Recent Topics - Rotate.Aero Sat, 17 Mar 2018 10:43:10 +0000 MYOB en-gb Master Caution Lights - by: Mark

Hopefully that question will be answered when I have the time to mess with it.]]>
ATA 31 Indicating and Recording Fri, 16 Mar 2018 01:47:45 +0000
ATA 33-51, Emergency Lights - by: Mark
Boeing 727, 737, 757 and 767's use battery packs for powering the emergency lights. Two packs are used on 737's. Both signage and floor track power come from these units. Typical output voltage is 6 VDC.

There are fuses shown on the unit's schematic manual. I've never opened one up to view them and I'm not sure they're line replacable.

On this aircraft, pin 9 of the forward pack is used for the L1 door exit sign. When shot for continuity to ground (this would verify power input wiring, light assembly, and ground wiring) a high resistance over 250 ohms was noted.

To isolate the actual problem, the power input wire was disconnected from the light assembly. Shooting the assembly's input wire to ground verified that both the light and ground wire were functional.

This only left the power wire from the pack to light as suspect. With the pin pushed out of plug and examined (wiggled and tugged on), the high resistance remained. At this point, the ceiling panel was accessed and the wire replaced (total length 2.5 feet). No evidence of wire damage on the old wire was noted.

6 VDC does not have much punch to it. 270 ohms was probably enough to drop that voltage to 0 at the light assembly.

737-400 WDM 33-51-12]]>
ATA 33 Lights Tue, 13 Mar 2018 23:04:03 +0000
ATA 24-30, DC Power - by: Mark
The sound was emanating from the right sidewall behind the jump-seat. After gaining access to to distribution panel, the sound was isolated to a relay on circuit card 809J. The relay cycles were extremely fast, I would guess about 5 per second.

We also noted the sound in the hell-hole and we removed the covers to the electrical components there.

The relay chatter heard in the back was not always present as noted in the front. When it did act up, the repetitions were the same rate.
We were able to determine that a relay on circuit card 42P was cycling also.

Falcon electrical prints can be extremely difficult to follow and I'm not "even" going to attempt to draw out what we found, on this post.

Shotgun troubleshooting was attempted first by removal of the 4 GCU'S and APU control box from the system. Those efforts did not help.

The problem was discovered to be one of three capacitors in the "Ground Connector Box" which is mounted just forward of the external power receptacle.

This box has a 5 amp breaker that extends out to the area where the cord plugs into the aircraft.

28VDC external power is picked off by this breaker and split numerous times to feed the four respective starter generator systems. 3 engines and the APU. I "did not" want to permanently ruin my eyes trying to trace the actual runs on the wiring prints, but I suspect the power inputs feed each GCU.

The purpose of the capacitors is unknown. Most likely a circuit protection function..... maybe. They are rather large and gold in color. None of the three showed physical signs of failure.

A quick way to remove the capacitors from the system is to disconnect a ground wire located right inboard of the box itself.

Falcon 50 WDM 24-30-01]]>
ATA 24 Electrical Power Fri, 09 Mar 2018 14:29:25 +0000
ATA 24-22, APU Generator - by: Mark

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 wires. 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 an alternate 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 removing 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]]>
ATA 24, Electrical Power Fri, 23 Feb 2018 02:56:05 +0000