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.
On the units with Rootes blowers, it was fascinating to see these big engines; I believe they were Fords, revving up with the Rootes blowers screaming like Banshees. The exhaust glowed not red hot but an incandescent near white heat. What was painful to watch was the stupid operator when he received the disconnect signal. He grabbed the throttle and yanked it right down to idle, dumped the compressor to unload it and while the manifolds were still glowing shut the engine down. You could hear the snap, crackle and pop of tortured metal but there was no sound from the red-hot valves distorting and twisting in the cylinders.
The combustor starters were installed in some aircraft that went to places where it was likely they might not have ground pneumatics. They stored very high pressure on the airplane. In the DC-8 it was a hollow chamber in each of the main gear struts and on the B707 they had flasks. For a combustor start the HP air was bled through a reducer and fuel was added plus ignition and this hot mass of air was fed to the starter on #3 engine. All very noisy but it was one shot and if the air was a little low or the engine a bit reluctant to start you got a hung start and a very nasty situation indeed with no means to motor it to cool things down. A couple of our customer airlines had them and would exercise them in New York but with ground pneumatics at the ready.
Training is Bottom Line
The next time the unit was called on to perform it would not start much to the amazement of the operators who I hasten to say worked for the ground handling company at the airport and were not mechanics. After some weeks spent in the GSE shop undergoing major repairs including engine valve replacements they reappeared but this time had a time delay built into the stop cycle. When you turned it off, it idled for about five minutes. These five minutes was precious time to the valves. It was also enough time for the rampie to go inside and get a cup of coffee and not spend time with some stupid engine that wouldn’t stop when the switch was turned off. Not to worry, the Emergency Shutdown Knob overrode the stupid time delay system and they wouldn’t have put it on the unit if it weren’t meant to be used. A meaty fist to the red knob and it shut down. Snap crackle and pop indeed. It just proves that nothing substitutes for training so the users understand the “why” of a procedure and the importance of getting the work force on board so they are not hostile to the company’s interests.