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  • Mark created a new forum post in ATA 24, Electrical Power
    757 with an APU generator that would only engage for about two seconds before dropping offline



    This problem is normally associated with the GCU (Generator Control Unit) sensing an out-of-tolerance condition. Over/Under Frequency, Over/Under Voltage, Missing Output Phase, or a Current Differential will cause the APU contactor to isolate the output from the aircraft distribution bus.

    It's a guess...... but I believe the GCU also removes the field current from the generator which basically stops the generator from creating power.




    I checked the output of the generator's PMG (Permanate Magnet Generator) with the APU running. This really shouldn't be the problem as the GCU can receive its power from a cockpit circuit breaker as an alternate source. The PMG should be primary power for the GCU. 62-64 VAC was observed on pins 2, 3, and 4 of the GCU's bottom plug. These measurements were taken to ground and not pin to pin.

    Next up was to check the field coil in the generator. From the GCU middle rack plug, pins 1A and 1B a resistance of 12 ohms was noted. Each leg was also checked for a short to ground which was not present. (If the field coil was open, the generator should not produce power at all. Not even for 2-3 seconds.)

    For the voltage sensing, pins 15A, 15B, and 15C of the middle plug were checked to ground. Pins A and B had readings of about 1-2 ohms to ground, pin C was open.



    It was noted later that taking these reading from the GCU was not a true test of the voltage sensing. The readings really should be taken from the "L" (or input) side of contactor C905 with the wires isolated from each other for each separate phase. There are some other pick-off wires that might provide a ground path that could possibly render the readings from the GCU rack as erroneous.

    C905 is located in the P34 panel. It is partially obstructed by a circuit breaker box. This image is the top (or power input side) of the contactor. A Current Transformer assembly is located above it.

    A reading from the contactor aft towards the generator verified the "C" phase power wire was open.

    Upon (trying) to remove the firewall plug, one broken wire and two fused pin/sockets were found.




    Unfortunately, both the firewall receptacle and the APU side harness will require replacement. These are big pins on 1/0 guage wire. A pneumatic crimper is required for replacement. The receptacle is replaced from the Stabilizer Trim area and then mounted to the firewall.




    757-200 ASM 24-22-05 AWM 24-21-41
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  • Mark created a new forum post in Faces
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  • Mark created a new forum post in 2018
    Incident was in 2016, but I'm going to throw them out as I hear about them.

    https://www.atsb.gov.au/publications/investigation_reports/2016/aair/ao-2016-012/

    One inop Flight Management Computer. Emphasis on "one".
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  • Mark created a new forum post in ATA 24 Electrical Power
    Lear 60 with no #1 inverter power output.



    The inverters are located in the ceiling of the aft bag pit.

    For troubleshooting purposes, the #1 (left inverter) is easier to deal with as the 28VDC power input and ground wires are inboard and accessible.



    The right side unit's inputs are somewhat tougher to get to.



    The first avenue for troubleshooting inverters is "is it getting power"? With the cockpit switch selected on, 28VDC should be present at the selected inverter's main power input lug. This is a large wire, I'm guessing 4 gauge. Power comes through a contactor from the main DC Power Bus.

    With this particular problem, power was not preset at the inverter. Starting at the source, there is a 100 amp fuse used to supply power. Visually, it looked burned and warped. A meter lead "carefully" placed through an opening at the top of box showed power at the bottom lug, nothing at the top.



    After the fuse was replaced, 28VDC was present at the inverter, but the A/C output was only 25VAC. The inverter's cooling fan was not turning either. Basically, the inverter was failing and the fuse blew in response to the high load.

    There are many other areas of failures that can be associated with inverter power. The input and output contactors along with switch controls and any control relays. This particular problem was pretty straight forward as to the failure.

    As a side note, I'm not a Lear guy. The batteries "should" be disconnected before EVER messing with the DC supply bus bars. From the access hole where the batteries are located....... getting to the disconnects looked damn near impossible. The left battery could be reached with a guy having a long arm. The right battery..... forget it. I did notice a possible access panel on the right side for accessing the right battery. This is NOT RIGHT. Any time you're working with power, the batteries should be disconnected. How can safe work be done if you can't reach the damn batteries?

    The fuse was replaced "hot". A ratchet and socket were used to break the fuse nuts. the rest was done with fingers (and "no" 28VDC will not hurt you). The ratchet was used to tighten the nuts after fuse replacement. Any misplacement of the ratchet causing a power path to ground is going to lead to some "very" interesting sequence of events that could end up killing you or burning up the aircraft. A serious "shame on you Learjet engineers" is fitting for the shitty battery disconnect design.

    Lear 60 AMM 24-20-01
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  • Mark created a new forum post in ATA 33 Lights
    Right nose taxi light inoperative. Simple enough problem, just power and a ground.



    Light removed. Power was verified good on one of the two wires. The ground was intermittent, but moving the leads just outside the bulb housing would give a reading to ground (not a solid ground).

    With the suspect area cut out...... no ground at all. What???



    Working the wire bundle aft and up from the light assemblies takes you to what appears to be a junction box.



    Just a wire guide assembly. At no point was a ground indicated when moving the bundle in this area.



    While moving up the bundle into the nose gear well, the meter had to have the ohm's "buzzer" turned on as it was impossible to watch the meter and shake wires at the same time.

    At the top of the gear the open was found. I should of guessed as the harness had been previously worked on as evident by electrical tape being wrapped around it.

    Upon gutting the harness two wires were found broken. The taxi light ground wire had an environmental splice that had snapped in half (although I was pleased to see someone actually went through the hassle of installing one there). The other wire probably broke when I was opening up the bundle.

    A repair was completed on both wires, but I suspect that more will break over time. This area is where the wire bundle moves and bends in response to gear retraction/extension. I'm not sure if any plausible solution is available other than changing placement and slack of the bundle itself. There is a pronounced bend aft of the lower adel clamp.

    If wires continue to break in this area, the complete bundle can be replaced as an assembly. A part number is located on the bundle at the gear well wall feed through plug.
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