When You’re Hot, You’re Hot – When You’re Not, You’re Not
But you still may spark! Part 2
To be a good trouble-shooter on aircraft, you sometimes need to be like Sherlock Holmes. Not that you have to wear a hat like his character or smoke a pipe, but you need to be a good investigator. We found out last issue that Mel and Ray had a lot of information about their airplane. What would you expect them to tell you if they brought you their Turbo Arrow III for repair? You have to search out the clues and solve the riddle. "When You’re Hot You’re Hot, When You’re Not Your Not; But You Still May Spark!"
Look for the clues
How much do you know about the aircraft that you are about to trouble-shoot? Do you have the maintenance manual available and do you have time to study it? I owned a Turbo Arrow III for 20 years, so I got to know the maintenance manual quite well. Not everything is in the maintenance manuals. Your logical sense of reasoning has to come into play. We have certain known information about the airplane like the hours on the airplane, who owns it, a history of the in-service use and damage history.
Often, the clues we need to repair the airplane come from the pilot. They get to know the individual airplanes that they fly. The pilot reports symptoms of a fault or failure. In Mel’s case he is flying solo and he is busy. Pilots usually develop a routine. This routine has them constantly checking and cross-checking everything. If anything is "not normal," it quickly gets the pilot’s attention. Pilots also perform checklists to determine if everything is normal. Subtle changes in noise, vibration and gauge readings are indicators to the pilot. Mel realized that during his flight, there were several things that were not normal.
We found out that the Number Two VOR navigation radio was passing the noise and interference generated somewhere in the aircraft into the navigation radio. Even though this situation is highly improbable, Mel could see that the navigation signal was unreliable. The circuits that supply power to the radio are filtered so that noise (static) from the aircraft should not pass into the radio.
Finding: The Number Two VOR navigation radio is connected through the coaxial cable to a dipole "Cat’s Whiskers" antenna. Upon inspection, it was found that the anchor screw in the micarta had broken allowing the right pole of the antenna to move freely in a seven-inch arc. This broke the wire at the wire crimp on the screw where it was mounted on the antenna. The signal was most probably somewhat less with only one pole of the dipole antenna connected.
This antenna coax had been routed through a diplexer coupling system that was also connected to a Number Two glide slope receiver for the HSI (Horizontal Situation Indicator). This is not a good practice because the Glide Slope receiver is UHF (Ultra High Frequency) with its signal being received from a VHF (Very High Frequency) antenna. Note: The duplexer is a component of the radar system that permits it to transmit and receive simultaneously. This broken wire would not have affected the Number One VOR navigation receiver. Sometimes, air turbulence will finally break a wire that has been breaking, strand by strand, for some time. Poor connections in wire crimps or wire with long unsupported runs between clamps sometimes break or wires pull out of crimped connections due to the constant vibrations in the aircraft.
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.
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.
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.
Look for the signs
Often, you won’t find any guidance in the Aircraft Operating Manuals or in Aircraft Maintenance Manuals. Instead, the information about handing the units is found in the Component Maintenance Manuals that you may not have available for reference. What if you saw a circle with a diagonal bar with a lightning strike through the bar? What does a triangle with a diagonal bar and a hand with grasping fingers mean? Lastly, what if you saw a segmented circle with three, inward pointing arrows? These are the JEDEC International Symbols for ESDS units. Sometimes, there is a placard that says "STATIC SENSITIVE."
Some avionics and electronic installations have placards that have "CAUTION" or "ATTENTION" placards. There may be specific instructions for handling Electro Static Sensitive Devices. In other cases, the placards may call for the use of a grounding wire or personnel wrist strap connected to a specific point in the airplane. In other avionics installations, it may not be readily apparent that the units are static-sensitive. If you remove and install old tube radios, be aware the rules have changed for newer radios. Only one static discharge can cause damage to components in a static-sensitive avionics/electronic unit or LRU plug-in circuit board. Over time, system functions may change with several static discharges. Of concern is the loss of system functions that may not be readily identified without some functional or diagnostic check.
The body electric
Let’s talk about electrostatic charges that are generated by your body, your hair, clothing, floor coverings, avionics/electronics units and the equipment racks. Any time there is a difference of the potential energy between two substances, there is a potential for an electrostatic discharge. An example would be electrostatic discharges between nylon fabric or human hair into steel or polyethylene. Never place these units in a plastic sack or wrapper. You must use conductive bags. When removing metal-encased units, care should be taken to ground them so that no potential energy difference exists between the airplane structure, the rack, or any associated control panel. Remember, the control panels are wired to the unit.
Never touch the electrical pin with anything. Unprotected electrical connectors can be a direct connection to sensitive internal components. You must use conductive electrical dust caps and connector covers. Conductive dust caps and connector covers are either black or gray in color. On ITT Cannon covers, look for the word "CONDUCTIVE." I recommend that if anti-static dust caps and connector covers are not available that you still use dust covers but spray them with anti-static solution and date them according to the instructions on the solution container. If you are replacing a unit, use the covers that come on the new serviceable unit. Never remove the covers until you have properly grounded the unit to the aircraft structure.
A precaution is to always completely remove electrical power when removing and installing ESDS units. Even some unrelated units may provide power through feedback circuits. Make sure that power is off for the whole system. Eliminate any contamination, including dirt or metal shavings, that might cause a connector pin to short and cause ESDS damage. Grounding the unit can now be effective. The grounding device or wrist strap may have to be checked with an ohmmeter. The wrist strap should have a resistance of less than 10 megohms. The grounding assembly that you use should have a minimum resistance of 250 kilohms and a maximum resistance of 1.5 megohms.
When You’re Hot You’re Hot, When You’re Not You’re Not, But You Still May Spark!
What did we find wrong with this airplane and what repaired it? I have probably asked more questions than I have answered. Those of us who have flown a Piper J-3 Cub know that the engine doesn’t need a starting or electrical system since it generates it own spark in the magnetos and we hand prop the engine to start it. If the wiring in the airplane has deteriorated, there are multiple problems that could present themselves to the pilot. As aircraft age, wiring becomes a more significant part of our inspections.
As we have seen, maintenance practices can also cause problems in the aircraft wiring and systems. Harsh detergents may cause corrosion in the fuselage and wiring and eventually system faults. Poor maintenance practices, like not protecting radios and line replaceable units from static discharge, may also lead to system faults.
We still have many unanswered questions. What should the maintenance record say after all this maintenance activity? Whose name should appear on the maintenance entries? Was the aircraft repaired or was it altered?
No matter what, our objective is to ’Keep ’em Flying."