An external power cart is a great tool to have for everything from engine starting to preflight checks. It can keep your battery charged, ensure a full 28 volts for cranking (batteries are typically around 24 volts) and run air conditioning on King Air aircraft with electrically driven compressors without starting the engines.
King Air flight manuals provide some information on external power systems, but they leave a bit to the imagination. I hope this article clears up any subtle misunderstandings.
Power carts and the aircraft receptacle
One universal concept in the flight manuals for various models is that the ground power should be capable of high current for engine starting but should still be limited to 1,000 amps for a very short duration. A direct current motor is a virtual short circuit until it starts to rotate. That creates a huge starting surge of current. Without power-cart limiting, the starter-generator’s starting torque could shear the drive or spines before the mass of the engine’s rotating components has time to accelerate. Small carts for ground operations other than starting are fine but start carts should have some limiting features.
For those folks purchasing smaller power carts for your hangars, keep the DC motor surge theory in mind. Even if it seems your air conditioning and blower don’t pull all that much current during operation, the motor starting surge is on the order of hundreds of amps. Newer carts that self-protect by shutting off are not going to make you happy. If your goal is only to run avionics, then press on.
Another common concept or rule: Do not connect external power when the aircraft battery is below approximately 20 volts. No current-limiting circuitry exists between the external cart and the battery, and a battery discharged to that low a voltage is hungry. The charging current could easily be high enough to bake aircraft wiring. High starter current is short-lived and the wiring will be fine. Battery charge current draw can last longer, and your wiring can cook. The only safe resolution is to remove the battery and perform a bench charge using a charger with limited output.
All models recommend (or in some cases require) the battery switch to ON. The common concept is that the battery can help suppress transients, but under real conditions the aircraft systems will still feel a voltage spike regardless of the battery’s presence. The battery will, however, become a replacement source if the externally supplied power drops or fluctuates.
One major advantage of turning the battery on before supplying external power comes from how avionics relays are wired. By default, avionics relays are closed and supply power when de-energized. That’s a fail-safe circuit for bad relays. So, to isolate your avionics, the relays must pull energized when the avionics power switch is placed in the OFF position. No battery power? No avionics off. Then a spike or surge on external power initiation becomes a race between relays. That’s not a great gamble.
External power evolution
In the earliest of the 90 series, the act of plugging in the power cord to the external receptacle was all it took to power the aircraft buses from the cart. No relay, no protection. Assuming the power cart was properly wired with a negative ground, power would be immediately applied to the aircraft buses. If the cart was wired backward, or provided an overvoltage, then smoke emission was assured.
Until LJ-154, LJ-155, LJ-185 and after, there was no protection whatsoever, even from reverse polarity. Then, the third short pin of the power cord came into use for an external power relay, and we at least gained polarity protection for the aircraft and electronics. As King Airs and electrical systems evolved, the protection became more sophisticated, and in some models, we gained a dedicated switch, annunciation and meter to monitor the external input.
Another advantage of the short pin in the connector is that the two primary current-carrying pins are at least 50% engaged before the short pin closes the external power relay and allows power to the aircraft. That makes it impossible to draw an arc and burn out the primary pins and sockets. That’s added protection if a hot power plug is inserted into the connector.
Starting with LJ-773 in 1978 for the C90s and LW-278 for the E90s, we gained overvoltage protection circuits that would monitor the third (short) pin of the power plug before allowing the external power relay to close. A kit (S.I.-1070) was available to add overvoltage protection to the earlier aircraft back to LJ-502/LW-1. These overvoltage circuits also required setting the battery switch to ON to get external power to the buses. The 200 series had the external power relay from the start and adopted overvoltage protection by BB-364/BT-4.
External power annunciation
Gaining annunciation was a bonus; before it was available, there were numerous instances when the power cart used to start the aircraft became a pull toy when it taxied away. Expensive sheet metal repairs are assured. Annunciation has reduced the number of occurrences, but it hasn’t completely solved the problem.
The evolution of the annunciation for external power is another confusing path to follow. Most of the 90 series until the C90A and F90 simply had no annunciation. The 200 and 300 series had an EXT POWER annunciator since the models were born, but the operation can be quite different.
One glaring difference that haunts operators and technicians, until Collins’ Pro Line Fusion avionics with the 200 series, is that the annunciator lamp is powered by the hot battery bus. That means that with no other switches on in the aircraft, if the external power plug is in the hole, then the external power annunciator is on. That’s unfortunately led to many dead-battery discoveries after a night in the hangar when the plug was left in the external power jack.
That’s a detail worth noting: The annunciation signifies a plug is in the jack, regardless of whether there is power to that plug. We’ll cover that distinction later when we discuss the triple-fed bus aircraft. The earliest version of a plug detection was a microswitch in the plug housing on the wing. That was used in the 200s until BB-88, when what I call the “continuity detection” idea came about. From then on, all models used some form of circuit board to detect continuity between the long and short power pins on the power plug. In the power cord there are only two wires for power and ground and the long and short power pins are wired together. In the aircraft, these two are separate circuits. Since they become connected by the plug, that connection could be used for the annunciation – no more physical switch required.
The detection circuits migrated to printed circuit boards mounted farther away, and circuit breakers were installed near the external power receptacle to protect those longer small wire runs. In the triple-fed bus aircraft (C90A, F90, 300, B300), the circuits also became more sophisticated. The annunciator is illuminated only if the battery is selected ON, preventing battery discharge if the plug is left in. These annunciators also provide better signaling to the crew. If the plug is in, the annunciator will flash. If the now-included external power switch is turned ON, and power is successfully applied to the aircraft, then the annunciator will be on steady-state. This also helps prevent taxiing away, since when the master caution light flashes, the circuit is also continuously tripped and that flasher can’t be canceled either.
External power protection
Only with the advent of the triple-fed bus King Airs did we gain additional sophisticated protection circuitry. Before these, overvoltage protection and reverse-current protection were available based on serial number and maybe kit installation. In these aircraft, the printed circuit board that provides the annunciation distinction and flashing also is responsible for sending power to energize the external power relay. Of course, the circuit still rejects reverse polarity.
For overvoltage, even if you select the external power switch to ON, it won’t happen if the voltage from the power cart is over 32 volts. That ensures your battery will charge, but at 32 volts the battery or aircraft circuits won’t get cooked. If there is a transient excursion above 32 volts, the circuit will interrupt the external power, but even if the power drops back to something reasonable, the EXT PWR switch will need to be cycled off then back on.
If the voltage is under 25 volts, the circuit will close the relay and apply power, but the annunciator will continue to flash. That should prompt the crew to check the voltmeter and realize the battery may not be getting much charge. One condition for automatic bus tie closure in these systems is the presence of external power, and the bus closure signal is the same as the signal that closes the external power relay.
External power voltage
In these triple-fed bus aircraft, the electrical systems also include an overhead voltmeter that, in addition to the bus selections, allows you to view the external power voltage before committing to the ON selection. That’s always a good idea, since power cart voltmeters will get little or no attention throughout their lifespan, so accuracy is a bit questionable. This is also the first place to check if your annunciation continues to flash after selecting external power ON.
B300 Fusion
I believe we gave up some information with the Fusion crew alerting system messaging. Although the same circuit card is in use, the output that once gave us the flashing annunciator has been abandoned, so there’s no indication for an undervoltage condition without consulting the meter on suspicion. The CAS messages are simply EXT PWR-CART or EXT PWR-ACTIVE. Although standard cockpit practice should be to ensure all CAS messages are removed from the multi-function display before taxi, I think the MASTER CAUTION flasher protected a few wings and power carts.
As always, reach out with your questions or comments!