Friday, December 9, 2016

Should We Turn Off PACS During Take-Off?

For sure many of you have this problem in your minds. Do we want to turn off aircraft pressurizing and air conditioning(PACS) system during take-off?

The answer might be YES or NO depending on the situation. To be more precise turning off PACS depends on few factors. If they are fulfilled, yes we should turn off the PACKS.

As we all know pressurizing and air conditioning in aircraft is done by the air taken from the engines, APU or from a ground cart(if the aircraft is on ground). In flight needed air is bleed out from the engines or APU to feed PACS. In some aircrafts like MD-80, DC-9, B727 and B747 using APU for the supply of bleed is something that can't be done. So engines are the sole supplier for the PACS.

In order to find an answer for this let's turn to basics of thrust production. How the thrust is produced in a turbo fan engine? Newtons laws of motion!! In order to generate more thrust from the engine it should accelerate an air mass to a certain velocity. If this air mass and acceleration velocity increases the engine is capable of producing more thrust.

Assume during the acceleration process some air mass is taken out from the engine, what a waste??
Normally bleed air is taken out from the both Low Pressure and High Pressure compressors. We know that due to the tapping of bleed air from the engine it is not capable of giving out its maximum thrust.

Which conditions decide whether PACS ON or OFF?
If our aircraft is taking-off with a heavy load we want to reach a lifting velocity slightly greater than normal value, when the runway is short, when the airport altitude is high and in hot days we try to extract maximum from or engines. For that engine bleed should be not be taken out. Pilots simply turn off PACS and bring them alive just after being airborne. Cabin remains un-pressurized for a slight period and passengers might feel or not feel the difference.

As a solution for this, APU can be selected for supplying bleed air for PACS and then engines can be isolated for sole thrust production. But of de-icing and anti-icing has carried out APU bleed should not use to feed the PACS because de-icing and anti-icing fluids might enter to the APU intake through the flap and might make cabin toxic or uncomfortable for the passengers.

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