Pressure Inside An Aircraft- Role of Bleed Switch 

For those who have travelled by airplane, the demonstration of security arrangements inside an aircraft and display of oxygen mask and other security measures by flight attendants are familiar. But, even frequent passengers might have experienced pressure drop and use of oxygen mask  very rarely. This is because the pressure inside an aircraft and oxygen supply are properly regulated through a mechanism called bleed switch which is part of the pressure maintenance system of an aircraft. 

What is bleed switch? What is Barotrauma?

Bleed switch is a control switch used in airplane to maintain proper supply of oxygen inside the aircraft. The bleeding switch  is switched on to release air from the engine to maintain proper level of oxygen, pressure and air conditioning inside the cabin.  In case of failure to switch on bleed switch, cabin pressure drips and actual bleeding from the noses and ears of passengers occur.  The injury caused by a change in air pressure and consequent bleeding from the ear as well as the nose is termed  Barotrauma. 

Is level of oxygen in air less at high altitude? 

The level of oxygen does not decrease as the height (altitude) increases.  The percentage of oxygen in air is the same at both high altitude and sea level.  However, as altitude increases, the pressure dwindles thereby leading to less oxygen in the air in a given breath. To ensure smooth breathing, cabin of an aircraft is pressurized as it moves upward to maintain the pressure as at a lower altitude. As the airplane descends, it is gradually de-pressurised.  

The hot and high pressure air from the engine of an airplane is known as bleed air. To maintain the pressure level inside the cabin, this bleed air is cooled in a multi-step process, and mixed with air that is already present in the cabin.  There are many pressure sensors inside an airplane.  These sensors continuously monitor the pressure inside the cabin and hold the incoming air by opening and closing an outflow valve. This valve releases the incoming air at a rate regulated by pressure sensors.  This function in an airplane happens only if the bleed switch is turned on. 

What is the impact of air pressure drop inside an aircraft?

If the air pressure inside the cabin goes down, passengers find it difficult to breath as oxygen intake lowers.  A sudden drop in in pressure can lead to release of excess nitrogen from the blood stream as bubbles causing joint pain, paralysis and even death.   The normal causes for pressure drop are technical faults with the pressurization system, cracks in windows or body of aircraft, improper sealing of doors and breaches in aircraft security measures. Normally, when the pressure inside an aircraft drops, oxygen masks come out for the rescue of passengers.   

Why is bleed switch and Jet Airways  in news?

The bleed switch caught attention as the  Jaipur bound  Jet Airways flight 9W 697 had to return to Mumbai shortly after the take-off. 30 passengers,  of the 166 passengers had to be given first aid at Mumbai airport immediately after landing.  Initial reports states that the mishap occurred as the pilots forgot to switch on bleed switch.  

Normally, when a flight with full capacity takes off, a trained pilot switches off the bleed switch to ensure full power of engine for take off. Immediately after take off, the pilot turn on bleed switch ensuring flow of bleed air to cabin. In this incident, it appears that the pilots started with bleed switch off procedure, but forgot to switch on after take off. 
 

Devices used by pilots to control an airplane.

Why is the gap between jet aircraft and contrails?