One particular piece of equipment that came into being with the advent of the jet age was the “jet starter” also known as the huffer, blowcart, ground pneumatic source and some unprintable names when they didn’t do their job. They reached their zenith of importance when the B707 and DC-8 aircraft ruled the airways. These aircraft did not luxuriate in having an “airborne power unit” or APU. Instead these aircraft were totally dependent on both ground power and ground pneumatics and also external air conditioning when their engines weren’t turning. They rather resembled “a whale on a beach” being totally helpless when out of their environment or in this case with the engines off. Actually, some of these birds were fitted with combustor starters of negligible reliability of which more later.
The piston birds of the DC-6 and Connie vintage were 28V DC powered airplanes with electric starters for the engines and in a pinch could crank their own engines off the battery. The 707 and DC-8 needed pneumatic air, lots of it at around 35-45psi to get the first engine started. After that with a little goose of the throttle they could generate their own pneumatic air for starting the rest.
Since a turbine engine is basically an air pump with a hot end, it was natural to use a small gas turbine engine to provide ground pneumatics. AirResearch jumped in having its series GTPC 85 engines sized just right. These Ground Power Units (GPU) were small enough to fit on a tow-around cart or inside a light truck. Some provided air only with no electrical output while others had one alternator for aircraft ground power and a very few had two alternators. The last were for some of the Boeing 707/720 that had an electrically driven Freon cooling pack.
Not Always Cut and Dry
I learned a lot about these units for suddenly around 1968 Eastern realized that their GSE people were maintaining these engines without the benefit of recent training. Since I was instructing aircraft mechs on the airborne version of the engine it was decided that all they had to do was send me out to train the auto shop folks (sorry — it’s force of habit to call them that). After all a model 85 was a model 85 — right or maybe not. The engine was very similar but the ratty control wiring that had suffered from years of ramp abuse was not.
I buried myself in wiring diagrams tracing out start and control circuits. Worse yet was the paucity of information on the alternator control circuits. And worst of all were the two generator models. With coffee and donuts I approached the auto shop where there were a few greybeards who had attended GPU factory school back when the Electras were being delivered. The irony of their future instructor begging for information was not lost on them and I comforted myself with the knowledge that I could put on my “know-it-all” hat anywhere but JFK. They actually were quite cooperative in training me. I ended up hitting a lot of the northern region stations and made a lot of noise to some people. They bought new trucks to put the old units in and rewired the lot so they mostly all worked the same — except the two generator models. Those had the second generator stripped out simplifying life for all.
The Same But Different
The turbine ground power units did not have it all their own way of course. Various manufacturers came out with gasoline-engined air compressors. Some were screw compressors and early on there were a few with Rootes blowers. These were particularly notable for the noise they produced. Not that any of them were quiet of course. It took a big industrial engine screaming at high revs to get up the air pressure and volume needed to crank a JT-4. And it had to maintain it for several minutes or more, depending if the cockpit crew had gotten seated comfortable enough to finally get down to starting engines.
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