Ideally, the cart was then returned to a ground charger for charging. That, of course, was a weak point. People forgot to put them on charge just like they forget to gas ground equipment. Just when you needed power the most, you’d have a dead battery.
So GPUs became standard ground equipment. The older aircraft, DC-4, DC-6 and 7 and the Connies, were quite tolerant of the quality of the external power. Some had over-voltage cutouts so you couldn’t fry the electronics by a power cart gone wild. Others did not. A DC-4 one day was happily lapping up 35 volts with only a pair of boiling batteries to be noted.
Ground power carts came in various flavors. In the hangars, they were usually motor generators. The larger ones were powered by 440V commercial power. Sometimes things went wrong inside them, probably due in this case to some of them being used outside in the wind and rain with a canvas cover to protect them.
One day, a GPU was happily eating 440V AC and pumping out 28V DC into an L-1049 Connie. One of our mechanics picked up a ground wire and clipped it to the ground stake in the ramp and stretched it over to the aircraft. Crouching slightly he bent over to attach it to a ground lug on the nose gear. He reached out to steady himself and touched the aircraft while holding the ground wire and executed a perfect half a back flip from a crouch.
This seemed extraordinary to us as he just lay there. He was unconscious, but breathing. I took the ground wire and tossed a loop of it onto the nose gear. It instantly smoked from all the oil on it and then burst into flames and finally the wire glowed red and burned through.
There was an internal short inside the power cart due to moisture and one leg of the 3 phase 440 was laid on the aircraft. The aircraft didn’t notice as it was on the ground side.
The gasoline-powered units were either tow-around or self-propelled. Some were dual use and could be used for light towing. I was in Cleveland one day teaching a course. Naturally, it was snowing and they decided to pull one of our Convair 440s into the huge hangar. I was called to evaluate the damage caused by the aircraft nose striking a column deep in the hangar.
The generator/tow unit was slowly pulling the aircraft in when the worn throttle linkage flipped over-center and the engine surged to the “GENERATE” setting. The driver tried to pull it out of gear, but the gearshift was stuck. While fighting it, he allowed the unit to veer left and the aircraft struck the post. It tore a pretty big hole. You could see the rudder pedals inside.
Meanwhile, the British favored 112V DC for the muscle power and 28V DC for the control. The power unit thereby had two cables.
One dark and stormy night, I was the only one on the ground while a taxi crew started a Bristol Britannia. This meant I had to pull the crew stand, give them signals to start the engines, pull the ground power unit away, pull chocks and then marshal the aircraft out of the parking spot.
After engine start, I pulled the ground power cable and stowed it on the tow-around generator. Note I said cable, not cables. I towed the generator away with the 112V cable still plugged in. It damaged the receptacle on the aircraft. Lesson learned.
With the advent of AC as the prime on aircraft came a need for ground power that required not only voltage control but also frequency control. We found early jets, such as the B707, DC-8 and Electra, were finicky about those two, but didn’t care much about the electrical cleanliness of the juice. We could not engage some of the earlier GPUs on later aircraft since there was too much electrical hash that could only be seen on an oscilloscope. The solid state units on board were intolerant of this.
And then there was the muscle for engine starting. No longer electrical (although we are swinging back to that), you needed pneumatic air, and lots of it to spin up the engines. There were many units around. Some were simply aircraft APUs, gas turbines, truck or cart mounted. They worked well, but were voracious consumers of kerosene. Noisy, too, but all pneumatic blow carts are noisy.
For a while there were some units using a Roots Blower powered by a big Ford industrial gasoline engine. These had to crank up to a very high rpm and so were extremely noisy.
I have seen airports change from the times I worked at them or visited as a starry-eyed airplane lover.
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