While travelling on different airplanes of various airlines across the globe, many of us may have noticed this phenomenon, before departure and before engine start, the cabin becomes quiet.
If you have ever experienced this, have you ever wondered why? If yes, then we are going to cover this in today’s blog, If not, then do try and notice it on your next flight. I observed this phenomena when I travelled for the first time on an Airbus A320, Indigo flight. I found this rather intriguing and on further investigation this is what I learnt.
The air-conditioning packs are turned-off before accelerating the engines to take-off power, they are also momentarily turned off before engine start, that is why the cabin becomes somewhat quiet before start-up and take-off. Actually, what happens is that the air for the air-conditioning packs comes from the engines, modern turbofan engines bleed-off compressed air from the compressor stages for cabin pressurisation and for anti-ice systems.
The air conditioning packs are switched off so that, engines can get the full take-off power.
Let’s see how it works….
Bleeding off more air from the compressor stages means less mass of air available for combustion and for producing thrust. Switching off the ACs cuts off the flow of bleed air from the engines, this increases the mass and amount of air available for producing the necessary take-off thrust.
The air conditioning packs are again turned on at around 500-1,000 ft. above ground level (AGL), and one must always remember to turn them on, If pilots forget to turn it on, it can lead to gradual cabin depressurisation and discomfort to passengers. This is included in the after-take-off checklists for the pilots.
Did you know???
Some airlines don’t completely switch off the air conditioning packs. They just turn it to AUTO position. This doesn’t cut-off the flow of bleed air completely, but reduces the flow of the air to its minimum. After take-off, and after reaching around 500-1,000 ft. it automatically increases the flow to normal.