Electrical Performance Deterioration
Electrical gremlins have been teasing us since Benjamin Franklin hung that key on his kite string so long ago. With that in mind, I thought a quick refresher on the importance of proper grounding was in order. How many times has troubleshooting led us down the wrong path because of either a poor or non-existent grounding point? It is easy to forget the simple things when we are looking for power supply or component integrity, a blown fuse, or a popped circuit breaker. A simple ground connection overlooked can be a missed opportunity to look like a troubleshooting genius. Our venerable fleet of Westwind Aircraft has been in service now for quite sometime with the newest aircraft having more than 20 years of duty. With time, corrosion, dirt, vibration, and/or combinations thereof can play havoc on the overall performance of an electrical system. Anything that will increase the ground path resistance is a gremlin’s paint brush slowly eliminating the required bonding. It is important that electrical bonds are properly maintained in order to minimize radio interference due to precipitation of static electricity and to provide effective electrical grounding of the aircraft. Static discharge build-up, St. Elmo’s fire, and lightning strike damage potential are all affected by poor electrical bonding. Good electrical continuity is required between all wing panels, fuselage panels, doors, stabilizers, and flight controls. When inspecting the aircraft for continuity, the use of stainless steel test probes is recommended to ensure penetration of protective metal coatings and accurate resistance readings. It is also important to use calibrated testing equipment that is in good repair. Resistance of test leads and jumper cables must be subtracted from readings obtained to get the actual bonding resistances. For surfaces fitted with bonding straps, measure resistance across structures adjacent to the bond strap attachment points. Do not measure at either end of the strap at fasteners or terminal lugs. Visually check bonding jumpers to assure that they are not broken, frayed, or missing. If you’re new to the aircraft, you may not catch a missing jumper, but if the area looks to you like it should probably have one, check the manuals. The Aircraft Maintenance Manual (AMM) has the specific details for the correct resistance between each measurement point and the zero reference point; it will vary between bonded and un-bonded components and surfaces. Never leave it to memory. Always use the latest revision of the appropriate manual when working on an aircraft, as we continue to update and improve the manuals on a regular basis.
Friday, September 29, 2006
ATA 28
Fuel Dumping From Tip Tanks
Scenario
During a long cross-country flight, you land with 2,250 pounds of fuel and request a full load of fuel including tip tanks. You leave electrical power on the buses during the refueling, in order to enter flight plans and shorten the time on the ground. As the fuel truck pulls away, you confirm a full fuel load on board, and get ready to restart engines and continue flight operations. During the start procedures, soon after the fuel boost pumps are selected to “ALT”, fuel begins to pour out of the wing fuel vents.
Correction
During the previous leg, tip tank fuel automatic transfer occurred at the normal point (approximately 6,600 pounds remaining). The control circuit opened the transfer valves and fuel was transferred from the tips to the wing tanks. This control circuit electrically latches in and remains in that configuration until power is removed from the auto transfer relay. If the transfer system remains active (valves open) and the fuel booster pumps are turned on, tip tank fuel will be forced into the wing and fuselage fuel storage areas, overflow into the fuel vent plumbing and exit through the wing fuel vents. Momentarily removing #1 distribution bus power or simply opening the AUTO TRANSFER circuit breaker will unlatch the holding circuit and prevent the situation. Service Bulletin 1124-28-098 modifies the control circuit. The electrical latch will be defeated when the aircraft has landed, allowing the auto transfer relay to de-energize. This service bulletin is effective for all serial numbers prior to 391.
Yes, this can occur during regular maintenance. If the transfer valves are open, all tanks are full, and the fuel boost pumps are activated, fuel will be forced from the vent system.
Scenario
During a long cross-country flight, you land with 2,250 pounds of fuel and request a full load of fuel including tip tanks. You leave electrical power on the buses during the refueling, in order to enter flight plans and shorten the time on the ground. As the fuel truck pulls away, you confirm a full fuel load on board, and get ready to restart engines and continue flight operations. During the start procedures, soon after the fuel boost pumps are selected to “ALT”, fuel begins to pour out of the wing fuel vents.
Correction
During the previous leg, tip tank fuel automatic transfer occurred at the normal point (approximately 6,600 pounds remaining). The control circuit opened the transfer valves and fuel was transferred from the tips to the wing tanks. This control circuit electrically latches in and remains in that configuration until power is removed from the auto transfer relay. If the transfer system remains active (valves open) and the fuel booster pumps are turned on, tip tank fuel will be forced into the wing and fuselage fuel storage areas, overflow into the fuel vent plumbing and exit through the wing fuel vents. Momentarily removing #1 distribution bus power or simply opening the AUTO TRANSFER circuit breaker will unlatch the holding circuit and prevent the situation. Service Bulletin 1124-28-098 modifies the control circuit. The electrical latch will be defeated when the aircraft has landed, allowing the auto transfer relay to de-energize. This service bulletin is effective for all serial numbers prior to 391.
Yes, this can occur during regular maintenance. If the transfer valves are open, all tanks are full, and the fuel boost pumps are activated, fuel will be forced from the vent system.
ATA 32
Proper Nose Landing Gear Strut Servicing Prevents Towing Damage
Air crews and ground personnel are reminded to monitor the service height of your Westwind's nose landing gear. With the crew and passengers out of the aircraft and the nose strut properly services, the upper scissor should be just above the nose wheels.
Although the scissors are disconnected for towing, a low strut will allow the upper scissor to become trapped between the nose tires. If you handle an aircraft in this condition, you can exceed it;s steering limit, resulting in damage to the nose gear components and airframe structure. If you suspect the plane was moved while you were away, look for scuff marks (on the inside of the tires and rims, and on the scissor) that may suggest the aircraft was improperly handled.
Air crews and ground personnel are reminded to monitor the service height of your Westwind's nose landing gear. With the crew and passengers out of the aircraft and the nose strut properly services, the upper scissor should be just above the nose wheels.
Although the scissors are disconnected for towing, a low strut will allow the upper scissor to become trapped between the nose tires. If you handle an aircraft in this condition, you can exceed it;s steering limit, resulting in damage to the nose gear components and airframe structure. If you suspect the plane was moved while you were away, look for scuff marks (on the inside of the tires and rims, and on the scissor) that may suggest the aircraft was improperly handled.
ATA 27
When the rudder is removed for any reason, take a look inside the torque tube (rudder post). there has been found quite a bit of debris accumulated at the bottom. There is no drain for this area, so any water, paint stripper, etc. will remain, possibly leading to corrosion. We have removed several for cleaning, inspection. and corrosion treatment and have had to replace one.
ATA 32
If your aircraft decides not to steer the next time you get in it, have your maintenance personnel place a jumper across the terminals on the little silver switch (not the round one with the connector)in the top of the nose wheel well on the steering control valve. If it steers, the switch is bad or out of rig, and you just eliminated a lot of troubleshooting time.
ATA 21
When you are pressurizing the aircraft on the ground, remove the inlet duct from the ram-air valve before starting the engines. When you get the aircraft pressurized, see how much air is blowing out of the valve inlet. If it is a lot, the butterfly seal in the valve is gone or deteriorated and the valve must be replaced.
This is also a good time to check the regulating pressure on the Fluid Pressure Regulating Valve. If it is low, the air cycle machine will not see enough inlet air pressure to come up to the correct speed.
This is also a good time to check the regulating pressure on the Fluid Pressure Regulating Valve. If it is low, the air cycle machine will not see enough inlet air pressure to come up to the correct speed.
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