Use GTC to operate packs B737
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Use GTC to operate packs B737
I have question, can I use GTC to operate the packs B737NG? Is this what supplement procedure "external air cart" is talking about?
Last week my ground crew said I can only use GTC to start the engine, not to operate the packs. Is this correct and why?
Thanks for info..
Last week my ground crew said I can only use GTC to start the engine, not to operate the packs. Is this correct and why?
Thanks for info..
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In the Hercules world GTC would mean Gas Turbine Compressor. Small gas turbine engine that produces nothing but compressed air, and noise. A lot of noise. Compressed air is used for engine starting, air conditioning in the ground, and spinning an Generator via an air turbine motor. No idea what it would mean in the context of a 737.
Forgot to add: the GTC I'm talking about is a part of the airplane, vs the ground support cart plhought is describing. Although the turbine is essentially the same Garrett turbine in the carts.
Forgot to add: the GTC I'm talking about is a part of the airplane, vs the ground support cart plhought is describing. Although the turbine is essentially the same Garrett turbine in the carts.
Last edited by A Squared; 7th Aug 2016 at 17:15.
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There's two kinds of air-start carts used nowadays:
The less prevalent (nowadays) are actually just turbine APU's on carts basically (called "huffer carts" in my part of the world). Examples of these are units like theA/M32-60's which are basically Garret GTCP85's mounted on a cart.
https://www.youtube.com/watch?v=CUnHAv123ys - I was repairing and reconditioning these units for civil use before I got into aircraft maintenance.
They put out just as much air as most aircraft APU's and would have no problem running the packs on a 737 for example. Running both packs may push it a bit, but it's capable depending on age & cycles on the cart. Your operator, if they are using carts like these, may prohibit operation to extend the life of the unit (just like your APU in the airplane, it runs hotter with bleed extraction/load). Also - as these units aren't typically maintained to 'aviation-standard' - they may want to reduce the risk of contaminating the whole air conditioning system if the unit should fail.
The other type of air-start carts (and more prevalent now than the above units) are traditional gas or electric (or hybrid on newer units) air compressors which charge a large tank. The individual compressor is a very hefty unit, but incapable of producing a sustained 45-50PSI at the flow required to start an engine on it's own. The compressor simply charges the large tank. The pressurized tank provides the pressure & flow required for usually around 60ish seconds, enough to do a start only. Obviously, if you tried to utilize the packs with these units, you'd quickly discharge the tank. These units are more reliable, cheaper to run, and a heck of a lot quieter (for those fancy European airports with their draconian noise limitations
).
System's wise, there's nothing preventing you from using external air (different then external conditioned air) to run the packs. It's teed-in right into the bleed system. The ECS doesn't really care where the bleed air is coming from. There is a discrete going to the APU ECU to build up the pressure when packs are selected on, or engine start is selected (as the NG APU uses a separate load compressor with variable vanes yadda yadda yadda...), but other then that - if there's duct pressure showing on the gauge, it'll run the AC packs.
The less prevalent (nowadays) are actually just turbine APU's on carts basically (called "huffer carts" in my part of the world). Examples of these are units like theA/M32-60's which are basically Garret GTCP85's mounted on a cart.
https://www.youtube.com/watch?v=CUnHAv123ys - I was repairing and reconditioning these units for civil use before I got into aircraft maintenance.
They put out just as much air as most aircraft APU's and would have no problem running the packs on a 737 for example. Running both packs may push it a bit, but it's capable depending on age & cycles on the cart. Your operator, if they are using carts like these, may prohibit operation to extend the life of the unit (just like your APU in the airplane, it runs hotter with bleed extraction/load). Also - as these units aren't typically maintained to 'aviation-standard' - they may want to reduce the risk of contaminating the whole air conditioning system if the unit should fail.
The other type of air-start carts (and more prevalent now than the above units) are traditional gas or electric (or hybrid on newer units) air compressors which charge a large tank. The individual compressor is a very hefty unit, but incapable of producing a sustained 45-50PSI at the flow required to start an engine on it's own. The compressor simply charges the large tank. The pressurized tank provides the pressure & flow required for usually around 60ish seconds, enough to do a start only. Obviously, if you tried to utilize the packs with these units, you'd quickly discharge the tank. These units are more reliable, cheaper to run, and a heck of a lot quieter (for those fancy European airports with their draconian noise limitations
).System's wise, there's nothing preventing you from using external air (different then external conditioned air) to run the packs. It's teed-in right into the bleed system. The ECS doesn't really care where the bleed air is coming from. There is a discrete going to the APU ECU to build up the pressure when packs are selected on, or engine start is selected (as the NG APU uses a separate load compressor with variable vanes yadda yadda yadda...), but other then that - if there's duct pressure showing on the gauge, it'll run the AC packs.
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Dear plhought,
Thanks for the extended explanatian. This exactly what I mean, normally indeed we use ground conditioned air, but unfortunately on arrival this cart was unservicable.
I requested to use the GTC( indeed very loud but effective) to operate the packs. So I keep the aircraft cool as the oat was 33 degree celsius.
Thanks for the extended explanatian. This exactly what I mean, normally indeed we use ground conditioned air, but unfortunately on arrival this cart was unservicable.
I requested to use the GTC( indeed very loud but effective) to operate the packs. So I keep the aircraft cool as the oat was 33 degree celsius.
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This reminds me of watching ground crew in 1980's Central America, having managed to somehow plumb an umbilical between a 727 cargo bird and a 737 Jurassic, proceed to attempt a start on the 3-holer with air from the 37. It worked. I recall more than a few rolls of speed tape and chocks for 'hose ballast & safety' being part of the 'rustic' process.
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In the "other" kind of pneumatic start carts, yes - there is a risk of contamination from the compressor. But, as explained above, those units will not provide sufficient air long enough to run the packs anyways.
There's a greater risk of contamination from a bung AC pack or exhaust from nearby ground support equipment being drawn into the buffet cart intake.
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99% of the turbine-powered external air carts are the same as any aircraft APU or engine. Air is bled off just like an engine. In the common Garret GTCP series, the lubrication system is a pretty simple two bearing system with little risk of oil contamination, especially because of the design of the annular compressor. The hot bleed air is really no different than bleed air off the aircraft's power plant.
