This is an excerpt from a report made to the Aviation Safety Reporting System. The narrative is written by the pilot, rather than FAA or NTSB officials. To maintain anonymity, many details, such as aircraft model or airport, are often scrubbed from the reports.
I departed ZZZ in the Cessna 414 on an IFR flight plan with a planned cruising altitude of 17,000 feet.
I was the Pilot in Command (PIC) and the pilot flying (PF). I had a fellow pilot sitting in the copilot seat performing radio, checklist, and pilot monitoring (PM) assistance. We both possess an Airline Transport Pilot (ATP) certificate and we both fly for Part 121 airlines.
During vectors around multiple storm cells, we established a stable climb profile out of ZZZ.
The aircraft’s cabin was hot and stuffy so I turned on the electrically operated air conditioning (AC) unit which caused a significant draw on the electrical system. I also had the anti-collision, position and instrument panel lights on that are required for night navigation along with a BFG Stormscope unit, a communications/navigation radio, a Garmin 430 GPS unit, a DME unit and a transponder unit.
Climbing through 10,000 feet, I turned on the pitot heat anticipating a freezing temperature at 17,000 feet. At this point in the flight, the electrical system was operating near maximum capacity.
Climbing through 15,500 feet, my 6-pack instrument panel lights all went out at the same time without any circuit breaker tripping.
About 30 seconds later, the PM and I both smelled what we concurred to be burning wire fumes.
Although we did not see smoke or fire, we declared a “PAN PAN PAN” and diverted to ZZZ2.
The controllers at ZZZ2 re-designated our aircraft as a priority aircraft and committed Crash, Fire, Rescue (CFR) and police resources to our landing.
During the descent, we turned off the AC and pitot heat and noticed that the burning fumes smell was gone.
We landed at ZZZ2 without any problem.
Primary Problem: Aircraft
ACN: 1903512
I doubt many aviation professionals are aware of heat impact vs wire sizing and bundling.
The wiring should be protected by the appropriate rated breaker, as well as the alternator output, which may be a 50 or 60 amp breaker. If the 60 amp breaker is carrying more than 66 amps, it will trip.
I suspect either some undersized wiring vs the breaker in a circuit,
or, as I found on a Warrior, the landing light terminal at the breaker was loose, so that with the 10 amps load it overheated and burned the insulation off about 1 inch of the wiring.
During an annual, I check all the screw connections to all the breakers for tightness.
BTW, as an electrical engineer and pilot, I’d recommend not loading an alternator to more than 60% of it’s rated current. So, if the max. load is 60 amps, a 100 amp rated alternator and appropriate gauge wire used to the breaker and buss.
Also, an alternator is about 50% efficient, so it dissipates a lot of watts and can overheat itself, damaging the diodes or internal regulator [ experimentals ].
Seems to me that what the pilots did with using the craft’s electrical system is akin to operating your car with headlights, heater on max, wipers, defroster, high-powered audio amplifier and everything else available using electricity. Those accessories would max-out the standard rated car alternator and start to run the battery down. But nobody advises us not to use all that stuff simultaneously, so we do it. Not me. As an electrical engineer with factory cost-conscious (overly so) experience, I know not to overload a system. But the average non-engineer person wouldn’t think that way. Somewhere under that airplane’s dashboard is a red wire that wasn’t thick enough to handle all that amperage and started to fry. Agree with above comments: a COMPLETE wire-by-wire harness inspection is dictated lest something decide to fail in the future in that airplane and cause a smoking crater and funerals. /J
A burning smell means wires are over heating. It does not indicate an overloaded generator. On a twin how would you smell the generators/ alternator or battery that is in the wing?
The burning wire( s) MUST. be found and replaced.. The entire electrical system needs to be examined for burnt wires, loose connection and poor grounds or shorts.
A complete load analysis to determine the load is not excessive and a list of what must be turned off with an engine or alternator failure..
I wonder how much of that equipment was original such that all of it operating would only take 60% (?) of the alternator/generator output. Now add the pitot heat and a Garmin 430 and…
Did a test of a Piper Saratoga with a G530, and we pegged the load meter at 60 amps. In an effort to cut power demand, we replace all lights with LEDs. That helped, but turn on the pitot heat and we were back to being pegged. So there is no excess to charge the battery from doing a start. I wonder how many GA aircraft, with the advent of GPS units and any additional equipment are running on the edge, at night when they have to have on the nav lights and some interior lighting, with a tablet plugged in…
The recap of the situation of the 414 included no after event debriefing, such as what any investigation might reveal. Was any load analysis done to point to under capacity of the alternator or whatever? It would seem unlikely that any fumes from same would reach the cockpit. It is speculation, but it more likely seems that something installed used undersize wire or possibly the wrong kind of wire insulation.
Relating such a story without any details of after action seems like a wasted chance for issuing any caveats