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 air conditioning and pressurizing system depends on few factors. If they are fulfilled, yes we should turn them off.

As we all know pressurizing and air conditioning of an aircraft is done by the air taken from the engines, APU or from a ground cart (if the aircraft is on ground). During the flight needed air is bled out from the engines or APU to feed PACS. But in some aircraft like MD-80, DC-9, B727 and B747 it's not possible to tap bleed air from the APU. Hence engines are the sole supplier for PACS.

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

Normally bleed air is taken out from both Low Pressure and High Pressure compressors. Due to this tapping of air, the mass of air that passes through the engine is reduced, hence we might except a reduction in thrust.

Which conditions decide the status of PACS?

If the aircraft is taking-off with a heavy load, it has to go for a higher lift-off speed compared to low weight take-offs. Addition to that, the field elevation of the airport too plays a role, at high altitudes air become less dense and the engines should deliver at their maximums. Runway conditions and the outside atmospheric conditions are also key factors which decide the status of the PACS. Aircraft taking-off from shorter runways need higher accelerations to be airborne with the available runway. As air become less dense in hotter days and in higher elevations engines should operate at their maximum power to deliver adequate amount of thrust.

In all the conditions described above, an engine should deliver it's maximum thrust to be airborne. Hence tapping bleed air for air conditioning and pressurizing should be terminated during the above conditions.

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