Maintenance Safety Tips
SMOKE IN THE COCKPIT
One of the most frightening preventable occurrences would be fire and / or smoke in flight. A pilot’s initial reaction might be to try to determine the cause of the fire rather than fly the aircraft, leading to further problems for the aviator. An electrical problem is more than likely going to be identified by a distinct smell of burning wire or component.
With today’s much more relaxed aviation safety culture, we see many aircraft wired with automotive cables. There’s a reason why aircraft manufacturers use PTFE cabling. PTFE cables have superb fire, smoke and chemical resistance properties. Automotive cables do not. Yes, of course PTFE is more expensive, but if the pilot does not have acrid smoke to deal with, there’s a far better chance of landing the aircraft the right way up.
What is an electrical fault?
Basically, all the energy at the Positive terminal of an aircraft battery wants to get to the Negative terminal. The current passes through any ‘ON’ electrical components to find it’s way back to ground (-ve). The most likely occurrences of fire or smoke might be either a component fault or a wiring fault.
Component Faults. A component can be anything such as a bulb, a capacitor in the radio, a relay or part of an instrument. If a bulb fails, there is an extremely low risk of danger, the circuit is broken and current can no longer pass through the bulb. If an electronic component like a power regulator or a capacitor fails, there is a higher risk that the component becomes a blob of melted carbon, allowing excessive current to pass through it. At this point, the correctly rated fuse in the power circuit should blow, breaking the circuit.
Circuit Breakers (CB) and Fuses. Every component on the aircraft should be protected with a fuse or CB of the correct rating. A 2 Amp fuse will blow if more than 2 Amps passes through it, and the circuit will safely break. A 2 Amp CB will ‘pop’ if more than 2 Amps passes through it. A CB can be easily reset. If a 10 Amp fuse is fitted where there should be a 1 Amp fuse, then the aircraft wiring can become the sacrificial burning fuse in the event of an electrical fault occurring.
Fuses and CB’s protect other components, wiring, passengers and pilots.
Wiring Faults. Wiring faults are found in aircraft much more frequently than component faults. Wires might be cheap automotive cable, be the wrong size (current rating) for the component, be poorly run, found chafing on metal or the airframe, be poorly connected, be inadequately insulated, be corroded, or bunched together producing too much heat. Sadly, these are common finds in poorly maintained and ageing aircraft.
If a live wire does break or wear and electrical contact is made to ground (-ve) then the rated current limit will be exceeded and the fuse should blow, or the CB should pop.
Try to Prevent Electrical Faults
As usual, problems occurring in the air are much easier to prevent on the ground.
- Check your aircraft wiring thoroughly. All the instrument and lighting wiring at least should be PTFE, which is usually identified as being white, hard coated, shiny and a little less flexible than automotive cables. Thicker cables like battery and starter motor cables are generally not PTFE. If there is any automotive cable in your wiring, have it checked and / or removed by a qualified person.
- Ensure that every electrical component is protected by a fuse or a CB. This is easy to check, simply by removing all the fuses (one at a time) or popping all of the CB’s and ensuring that nothing in the aircraft can be switched on. If all the fuses are removed and / or all of the CB’s are ‘OFF’ then no electrical devices should operate. The aircraft should have a main fuse of a much higher amperage, which is often close to the battery.
- Physically check where every wire runs throughout the aircraft. Where does the tail light wire go when the rudder moves? What edges or corners are cables running past? Are the cables secured in place? Are rubber grommets intact and keeping wires away from sharp edges in penetrations?
Before working on any aircraft electrical system, always remove the -VE battery terminal (for -VE ground aircraft) to isolate the power.
Btw, Carbon Fiber is conductive so it's just as important to disconnect the battery on a CF aircraft as it is a metal one.
At Flight Safety Solutions, we see dreadful cases of electrical wiring in aircraft, even in brand new aircraft. In a recent case where “, the pilot sensed a smell of burning wire and shortly after cabin started to fill with smoke”, we would suggest that this was likely to have been a preventable occurrence and certainly one that the pilot would have preferred not to have happened.
Extracts from RAAus OMS
Note: The RAAus OMS contains only a tiny percentage of incidents globally, and only those events that are actually reported. There are many many more...
"OUTCOME: Inspection of the aircraft found that the strobe light fuse had blown which led to the Strobe Controller Box, situated under the pilot's left-hand seat. Inspection of the grill over the cooling fan found black marks and smelt of burning. A new fuse was inserted, ignition key turned on and strobe switch turned on. The fan in Strobe Controller Box started humming and smoke started to exit from the cooling fan grill, confirming where the smoke came from. The fuse was disconnected and maintainer removed the Strobe Controller Box."
"Determined Outcome: A failed electrical join at the fuse box caused the engine pump to fail."
"DETERMINED OUTCOME: This was an electrical fault caused by a voltage regulator becoming inoperable. This is an issue that cannot be detected and can happen at any time. Rotax are aware of the continual voltage regulator issues. Due to time in service its impossible to determine the length of service or operational conditions this part has been exposed to."
"The aircraft was destroyed in a hangar fire and another aircraft that was parked next to this aircraft was heat damaged and written off. There was also smoke damage to the hangar. Investigation showed fire started in the engine bay. It is suspected that the voltage regulator failed. All aircraft owned by the Club have now been fitted with solenoids, to isolate the battery from the electrical system."
"OUTCOME: RAAus have spoken with the Australian agent. It appears that an electrical connector for the trim system may have failed. In discussion with the Australian agent a modification to remove the suspect connector will be completed and a letter from the manufacturer will be supplied in due course. RAAus will review and investigate with the engineer and ask that any other model with the same type of connector be inspected and if required have a change completed."
"DEFECT: Pilot noticed a strong smell of smoke and carried out emergency checks. It was discovered that the avionics 10 amp circuit breaker had popped. Prior to the smoke smell, the radio had became unserviceable.
OUTCOME: After inspection of the aircraft by a factory rep, various parts were replaced with others remained in situ. The aircraft the conducted a further flight of 100 NM before the aircraft's electrical system failed again which appear to be due to the radio and tacho. Awaiting further inspection report from the manufacturer."
"OUTCOME: On later examination of the aircraft it was revealed that an electrical connector had become disconnected allowing the connector (on the free end of the cable) to lodge against with the side of the LH carburetor and prevent the throttle linkage closing completely. The reporter stated that the connector has to be disconnected every time the lower engine cowling is removed and may not have been properly latched together after the last removal. The cable has been re-routed to prevent any possibility of it affecting the throttle linkage on the carburetor should it become disconnected."
"During the flight all electrical gauges went to zero. The pilot elected to make a precautionary landing to check the problem.
OUTCOME: Aircraft electrical issues were investigated and the issue was identified as a faulty electrical plug that has since been rectified with no further problems."
"Technical outcome: The aircraft is confirmed destroyed and the owner was not able to identify the cause of the failure however it was believed to be electrical due to the instantaneous failure."
"Review determined failure was caused due to failed alternator and suspect earth grounding issue. New firewall forward package installed."
"After take-off at 700 ft there was a toxic smell and smoke in the cockpit... ...At or before the half way point of the approach, it became difficult to breathe and see due to smoke and flames... ...The aircraft was destroyed by the fire."