A Jet Airways flight on a Boeing 737 en route to Jaipur from Mumbai had to turn back Thursday morning, shortly after takeoff. The reason: an error in maintaining the cabin air pressure beyond 10,000 ft, due to which oxygen masks were deployed and some passengers began bleeding from the nose and ears. A look at the reasons, and norms, for maintaining cabin air pressure at higher altitudes:
Altitude & pressure
In the early days of aviation, aircraft only flew at lower altitudes. In the late 1930s, Boeing introduced the 307 Stratoliner as the first commercial airliner with a pressurised cabin that could fly up to 20,000 ft while maintaining “cabin altitude” below 10,000 ft, which means that conditions in the cabin would match those that would have prevailed if the aircraft had been flying at 10,000 ft.
When at higher altitudes, it is important to maintain certain parameters within the cabin for passenger comfort. These include temperature, humidity, air circulation and cabin pressure. Most modern jets fly at altitudes between 25,000 and 51,000 ft. These have systems in place to ensure the cabin is pressurised enough to correspond with a flight altitude between 6,500 and 8,000 ft.
To pressurise the cabin, ambient air is introduced into a compressor inside the aircraft’s engine, and heated up rapidly. This air, set aside for pressurisation and air-conditioning, is called “bleed air”. The heated air is sent to a cooling unit, then sent into the cabin. An overflow valve towards the rear of the aircraft ensures that air enters the cabin faster than it leaves, creating a pressurised ecosystem. Under takeoff procedures, the bleed air is sometimes turned off when the plane is carrying a payload above a limit. This ensures that the engine does not leak the air and uses all of it to create enough thrust for a successful takeoff. In such situations, the bleed is switched on after takeoff. Until then, air-conditioning is maintained by the auxiliary power unit — which, in Boeing 737s, works until 17,000 feet and then trips. The pilot needs to ensure that after takeoff, bleed is switched on to maintain cabin pressurisation.
The human body cannot endure an environment above certain altitudes and would start reacting above 10,000 ft. At of 18,000 ft, for example, passengers would experience a pressure of 7.3 pounds per square inch, and fall unconscious within 30 minutes. If cabin altitude is not maintained below 10,000 ft, it would lead to various effects. Gases in the middle-ear, sinuses and digestive tracts would start to expand, causing bleeding from the ears and nose. As the altitude rises, temperature and oxygen levels decrease. This means a risk of frostbite, hypothermia and deficiency of oxygen in the blood, leading to hypoxia.
In a 2005 flight of the Cypriot airline Helios Airways, loss of cabin pressurisation led to hypoxia in the crew. The crew failed to recognise that the pressurisation switch was set to manual. After the plane crossed 30,000 ft, most crew and passengers were unconscious. Eventually the aircraft depleted its fuel and crashed in Greece, killing all 121 on board.
In 2008, a Ryanair flight from Bristol to Barcelona-Girona experienced a drop in cabin pressure, leading to an emergency descent and hospitalisation of 16 passengers. In 2017, an AirAsia flight from Australia to Indonesia was diverted to Perth because of low pressure.
Checks & balances
Among safety measures used by manufacturers and airlines, one involves a checklist of tasks for crew. For a Boeing 737, for normal circumstances, there are 12 steps for pilots at various stages — pre-flight, before engine start, after engine start, before taxi, before takeoff, after takeoff, descent, approach, landing, after landing, engine shutdown and aircraft secure. Airlines sometimes add more tasks to these basic checklists.
The Jet Airways flight descended from 11,000 ft. Bleed air had not been turned on after takeoff, a fact attributed, prima facie, to the pilot’s forgetfulness. In such cases, another possible cause is a failure of instruments to act according to instructions from the crew. Civil Aviation Minister Suresh Prabhu has ordered a complete safety audit of the sector.
Inputs: Tabassum Barnagarwala & Neha Kulkarni in Mumbai