When You're Hot, You're Hot-When You're Not, You're Not

When You’re Hot, You’re Hot – When You’re Not, You’re Not But you still may spark! Part 2 By Fred Workley May/June 2001 To be a good trouble-shooter on aircraft, you sometimes need to be like Sherlock Holmes. Not that you...


Noise Source
What was the source of the noise and static in the audio of the Number Two navigation receiver? The engine alternator was operating properly and the ammeter indicated that it was charging properly. All the alternator diodes were good. When Mel and Ray placed the aircraft on jacks and performed a gear retraction check, everything worked properly for one up-and-down cycle. They inspected all the wiring to the micro switches on all three gears.
Finding: At the left gear trunion, the tubing over the wires that goes to crimps and then the wing root was brittle and cracked. They also found that moisture was inside the tubing and that one wire going to the down micro switch was loose in the crimp. This might have been what Mel saw with the delay of the left green light. Another possibility is that the gear was down with an intermittent green light.
Mel and Ray did not have an auxiliary power supply to plug to the external power receptacle on the airplane. So, they were performing the gear retraction test with the aircraft battery as the source of power for the electric hydraulic pump. By using an oscilloscope, it was shown that when it was running, the electric motor on the hydraulic pump generated a considerable amount of noise on the DC current. It was decided that this was normal and that the radios would normally filter this noise out through their power supply circuits. Later, it was found that when the Number Two navigation radio power supply filter was repaired, running the electric hydraulic pump no longer could be heard in the audio.

Gear Swing
Mel and Ray did observe that when the battery charge got low, the electric pump got slower and the left gear would not go down and lock into position until the right and nose where down and locked. This is what Mel saw during his flight related to gear sequencing.
Finding: The "O-rings" in the left landing gear hydraulic actuating cylinder were worn allowing pressure to be sent to the upside of the cylinder. The cylinder also had a lot of black debris that was packed around the worn "O-rings." Once the cylinder had new "O-rings," the gear operated in the normal sequence even with a low battery charge. I want to remind you that the battery in the Turbo Arrow is on the same shelf as the electric hydraulic pump and that both are located behind the access door on the aft wall of the cabin.

It’s in the wash
Since the radios weren’t back yet and thinking that they had repaired everything, Mel and Ray decided to wash the airplane. Ray installed the spare battery that had been fully charged on their battery charger. They phoned the tower to coordinate light signals and headed for the wash rack. They were noted for keeping their airplane spotless. While they were at the wash rack, they would even wash the engine and firewall occasionally with degreaser. When they were done at the wash rack, they phoned the tower for light signals. When they attempted to start the engine, nothing happened. Mel said it must be the starter solenoid again. They removed the top cowling and Ray listened and heard the starter solenoid close and the starter turned the engine slowly. The airplane was towed back to the hangar for more troubleshooting.
On inspection, the starter commutator was found to be very dirty, but there was plenty of brush wear left and there was no sign of the segments being overheated or throwing lead (silver solder).
Finding: The aluminum cable that connects to the starter solenoid running through the firewall and back to the battery had a very high resistance. The end of the aluminum cable was not sealed, and it is suspected that water had wicked into the cable. To the left side of the pilot in the sidewall was an unsupported cable run between fuselage frames that sagged about ten inches. This was the lowest point in the cable run. Inside the insulation, that section of wire had work-hardened and corroded. Moving the cable at that location, you could hear a crunching sound and the resistance of the wire would vary. There was evidence on the insulation that the wire might have gotten very hot. All of the aluminum wire in the aircraft was replaced with copper wire. The ground strap between the engine and airframe was replaced with a larger copper-braided strap. Eight ground points to the aircraft fuselage were found to have high resistance. Bonding straps were installed on all moving surfaces and static wicks were installed. Voltage regulation from the alternator was checked and found to be on specification.

Blanket protection?
In the last issue of this magazine, we also found out that Mel had removed the radios from the airplane and transported them on a thick, nylon blanket in the trunk of his car. It is possible that he induced the damage to internal circuits that caused failures in certain radio functions. Avionics, and many electronic line replaceable units, can be damaged by electrostatic discharges because they contain microcircuits and other sensitive devices. With the use of glass cockpits in general aviation airplanes and the desire of manufacturers to increase the use of line replaceable units (LRUs) to decrease aircraft maintenance down time, there are some things you need to do to protect these units. If you remove, install or move these units, you have to protect these Electro Static Discharge Sensitive (ESDS) units.

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