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OldNAESUrep
Topic Author
Posts: 3
Joined: Sat Jun 04, 2016 12:06 am

A/C Ground Power connection issues

Tue Jun 14, 2016 10:47 pm

In my final hangar visit just prior to retirement, I was called to an Air National Guard base in Minnesota. It seems that an F-16; while up on jacks to test cycle the landing gear, nearly caught fire. The indicator was that the high pressure supply hose
connection at the a/c from the hydraulic "jenny" had turned red-hot.
The a/c was powered from the hangar 400Hz ground power system. The hydraulic cart was powered from the building 60HZ, 480V three-phase power system.
My investigation determined that a key factor in the root cause chain, was that one of the phase voltage supply pins, at the a/c bulkhead connector had worn past the point in which a reliable connection could be maintained. When this occurs, the Neutral supply pin voltage rises to the missing phase voltage, thus placing 115v, 400Hz on the airframe. This is the root cause of most of the static ground cable fires, in my opinion.
At any rate, when the F-16 Maintenance Manual was consulted, we could not find any requirement to periodically gauge the pin diameters using any type of Go-NoGo gauge.

So here is the reason for this post; this subject has been nagging for some time now. Question to all the Mil A/C maintainers: is there any known maintenance requirement on the a/c ground power connector?

Thanks.
 
rcupp
Posts: 1
Joined: Tue Jun 14, 2016 11:44 pm

Re: A/C Ground Power connection issues

Tue Jun 14, 2016 11:55 pm

Sure there is, I can't remember the manufacturer we were using for the replacement cord plugs but they sell go/nogo gauges for the ABCefn pins as well as ones for the aircraft side pins.
http://mcmeng.com/pdf/400Hz/400Hz%20Air ... 0Gauge.pdf found on Google
 
KC135Hydraulics
Posts: 443
Joined: Sat Nov 03, 2012 2:05 am

Re: A/C Ground Power connection issues

Wed Jun 15, 2016 2:15 pm

I have consulted with some friends in the ELEN shop next door, and they have never heard of a check/inspection of the power receptacle pins on any of the aircraft they've ever worked (B-1, C-130, C-17, KC-135).
MSgt, USAF
KC-135R / C-17A Pneudraulic Systems Mechanic Supervisor
 
boeingfixer
Posts: 573
Joined: Fri Jul 29, 2005 2:02 am

Re: A/C Ground Power connection issues

Wed Jun 15, 2016 4:08 pm

rcupp wrote:
Sure there is, I can't remember the manufacturer we were using for the replacement cord plugs but they sell go/nogo gauges for the ABCefn pins as well as ones for the aircraft side pins.
http://mcmeng.com/pdf/400Hz/400Hz%20Air ... 0Gauge.pdf found on Google


That tool is for checking the external power connector on the supply cable side, not the aircraft receptacle.

As far as inspecting the plug receptacle on the aircraft I don't recall anything other than a visual inspection for wear. This was on the CC-130.

Cheers,

John
Cheers, John YYC
 
OldNAESUrep
Topic Author
Posts: 3
Joined: Sat Jun 04, 2016 12:06 am

Re: A/C Ground Power connection issues

Wed Jun 15, 2016 5:42 pm

Yes, the good guys at MCM Engineering can supply the TWO go-nogo gauges. But the interesting thing here is that so far, no MIL-maintainer has seemingly heard of them.

Well, the effect of a worn pin is that when it opens, and sometimes arcs, this voltage; as much as 115V, 400HZ, is suddenly applied to the airframe. And recall, all the electronics in the A/C is grounded to the airframe. So consider the effect of placing 115V on the ground pin of every chip in the A/C. This is a major cause of Avionics failures in my experience.
 
JohnM
Posts: 395
Joined: Sat Feb 24, 2001 12:35 pm

Re: A/C Ground Power connection issues

Wed Jun 15, 2016 8:57 pm

I do not have experience with small pointy nose jets. However everything I have dealt with from the C-141, c-5, and the H-60 family all have (in C-5 A/B talk) a bus protection panel. If the incoming 400 hz is not the correct voltage, phase rotation, etc, it will not let the power on the airplane, and will disconnect A/C power immediately upon any issues with the same. Your example of an open input phase will result in an automatic external power trip in fractions of a second,due to an under voltage condition on A, B or C pin. Also, the D pin of the external power receptacle on the aircraft is connected to aircraft ground. D pin of the external power cord is grounded on it's side. The aircraft is static grounded to earth ground. If an hydraulic mule hose is flowing current, there are some major things wrong besides one of the 3 phase input pins being open.

The only way to get 115VAC present on the airframe is not having the airframe structure grounded. At that point a differential fault will exist, which kills power, or the external power source will trip.

I have encountered several worn external power receptacle male pins and cords, which would require lots of effort to reseat to get an external power "ready" light. "Ready" means the aircraft thinks the power is ok for application, after looking at it, and only then will the aircraft accept it. The C-5 family might hold the DOD record for most time on external power. I think the average jet is on external power 16-20 hours a day, some 24 hours a day for weeks at a time. Only down time is between power carts. So those external receptacles get the ultimate workout.

Off topic, but how the power cart voltage is adjusted, even "within AGE limits" can allow the jet to take the power, but some systems on the plane will not be happy with it, and will need a tweak of a few volts up or down.
 
OldNAESUrep
Topic Author
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Joined: Sat Jun 04, 2016 12:06 am

Re: A/C Ground Power connection issues

Thu Jun 16, 2016 12:20 am

JohnM, thanks for your inputs. We checked the Power Monitor LRU and the MIL-PRF-24021K specification does not indicate that these units monitor loss of phase.

I have also seen this issue with the MH-60s, that turned out to be a bad power monitor.

The E & F pin of the service cable are the interlock pins. They detect cable to bulkhead insertion and enable closure of the gate box line contactor. There is also a timer associated with the gate boxes, allowing power to be applied for a pre-determined period. If no interlock closure is detected by the end of the timed cycle, the contactor is opened. In our case, this took over 2 minutes before the line contactor dropped out.

Bear in mind that application of 400HZ to the airframe for even a few milliseconds can damage the electronics.

I think that everything you have said is correct, but only if all components are working properly. I thing the P/M should be periodically tested, and all connection pins should be gauged periodically.
interesting to note, there is no detailed specification for hangar or flight line gate boxes other that what NAVFAC publishes. But in my experience, this is seldom adhered to. I think exploring this issue would improve safety.
regards,
 
JohnM
Posts: 395
Joined: Sat Feb 24, 2001 12:35 pm

Re: A/C Ground Power connection issues

Thu Jun 16, 2016 1:01 am

I am assuming that the smaller tactical aircraft are space & weight limited to some extent, so that your external power system is not chalk full of bells and whistles. I'm not sure how any plane can be maintained without 2 auxiliary power units, 2 galleys, and a nice table to sit at for forms work and general BS once the work is done! The fighter guys have such barbaric conditions... Thanks for the interesting info.

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