowing the fix, it's preventing the problem.




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Plain BAD Design, bad response too.
Amclaussen   9/9/2015 10:59:15 AM
Grouping and bundling together critical wiring (ALL Flight controls MUST be considered critical) with a high powered antenna is inviting disaster.  This article should be published in the "Made-by-Monkeys" section, and taken seriously.  Aviation is aviation, even in the military.  Consider the high probability of a pilot in combat of receiving wounds, such a diminished capability pilot could be easily killed by an unspected control glitch such as that.  Even a highly capable and trained pilot can be taken by surprise when flying low, such as when landing on a carrier at high seas or at night.

The response of the technicians, mechanics and rest of the personnel was not proper. Laughing at the complaint of a fellow pilot means that the prevailing attitude was the typical "Macho-men" thinking their fellow was pannicking or cowed.  That kind of incident should have been duly reported not only by the pilot (who acted responsibly) but by all along the command chain, and analyzed together with the manufacturer, the design changed or improved, and all the aircraft of that type modified.

I know, my late father was an Admiral in the Navy, and very respected for his rare combination of rigorous strictness, sensibility, gift of command and leadership.  He would have laughed even more LOUDLY for several seconds to attract attention, then he would have shouted: "SHUT-UP everyone that laughed..." and then would have explained the seriousnes of the situation.  After checking that everyone had understood clearly, he would return to his amiable and friendly attitude instantly, signaling everyone to continue working.

Wayne Eleazer
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Re: Plain BAD Design, bad response too.
Wayne Eleazer   9/9/2015 11:23:22 AM
This reminds me of something a fellow amateur radio operator told me.  "I was driving down the freeway on cruise control, picked up the mike, keyed my 2 meter radio on 146.94 MZ and very shortly was going 94 miles an hour!" 

Same problem, RF getting into the cruise control circuits.  GM had not considered that as a possible problem.

On my own airplane I found that the VHF comm radio was not working very well, poor receive, weak scratchy transmit..  The radio shop pronounced the radio itself Okay so I decided to replace the coax to the antenna.  Eventually I found that the coax to the comm antenna that had been installed with the radio was fine - but had never even been hooked to the radio.  Years before, the technician had accidentally hooked the radio not to the new antenna he had just installed but to the old tail-mounted NAVCOM antenna, which I had disconnected during a recent inspection.  The radio was trying to transmit and receive through nothing but a length of coax extending back to the tail of the airplane - it's amazing that it worked as well as it did! 


William K.
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Re: Plain BAD Design, bad response too.
William K.   9/10/2015 4:52:50 AM
At least on some USN carriers every pilot complaint was taken VERY seriously. But that was in an area where hostiles were met frequently. 

The CO always required reporting of what was done to fix the problem, which the reports were intended to build up our "stack of knowledge." I don't know what finally happened to the project, but that group did get high ratings for efficiency and accuracy.

The biggest problems came from things that "fixed themselves" when an attempt was made at finding the problems.

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The new norm
SherpaDoug   9/9/2015 11:35:40 AM
With high employee turnover as the new normal, tribal knowledge like this is sure to get lost.

My dad spent 30+ years at the same company and the last few weeks befor retirement they left him free to document anything he though they should know about any of the projects he had been a draftsman (now mechanical  engineer) on.

I was laid off a few years ago leaving no one in engineering for the whole product line that had been there more than 6 months.


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Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
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