An aircraft cabin PHOTO:Google
The sudden or gradual loss of pressure at high altitude is not unusual in aviation, though not a normal occurrence. Experts explain the phenomenon, why passengers should panic less and operators cum regulators should be more responsive in handling related matters. WOLE OYEBADE reports.
Frequent fliers would have heard and seen it displayed dozens of times just before take-off: “Should the cabin experience sudden pressure loss, oxygen masks will drop down from above your seat. Place the mask over your mouth and nose…” Perhaps for neophytes that are often awed by such eerie warnings, not too many frequent travellers pay attention. Fact is that no one looks forward to any emergency or seeing the oxygen masks drop down. Most Nigerians would rather reject it prayerfully.And so it was on Air Peace’s Enugu-bound airplane a couple of weeks ago. At high altitude, the cabin lost pressure, oxygen mask dropped and passengers went into a frenzy of panic.
Video clips making the rounds on social media showed an emotional charged cabin, with some passengers singing praises to God “who saved them from air crash”. But did that really happen?
Fact is that all human being needs a good amount of oxygen to survive and any venture above the sea level puts the oxygen supply and human survival at risk.On account of adventure, modernity and inventions, man invented flying machines that need higher altitude (far above 10,000 feet or 3,000m) to perform.
So, pressurisation or alternative supply of oxygen of sea level’s equivalence becomes a necessity in the airplane cabin at high altitude. This dated back to 1921 and continues till date. Pressurisation aims to avert physiological problems, altitude sickness and related deaths on account of short or nil supply of oxygen flow.
According to experts, aircraft cabins are pressurised using cooled and filtered air bled from the engines, keeping the air pressure inside the cabin at the equivalent of an altitude of 8,000 feet. Boeing’s Dreamliner technology has lowered that to 6,000 feet, making the cabin atmosphere more pleasant – even though commercial aircraft often fly at 40,000 feet.
George C. Larson of Air & Space magazine explained that turbofan engines compress intake air with a series of vanned rotors right behind the fan. At each stage of compression, the air gets hotter, and at the point where the heat and pressure are highest, some air is diverted. Some of the hot, high-pressure air, called bleed air, is sent to de-ice wings and other surfaces, some goes to systems operated by air pressure, and some starts its journey to the cabin.
Larson, who is also a pilot, added that to maintain a comfortable temperature for the passengers, automatic systems regulate the mixture of heat from the engines and cold from the air packs. To maintain the pressure in the cabin equal to that at low altitude, even while the airplane is at 30,000 feet, the incoming air is held within the cabin by opening and closing an outflow valve, which releases the incoming air at a rate regulated by pressure sensors. Think of a pressurized cabin as a balloon that has a leak but is being inflated continuously.
He stressed that pressurisation becomes increasingly necessary at altitudes above 10,000 feet (3,000 m) above sea level to protect crew and passengers from the risk of a number of physiological problems caused by the low outside air pressure above that altitude. For private aircraft operating in the U.S. for instance, crew members are required to use oxygen masks if the cabin altitude stays above 12,500 ft for more than 30 minutes, or if the cabin altitude reaches 14,000 ft at any time. At altitudes above 15,000 ft, passengers are required to be provided oxygen masks as well.
On commercial aircraft, the cabin altitude must be maintained at 8,000 ft or less. Pressurisation of the cargo hold is also required to prevent damage to pressure-sensitive goods that might leak, expand, burst or be crushed on re-pressurization. The principal physiological problems are listed below.
Cabin loss of pressure do happen, but not normal
According to experts, unplanned loss of cabin pressure, be it sudden or gradual, is rare and not normal. Failures range from sudden, catastrophic loss of airframe integrity (i.e. explosive decompression) to slow leaks or equipment malfunctions that allow cabin pressure to drop.
Cabin loss of pressure rarely results to plane crash, except in one major case. Findings revealed that in 2005, a Helios Airways plane en route Cyprus to Athens crashed into a mountain after a loss of cabin pressure, killing all 115 passengers and six crew on board.
The official investigation into the incident found that the pressurisation system had been left in manual gear, causing cabin pressure to drop and the pilots to fall unconscious as they suffered hypoxia. Oxygen supplies for passengers ran out after 15 minutes, and when the plane, running on autopilot, ran out of fuel, it crashed 33km from Athens airport.
Other cases on record showed that the airplanes landed at their destinations safely with oxygen masks dangling over calmed passengers.However, any failure of cabin pressurisation above 10,000 feet (3,000 m) requires an emergency descent to 8,000 feet (2,400 m) or the closest to that while maintaining the Minimum Safe Altitude (MSA), and the deployment of an oxygen mask for each seat. The oxygen systems have sufficient oxygen for all on board and give the pilots adequate time to descend to below 8,000 ft (2,400 m).
Corporate Communications Manager of Air Peace, Chris Iwarah, explained that the Enugu bound plane experienced a change in cabin pressure, “the oxygen masks on the aircraft automatically deployed and in line with standard safety procedure, our flight crew performed a technical manoeuvre by descending to a level comfortable enough for passengers on board.”
Iwarah added that “We, however, observed cases of some passengers who did not properly fit their oxygen masks. A few others, perhaps out of panic, pulled too hard on the oxygen masks, causing the equipment to snap. This led to the unfortunate claims that there was no flow of oxygen from the masks. For the avoidance of doubt, the oxygen generators are part of the items of interest during C-checks. So, it could not have been right that there was no flow of oxygen.
“While it is quite expected and understandable that passengers would panic on occurrence of incidents such as a change in cabin pressure, the claims of a near-crash were a regrettable misrepresentation of facts which really should not be. The aircraft was ferried from Enugu back to Lagos in less than an hour of safely landing at destination.”
Indeed, small air leaks would likely cause a slow loss of pressurisation, in which case the pilot would have time to make an emergency bumpy descent to a safe altitude, of between 8-10,000 ft.In 2007, Airbus issued a “cabin decompression awareness” note that said that at 40,000 ft, people have as little as 18 seconds of “useful consciousness” if they are starved of oxygen. It stressed the risks of hypoxia – oxygen starvation – are all the greater as people may not realise they are suffering until they can no longer breathe and fall unconscious.
Signs of sudden decompression include “a loud bang, thump or clap” as air inside and outside the plane meet, debris flying around the cabin, and “unsecured items in the immediate area” near the breach being “ejected” from the aircraft.
Oxygen masks ensure safety, use it!
A pilot and author of Cockpit Confidential, Patrick Smith, said: “Crashes or fatalities from pressure problems are extremely uncommon, even with a fairly rapid decompression brought on by a hole or puncture.“If cabin pressure falls below a certain threshold, the masks will deploy from the ceiling, exposing everybody to the so-called ‘rubber jungle’. Should you be confronted by this spectacle, strap your mask on and try to relax. The plane will be at a safe altitude shortly, and there are several minutes of backup oxygen for everybody.”
Smith added that up front, the pilots will don their own masks and commence a rapid descent to an altitude no higher than 10,000 feet. “If the emergency descent feels perilously fast, this isn’t because the plane is crashing: it’s because the crew is doing what it’s supposed to do.”Industry expert and consultant, Chris Aligbe, said depressurisation happens in the industry and where the reasons are known, there is no too much hullaballoo about such incidences.
But in our local environment, where knowledge is limited coupled with poor handling of issues, then it becomes a serious problem, Aligbe said.“Poor handling of such incidences says a lot. We were somewhere in a local airline and all the oxygen masks suddenly dropped. I was with Captain Nogie Meggison (the Chairman of Airlines Operators of Nigeria) and all he could say was that we were returning to base. People panicked a little and they started talking to them to calm down. The Captain immediately came on and explained the issues and we were returned to base.
Ojikutu said while some commentators kept describing the incident as common, “it does not make it normal.“So, NCAA should be alive to its responsibility. For me this is where the Accident Investigation Bureau (AIB) not NCAA’s investigation should start; to find out if inspections of this critical safety elements are regularly done by both the operators and oversight regulator,” he said.
But until NCAA disclosed the report of its findings, in the case of Air Peace airplane’s recent loss of cabin pressure, what really happened may still not be known. What is as clear as the daylight is that all stakeholders – operators, regulators and passengers – all have roles to play for safer operations without incidences or fatalities.
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