METEOROLOGICAL CONDITIONS / OXYGEN SUPPLY (Tayfun KAYIRAN)

METEOROLOGICAL CONDITIONS

In aviation, visual meteorological conditions (or VMC) is an aviation flight category in which visual flight rules (VFR) flight is permitted—that is, conditions in which pilots have sufficient visibility to fly the aircraft maintaining visual separation from terrain and other aircraft. They are the opposite of instrument meteorological conditions (IMC). The boundary criteria between IMC and VMC are known as the VMC minima and are defined by: visibility, cloud ceilings (for takeoffs and landings), and cloud clearances.

The exact requirements vary by type of airspace, whether it is day or night (for countries that permit night VFR), and from country to country. Typical visibility requirements vary from one statute mile to five statute miles (many countries define these in metric units as 1,500 m to 8 km).

Typical cloud clearance requirements vary from merely remaining clear of clouds to remaining at least one mile away (1,500 m in some countries) from clouds horizontally and 1,000 feet away from clouds vertically. For instance, in Australia, VMC minima outside controlled airspace are clear of cloud with 5,000 m visibility below 3,000 ft AMSL or 1,000 ft AGL (whichever is higher), and 1,000 ft vertical/1,500 m horizontal separation from cloud above these altitudes or in controlled airspace. Above 10,000 ft, 8,000 m visibility is required to maintain VMC. Air traffic control may also issue a "special VFR" clearance to VFR aircraft, to allow departure from a control zone in less than VMC – this reduces the visibility minimum to 1,600 m.

Generally, VMC requires greater visibility and cloud clearance in controlled airspace than in uncontrolled airspace. In uncontrolled airspace there is less risk of a VFR aircraft colliding with an instrument flight rules (IFR) aircraft emerging from a cloud, so aircraft are permitted to fly closer to clouds. An exception to this rule is class B airspace, in which ATC separates VFR traffic from all other traffic (VFR or IFR), which is why in class B airspace lower cloud clearance is permitted.

ICAO recommends the VMC minima internationally; they are defined in national regulations, which rarely significantly vary from ICAO. The main variation is in the units of measurement as different states use different units of measurement in aviation. The minima tend to be stricter in controlled airspace, where there is a lot of traffic therefore greater visibility and cloud clearance is desirable. The degree of separation provided by air traffic control is also a factor. For example, in class A and B airspace where all aircraft are provided with positive separation, the VMC minima feature visibility limits only, whereas in classes C–G airspace where some or all aircraft are not separated from each other by air traffic control, the VMC minima also feature cloud separation criteria.

It is important not to confuse IMC with IFR (instrument flight rules) – IMC describes the actual weather conditions, while IFR describes the rules under which the aircraft is flying. Aircraft can (and often do) fly IFR in clear weather, for operational reasons or when flying in airspace where flight under VFR is not permitted; indeed by far the majority of commercial flights are operated solely under IFR.

It is possible to be flying VFR in conditions that are legally considered VMC and have to rely on flight instruments for attitude control because there is no distinct external horizon, for example, on a dark night over water (which may create a so-called black hole effect) or a clear night with lights on the water and stars in the sky looking the same

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OXYGEN SUPPLY

Aircraft emergency oxygen systems are emergency equipment fitted to pressurized commercial aircraft, intended for use when thecabin pressurisation system has failed and the cabin altitude has climbed above a safe level. It consists of a number of individual oxygen masks stored in compartments near passenger seats and near areas like lavatories and galleys, and an oxygen source, like a centralized gaseous cylinder or decentralized chemical oxygen generator.

Most commercial aircraft that operate at high flight altitudes are pressurized at a maximum cabin altitude of approximately 8,000 feet, where it is possible to breathe normally without an oxygen mask. On most pressurized aircraft, if cabin pressure is lost when the cabin altitude is above 14,000 feet, compartments containing the oxygen masks will open automatically, either above or in front of the passenger and crew seats, and the oxygen masks will drop down in front of the passenger. Oxygen masks may also drop on extremely rough landings or during severe turbulence if the oxygen mask panel becomes loose. Rows of seats typically have an extra mask (i.e. 3 seats, 4 masks), in case someone has an infant in their lap, or someone in the aisle needs to grab one.

An oxygen mask consists of a yellow, soft, silicone facial cup with white elastic bands for securing the mask to the passenger's face. This band is adjustable by pulling two ends looped through the facial cup. The mask may also have a concentrator or re-breather bag that may or may not inflate depending on the cabin altitude, which has (in some instances) made passengers nervous the mask was not providing adequate oxygen, causing some to remove them, who thereby suffered hypoxia. All airlines now make a point in the safety video or demonstration to point out that the bag may not inflate. The bag is attached to a tube, connected to the oxygen source in the compartment, allowing for it to drop down and hang in front of the passengers. To operate on all aircraft except the L-1011, they must be pulled sharply toward the passenger who needs it to un-clip the flow pin and start the process of transporting the oxygen to the passenger. Passenger oxygen masks cannot deliver enough oxygen for sustained periods at high altitudes. This is why the flight crew needs to place the aircraft in a controlled emergency descent to a lower altitude where it is possible to breathe without emergency oxygen. While the masks are being used, passengers are not allowed to leave their seat for any reason until it is safe to breathe without the emergency oxygen. If there is a fire on board the aircraft, masks are not deployed, as the production of oxygen may further fuel the fire.

Aircraft safety cards and in-flight safety demonstrations shown at the beginning of each flight explain the location and use of oxygen masks.

Some aircraft, such as the SAAB Series Aircraft and the 1900D, have a mask system where either a mask is stored under the seat or is distributed by the cabin attendant. These masks are removed from packaging and plugged into the socket for oxygen supply.

·         A chemical oxygen generator system connected to all masks in the compartment. Pulling down on one oxygen mask removes the firing pin of the generator igniting a mixture of sodium chlorate and iron powder, opening the oxygen supply for all the masks in the compartment. Oxygen production cannot be shut off once a mask is pulled, and oxygen production typically lasts at least 15 minutes. During the production of oxygen, the generator becomes extremely hot and should not be touched. A burning smell may be noted and cause alarm among passengers, but this smell is a normal part of the chemical reaction. This system can be found on the MD-80 aircraft, whose system is also unique in the fact that the face masks are clipped to the inside of the compartment door and do not drop out and hang, by the oxygen tube, in front of the passengers. "For any aircraft which carries more than a very few passengers, the weight, complexity and maintenance issues associated with a compressed gas system would be prohibitive. As a consequence, the industry relies on chemical oxygen generators."^[1]

·         A gaseous manifold system, which connects all oxygen masks to a central oxygen supply, usually in the cargo hold area. Pulling down on one oxygen mask starts the oxygen supply for that mask only. The entire system can usually be reset in the cockpit or in some other location in the aircraft.

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                                            Tayfun KAYIRAN

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